bs 2 electrical installation 1

BUILDING SERVICES TWO (BLD60503)

ELECTRICAL INSTALLATION

Prepared by Tan Hee Chai

BUILDING | SERVICES | TWO (BLD60503)

Sources of Electricity


3 major classifications for power production in reasonably large scale are :-

1) Thermal power generation 2) Nuclear power generation. 3) Hydro-electric power generation.


Electrical supply and distribution

•Fuel converted to steam at high temperature and pressure, to drive turbines which generate electricity at 25kV alternating current.•AC changes polarity from positive to negative at 50 cycles per second (50 hertz)



Reasons power is supplied in Alternating Current (AC) at 50 Hertz:

•Facility of generating machines•Ease of transformation•Use of electricity powered AC motors


Electrical supply and distribution132, 275 or 400 kV: National transmission

33 000 V: Large/Heavy industries, cities, towns and railways

11 000 V: Light Industries, hospitals, towns and villages

400 V: Small industries, offices, farms etc

230 V: Housing, schools, small commercial premises.



• All but 230 V is supplied with 3 phases: domestic appliances and standard office equipment are designed to operate from only one phase.

• 33 000 V, three phase supply to railway operators via sub-station where it is transformed to 25 000 V DC


Local distribution-Three phase and single phase supply


Local distribution-Three phase & single phase

Generators at power station are wound with three conductors.•Each generate one phase•Three phases into 3 wires

Called ‘three phase three wire’ supply


Local distribution-Three phase form

At secondary side of transformer at sub-station:•Any pair of three wire is 415 volts•Any of three conductors is 240 volts

Called ‘three phase four wire; supply


Local distribution

Service cable supply to external meter cupboard


Local distribution

Incoming cable


Local distribution Overhead cable


Local distribution -TT System


TT System• Supplied overhead without earth terminal• Consumer to provide earth electrode.• Residual current circuit breaker (or trip switch) provided

between meter & consumer unit.• RCCB operate on imbalance between the phase or live

conductor and the neutral.• A fault will energise the core coil to effect the

electromagnetic trip.


TN-S System


TN-S System• Applied to underground supplies • Metal sheathed and/or armoured cable.• Consumer’s terminal connected to metal sheath


TN-C System• Earth and neutral

conductors combine to provide a protective earth and neutral (PEN) or Protective multi earthing(PME)


Consumer unit


Electrical Meter

• One rate (tariff) – domestic consumers

• Off peak & on peak meters


Electrical Meter•Two coils: current coil connect to phase and & voltage coil across phase & neutral•Interaction of two energises a dics which rotates at speeds proportional to power consumed•Gears driven disc effect a digital display of energy consumed in KWh


Overload Protection

• Need to prevent current leaks from electrical installation due to insulation failure in the electrical circuit.

• Earth Leakage circuit breaker detects earth leakage current and makes the power supply off by opening the associated circuit breaker.


Overload Protection

This can be provided by:

1. Semi-enclosed rewirable fuses2. Cartridge fuses3. Miniature circuit breakers (MCB)


Overload protection


1. Semi-enclosed rewirable fuses

• Advantages • Low capital cost• Cheap to replace element• Simple in concept• No moving parts


Disadvantages

•Prone to abuse. Incorrect wire can be used•Deterioration with age•Reliability can vary, with temperature & fixing method.•Cannot be tested•Cannot be replaced quickly•Fusing factor higher than current rating


2. Cartridge fuses

Advantages•Small and compact•No moving parts•Accurate and consistent to declared rating•Will not deteriorate in protective tube


2. Cartridge fuses

Disadvantages•Cannot be repaired or reset, must be replaced•Prone to abuse (with metal foil)•Much more expensive than fuse wire


3. Miniature Circuit breaker

Advantages•No moving parts•Accurate and consistent to declared rating•Durable•Can be reset


3. Miniature Circuit Breaker

Disadvantages•Cannot be repaired, must be replaced•Much more expensive than fuse wire


The End

bs 2 electrical installation 1

Investor Relations