Electrical Installations

Electrical safety rules protect people and property from electric shock, burns, fire and equipment damage.

General rules:

1. Treat every wire as live until it is tested and proved dead.
2. Switch off and isolate the supply (and lock out the switch) before working on any equipment.
3. Never touch electrical equipment with wet hands or while standing on a wet floor; use insulating mats and rubber-soled shoes.
4. Use properly insulated tools, gloves and switches of correct rating; keep the work area dry.
5. Earth all metallic frames of appliances and equipment properly. Use three-pin plugs.
6. Install fuses, MCBs, ELCB/RCCB of the correct ratings; never bypass or replace a fuse with a wire of higher size.
7. Do not overload sockets or use damaged, joined or loose cables; replace worn insulation.
8. Do not work alone on live equipment; keep a trained person nearby who knows first aid and where the main switch is.
9. Never use water on an electrical fire; use CO2 or dry powder extinguishers, and switch off the supply first.
10. Keep warning signs (danger, high voltage), keep children away from sockets and use shuttered sockets.
11. In case of shock, switch off the supply first, or push the person away with a dry insulating object (wood, rubber), and then give first aid/CPR and call for help.
12. Follow the Nepal Electricity Authority rules and standard wiring regulations.

Household wiring is the arrangement of cables and accessories used to supply electricity to lights, fans and sockets. The main types are:

1. Cleat wiring. Insulated (VIR/PVC) cables are held by porcelain or wooden cleats fixed to walls and ceilings. It is cheap and temporary, easy to inspect, but unsafe and unsightly. Used in temporary sites and exhibitions.

2. Casing and capping wiring. Cables run in grooves of a wooden/PVC casing that is covered by a capping. It is cheap and fairly neat but liable to fire and damp. It is now rare.

3. Batten wiring. Cables are fixed with brass clips on teak wood battens. It is simple and cheap, but it is not strong against moisture, so it is seldom used.

4. Conduit wiring. Cables are drawn through PVC or steel pipes (conduits), installed on the surface or concealed in walls and slabs. It gives the best protection against moisture, mechanical damage and fire, and a neat appearance. It is the standard for modern buildings, though costly and difficult to modify.

• Surface conduit: pipes on the wall surface.
• Concealed conduit: pipes buried in plaster, hiding the wires.

5. Lead-sheathed (TRS/CTS) wiring. Cables with a tough rubber/lead sheath are fixed directly on walls with clips. Good for damp areas and used in small or old houses.

An MCB can also be used as an ON/OFF switch, and it can be re-set without opening the board.

A Miniature Circuit Breaker (MCB) automatically opens the circuit when the current exceeds its rated value, and can be closed again by hand after the fault is cleared. It uses two tripping mechanisms.

 In --o--[ bimetal strip ]--[ solenoid coil ]--o--- Out
         (overload trip)    (short-circuit trip)
                \              /
                  trip lever -> releases moving contact

1. Thermal (bimetallic) trip for overload.

• A bimetallic strip (two metals with different expansion rates) carries the load current.
• With a prolonged overload, the strip heats up and bends, and at a certain point it releases the latch.
• The spring then opens the contacts. The time delay is inversely related to the current: a small overload takes some time to trip, a larger one trips faster. This avoids false tripping on motor starting surges.

2. Magnetic (electromagnetic) trip for short circuit.

• A solenoid coil is in series with the load.
• During a short circuit, the very large current produces a strong magnetic field which pulls a plunger instantly, striking the trip lever and opening the contacts within a few milliseconds.

Arc extinction. When the contacts separate, the arc is drawn into an arc chute, where it is split and cooled, and quenched.

Ratings are given in amperes (6 A, 10 A, 16 A, 32 A, etc.) with B, C or D curves for the magnetic trip level.

Earthing (grounding) is the connection of the non-current-carrying metal parts of an electrical installation (or the neutral of the supply) to the general mass of the earth through a low-resistance conductor.

Purposes:

• To protect people from electric shock if the insulation fails and the metal body becomes live.
• To give a path for fault current, so that fuses/MCBs/ELCBs operate quickly.
• To keep the voltage of the system stable with respect to earth, and to protect equipment from lightning and surges.

 Appliance body --- earth wire (green/yellow) ---+
                                                  |
                                 Earth electrode (pipe/plate)
                                 buried in soil with salt + charcoal

Methods of earthing:

1. Plate earthing: a copper plate (60 cm x 60 cm x 3 mm) or GI plate (60 x 60 x 6 mm) buried vertically 3 m deep in alternate layers of charcoal and salt.
2. Pipe earthing: a GI pipe (about 38-50 mm diameter, 2-3 m long) with holes, driven vertically into the ground; the most common type; water is poured to keep the soil moist.
3. Rod earthing: a copper or GI rod driven into the ground.
4. Strip/wire earthing: a GI strip laid in a trench.

The earth resistance should be as low as possible, typically below 5 Ω for a house (1 Ω for power stations) and is checked using an earth tester, with seasonal watering.

A power supply system carries electrical energy from generating stations to consumers. It has four main stages.

 Generating  -> Step-up   -> Transmission -> Step-down  -> Distribution -> Consumer
 station        transformer    line           substation     (11 kV/400 V)
 (11 kV)       (132/220 kV)   (HV)           (33/11 kV)

1. Generation: power is generated at 11 kV, 3-phase, 50 Hz by synchronous generators in hydro, thermal, solar and other plants (in Nepal, mostly hydro).
2. Transmission: the voltage is raised by transformers to 66, 132, 220 or 400 kV so that current and $I^2R$ loss are small for long distances over overhead lines.
3. Sub-transmission/primary distribution: step-down substations reduce voltage to 33 kV or 11 kV for towns and industries.
4. Secondary distribution: distribution transformers step down to 400 V (line) / 230 V (phase), supplied by a three-phase four-wire system, with 230 V single-phase for homes.

Systems of supply: AC (usually) or DC; single-phase two-wire; three-phase three-wire (transmission); three-phase four-wire (distribution). The distribution layout may be radial, ring or interconnected, in order to give reliability.

In Nepal the Nepal Electricity Authority (NEA) operates the grid at a standard of 230 V, 50 Hz.

The size (cross-sectional area) of a conductor must carry the load current safely without overheating and without an excessive voltage drop. The steps are:

1. Find the total load. Add the power ratings of all appliances on the circuit:

$$P_{total}=\sum P$$

2. Calculate the load current.

Single phase:

$$I=\frac{P}{V\cos\phi}$$

Three phase:

$$I=\frac{P}{\sqrt3\,V_L\cos\phi}$$

3. Apply a safety margin and diversity factor. Increase the current by 20-25% for future expansion (or use a diversity factor if not all loads operate at once).

4. Select the wire size for current-carrying capacity. From the manufacturer's or standard table (such as IS/IEC), choose a conductor whose rated ampacity is at least the design current, considering the ambient temperature, the method of installation (conduit, open) and the number of cables grouped together. Typical copper sizes: 1.5 mm² (lighting, up to 10 A), 2.5 mm² (power sockets, 16 A), 4 mm² (geysers, ACs, 25 A), 6 mm² (32 A).

5. Check the voltage drop. The drop in the cable should not exceed about 3-5% of the supply voltage:

$$\Delta V=I\cdot R=I\cdot\frac{\rho L}{A}$$

If it exceeds the limit, choose a larger size.

6. Check short-circuit withstand and coordinate with the protective device (MCB rating at or above the load current and below the cable rating).

Example: A 2 kW heater at 230 V, pf 1 gives $I=8.7$ A; with 25% margin, 10.9 A, so a 2.5 mm² copper wire with a 16 A MCB is selected.

Protective devices disconnect a faulty circuit automatically to prevent damage to cables, appliances and persons. Their sizing must match the load and the cable.

Common devices:

1. Fuse: wire that melts at overload. Rated in amperes.
2. MCB: thermal-magnetic breaker for overload and short circuit; ratings 6, 10, 16, 20, 32, 40, 63 A.
3. MCCB: moulded-case breaker for higher currents (up to a few hundred amperes).
4. RCCB/ELCB: detects earth leakage (residual current, typically 30 mA for people) and trips quickly.
5. Isolators and surge protectors for switching and voltage surges.

Sizing rules:

• Rated current of the device $I_n$ must be at least the design (load) current $I_b$, and not more than the current-carrying capacity of the cable $I_z$:

$$I_b\le I_n\le I_z$$

• The breaking capacity must be greater than the prospective short-circuit current at the point of installation (e.g. 6 kA for a house).
• The rated voltage must match the system (230/400 V).
• Select the curve type: B for resistive loads, C for mixed/inductive, D for motors with high inrush current.
• For a motor, the fuse/breaker is chosen for about 1.5-2.5 times the full-load current to allow for starting.

Example: a geyser of 3 kW at 230 V draws 13 A, so a 16 A MCB with a 4 mm² cable is chosen, and a 30 mA RCCB is added.

Importance of protective devices

In homes, faults such as short circuits, overloads and earth leakage may occur. Without protection, they can cause:

• Fire, as cables overheat and insulation burns.
• Electric shock and loss of life from live metal bodies.
• Damage to costly appliances (TV, refrigerator, computer) from surges.
• Interruption of supply to the whole house.

A protective device detects the abnormal condition and disconnects the circuit quickly, so that damage is limited and safety is assured. It also isolates the faulty portion, and the rest remains healthy.

Types of protective devices

1. Fuse: a thin wire/strip of low melting point (tin-lead, copper) that melts when current exceeds its rating. Types: rewirable (kit-kat), cartridge (HRC). Simple and cheap but must be replaced.
2. MCB (Miniature Circuit Breaker): trips automatically on overload (thermal) and short circuit (magnetic) and is resettable.
3. ELCB / RCCB (Residual current device): compares phase and neutral current; if the difference (leakage to earth) exceeds 30 mA, it trips within about 30 ms, protecting against shock.
4. RCBO: a combination of an MCB and RCCB in one unit.
5. Surge protection device (SPD): diverts voltage spikes (lightning, switching) to earth.
6. Overvoltage/undervoltage relay or stabiliser: protects appliances from supply fluctuations.
7. Earthing: the basic protective measure that gives a low-resistance path for fault current.