electrical installation 21 overcurrent protection (note: all the mentioned tables in this course...

Electrical Installation 2 1

Overcurrent Protection(Note: All the mentioned tables in this course refer to, unless otherwise specified, Low

Voltage Electrical Installation Handbook, by Johnny C.F. Wong, Edition 2004)

Chapter 6


General

Purpose– Safety of Personnel (Shock) and Property (Fire Hazards)

– Maintain reliable life of equipment and systems Overcurrent

– a current exceeding the rated value of a circuit or the current-carrying capacity of a conductor

– Overload

– Fault• Short-circuit fault

• Earth fault

– This part, we are concerned with the short-circuit fault only.


Devices for Overcurrent Protection

Examples are:

– Fuses (HBC/HRC)

– Miniature circuit breakers (MCBs)

– Combined MCB and RCD (RCBOs)

– Moulded case circuit breakers (MCCBs)

– Air circuit breaker + IDMTL relay


Devices for Overcurrent Protection

Protection for the NEUTRAL conductor is NOT required for TT and TN systems– 100% Neutral should be used

– Protection already provided by the live conductor protective device

– Neutral link (not protective device)

– If the neutral breaks, the live supply must break too

– LOSS OF NEUTRAL must be avoided to eliminate the risk of raising the potential of the load star point to dangerous level


Protection against Overload

Main purpose is to avoid sustained temperature that causes deterioration of insulation

e.g. only a short duration of overload current is allowed to flow in a motor circuit - the starting duration should be short. Otherwise larger cables shall be installed


Selection of Overload Protective Device

design current Ib nominal current or rated current In lowest CCC, Iz


Position of Overload Protective Device

At the point where there is a reduction of Iz (CCC) such as– CSA of conductor is reduced

– Worsening of environmental condition

– Change of cable type or installation method Overload protective device and fault current protective

device may be the same device and may be 2 different devices


Overload Protection of Conductors in Parallel

The Iz in this case is the sum of Iz of the individual cables provided they are in accordance with the conditions for parallel running cables.

Standard ring final circuits are not in this context.


Omission of Overload Protective Device

Overload current is unlikely to flow Refer to Fig. 6.5 for illustration



Unexpected loss of supply is more dangerous than overloading of circuit

Refer to Fig. 6.6 for illustration



CT secondary circuit should not be broken. If this is the case, dangerous high voltage will appear at the CT secondary side




Protection is afforded by electricity supplier’s protective device (not normally accepted by power companies in Hong Kong)



Protection against Fault Current

Cause - Insulation failure, faulted switching operation and invariably associated with arcs

Effect - Thermal and mechanical stress produced in conductors, associated support and plant components

Fault current protection is to prevent this


Protection for Maximum prospective fault current, Isc

Maximum prospective fault current, Isc

– 3-phase : calculation based on symmetrical fault impedance,

Isc = Up / Z

where Up = phase voltage Z = phase conductor impedance at supply source– 1-phase : calculation based on line-neutral impedance at 20oC,

Isc = Up / (Z + Zn)

where Zn = neutral conductor impedance at supply source– The above should base on fault appeared just after the protective device

– Breaking capacity of fault current protective devices should exceed the max. prospective fault current, Isc


Minimum Prospective Fault Current, I

Minimum prospective fault current, I

– Calculation bases on total phase-neutral impedance values, up to the remote end

I = Up / (Z + Zn+ Z1 + Z2)

where Z1 = phase conductor impedance at consumer side

Z2 = neutral conductor impedance at consumer side

– Significant in determining fault disconnection time, t


Protection for Minimum Prospective Short Circuit, I

Basic equation to satisfy– k2S2 > I2t

Where– k - a constant associated with the type of conductor + in

sulation– S - Cross-sectional Area (CSA) of conductor– I - minimum prospective fault current (fault occur at re

mote end)– t - disconnection time– I2t - let-through energy


Guidelines in fault current protection

Max. prospective 3-ph symmetrical short-circuit at the l.v. source of supply provided by the supply company is 40kA.– All fuses and MCCBs at source of energy must have breakin

g capacity > 40kA

– Fault current protective devices with smaller breaking capacities are generally acceptable if they are backed up by fuses to BS88-2.1 or BS88-6 (Backup protection will be discussed later in Chapter 10)

The further away from the source of supply, the smaller the prospective short circuit current.


Fault Current Protection in General

Example: The following single phase circuit is protected by 63A BS88 fuse, the prospective short circuit current at the fuse is known to be 3 kA. A connected load, with circuit distance 87m from the fuse, is to be supplied by using 16mm2 1/C PVC copper cable. Please check whether the fuse can provide short circuit protection for the cable.

ZSource voltage Up

Zn

Z1

Z2

Load

63A fuse

Source Installation side

1.68 Ω / km



At fuse position, it is given that the 1-Ф prospective short circuit current is 3 kA,

i.e. Isc = Up / (Z + Zn) Z + Zn = 3000 / 220 = 0.073 Ω

The total impedance from the fuse to the remote load end, Z1 + Z2 = 2 x 87m x 1.68 Ω/km = 0.292 Ω

So, the minimum short circuit current at the load end, I = Up / (Z + Zn+ Z1 + Z2) = 220 / (0.073 + 0.292) = 603 A



Whether k2S2 > I2t ??

From I-t characteristic of BS88 fuse, t = 0.18 s when I = 603 A

PVC copper cable is used k = 115

S = 16 mm2

k2S2 = 1152 x 162 = 3,385,600 A2S

I2t = 6032 x 0.18 = 65,450 A2S

k2S2 > I2t O.K


Fault Current Protected by Overload Protective Device

The protective device is assumed to be adequate if it– satisfies conditions for overload protective device. That is, w

e sizes cable and protective device by using the principle

Ib ≤ In ≤ Iz ; and

– Breaking capacity of protective device ≥ Maximum prospective fault current, Isc

This is the most common way to protect a circuit, since only ONE protective device is needed.


Position of fault current protective device

Normally placed at or before the point where a reduction in the conductor’s current-carrying capacity (Iz) occurs. Such change may be due to a change in:– cross-sectional area, method or installation, type of cable or

conductor, or in environmental conditions


Fault current protection of conductors in parallel

A single device may provide protection against fault current for conductors in parallel provided the parallel conductors are in accordance with Section 5.8


Omission of short-circuit protective devices

Conductor between a transformer and its control panel Refer to Fig. 6.18 for detailed illustration

electrical installation 21 overcurrent protection (note: all the mentioned tables in this course...

Documents

parallel n

overloading of circuit

overload n main purpose

systems n overcurrent

z slide

current i n lowest ccc

deterioration of insulation

conductor overload fault