ELECTRICAL SERVICES IN BUILDINGS | SINGLE AND THREE-PHASE SUPPLY | DISTRIBUTION CIRCUITS

  

ELECTRICAL SUPPLY

Electrical power is provided to the building by the Government. Electricity is distributed in small towns and villages by overhead wires. But in large cities and towns, the distribution is done through underground cables. The supply of electricity is including the cables up to

 

1. Outer casing with coarse sand (30 cm thick).

2. Lining with coarse aggregate (15 cm thick).

3. Brickwork with mortar joints.

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the electric metre. In order to isolate the main supply from the building, a system of fuses is installed between the supply inlet and the electric metre board. From the fuses, the wires are led to the metre and from there to the consumer unit. From this point, the electricity is distributed to the various parts of the building. It is the responsibility of the owner of the building to provide wiring and other fixtures from the metre to the distribution board and other parts of the building (Varghese, 2012).

 

SINGLE AND THREE-PHASE SUPPLY

The electricity from the sub-stations are stepped down through local transformers and supplied to nearby buildings as a single-phase or three-phase supply (50 cycles per second at 220 volts between the phase and the neutral). Then the supply is 230 volts between a phase and a neutral and it is 400 (= √3 × 230) volts between the phases. In a single-phase supply,

there shall be one live-phase wire and a neutral. In a three-phase supply, there shall be three live phases and a neutral.

When the electrical power load is small and the total current drawn is less than 30 amp, a single-phase may be sufficient. That is only for lighting and use of fans a single-phase may be enough. However, when the load is heavy, as when the use of any type of equipment such as air conditioners, cooling ranges, etc., it is necessary to draw current from three live phases and a neutral. The electric current metre for a three-phase supply is different from that of single-phase. It is a condition that all equipment used in residences must be 220–230 volts. Instead of underground cable, if supply is drawn from overhead insulated cable special care has to be taken. In this case it is to be first run down the walls with insulated cables and then turned up at the entry point of the building to prevent rainwater from running along the cable into the electric board.

 

DISTRIBUTION CIRCUITS

In an ordinary building, for distribution of electricity from the consumer control unit to other points, the supply is divided into three separate types of circuits, viz.,

1. Lighting circuit of low capacity

2. Power circuit through ring circuit

3. Fixed appliance circuits

Red wires are used for the live wire whereas the neutral is black or blue and earth wires are green and yellow. In the circuit, a phase changer may also be incorporated.

 

MATERIALS AND DEVICES USED IN WIRING

Following are the materials and devices used in the wiring of a building.

 

1. Cables

Cables consist of copper or aluminium conductors surrounded by insulation. Aluminium conductors are used to reducing the cost but they are inferior in quality compared to copper cables. Now-a-days stranded wires are commonly used for better performance.

Rubber or PVC insulation may be made. In moist or wet conditions PVC insulation may fare better than rubber.

 

2. Fuses

Basically, a fuse protects appliances and cables from damage by electrical faults. The current rating of fuses should be lower than that of the cable it has to protect. Two types of fuses are in use, viz., Rewirable or semi-enclosed fuses and cartridge fuses.

 

3. Miniature Circuit Breaker

Now-a-days miniature circuit breakers (MCB) are used instead of fuses. Miniature circuit breakers are single-pole switches that automatically switches off when excess current flows. An MCB trip at 1.25 times its rated current capacity. It is only an overload device. But for the initial cost, it has many advantages.

 

4. Earth Leakage Circuit Breaker (ELCB)

An electric appliance with a fault is connected to a line and if the metal parts are not earthed the metal may become live without the fuse blowing. Such a situation may cause a shock to the person touching it. On the other hand, if the metal body is earthed, then the current will flow through the earth wire. In such a condition, it is advisable to use a tripping device called the earth leakage circuit breaker. It automatically trips even where a small current flows through the earth wire. This switch is connected to the neutral line of the electric supply. When it trips, the supply to the building will be completely cut off.

 

5. Residual Current Circuit Breaker (RCCB)

This type of breaker is a much more positive protection device that can be provided instead of an earth leakage circuit breaker to avoid electric shocks and current leakages. An RCCB is connected to all three phases and the neutral. It is more expensive but provides excellent protection.

 

WIRING OF BUILDINGS

The following are three types of wires:

1. Sheathed wiring by surface fixing

2. Conduit installation on walls and ceilings

3. Concealed conduit wiring

Nowadays in most of the buildings concealed PVC conduit wiring is used. The PVC conduits may be laid on the surface or concealed. In both cases, care has to be taken to see that no water enters the pipes. In the case of a concealed pipe, it is safe to surround the conduit in chicken mesh reinforcement.

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