Electrical InstallationAn electrical installation is a combination of electrical equipment installed to fulfill a specific purpose and having coordinated characteristics. In dealing with the electrical installation, it is necessary to ensure the safety of personnel as well as the protection of equipment from electrical faults.

The most common types of faults in domestic systems are

1. The short circuit faults (phase to neutral faults)- as a result of which large currents will flow and damage may occur to wires, insulators, switches, etc..

2. insulation failure (fault between the phase conductor and non-current carrying metallic parts)- as a result of which high voltages may appear on the frames of equipment and may be dangerous to a person coming in contact with it.

The most popular methods of protection

1. Use of fuses or circuit breakers (such as the Miniature Circuit Breaker – MCB).

2. Use of earth leakage and residual current circuit breakers(RCCB).

3. Earthing or grounding of equipment.

FuseThe fuse consists of a short length of thin wire. When the

current flow is greater than the fusing current of the fuse, it will get hot and burn (melt), thus interrupting the fault current before damage could be caused.

The time current characteristic of a fuseFusing time (ms)

Current (A) Rated Current

There are 3 general types of fuses.

1) Re-wirable (semi-enclosed) fuse

2) Cartridge fuse

3) High-rupturing capacity (HRC) fuse – a

development of the cartridge fuse

Re-wirable (semi-enclosed) fuse

Advantages

• The re-wirable fuse is cheap,

Semi-enclosed (re-wirable) fuse is a

simple device. It consists of a short

length of wire, generally of tinned

copper. The current at which the

wire melts depends on the length of

the wire and its cross sectional area

(R=ρl/A).

Disadvantages

Deterioration with time due to oxidation - may operate at lower currents than expected due to the reduction in cross sectional area and hence increase in resistance.

Very easy for an inexperienced person to replace a blown fuse-element with a wire of incorrect size or type.

Calibration of re-wirable fuse can never be accurate

The time taken for the fuse to blow may be enough to bring damage to circuit conductors and the equipment being protected.

Fully enclosed (cartridge) fuse

• fuse wire is enclosed in an evacuated glass tube with metal end caps

Advantages• Non-deterioration of the fuse element• Usually more accurate

Disadvantages• More expensive to replace

Fully enclosed (cartridge) fuse Developed to overcome the disadvantages of the re-wirable type of fuse.

High rupturing capacity (HRC) fuses

The HRC fuse is usually a high-grade ceramic barrel containing the fuse element. The barrel is usually filled with sand, which helps to quench the resultant arc produced when the element melts.

Miniature Circuit Breaker (MCB)

Magnetic mechanism

The magnetic mechanism uses a solenoid with an iron piece. It is used for short circuit (fault) protection.

Can operated by hand or automatically.

The automatic operation is done by using magnetic or thermal mechanism.

Thermal mechanism

This mechanism uses a heat sensitive bimetal element.

It is used for overload circuit protection.

Advantages of mcb s over fuses areNon destructive determination of tripping

characteristicsShorter tripping times under moderate over currents

than with fusesImmediate indication of faulty circuitReclosing can be effected at once after the fault has

been clearedNo stock of fuses are requiredCan be easily used as a circuit control switch when

needed

Circuit Breakers for earth leakage circuit protection

These are used to detect electrical faults to earth in equipment and to clear the fault by tripping.

Classified into two types:

Voltage operated protection − Earth Leakage Circuit Breaker (ELCB)

Current operated protection − Residual Current Circuit Breakers (RCCB) or Residual Current Device (RCD)

Earth Leakage Circuit Breaker (ELCB):

For the proper operation two earth terminals are required.i. Frame earth (which all non-conducting metallic

parts of equipment are connected) ii. ELCB reference earth.

The ELCB will normally operate when the voltage across the coil exceeds about 40 V with respect to the reference earth.

Residual Current Circuit Breaker (RCCB):

The operation is based on a fault current Difference between the line current and the neutral current is

used to energize the solenoid, which causes the switch to open.

Basic Domestic Wiring System(single phase)

Up to the energy meter belongs to the supply authority. The consumer’s installation starts from the Main switch. Next followed by usually one (or more) RCCBs. Then followed by a Distribution board (with different rated MCBs). Final circuits are connected to the distribution box. ( circuit for

Lighting and circuits for socket outlets )

Service Fuse Box

The electricity supply into your house is controlled by a service fuse, usually found before the electricity meter.

Electricity Meter

An electrical meter or energy meter is a device that measure the amount of electrical energy supplied to or produced by a consumer.

Unit of measurements kWh

One unit is equal to amount of energy used by a load of 1kW over a period of one hour,

1kW × 1h = 1000 Js-1 × 3600 s = 3,600,000 J

Main Switch

The main switch is use to switch off and on the supply of electricity to the entire house.

Both live and neutral wires are can disconnected. When electrical inspection carried out it is vital to

turned off the main switch.

Trip Switch

RCCB or ELCB use as trip switch.

Final circuit for lighting:

Loop-in method is use for wiring a final circuit for lighting.

Loop-in Method:

o This method enables all joints and terminations in a single final circuit to be made at switches or other accessories.

o Lamp circuits do not normally need an earth wire.

Type of switches

1. Single pole switches

This is used turn a light, receptacle or device on and off from a single location

2. Two way switches

This is used when it is necessary to operate a lamp from two positions such as at the top and bottom of a staircase and at the ends of a long corridor.

Final circuits for socket outlets:

Socket outlets (plug points) are wired in two ways

I. Ring circuit connection method.

II. Radial connection method.

Ring circuit connection method

Looping is done for the live conductor, neutral conductor and the protective conductor in separate rings.

Radial circuits

Commences from the distribution board. through an MCB/fuse of specific rating (e.g.20A).

Loop into each socket outlet but ends at a socket outlet.