ecv211

ECV 211:

Course Outline

Electromechanical Engineering: ECV 211

Introduction The objective of this course is assist the civil engineering student toprovide for electrical and mechanical services

Expected Outcomes At the end of the course the student should be able to:

Understand current, voltage and resistance. Understand alternative and direct current. Understand single phase and three phase alternating current. Carry out electrical installation. Design for fire protection. Design for mechanical systems e.g. pumps lifts etcCourse Structure

Lectures: 2 hours/week (Mondays 1100hrs to 1300 hrs) Tutorial: 1 hour/week (Thursdays 0900hrs to 1000 hrs) Practical: 3 hours/week (Wednesdays)

CATs:

Week 6 Week 9 Week 12

Assignments:Three Assignments, each to be handed in two weeks after date issued.Assessment:

Continuous assessment: Tests- 10%

Assignments- 5%

Practicals- 15%

Examination- 70%

L.A. KAdoyo, 2013 i

ECV 211:

Course DescriptionCurrent, voltage and resistance, Ohms law. Electric power. Joules law. Power

sources. Series and parallel circuits. Kirchhoffs laws. Direct Current (D.C.) circuitanalysis.

Alternating voltage and current. Impedance and admittance. Single phase A.C.circuits. Three-phase alternating current.

Electrical measurements and measuring instruments.Electrical installation; wiring facilities and ducting.Fire protection: means of escape, fire regulations, grading, resistance, fire detection

and fighting facilities.Refuse disposal systems: Chutes, incinerator, garchey systems, macerator equipment.

Solid waste and soil drainage disposal.Mechanical systems, pumps sizes and location. Provision of specialized services such

as lifts, excavators and ventilation, air conditioning and refrigeration.Practical Work/Laboratory ExercisesEach student is required to conduct the four experiments listed below. The practical

work/laboratory exercises are to cover the following topics

(a) Speed control of different types of electrical machines

(b) Principles of voltage stepping down

(c) Design and draw an electrical installation for a small building

(d) Carry out electrical installation in buildings

Literature

1. Electrical and Electronic Technology, Edward Hughes, Publisher: Pearson PrenticeHall; 10th edition (June 2008) ISBN-10: 0132060116

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Chapter 1

BASIC ELECTRIC PRINCIPLES

1.1 Definitions

1. Current: The amount of charge(Q) flowing in a circuit. Is measured in Amperes.An Ampere is defined as one Coulomb of electrons (1 Coulomb = 6.25 1018electrons) passing through a given point in a circuit in 1 second.

2. Voltage: A measure of specific potential energy between two locations. Voltageis always relative between two locations. Voltage is measured in volts. The voltagebetween two points is one volt if it requires one joule of energy to move one coulombof electrons from one point to the other. In equation form, the voltage, V, in volts,is found as:

V =W

Q(1.1)

where W is the energy in joules and Q is the charge in coulombs.

3. Resistance: Is the opposition to flow of current. Is measured in Ohms.

4. Short Circuit: A circuit offering little or no resistance to the flow of electrons.Short circuits are dangerous with high voltage power sources because the highcurrents encountered can cause large amounts of heat energy to be dissipated.

5. Open Circuit: A circuit whose continuity has been broken by an interruption inthe path for electrons to flow.

6. Closed Circuit: A complete circuit with good continuity throughout.

1.2 Ohms Law

Gives the relationship between current, voltage and resistance. Ohms law is stated as:The potential difference across the ends of a conductor is proportional to

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ECV 211: CHAPTER 1. BASIC ELECTRIC PRINCIPLES

the current flowing through the conductor, provided the temperature is heldconstant. This is expressed as:

V I (1.2)The value relating voltage and current is known as the resistance of the conductor, thus

I =V

R

orV = IR

and

R =V

I

1.3 Electric Power

Power is a measure of how much work is done in a given amount of time. In electriccircuits, power is obtained as a product of voltage and current, such that:

P = IV (1.3)

The unit of electric power is the Watt.The relationship between power and resistance is related using Joules Law, and

may be derived from the Ohmic relationship between voltage and resistance. Thus,

P = I2R (1.4)

Electrical Energy is a measure of the work done by the electric power, and is aproduct of power and time.

The unit of electrical energy is the Joule or the watt second. For practical purposes,power companies measure energy in Kilowatt hours (kWh).

1.3.1 Power Sources

A source of electrical energy can be represented by a source of e.m.f in series with aninternal resistance. It can also be represented by a source of current in parallel with aninternal resistance.

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A constant voltage source supplies the same voltage to all loads, regardless of thecurrent flowing through the load. An ideal constant voltage source is one with zerointernal resistance.

A constant current source supplies the same current to all loads, regardless of thepotential difference across the load. An ideal constant current source is one with infiniteinternal resistance.

1.4 Series and Parallel Circuits

In a series connection, components are connected end-to-end in a line, forming a singlepath for current to flow, as shown in Figure 1.1. Thus the current across all the com-ponents in a purely series circuit is common. The order of series components may bechanged without affecting the operation of the circuit.

Two elements are said to be connected in series if they are connectedat a single point and if there are no other current carrying connections atthat point .

Figure 1.1: Simple series connection

In a parallel connection, the components are connected across each others leads.This is as shown in Figure 1.2. In a purely parallel circuit, there are never more thantwo electrically common points, no matter how many components are connected. Thereare many paths for current to flow but only one voltage across all the components.

Figure 1.2: Simple parallel connection

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1.4.1 Properties of Simple Series Circuits

(i) Components in a series circuit share the same current, thus ITotal=I1 = I2 = I3. . . = In and so on for all components.

(ii) Total resistance in a series circuit is equal to the sum of the individual resistances,i.e. RTotal = R1 +R2 +R3 + ...+Rn

(iii) Total voltage in a series circuit is equal to the sum of the individual voltage drops:VTotal = V1 + V2 + V3 + . . . Vn

1.4.2 Voltage Divider Circuits

Series circuits are also referred to as voltage divider circuits because they effectivelydivide the total voltage into fractional portions of constant ratio, dependent on the valueof the components in the circuit.

For a voltage divider circuit:Voltage drop across any resistor is obtained as

Vn = InRn (1.5)

Current in the circuit is obtained as

ITotal =VTotalRTotal

(1.6)

Substituting the value of current from equation 1.6 into equation 1.5,

Vn =VTotalRTotal

Rn (1.7)

or

Vn = VTotalRn

RTotal(1.8)

This is known as the Voltage Divider Formula .

1.4.3 Properties of Simple Parallel Circuits

(i) Components in a parallel circuit share the same voltage, thus VTotal=V1 = V2 = V3. . . = Vn and so on for all components.

(ii) Total resistance in a parallel circuit is less than any of the individual resistances,i.e. RTotal = 1/(1/R1 + 1/R2 + 1/R3 + ...+ 1/Rn)

(iii) Total voltage in a series circuit is equal to the sum of the individual voltage drops:VTotal = V1 + V2 + V3 + . . . Vn

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1.4.4 Current Divider Circuits

Parallel circuits are also referred to as current divider circuits because they effectivelydivide the total current into fractional portions of constant ratio, dependent on the valueof the components in the circuit.

For a current divider circuit:Current through any resistor is obtained as

In =VnRn

(1.9)

Voltage in a parallel circuit is obtained as

VTotal = Vn = ITotalRTotal (1.10)

Substituting the value of voltage from equation 1.10 into equation 1.9,

In =ITotalRTotal

Rn(1.11)

or

In = ITotalRTotalRn

(1.12)

This is known as the Current Divider Formula .

1.5 Kirchhoffs Laws

1. Kirchhoffs Voltage Law states that the algebraic sum of all voltages in a loopmust equal zero.

2. Kirchhoffs Current Law states that the algebraic sum of all voltages enteringand exiting a node must equal zero.

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TUTORIAL 1

1. Rewrite the following as indicated:

(a) 1000 pF = . . . . . . nF

(b) 0.02 F = . . . . . . pF

(c) 5000 kHz = . . . . . . MHz

(d) 47 k = . . . . . .M

(e) 0.32 mA = . . . . . . A

2. (a) What quantity of charge is carried by 6.24 1021 electrons?(b) An initially neutral body has 1.7C of negative charge removed. Later, 1.87

1012 electrons are added. What is the bodys final charge?

(c) After 1.0611014 electrons are added to a metal plate, it has a negative chargeof 3C. What was its initial charge in coulombs?

(d) Body A has a negative charge of 0.2C and body B has a positive chargeof 0.37C. If 8.7 1013 electrons are transferred from A to B, what are thecharges in coulombs on A and on B after the transfer?

(e) A metal plate has 1.461014 electrons added. Later, 1.3C of charge is added.If the final charge on the plate is 5.6C, what was its initial charge?

3. (a) A current of 3A flows for 5 minutes. How many electrons are transferred pastany given point in the circuit?

(b) If 7.4881021 electrons pass through a point during a time interval of 2 minutes,what is the current?

(c) How long does it take 100C to pass through a point if the current is 25 mA?

(d) The charge passing through a wire is given by the equation

q = 10t+ 4

where q is in coulombs and t is in seconds.

(i) How much charge has passed at t = 5 seconds?

(ii) How much charge has passed at t = 8 seconds?

(iii) What is the current in Amperes?

(e) The charge passing through a wire is q = (80t + 20)C. What is the current?

4. (a) The voltage between two points is 19V. How much energy is required to move6.7 1019 electrons from one point to the other?

(b) The potential difference between two points is 140 mV. If 280J of work re-quired to move a charge, Q from one point to the other, what is Q?


(c) A circuit consists of a load connected to a 12V battery. The switch is closed fora short interval then opened. If the current, I = 6A, and the battery expends230,040 J moving charge through the circuit, how long was the switch closed?

(d) How much energy is gained by a charge of 0.5C as it moves through a potentialdifference of 8.5 kV?

(e) If the voltage between two points is 100 V, how much energy is required tomove an electron between the two points?

(f) If 1353.6 J are required to move 4.7 1020 electrons through a lamp in 1.3minutes, what are V and I?

5. (a) A lamp draws 25 mA when connected to a 6V battery. What is its resistance?

(b) The current through a 2M resistor is 0.15 mA. What is the voltage across it?

(c) How much voltage can be applied across a 560 resistor if the current throughit must not exceed 50 mA?

(d) A relay with a coil resistance of 240 requires a minimum of 50 mA to operate.What is the minimum voltage that will cause it to operate?

6. (a) A 100 resistor dissipates 169 W. What is the current flowing through it?

(b) A 3 resistor dissipates 243 W. What is the voltage across it?

(c) An electric heater consumes 1.728 MJ when connected to a 240 V supply for30 minutes. Find the power rating of the heater and the current taken fromthe supply.

(d) Your power use per month is as follows: a 1.5 kW water heater for 7.5 hours,a 3.6 kW grill for 17 minutes, three 100 W lamps for 3 hours, a 900 W toasterfor 6 minutes and a 1 kW iron box for 4.5 hours. The power utility companycharges are as follows:

Fixed charge: Ksh 120.00 Consumption charge: Ksh 2.00 per kWh Fuel cost charge: Ksh 5.35 per kWh Forex Adjustment: Ksh 1.35 per kWh(i) Calculate your power bill for the month.

(ii) If you replace the lamps with energy saving bulbs each consuming 15Wand use solar water heating for 60% of your water heating needs, calculatethe new power bill per month.

(e) The load on a 120 V circuit consists of six 100 W lamps, a 1.2 kW electricheater and an electric motor drawing 1500 W. If the circuit is fused at 30 A,what happens when a 900 W toaster is plugged in? Justify your answer.

7. A 120 V DC motor draws 12A and develops an output power of 1194 W. Calculatethe efficiency of the motor and the power wasted.

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8. Given a series circuit consisting of three resistors and a single battery, fill in thetable below for the following values of Vs, R1, R2 and R3.

Vs

R1

R2

R3

R1 R2 R3 TotalV VoltsI AmpsR OhmsP Watts

(i) Vs = 9 V, R1 = 3k, R2 = 10k, R3 = 5k

(ii) Vs = 11.1 V, R1 =5k, R2 = 12k, R3 = 20k

L.A. KAdoyo, 2013

ECV 211: CHAPTER 2. DC CIRCUIT ANALYSIS

Chapter 2

DC Circuit Analysis

2.1 Example 1

Consider the Series-Parallel circuit shown in Figure 2.1

Figure 2.1: Series-Parallel Circuit

The circuit is composed of two parallel branches, which are in turn connected in serieswith respect to one another. In order to analyse the circuit, it is necessary to obtain thecurrents and voltages for each component.

A table format can be used to simplify analysis, as given in Table 2.1.

R1 R2 R3 R4 TotalV 24 VoltsI AmpsR 100 250 350 200 Ohms

Table 2.1: Table for solving DC circuit

The first step is to obtain the total resistance in the circuit. This can be expressed

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as:RTotal = (R1R2) + (R3R4) (2.1)

To obtain the equivalent resistances in parallel:

1

(R1R2) =1

R1+

1

R2(2.2)

and

1

(R3R4) =1

R3+

1

R4(2.3)

The table is then redrawn to include the parallel resistances, as shown in Table 2.2

R1 R2 R3 R4 R1||R2 R3||R4 TotalV 24 VoltsI AmpsR 100 250 350 200 71.429 127.272 Ohms

Table 2.2: Table for solving DC circuit (cont...)

Since the parallel branches are in series, the total resistance is obtained by summingup the parallel equivalents. The total current flowing into each set of parallel connectionscan be obtained by applying Ohms law to the source voltage and the total resistanceobtained above. The current is then used to obtain the voltage across the parallelbranches. These four steps result in Table 2.3:

R1 R2 R3 R4 R1||R2 R3||R4 TotalV 8.627 15.372 24 VoltsI 120.78m 120.78m 120.78m AmpsR 100 250 350 200 71.429 127.272 198.701 Ohms


Since voltage across parallel branches is equal, it is now possible to obtain individualvoltages across R1, R2, R3 and R4, and through application of Ohms law, to find currentthrough each resistor. The final table is as shown in Table 2.4:

The final circuit is as shown in Figure 2.2, with all currents and voltages indicated.

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R1 R2 R3 R4 R1||R2 R3||R4 TotalV 8.627 8.627 15.372 15.372 8.627 15.372 24 VoltsI 86.27m 34.51m 43.92m 76.86m 120.78m 120.78m 120.78m AmpsR 100 250 350 200 71.429 127.272 198.701 Ohms


8.627 V

15.372 V

+

-

+

-

86.27 mA

43.92 mA76.86 mA

34.51 mA

Figure 2.2: Series-Parallel circuit after analysis



2.2 Example 2

For figure 2.3, given that R1 = 100, R2 = 250, R3 = 470, R4 = 56, R5 = 330, R6= 560 and R7 = 120, and that the source voltage is 12V, find the currents throughand voltages across each resistor.

Figure 2.3: Series-Parallel circuit: Example 2


TUTORIAL 2

1. Figure Q1 shows a network of resistors.

(a) Find the total circuit resistance, RT

(b) Determine the current IT through the voltage sources.

(c) Solve for the currents I1 and I2.

(d) Calculate the voltage Vab

Figure 2.4: Figure Q1

2. Find all the currents and voltages through the resistors in Figure Q2.

3. Find the currents through and the voltages across R1 and R2 in Figure Q3.

4. A current of 6A flows in the circuit in Figure Q4. What is the value of R?





5. From the circuit in Figure Q5, find I1, I2 and VA.

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L.A. KAdoyo, 2013

ECV 211: CHAPTER 3. AC CIRCUIT ANALYSIS

Chapter 3

AC Circuit Analysis

3.1 Definitions

1. AC: Alternating currents (ac) are currents that change direction during the courseof their cycle. In general, ac is used to refer to the alternating waveform. Thus,we have ac currents and ac voltages.

2. Amplitude: The amplitude of a waveform is the distance from its average to itspeak. The Peak Value is the maximum value of the waveform with respect tozero. The Peak-to Peak value is the distance between the highest and the lowestpeaks.

3. Period (T): Is the duration of one cycle. Is measured in seconds.

4. Frequency (f): Is the number of cycles per second. It is the inverse of the period.Is measured in Hertz (Hz).

5. Average (dc) Value: Is obtained by dividing the area under a waveform by thelength of its base. This is the value that a dc meter connected to the waveformwill read as its measurement.

6. rms Value: Is obtained by squaring the expression for the waveform, obtainingthe mean then finding the square root of the mean. This value represents theequivalent dc voltage/current that would be required to dissipate an equal amountof power.

7. Phase Difference: Is the angular displacement between different waveforms ofthe same frequency.

8. Phasor: A rotating vector that can be used to represent a sinusoidal waveform,indicating the position of the waveform at time t = 0.



3.2 Impedance and Admittance

Ac impedance is defined as the ratio of ac voltage to current, and is denoted Z, thus:

Z =V

I(3.1)

3.2.1 Resistor Impedance

v = iR (3.2)

Let v = A cost = A6 0

i(t) =A 6 0

R(3.3)

Thus, the impedance, ZR, is obtained as:

ZR =VRIR

(3.4)

=A 6 0

A 6 0R

ZR = R (3.5)

3.2.2 Impedance of an Inductor

vL = Ldi

dt(3.6)

iL =1

L

vLdt (3.7)

iL =1

L

A costdt (3.8)

=A

Lsint

=A

Lcos(t pi

2) (3.9)

Thus:vL = A6 0

iL =A

L6 pi

2

ZL = L6pi

2(3.10)

= jL



3.2.3 Impedance of a Capacitor

iC = CdVCdt

(3.11)

= Cd

dt(A cost) (3.12)

= C(A sint) (3.13)

= CA cos(t+pi

2) (3.14)

vC = A6 0

iC = CA6pi

2

ZC =1

C6 pi

2(3.15)

=jC

=1

jC(3.16)

The impedance of a circuit, Z, is expressed as:

Z(j) = R(j) + jX(j) (3.17)

where R is the ac resistance and X is the reactance.The reactance component of the impedance is made up of the inductive reactance,

XL, equal to L, which is always positive, and the capacitive reactance, XC , equal to 1

C, which is always negative.

3.2.4 Admittance

Admittance, Y, is the reciprocal of the impedance, and its unit is the Siemens.

Y =1

Z= G+ jB (3.18)

G is known as the ac conductance, and B is referred to as the susceptance.To obtain G and B from Z:

Y =1

Z=

1

R + jX(3.19)



=(R jX)

(R + jX)(R jX) (3.20)

Y =R jXR2 +X2

(3.21)

Equating the real and imaginary terms, we get:

G =R

R2 +X2(3.22)

and

B =X

R2 +X2(3.23)

3.3 Power in ac Circuits

(a) For a purely resistive ac circuit, the average power dissipated is given by:

P = V I = I2R =V 2

RWatts (3.24)

(b) For a purely inductive circuit, average power dissipated is ZERO.

(c) For a purely capacitive circuit, average power dissipated is ZERO.

(d) For an R-L, R-C or R-L-C circuit, average power dissipated is given by:

P = V I cos (3.25)

where cos is known as the power factor.

3.4 Three Phase AC Circuits

Single phase voltage is induced when a single coil is rotated in a uniform magnetic field.It makes use of two wires- a live wire (usually red) and a neutral wire (usually black).The neutral is usually connected to the earth, with the earth wire being coloured green.The standard voltage for a single phase ac supply is 240V.

A three phase voltage is generated when three coils are placed 120 apart and thewhole setup rotated in a uniform magnetic field. The result is three independent suppliesof equal voltages displaced from each other by 120.

The convention adopted to identify each of the phase voltages is Red, Yellow andBlue, and the phase sequence is Red-Yellow-Blue.

A three-phase ac supply is carried by three conductors, called lines, which are colouredred, yellow and blue. A fourth conductor, the neutral, usually coloured black, may be



Figure 3.1: Single phase ac generator

Figure 3.2: Three phase ac generator

used with a three phase supply. The current flowing through the conductors is knownas line current (iL), and the voltages between them are known as line voltages (vL).

If the three voltages are connected independently, it would be necessary to use atleast six wires. However, to reduce the number of wires required to connect loads tothree-phase supplies, there are two common types of connection used- star (or wye) anddelta.

3.4.1 Star Connection

(a) In a star connection, the three conductors are joined together at a common pointknown as the star point. If a neutral conductor is used in the connection, it is alsoconnected to the star point, resulting in a 4-wire connection.

(b) VR, VY and VB, are known as phase voltages, denoted Vp.

(c) VRY , VY B and VBR are called line voltages.

(d) The phase currents Ip are equal to their respective line currents, IR, IY and IB.

(e) For a balanced system, IR = IY = IB, VR = VY = VB, VRY = VY B = VBR and



Figure 3.3: Star connected three-phase load

ZR = ZY = ZB, and the current in the neutral conductor, IN = 0. When a starconnection is balanced, the neutral conductor is unnecessary, and is often omitted.

(f) For a balanced star system, VL =

3Vp

(g) A 4-wire star connection allows for the use of 2 voltages. The standard electricitysupply in Kenya from a three-phase voltage supply is 415/240 V, 50 Hz.

3.4.2 Delta Connection

Figure 3.4: Delta connected three-phase load

(a) In a delta (or mesh) connection, the end of one load is connected to the start of thenext load.

(b) The line voltages are the respective phase voltages, i.e. VL = Vp.

(c) IL =

3Ip



3.4.3 Power in Three Phase Circuits

Power dissipated in a three-phase system is given by the sum of the power dissipatedin each phase. If the system is balanced, the power dissipated is 3 times the powerconsumed by one phase.

For a star connection, Vp =VL3

and IL = Ip. Thus,

P = 3

(VL

3

)IL cos (3.26)

=

3VLIL cos

For a delta connection, Ip =IL3

and VL = Vp. Thus,

P = 3VL

(IL

3

)cos (3.27)

=

3VLIL cos

Hence, for either a star or delta connection, power is given by

P =

3VLIL cosWatts (3.28)

orP = 3I2pRpWatts (3.29)


Chapter 4

Electrical Measurements andMeasuring Instruments

4.1 Introduction

A meter is any device built to accurately detect and display an electrical quantity in aform readable by a human being. This readable form is usually visual and may involvemotion of a pointer on a scale, a series of lights arranged to form a bargraph or somesort of display composed of numerical figures.

Meters may be divided into analogue and digital types. Analogue meters give anoutput action that directly represents the quantity being measured. Digital meters giveoutput in the form of digits displayed on an output device.

Advantages of Analogue Instruments

They are cheaper People find it easier to visualize the output using an analogue display, thus many

prefer it to a digital one.

Disadvantages of Analogue Instruments

They are not very accurate They tend to distort the circuit in which they are applied

4.2 Analogue Instruments

All analogue instruments require three essential devices:

(i) A deflecting or operating device: A mechanical force is produced by the cur-rent or voltage which causes the pointer to deflect between its zero position atzero current/voltage and its f.s.d (full scale deviation) position at the maximumcurrent/voltage rating of the device.

23

ECV 211: CHAPTER 4. ELECTRICAL MEASUREMENTS AND MEASURINGINSTRUMENTS

(ii) A controlling device: It acts in opposition to the deflecting force and ensuresthat the deflection shown on the meter is always the same for a given measuredquantity. It also prevents the pointer always going to the maximum deflection.

There are two main types of controlling device- spring control and gravity control.

(iii) A damping device: A damping force ensures that the pointer comes to rest in itsfinal position quickly and without undue oscillation. There are three main typesof damping used- eddy current damping, air friction damping and fluid frictiondamping.

There are two types of scale- Linear Scale, in which the divisions or graduationsare evenly spaced, and Non-Linear Scale, which is cramped at the beginning and thegraduations are uneven throughout the range.

4.2.1 Moving Iron Instrument

Moving iron instruments are of two types: attraction type and repulsion type.

Figure 4.1: Moving Iron Instruments



(a) In the attraction type moving iron instrument, when current flows in the solenoid,a pivoted soft iron disc is attracted to the solenoid and the movement causes thepointer to move across a scale.

(b) In the repulsion type moving iron instrument, two pieces of iron are placed in-side the solenoid, one being fixed and the other attached to the spindle carryingthe pointer. When current passes through the solenoid, the two pieces of iron aremagnetized in the same direction and therefore repel each other. The pointer movesacross the scale.

The force moving the pointer is, in each type of moving iron instrument, proportionalto I2, thus the direction of current does not matter. For that reason, moving ironinstruments can be used to measure both dc and ac. The scale, however, is non-linear.

4.2.2 Moving Coil Instrument

Figure 4.2: Moving Coil Instrument

The rectangular moving-coil consists of insulated copper wire wound on a light alu-minium frame fitted with steel pivots resting on bearings. Current is led into and out ofthe coil by spiral hairsprings AA, which also provide the controlling torque. The coil isfree to move in airgaps between the soft-iron pole pieces PP and a soft-iron cylinder Bsupported by a brass plate (not shown). The functions of core B are:



to intensify flux by reducing the air-gap length to give a radial magnetic flux of uniform density, thus enabling the scale to be

uniformly divided.

Figure 4.3: Distribution of Flux in Moving Coil instrument

The manner in which a torque is produced when the coil is carrying a current maybe understood more easily by considering a single turn PQ. Suppose P to carry currentoutwards from the paper; then Q is carrying current towards the paper. Current inP tends to set up a magnetic field in a counterclockwise direction around P and thusstrengthens the magnetic field on the lower side and weakens it on the upper side. Thecurrent in Q, on the other hand, strengthens the field on the upper side while weakeningit on the lower side. Hence, the effect is to distort the magnetic flux. Since the fluxbehaves as if it was in tension and therefore tries to take the shortest path between polesNS, it exerts forces FF on coil PQ, tending to move it out of the magnetic field.

4.2.3 Moving Coil Rectifier Instrument

Consists of a moving coil instrument used in conjunction with a bridge circuit to pro-vide an indication of alternating currents and voltages. This meter is calibrated in rmsvalues rather than average values, and it is assumed by the manufacturers that the ac issinusoidal.

4.2.4 Comparison between Moving Iron, Moving Coil and Mov-ing Coil Rectifier Instruments

4.3 Voltmeter Design

Most meters are sensitive devices, thus it is necessary to reduce the quantity beingmeasured to a value the instrument can handle.

For voltmeters, the range of voltage that the instrument can measure is increased byuse of the voltage division principle. A resistor is connected in series with the instrumentsuch that the greater voltage appears across the resistor, known as a multiplier, and a



Figure 4.4: Moving coil rectifier instrument

Table 4.1: Comparison of Instrument types



smaller voltage appears across the instrument. The scale on the meter face is also cali-brated in proportion to indicate the new range. The value of the multiplier is calculateddepending on the range required and the characteristics of the instrument, i.e. its fullscale deviation and its internal resistance.

Multi-range voltmeters are designed using a multi-pole switch and several multiplierresistors, each one sized for a particular range.

4.4 Ammeter Design

In ammeter design, the current division principle is applied to reduce the current flowingthrough the instrument. Thus, a low value resistor known as a shunt is connected inparallel with the instrument to carry the larger proportion of the current, while a smallerproportion of the current to be measured flows through the instrument.

4.5 Ohmmeter Design

An ohmmeter measures the value of resistance between its leads. This resistance readingis indicated by a pointer movement which is operated by electric current. Analogue ohm-meters are designed with an internal voltage source to provide the current for moving thepointer and a protective resistor in series to allow for the required range of measurementsto be included in the calibrations.

The ohmmeter scale ranges from 0 to infinity, and is reversed. The scale is non-linear,with the calibrations calculated based on the current drawn from the meter.

Ohmmeter reliability is highly dependent on the reliability of the voltage source, andthe ohmmeter can only function correctly when used to measure a resistance that is NOTconnected to a live circuit.


TUTORIAL 4

Voltmeter Design

ExampleA moving coil meter has a full scale deviation of 1 mA and a coil resistance of 500.

Use the meter to design a voltmeter with a range of:(a) 10 V(b) 100 V.

Solution:The full scale deviation of the meter unaltered allows for the measurement of a range

given by:V = IR = 1 103 500 = 0.5V

To allow for measurement of more than 0.5 V, a multiplier must be connected inseries with the meter to take up the extra voltage, as shown in the Figure 4.5.

Rm

Figure 4.5: Voltmeter design from moving coil instrument

Using the voltage divider formula, it is possible to calculate the value of resistancerequired for the multiplier.

(a) For the 10 V voltmeter, the multiplier is required to have a potential difference of(10-0.5) V across it at f.s.d, which works out to 9.5 V. Using the voltage dividerformula, the value of resistance required for the multiplier can now be worked outthus:

9.5V =Rm

(500 +Rm)10V

9.5(500 +Rm) = 10Rm

10Rm 9.5Rm = 9.5 500

0.5Rm = 4750

Thus, Rm is found to be equal to 9500 or 9.5k.


(b) Working as for (a), for the 100V voltmeter, the voltage across the multiplier will be99.5 V. The resistance required for the multiplier will be:

99.5V =Rm

(500 +Rm)100V

99.5(500 +Rm) = 100Rm

100Rm 99.5Rm = 99.5 500

0.5Rm = 49750

Thus, to create a 100V voltmeter, the value required for the multiplier is 99500,which is equivalent to 99.5k.

It is possible to design a multi-range voltmeter by including a range of multiplierresistors, as shown in Figure 4.6.

Figure 4.6: Multi-Range voltmeter

However, since the values obtained for the multipliers are often uncommon values, notusually easily obtained commercially, the usual configuration for a multi-range voltmeteris as shown in Figure 4.7.

L.A. KAdoyo, 2013


Figure 4.7: Multi-range voltmeter using practical resistor values

Exercises

1. A moving coil instrument gives f.s.d for a current of 10 mA. Given that the internalresistance of the instrument is 250, calculate the value of the multiplier requiredto enable the instrument to measure up to:

(a) 20V

(b) 100V

(c) 250V

2. A moving coil instrument having a resistance of 20 gives a f.s.d when the currentis 5 mA. Calculate the value of multiplier required so that it can be used as avoltmeter with a range of up to:

(a) 10V

(b) 25V

(c) 100V

(d) 250V

Ammeter Design

ExampleUsing the same instrument used in voltmeter design, we wish to create an instrument

with a full scale deviation of:(a)100 mA(b)5 Amps

SolutionThe current that causes f.s.d in the instrument is 1mA. To allow the instrument to

measure currents greater than that, a shunt resistance is connected in parallel to carrythe extra current, as shown in Figure 4.8.

The current divider formula is used to calculate the resistance of the shunt used.

L.A. KAdoyo, 2013


Figure 4.8: Ammeter design from moving coil instrument

(a) To calculate shunt resistance for a 100 mA ammeter, we use current division, asfollows: Working in milliamperes:

1mA =Rs

(500 +Rs)100mA

500 +Rs = 100Rs

99Rs = 500

The value of shunt resistance required is 5.05051.

(b) For a 5A ammeter, the required shunt is:

1mA =Rs

(500 +Rs)5000mA

500 +Rs = 5000Rs

4999Rs = 500

The value required for the shunt is thus found to be 0.10002.

Multi-range ammeters can be made by using a range of shunt resistors and a rangeselector switch, as shown in Figure 4.9.

Since the values used for shunt resistances are very low, they usually have to bespecially fabricated from relatively large diameter wire or solid pieces of metal.

L.A. KAdoyo, 2013


Figure 4.9: Multi-Range ammeter

Exercises

1. A moving coil instrument gives f.s.d for a current of 10 mA. Given that the internalresistance of the instrument is 250, calculate the value of the shunt required toenable the instrument to measure up to:

(a) 2 A

(b) 10 A

(c) 25 A

2. A moving coil instrument having a resistance of 20 gives a f.s.d when the currentis 5 mA. Calculate the value of multiplier required so that it can be used as avoltmeter with a range of up to:

(a) 100 mA

(b) 250 mA

(c) 1 A

(d) 2.5 A

(e) 10 A

(f) 25 A

Ohmmeter Design

ExampleAn ohmmeter is to be designed from a moving coil meter with a full scale deviation

of 1 mA, an internal resistance of 500 and a voltage source supplying 9V.To investigate how it serves as a simple ohmmeter, first consider the case when there

is infinite resistance between the leads, i.e. there is no continuity in the circuit. Currentflow will be zero and the pointer will be at the extreme left of the scale.

If the test leads of the ohmmeter are directly connected, the meter will have maxi-mum current flowing through it, obtained by dividing the supply voltage by the internal

L.A. KAdoyo, 2013


Figure 4.10: Simple ohmmeter

resistance of the meter, resulting in a current flow of 18mA. This is higher than thefull scale deviation of the meter, and would likely damage the meter. To accommodatethe extra current, it is necessary to add a series resistance which ensures that full scaledeviation is just accomplished by zero resistance between the leads.

The total resistance required in the circuit is obtained by Ohms law as:

Rtotal =V

I=

9

1 103 = 9k

The value of the series resistance is obtained by simple arithmetic: R = Rtotal500 =8.5k.

In order to graduate the scale, it is necessary to determine the values at half, quarterand three-quarters of the full scale deviation. Since the resistance ranges from infinityon the left to 0 on the right, the scale is non-linear.

To determine the half-scale value, the calculation is as follows:If the full scale deviation is achieved by a current of 1 mA, then 0.5 mA through the

instrument causes half-scale deviation. Using the design with the 9V battery,

Rtotal =V

I=

9

0.5 103 = 18k

Since there is a series resistance of 8.5k in addition to the instrument resistanceof 500 in the circuit, the resistance connected between the leads is equal to 18k 8.5k 500 = 9k

At quarter scale deflection, the current is 0.25 mA. The total resistance is:

Rtotal =9

0.25 103 = 36k

The value of the resistance connected between the leads for a quarter scale deflectionwould then be 36k 8.5k 500 = 27k.

Using similar calculations, the three-quarters scale deviation is calculated to be 3k.

L.A. KAdoyo, 2013


Exercises

(a) A student wishes to design an ohmmeter from a moving coil meter and a voltagesource. Given the following specifications for the instruments available, show theohmmeter as designed, and indicate the values that will be required to mark thehalfway point, the quarter-way and the three-quarter mark on the scale.

(i) A moving coil meter with a full scale deviation of 2.5 mA and an internalresistance of 100 and a voltage source supplying 20V.

(ii) A moving coil meter with a full scale deviation of 10mA and an internal resis-tance of 250, and a voltage source supplying 15V.

(b) An ohmmeter is to be designed from a moving coil meter with a full scale deviationof 5 mA and an internal resistance of 500.

(i) Given that the half-scale deviation is to be 15k and the three-quarter scaledeviation is to be 3.5k, calculate the voltage and series resistance required.

(ii) If the instrument was designed using a 20V voltage source, calculate the seriesresistance required and the half, quarter and three-quarter scale values.

L.A. KAdoyo, 2013

ECV 211: CHAPTER 5. ELECTRICAL INSTALLATION

Chapter 5

Electrical Installation

5.1 Wiring Facilities

5.1.1 Introduction

Electrical services in a building may be provided for different kinds of loads- lighting,heating, motors, communication equipment, etc. These loads may vary in voltage andtime of service, as for example, continuous lighting and intermittent elevator motors. Itis highly improbable that all of the intermittent loads will occur at once. To determinethe probable maximum load, diversity factors, consisting of coincidence factors anddemand factors are applied to the total connected load.

The coincidence factor is a ratio of the maximum demand load of a system to thesum of its individual components, and indicates the largest portion of all the electricalloads likely to be operating at any one time. The demand factor is the ratio of theactual peak load equipment or system to its maximum rating.

5.1.2 Electrical Plans

Electrical plans should be drawn to scale, traced or reproduced from the architecturalplans. Floor heights should be indicated if full elevations are not given. Locations ofwindows and doors should be reproduced accurately and door swings shown, to facilitatelocation of wall switches. Length of wiring required may be estimated from the planswith sufficient accuracy.

Location of all electrical equipment should be indicated on the plan by use of symbols-ceiling outlets, wall receptacles, switches, junction boxes, panel boards, telephone andinterior communication equipment, fire alarms, television master-antenna connections,etc.

5.1.3 Branch Circuits

It is good practice to limit branch runs to 50 ft for 120V circuits and 100 ft for 277Vcircuits by installing sufficient panelboards in efficient locations.



General lighting branch circuits with a 15A fuse or circuit breaker in the panelboardare usually limited to 6 to 8 outlets, although most building codes permit up to 12. Nomore than 2 outlets should be connected in a 20A appliance circuit.

5.1.4 Service Entrance Switch and Metering Equipment

Fused switches and circuit breakers must be provided near the entrance point of electricalservices in a building for shutting off the power. Each incoming service in a multi-occupancy building must be controlled near its entrance by not more than 6 switches orcircuit breakers.

Metering equipment must be located near the point of service entrance, unless oth-erwise permitted by the utility company. Tenant meter closets on upper floors, openingon public halls, may also be permitted.

5.1.5 Electric Wiring Circuits

Power Sockets

Power sockets should be positioned between 150 mm and 250 mm above floor level andwork surfaces. An exception is in buildings designed for the elderly and the infirm, wherethe socket heights should be between 750 mm and 900 mm above the floor.

Every socket terminal should be fitted with a double socket to reduce the need foradaptors. Positioning of sockets should reduce the need for lead lengths to no more than2 m.

Ring Circuits

Ring circuits (see Fig. 5.1) are used for single phase supply to 3 pin sockets or to lightingcircuits. It consists of a PVC sheathed cable containing live and neutral conductors inPVC insulation and an exposed earth looped into each socket outlet or ceiling rose (seeFig. 5.2).

In a domestic building, a ring circuit may supply an unlimited number of sockets upto a maximum floor area of 100m2. A separate circuit is also provided solely for thekitchen, which contains relatively high-rated appliances.

The number of socket outlets on a spur should not exceed the number of socket outletsand fixed appliances on the ring.

Radial Circuits

A radial circuit (see Fig. 5.3) may be used as an alternative to a ring circuit to supplyany number of outlets, provided the following limitations are effected:

For 2.5 mm2 cross-sectional area cable, minimum overload protection is 20A, andmaximum coverage is 17 m cable length over 20m2 floor area.



Figure 5.1: Ring circuit socket connection

For 4 mm2 cross-sectional area cable, minimum overload protection is 30A, andmaximum coverage is 21 m cable length over 50m2 floor area.

Since the ring circuit limitation for the 2.5 mm2 cable is 54 m cable length over 100m2 floor area, radial circuits are usually limited to the following applications:

lighting circuits immersion heaters cookers showers storage radiators outside extensions



Figure 5.2: Ring circuit lighting connection

Figure 5.3: Radial circuit socket outlet connection

5.1.6 Lighting Circuits

In a one-way switch circuit the single-pole switch must be connected to the live conductor.To ensure that both live and neutral conductors are isolated from the supply a double-



pole switch may be used, although these are generally limited to installations in largerbuildings where the number and type of light fittings demand a relatively high currentflow. Provided the voltage drop (4% max.) is not exceeded, two or more lamps may becontrolled by a one-way single-pole switch.

In principle, the two-way switch is a single-pole changeover switch interconnected inpairs. Two switches provide control of one or more lamps from two positions, such asthat found in stair/landing, bedroom and corridor situations. In large buildings, everyaccess point should have its own lighting control switch. Any number of these maybe incorporated into a two-way switch circuit. These additional controls are known asintermediate switches.

5.1.7 Testing of Completed Installation

Three tests are essential once electrical installation is completed:

1. Continuity: This test ensures integrity of the live, neutral and earth conductorswithout bridging (shorting out) of connections.

2. Insulation: This test is done to ensure that there is high resistance betweenthe live and neutral conductors, and between these conductors and the earth. Alow resistance would result in current leakage and energy wastage, which couldbe a potential fire hazard. The test to earth requires that all lamps and otherequipment be disconnected, all switches and circuit breakers closed and fuses leftin. Ohmmeters should read at least 1 M.

3. Polarity: This test ensures that all switches and circuit breakers are connectedin the phase or live conductor. Connection of switchgear to the neutral conductorcould lead to a very dangerous situation where apparent isolation of equipmentwould still leave it live.

5.2 Ducting

Before installing ducts for the entry of services into a building, it is essential to ascertainthe location of pipes and cables provided by the public utilities companies. Thereafter,the shortest, most practicable and most economic route can be planned.

For flexible pipes and cables, a purpose-made plastic pipe duct and bend may beused. For rigid pipes or large cables, a straight pipe duct to a pit will be required. Pipeducts must be sealed at the ends with a plastic filling and mastic sealant, otherwisesubsoil and other materials will encroach into the duct. If this occurs, it will reducethe effectiveness of the void around the pipe or cable to absorb differential settlementbetween the building and incoming service.

To accommodate horizontal services, a skirting or floor duct may be used (Figure5.5). These may be purpose made by the site joiner or be standard manufactured items.Vertical services may be housed in either a surface-type duct or a chase (Figure 5.6).



Figure 5.4: Ducts for entry of services into buildings

The latter may only be used if the depth of chase does not affect the structural strengthof the wall. The reduction in the walls thermal and sound insulation properties mayalso be a consideration.

Figure 5.5: Horizontal ducts for small diameter cables

5.2.1 Floor and Skirting Ducts

A grid distribution of floor ducting is appropriate in open plan offices and shops wherethere is an absence of internal walls for power and telecommunications sockets. It is alsouseful in offices designed with demountable partitioning where room layout is subject tochanges. Sockets are surface mounted in the floor with a hinged cover plate to protectthem when not in use. The disruption to the structure is minimal as the ducts canbe set in the screed, eliminating the need for long lengths of trailing cables to remoteworkstations.



Figure 5.6: Vertical ducts for small diameter cables

Figure 5.7: Floor Ducting

For partitioned rooms, a branching duct layout may be preferred. The branches canterminate at sockets near to the wall or extend into wall sockets.

Where power supplies run parallel with telecommunications cables in shared ducts,the services must be segregated and clearly defined.

For some buildings, proprietary metal, plastic or laminated plywood skirting ductsmay be used. These usually have socket outlets at fixed intervals.

5.2.2 Medium and Large Vertical Ducts

The purpose of a service duct is to conceal the services without restricting access forinspection, repair and alterations. A duct also helps to reduce noise and protect theservices from damage.

When designing a service duct, the transmission of noise, possible build-up of heat inthe enclosure and accessibility to the services must be considered. The number of ductsrequired will depend on the variation in services, the need for segregation and locationof equipment served.

Vertical ducts usually extend the full height of a building which is an important factorwhen considering the potential for spread of fire. The duct must be constructed as aprotected shaft and form a complete barrier to fire between the different compartmentsit passes. This will require construction of at least 60 minutes fire resistance with access



Figure 5.8: Distribution of floor ducts

doors at least half the structural fire resistance.

5.2.3 Medium and Large Horizontal Ducts

Floor trenches are usually fitted with continuous covers. Crawl-ways generally haveaccess covers of minimum 600 mm dimension, provided at convenient intervals. A crawl-way should be wide enough to allow a clear working space of at least 700 mm and havea minimum headroom of at least 1 m. A trench has an internal depth of less than 1 m.

Continuous trench covers may be of timber, stone, reinforced concrete, metal or ametal tray filled to match the floor finish. The covers should be light enough to be raisedby one person, or, at most, two. Sockets for lifting handles should be incorporated inthe covers. In external situations, the cover slabs (usually of stone or concrete) can bebedded and joined together with a weak cement mortar. If timber or similar covers areused to match a floor finish, they should be fixed with brass cups and countersunk brassscrews.

In internal situations where ducts cross the line of fire compartment walls, a firebarrier must be provided within the void and the services suitably fire stopped.

5.2.4 Raised Access Floors

Raised flooring provides discrete housing for the huge volumes of data and telecom-munications cabling, electrical power cables, pipes, ventilation ducts and other services



Figure 5.9: Medium and large vertical ducts

associated with modern buildings (Fig. 5.11).Proprietary raised floors use standard 600 mm square interchangeable decking panels,

suspended from each corner on adjustable pedestals. These are produced in a variety ofheights to suit individual applications, but most range between 100 mm and 600 mm.Panels are generally produced from wood particle board and have a galvanised steelcasing or overwrap to enhance strength and provide fire resistance. Applied finishes varyto suit application, e.g. carpet, wood veneer, vinyl, etc.

Pedestals are screw-threaded steel or polypropylene legs, connected to a panel supportplate and a base plate. The void between structural floor and raised panels will requirefire stopping at specific intervals to retain the integrity of compartmentation.



Figure 5.10: Medium and large horizontal ducts

5.2.5 Suspended and False Ceilings

A suspended ceiling contributes to the fire resistance of a structural floor. An additionalpurpose for a suspended ceiling is to accommodate and conceal building services, whichis primarily the function of a false ceiling.

False ceiling systems may be constructed from timber or metal framing. A grid orlattice support system is produced to accommodate loose fit ceiling tiles of plasterboard,particle board or composites. As with raised flooring, the possibility of fire spreadingthrough the void must be prevented. Fire stopping is necessary at appropriate intervals(Fig. 5.12).



Figure 5.11: Raised Access Floor

Figure 5.12: False Ceiling


ECV 211: CHAPTER 6. FIRE PROTECTION

Chapter 6

Fire Protection

6.1 Introduction to Fire Safety

There are two distinct aspects of fire protection, which may overlap in some cases: lifesafety and property protection. Life safety may be ensured by a program that ensuresprompt notification and evacuation of occupants. Property protection is ensured byproper consideration of material properties and fire resistant structures.

The first obligation of designers is to meet legal requirements while providing the fa-cilities required by the client. Many clients will also require that their insurance providerbe consulted to obtain the most favourable insurance rate.

Fire risk assessment involves the following three steps:

Identifying fire hazards Identifying people at risk Evaluation, removal, reduction and protection from risk

6.1.1 Identification of Fire Hazards

A fire requires three ingredients:

A source of ignition A source of fuel A source of oxygen

Fire hazards are items or conditions that provide one or more of the ingredients listed.

Sources of Ignition

Possible sources of ignition may include:

(i) smokers materials, such as cigarettes, lighters and matches



(ii) naked flames, such as candles or gas or liquid-fuelled open flame equipment

(iii) electrical, gas or oil-fired heaters

(iv) hot processes, e.g. welding

(v) cooking equipment and activities

(vi) faulty or misused electrical equipment

(vii) lighting equipment placed too close to stored products

(viii) machines with hot surfaces

(ix) obstruction of equipment ventilation

(x) arson

(xi) boilers

Sources of Fuel

Anything that burns is fuel for a fire. Common sources of fuel may include:

(i) flammable liquid-based products such as thinners, paints, adhesives and varnishes.

(ii) flammable liquids and solvents such as white spirit, methylated spirit, cooking oilsand disposable cigarette lighters.

(iii) flammable chemicals such as certain cleaning products, photocopier chemicals anddry-cleaners that use hydrocarbon solvents.

(iv) packaging materials, stationery, advertising material and decorations

(v) plastics and rubber such as video tapes, polyurethane foam-filled furniture andpolystyrene-based display materials

(vi) textiles and soft furnishings such as hanging curtains and clothing displays

(vii) waste products, particularly finely divided material such as shredded paper andwood shavings, off-cuts and dust

(viii) flammable gases such as liquefied petroleum gas (LPG)

(ix) wall and ceiling hangings

(x) decorations for seasonal and religious occasions

(xi) water storage and refuse containers

(xii) materials used to line walls and ceilings such as polystyrene tiles.



Sources of Oxygen

The main source of oxygen for a fire is in the air around us. In an enclosed buildingthis is provided by the ventilation system in use. This generally falls into one of twocategories: natural airflow through doors,windows and other openings; or mechanicalair conditioning systems and air handling systems. In many buildings there will be acombination of systems, which will be capable of introducing/extracting air to and fromthe building.

Additional sources of oxygen may be from:

(i) Some chemicals (oxidizing materials), which can provide additional oxygen to afire.

(ii) Oxygen supplies from cylinders and piped systems

(iii) Pyrotechnics (fireworks)

6.1.2 Identification of People at risk

Everyone within a building is potentially at risk from fire, but there are those who areat greater risk, and they may fall in the following categories:

(i) employees who work alone and/or in isolated areas, e.g. cleaners, security staff,night staff, maintenance staff, etc

(ii) people who are unfamiliar with the premises e.g. guests, visitors, customers, sea-sonal staff, new staff and contractors

(iii) people asleep (they will be slow to respond and disorientated)

(iv) people with disabilities

(v) people with language difficulties

(vi) unaccompanied children and young persons

(vii) people who may have some other reason for not being able to leave the premisesquickly, e.g. elderly customers or parents with children

(viii) people who are sensorially impaired due to drugs, alcohol or medication

(ix) other people in the immediate vicinity of your premises

6.1.3 Evaluation, Reduction, Removal and Protection from Risk

Absolute fire safety is unattainable, thus, the objective of fire protection is to reduceoccurrence of preventable fires and to minimize the losses caused by fires that do occur.



Evaluation of risk of occurrence of fire

Evaluation of risk of fire occurrence requires knowledge of the ways in which fires start,i.e.

Accidentally, e.g. when smoking materials are not properly extinguished or whenlighting displays are knocked over.

Through negligence, e.g. when office electrical equipment is not properly main-tained or when waste material is allowed to accumulate next to a heat source

Deliberately, i.e. arson

Evaluation of risk to people from fire

Evaluation of risk to people involves a knowledge of methods by which fire and smokespread, i.e.

(i) Convection: is the most dangerous, resulting in the highest number of injuriesand deaths. When fires start in enclosed spaces, the smoke rising from the fire getstrapped by the ceiling and then spreads in all directions to form an ever-deepeninglayer over the entire room space. The smoke will pass through any holes or gapsin the walls, ceilings and floor into other parts of the building. The heat from thefire gets trapped in the building and the temperature rises.

(ii) Conduction: some materials such as metal shutters and ducting can absorb heatand transmit it to neighbouring rooms where it can set fire to combustible materialsthat are in contact with the heated material.

(iii) Radiation: radiation causes the air in the room to heat up. Any material close toa fire will absorb the heat until the material starts to smolder and then burn.

Smoke produced by a fire also contains toxic gases that are harmful to people. Thick,black smoke obscures vision, causes difficulty breathing and can block escape routes.

In evaluating risk to people, it is important to consider situations such as:

(a) Fire starting on a lower floor affecting the only escape route for people on upperfloors or people with disabilities.

(b) Fire developing in an unoccupied space that people have to pass by to escape fromthe building

(c) Fire or smoke spreading through a building via routes such as vertical shafts, serviceducts, ventilation systems, poorly installed, poorly maintained or damaged walls,partitions and ceilings affecting people in remote areas.

(d) Fire starting in a service room and affecting hazardous materials



(e) Fire spreading rapidly through the building because of combustible structural ele-ments and/or large quantities of combustible goods

(f) Rapid vertical fire spread in racked displays

(g) Fire and smoke spreading through a building due to poor installation of fire precau-tions, e.g. incorrectly installed fire doors or incorrectly installed services penetratingfire walls

(h) Fire and smoke spreading through the building due to poorly maintained and dam-aged fire doors or fire doors being wedged open.

Reduction and removal of sources of ignition

This can be done by taking the following precautions:

Wherever possible replace a potential ignition source by a safer alternative. Replace naked flame and radiant heaters with fixed convector heaters or a central

heating system. Restrict the movement of and guard portable heating appliances.

Separate ignition hazards and combustibles e.g. ensure sufficient clear space be-tween lights and combustibles.

Operate a safe smoking policy in designated smoking areas and prohibit smokingelsewhere.

Ensure electrical and mechanical and gas equipment is installed, used, maintainedand protected in accordance with the manufacturers instructions.

Check all areas where hot work (e.g. welding) has been carried out to ensure thatno ignition has taken place or any smoldering materials remain that may cause offire.

Ensure that no-one carrying out work on gas fittings which involves exposing pipesthat contain or have contained flammable gas uses any source of ignition such asblow-lamps or hot-air guns.

Take precautions to avoid arson.

Reduction and removal of sources of fuel

This can be accomplished by the following steps:

Ensure combustible items, such as furniture, laundry, decorations, are stored prop-erly and are separate from potential ignition sources, such as boilers.



Reduce stocks of flammable materials, liquids and gases on display in public areasto a minimum. Keep remaining stock in dedicated storerooms or storage areaswhere the public are not allowed to go, and keep the minimum required for theoperation of the business.

Ensure flammable materials, liquids and gases, are kept to a minimum, and arestored properly with adequate separation distances between them.

Reduce or protect combustible displays, furnishings and foliage. Keep areas containing flammable gases ventilated. Clean ducts and flues. Make sure staffs responsible for cleaning bedrooms are aware of potential fire haz-

ards (e.g. storage, use and disposal of aerosols/newspapers) that may be broughtinto rooms by guests and residents and left in a haphazard manner. There shouldbe a policy in place to deal with this constant hazard.

Do not keep flammable solids, liquids and gases together. Remove, or treat large areas of highly combustible wall and ceiling linings, e.g.

Polystyrene to reduce the rate of flame spread across the surface.

Develop a formal system for the control of combustible waste by ensuring thatwaste materials and rubbish are not allowed to build up and are carefully storeduntil properly disposed of, particularly at the end of the day.

Take action to avoid storage areas being vulnerable to arson or vandalism. Check all areas where hot work (e.g. welding) has been carried out to ensure that

no ignition has taken place and no smoldering or hot materials remain that maycause a fire later.

Reduce the amount of combustible materials, such as paper products and plastics.Keep spare items in storerooms or storage areas where the public are not allowedto go.

Reduction and removal of sources of oxygen

This can be accomplished by taking the following precautions:

Closing all doors, windows and other openings not required for ventilation, partic-ularly out of working hours.

Shutting down ventilation systems which are not essential to the function of thepremises;



Not storing oxidizing materials near or with any heat source or flammable materials Controlling the use and storage of oxygen cylinders, ensuring that they are not

leaking.

Reduction and removal of risks to people

Where travel distance is in excess of the norm for the level of risk determined, thefollowing precautions may be taken to reduce risk to people:

Provide early warning of fire using automatic fire detection. Revise the layout to reduce travel distances. Reduce the fire risk by removing or reducing combustible materials and/or ignition

sources.

Control the number of people in the premises. Limit the area to trained staff only (no public). Increase staff training and awareness.

6.2 Fire Load and Resistance

The nature and potential magnitude of fire in a building are directly related to theamount and physical arrangement of combustibles present, whether as contents of thebuilding or as materials used in its construction. Thus, buildings are classified by occu-pancy and construction.

The total amount of combustibles in a building is referred to as the fire load. Itis expressed as a ratio of mass per unit area. For highly combustible materials such aspetroleum, alcohols, waxes, fats and similar materials, their masses are taken at twicethe actual mass, because of their higher calorific value.

The fire load affects the severity of a fire, with an average fire load of 5 pounds persquare foot(about 24.41 kg per square metre) resulting in an equivalent fire severity of12

hour, and an average fire load of 60 psf (about 292.95 kg per square metre) resultingin a 71

2hour fire.

Fire resistance ratings are required for structural members, exterior walls, fire divi-sions, fire separations, ceiling-floor assemblies, and any other constructions for which afire rating is necessary. Ratings are also required for interior finish of walls, ceilings andfloors, classified as to flame spread, fuel contributed and smoke developed.

6.2.1 Fire and Smoke Barriers

Buildings should be designed to control fires and smoke so that they do not spread frombuilding to building. The following measures may be employed:



Zoning

Application of fire zones or fire limits that restrict types of construction or occupancythat can be used. Additional regulations establish minimum distance between buildingsand specify the types of construction that may be used for enclosing the exterior ofbuildings.

Building codes also require extending exterior walls as parapets at least 3 feet aboveroof level. Parapets shield fire fighters who may be fighting a fire on an adjacent roofand prevent flames from spreading from roof to roof.

Building codes also specify the level of fire-resistance required for roof coverings.

Fire Divisions/Barriers

Fire divisions are employed to prevent the spread of fire and smoke horizontally in build-ing interiors.

A fire division is any construction with the fire-resistance rating and structural sta-bility under fire conditions required for the type of occupancy and construction of thebuilding to bar the spread of fire between adjoining buildings or between parts of thesame building on opposite sides of the division. A fire division may be an exterior wall,fire window, fire door, fire wall, ceiling or firestop.

A fire wall should be built of incombustible material, have a fire rating of at least4 hours, and extend continuously from foundation to roof. Also, the wall should haveenough structural stability to allow collapse of construction of structures on either sidewithout the wall collapsing. The size of openings that may be provided in a fire wall arerestricted by building codes, and the openings must be fire protected.

A firestop is a solid or compact, tight closure set in a hollow, concealed space in abuilding to retard spread of flames, smoke or hot gases. All partitions and walls shouldbe firestopped at every floor level, at the top story ceiling level and at the level of supportfor roofs. Also, very large unoccupied attics should be subdivided by firestops into areasof 3000 ft2 or less. Similarly, any large concealed space between a ceiling and floor orroof should be subdivided. Firestops should extend the full depth of the space and beplaced along the line of support of structural members and elsewhere, if necessary, toenclose areas not exceeding 1000 ft2 when situated between a floor and ceiling or 3000ft2 when situated between a ceiling and roof.

Openings between floors for pipes, ducts, wiring, and other services should be sealedwith the equal of positive firestops. Partitions between each floor and a suspendedceiling above are not generally required to be extended to the slab above unless thisis necessary for required compartmentation. But smoke stops should be provided atreasonable intervals to prevent passage of smoke to noninvolved areas.

Height and Area Restrictions

Limitations on heights and floor areas included between fire walls in any story of abuilding are mainly placed to protect human life. Height and area restrictions are usually



ensured by placing requirements determining minimum number of exits, proper locationof exits and maximum travel distance (hence escape time) necessary to reach a place ofrefuge. The limitations are also aimed at limiting the size of fires.

Unlimited height and area may be permitted for the most highly fire-resistant types ofconstruction. Also, in general, installation of automatic sprinklers increases permissibleheight and area in all classes, except those allowed unlimited height and area.

6.2.2 Fire Resistance Classification of Buildings

Types of construction may be divided broadly into 5 classes, which may be furthersubdivided, depending on local building codes.

1. Type I: Fire resistive construction- The primary structural frame, both inte-rior and exterior bearing walls, interior and exterior non-bearing walls and parti-tions, roof and floor construction and associated secondary members are of non-combustible materials, except as permitted by local building codes.

2. Type II: Protected noncombustible construction- The primary structuralframe, both interior and exterior bearing walls, interior and exterior non-bearingwalls and partitions, roof and floor construction and associated secondary membersare of non-combustible materials, except as permitted by local building codes.

3. Type III: Unprotected noncombustible construction-The exterior walls areof non-combustible materials and the interior building elements are of any ma-terials permitted by local building codes. Fire-retardant wood framing materialis permitted within external wall assemblies with a 2 hour or less fire-resistancerating.

4. Type IV: Heavy timber construction- The exterior walls are of non-combustiblematerials and the interior building elements are of solid or laminated wood with-out concealed spaces. Fire-retardant wod framing is permitted within external wallassemblies with a 2 hour or less fire resistance rating.

5. Type V: Ordinary construction- The structural elements, interior walls andexterior walls are of any material permissible by local building codes.

Types of occupancy may be divided into the following classes:

1. Assembly (average fire load- 10.0 psf)

2. Business (average fire load- 12.6 psf)

3. Educational (average fire load -7.6 psf)

4. Factory and Industrial (average fire load- 25.0 psf)

5. High hazard



6. Institutional (average fire load- 5.7 psf)

7. Mercantile (average fire load- 15-20 psf)

8. Residential (average fire load- 8.8 psf)

9. Storage (average fire load- 30.0 psf)

10. Utility and Miscellaneous

6.3 Means of Escape

The aim of the building designer is to prevent panic in emergencies, especially in confinedareas where large numbers of people may assemble. Thus, the arrangement of exitfacilities should permit occupants to move freely towards exits that they can see clearlyand that can be reached by safe, unobstructed and uncongested paths.

There should be more than one path to safety, and the paths should be accessibleand usable by handicapped persons, including those in wheelchairs.

6.3.1 Corridors

Minimum floor to ceiling height permitted for corridors is generally 80 inches. Minimumwidth depends on type of occupancy and passageway. Codes may require subdivisionsinto lengths not exceeding 300 ft for educational buildings and 150 ft for institutionalbuildings. Subdivision should be accomplished with noncombustible partitions incorpo-rating smokestop doors. In addition, codes may require the corridor enclosures to havea fire rating of 1 to 2 hours.

6.3.2 Exit Doors

These are doors providing access to the street or to exit passageways. Doors at stairs orpassageways should have a fire rateing of at least 3

4hour.

6.3.3 Horizontal Exit

This is a passageway to a refuge area. The exit may be a fire door through a wall witha 2 hour fire rating, a balcony providing a path around a fire barrier, or a bridge ora tunnel between two buildings. Doors in fire barriers with 3 or 4 hour ratings shouldhave a 11

2hour rating on each face of the fire division. Balconies, bridges and tunnels

should be at least as wide as the doors accessing them, and enclosures or sides of thesepassageways should have a fire rating of 2 hours or more.



6.3.4 Interior Stairs

These are stairs inside a building and that serve as an exit. Should be built of noncom-bustible materials. Stairway enclosures should have a 2 hour fire rating.

6.3.5 Exterior Stairs

These are stairs that are open to outdoors and serve as an exit to ground level. Heightof such stairs is often limited to six stories. The stairs should be protected by a fire-resistant roof and be built of non-combustible materials. Wall openings within 10 ft ofthe stairs should have a 3

4hour fire rating.

6.3.6 Refuge Areas

A refuge area is a space protected against fire and smoke. When located within abuilding, the refuge should be at about the same level as the areas served and separatedfrom them by construction with at least a 2-hr fire rating. Access to the refuge areasshould be protected by fire doors with a fire rating of 11

2hr or more.

A refuge area should be large enough to shelter comfortably its own occupants plusthose from other spaces served. The minimum floor area required may be calculated byallowing 3ft2 of unobstructed space for each ambulatory person and 30ft2 per personfor hospital or nursing-home patients.

Each refuge area should be provided with at least one horizontal or vertical exit, suchas a stairway, and in locations more than 11 stories above the ground level, with at leastone elevator.

6.3.7 Location of Exits

Building codes usually require a building to have at least two means of escape fromevery floor.

Exits should be remote from each other, to reduce the chance that both will beblocked in an emergency.

All exit access facilities and exits should be located so as to be clearly visible tobuilding occupants or signs should be installed to indicate the direction of travelto the exits.

Signs marking the locations of exits should be illuminated. Floors of means of egress should be illuminated with artificial light whenever the

building is occupied.

If an open floor area does not have direct access to an exit, a protected, continuouspassageway should be provided directly to an exit. The passageway should be kept



open at all times. Occupants using the passageway should not have to pass anyhigh-hazard areas not fully shielded.

Maximum travel distance on corridors without sprinklers is 100 ft for storage andinstitutional buildings and 150 ft for residential, mercantile and industrial occu-pancies. With sprinklers installed, maximum permissible travel distance is 150 ftfor high hazard and storage buildings, up to 300 ft for commercial buildings and200 ft for other occupancies.

Corridors leading to dead ends are prohibited in high hazard buildings. In assembly,educational and institutional buildings, the maximum length of corridor leading toa dead end is 30 ft, for residential buildings, maximum is 40 ft, and for all otheroccupancies, except high hazard, maximum is 50 ft.

6.3.8 Required Exit Capacity

Minimum width of a passageway for normal one way travel is 36 inches. For two waytravel, 60 inches is required. Running slope should not exceed 1:20 and cross slope 1:50.

Capacities of exit facilities are generally measured in units of 22 inches, and thenumber of persons per unit of width is determined by the type of occupancy. If 12 inchesor more are left over, 1

2unit can be counted. Less than 12 inches is disregarded.

Exit capacities of different facilities per unit of width are as follows:

Level components, such as doors: 100 persons per unit Stairway: 60 persons per unit Ramps, 44 inches or more wide, not more than 10% slope: 100 persons per unit Narrower or steeper ramps- Up: 60 persons per unit Narrower or steeper ramps- Down: 100 persons per unit

6.4 Fire Detection Facilities

There are five general types of detectors:

fixed temperature rate of rise photoelectric combustion products ultraviolet or infrared (flame detectors)



6.4.1 Fixed Temperature Detectors

They detect the actual ambient temperature which is reflected by a rise in the temper-ature of the detector itself. Since there is a thermal lag between the time the ambienttemperature reaches the rated temperature and the detector is able to react, for greatersensitivity, spacing between detectors is reduced.

Examples of fixed temperature detectors are:

(i) Disk Thermostats: These are the cheapest and most widely used detectors. Theyconsist of a bimetallic assembly which closes an electric contact when rated temper-ature is achieved. These detectors are self-resetting. The contact is disconnectedwhen normal temperature is achieved.

(ii) Thermostatic Cable: Consists of two sheathed wires separated by a heat sensitivecoating which melts at high temperatures allowing the wires to contact each other.The assembly is covered by a protective sheath. When any section has functioned,it must be replaced.

(iii) Continuous Detector Tubing: Consists of a small diameter Inconel tube ofalmost any length, containing a central wire separated from the tube by a thermistorelement. At elevated temperatures, the resistance of the thermistor drops to a levelpermitting current to flow between the wire and the tube. The current can bemonitored, thus temperature changes over a range of up to 500C can be detected.The detector is self-restoring when normal temperature is achieved.

6.4.2 Rate of Rise Detectors

They are designed to operate when the temperature rises at a specific rate, usually 10or 15 degrees per minute, regardless of the original temperature. They are not affectedby normal temperature increases and are not subject to thermal lag.

6.4.3 Photoelectric Detectors

They indicate a fire condition by detecting the smoke. A light source is directed sothat it does not impinge on a photoelectric cell. When sufficient smoke particles areconcentrated in the chamber, their reflected light reaches the cell, changing its resistanceand initiating a signal.

These detectors are particularly useful when a potential fire is likely to generatesubstantial smoke before appreciable heat and flames erupt.

6.4.4 Combustion-Products Detectors

Combustion products detectors are designed for extremely early warning, and are usefulwhen it is desirable to have warning of impending combustion when combustion productsare still invisible. The detectors involve either ionization chambers or resistance bridge



circuits, which are disrupted by the presence of combustion products, resulting in currentflow sor the ionization type and change in impedance for the resistance bridge type.

The detectors are sensitive to air currents, humidity and temperature, and shouldonly be used in consultation with competent designers.

6.4.5 Flame Detectors

They discriminate between visible light and the light produced by combustion reactions.The effective distance between flame and detector is about 10 ft for a 5 inch diameterpan of petrol, but a 12 inch diameter pan can be detected from 30 ft away.

Infrared detectors employ the characteristic flame flicker to distinguish between heatfrom combustion and heat from other sources, such as humans, animals or machinery.They also have a built in delay to eliminate accidental detection.

6.5 Fire Fighting Facilities

The method and material used to extinguish a fire depends on the type of fire. Fires areclassified into four, based on the combustible materials involved.

1. Class A fires involve ordinary combustibles, and are readily extinguishable by wa-ter, or by cooling, or by coating with a suitable chemical powder.

2. Class B fires involve flammable liquids and are extinguished by smothering. Coolingagents, if used, must be applied with care.

3. Class C fires involve live electrical equipment. The extinguishing agent must benon-conductive. Since a continuing electrical malfunction will keep the fire active,circuit protection must operate to cut off current flow after which an electricallyconductive agent may be used with safety.

4. Class D fires involve metals that burn, such as sodium, potassium, magnesium andpowdered aluminium. Special powders are necessary to extinguish such fires as wellas special training for operators. Such fires must never be attacked by untrainedpersonnel.

6.5.1 Automatic Sprinklers

Sprinklers provide an automatic spray dedicated to an area of fire outbreak. Sprinklerheads have temperature sensitive elements that respond immediately, discharging thecontents of the water main to which they are attached.

Types of sprinkler heads include:

1. Quartzoid Bulb:Consists of a glass tube which retains a water valve on its seating.The bulb or tube contains a coloured volatile fluid, which when heated to a specifictemperature expands to shatter the glass and open the valve. Water flows on to



a deflector, dispersing as a spray over the source of fire. Operating temperaturesvary with a colour coded liquid:

Orange : 57C Red : 68C Yellow : 79C Green : 93C Blue : 141C Purple : 182C Black : 204 or 260C

Figure 6.1: Quartzoid Bulb type sprinkler head

2. Fusible strut: It has two metal struts soldered together to retain a water valvein place. A range of solder melting temperatures are available to suit variousapplications. Under heat, the struts part to allow the valve to discharge water onthe fire.

3. Duraspeed solder type: Contains a heat collector which has a soldered capattached. When heat melts the solder, the cap falls away to displace a strutallowing the head to open. Produced in a range of operating temperatures.

Advantages of sprinkler systems include rapid response which reduces and isolatesfire damage and lower water usage than conventional fire-fighting service, resulting inless damage from excess water.

Sprinkler systems can be used for all Class A fires, and in many cases, Class B andClass C fires as well. For Class B fires, a sealed (fusible) head system may be used if theflammable liquid is in containers or is not present in large quantity. For Class C fires,water may be applied in the form of a nonconductive fog-like spray.

There are several types of sprinkler systems based on the mode of operation:



Figure 6.2: Strut type sprinkler head

Figure 6.3: Duraspeed solder type sprinkler head

(i) Wet Type: This system is used in heated buildings where there is no risk of thewater in the pipework freezing. All pipework is permanently pressure charged withwater and the sprinkler heads attach to the underside of the range pipes. Whena sprinkler head is fractured, water is immediately dispersed. Water will also flowthrough a groove in the alarm valve to an alarm gong, sounding both an internalalarm and alerting the local fire service.



Where water is mains supplied, it should be supplied from both ends so that if anyrepairs are being undertaken on one side of the system, the branch on that side canbe closed and the system supplied from the other side.

(ii) Dry Type:In the dry system, installation pipework above the differential valve ispermanently charged with compressed air. When a sprinkler head is fractured, theair escapes to allow the retained water to displace the differential valve and flow tothe broken sprinkler.

(iii) Alternate wet and dry: Is operated as a wet system most of the year but duringthe winter season it functions as a dry system.

(iv) Tail End: This system is mainly wet, i.e. charged with water, with the exceptionof one section of pipework which is fitted with an air valve to maintain that sectionwith compressed air. It can be used where part of the building, such as a warehouse,is unheated.

(v) Pre-action: Is used where there is the possibility of mechanical damage to thesprinkler heads by tall equipment, for example, by forklift trucks. To avoid un-neccesary water damage, the system is dry. If a sprinkler head is damaged, com-pressed air discharges to effect an initial alarm. Water supply to the sprinkler isdependent on a fire detector which will operate a motoriz

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