Working of a Lead Acid Battery

A lead acid battery is a secondary (rechargeable) cell that converts chemical energy into electrical energy during discharge and reverses the reaction during charging. It consists of:

• Positive plate (anode in discharge): Lead dioxide (PbO₂)
• Negative plate (cathode in discharge): Sponge lead (Pb)
• Electrolyte: Dilute sulphuric acid (H₂SO₄, about 35% strength)

During discharge (when supplying current):

• At the negative plate, lead is oxidised and releases electrons: Pb + SO₄²⁻ → PbSO₄ + 2e⁻
• At the positive plate, lead dioxide gains electrons and is reduced: PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻ → PbSO₄ + 2H₂O
• Overall reaction: Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O

Electrons flow through the external circuit from the negative terminal to the positive terminal, while the electrolyte conducts ions internally. As discharge proceeds, both plates get coated with lead sulphate (PbSO₄) and the acid becomes dilute (water is formed), so the specific gravity of the electrolyte falls — this is used to check the state of charge.

During charging:

• An external DC source is connected with its positive terminal to the positive plate.
• The above reaction is reversed: PbSO₄ at both plates is converted back to PbO₂ (positive) and Pb (negative), and the acid is regenerated.
• Overall reaction: 2PbSO₄ + 2H₂O → Pb + PbO₂ + 2H₂SO₄

Thus the battery can be charged and discharged many times. Each cell gives about 2 V; six cells in series give a 12 V battery. Water lost by electrolysis during overcharging must be topped up with distilled water.

Uses of Lead Acid Battery in Household

1. Inverter / backup power supply — to run lights, fans and TV during load-shedding or power cuts.
2. Solar home systems — to store electricity generated by solar panels for use at night.
3. Starting (SLI) battery in vehicles — cars, motorcycles and tractors for ignition, lighting and horn.
4. Emergency lights and torch lights — for emergency illumination.
5. UPS for computers and routers — to keep them running during power failure.
6. Small DC appliances — radio, gate motors, and electric fencing units in rural households.

Ohm's Law

Ohm's law states that the current flowing through a conductor is directly proportional to the potential difference across its ends, provided the physical conditions (temperature, length, and cross-sectional area) remain constant.

If $V$ is the potential difference across a conductor and $I$ is the current flowing through it, then

$V \propto I \quad \Rightarrow \quad V = IR$

where $R$ is the resistance of the conductor. In SI units, $V$ is in volts (V), $I$ in amperes (A), and $R$ in ohms ($\Omega$).

Limitations of Ohm's Law

1. Temperature dependence: Ohm's law holds only when the temperature is constant. For metals, $R$ increases with temperature, so $V$ and $I$ are no longer proportional.
2. Non-ohmic devices: It is not valid for devices such as diodes, transistors, thermistors, filament lamps, and electrolytes, whose $V$–$I$ graph is not a straight line.
3. Non-linear conductors: It fails for semiconductors, vacuum tubes, and gas discharge tubes where resistance changes with applied voltage.
4. Extremely high/low conditions: It does not hold for very high voltages, very low temperatures (superconductors), or when the physical dimensions of the conductor change.

Diesel Power Plant – Working Principle and Operation [5 marks]

A diesel power plant is a standby or isolated generating station in which a diesel engine drives an alternator (generator) to produce electrical energy. It is used where load is small, fuel transport is easy, and grid supply is unavailable or unreliable. Its main components are: diesel engine, alternator, fuel supply system, air intake system, exhaust system, cooling system, lubrication system, starting system, and control/switchgear panel.

Working principle: The plant works on the Diesel cycle (constant-pressure cycle). Air is drawn into the cylinder and compressed to a high pressure and temperature; fuel is then injected and burnt, and the expanding hot gases push the piston to do mechanical work. This work rotates the engine shaft, which is coupled to the alternator, converting mechanical energy into electrical energy.

Step-by-step operation:

1. Starting: The engine is started by a battery-operated starter motor or compressed air. The crankshaft begins to rotate.
2. Air intake: During the suction stroke, the inlet valve opens and filtered atmospheric air from the air filter enters the cylinder.
3. Compression: The piston moves up, compressing air to about 30–40 bar, raising its temperature to about 500–600 °C. Both valves remain closed.
4. Fuel injection and combustion: At the end of compression, the fuel injection pump and injector spray diesel fuel into the hot air. The fuel ignites automatically (no spark plug needed), causing a sudden rise in pressure.
5. Power stroke: The expanding combustion gases push the piston down, doing work on the crankshaft. This is the only stroke that produces power.
6. Exhaust: The exhaust valve opens and burnt gases are expelled through the silencer to the atmosphere.
7. Cyclic repetition: The four strokes — suction, compression, power and exhaust — repeat continuously (four-stroke engine), or in two strokes in a two-stroke engine.
8. Generation: The engine shaft rotates the alternator; the alternator generates 3-phase electrical power, which is stepped up by a transformer and supplied through switchgear and transmission lines.
9. Auxiliary systems in operation:
   • Fuel system: storage tank → strainer → fuel pump → filter → injection pump → injector.
   • Cooling system: water/radiator cooling removes engine heat.
   • Lubrication system: reduces friction and wear of moving parts.
   • Exhaust system: silencer reduces noise; heat may be recovered by a waste heat recovery boiler.

Advantages: quick starting, low initial cost, simple and compact, needs less water, can be located anywhere. Limitations: high fuel cost, high maintenance, limited capacity, shorter life, noise and smoke pollution.

No Load Test on a Transformer (Open Circuit Test)

Circuit connection: The low-voltage (LV) winding is kept open and the high-voltage (HV) winding is connected to a rated-voltage AC supply through an ammeter, a voltmeter and a wattmeter. The voltmeter measures the applied voltage, the ammeter measures the no-load current, and the wattmeter measures the input power.

Procedure:

1. The connections are made as above, with the LV side left open (no load connected).
2. The supply voltage is gradually raised to the rated voltage of the transformer using a variac (auto-transformer).
3. At rated voltage, the readings of the voltmeter (V₁), ammeter (I₀) and wattmeter (W₀) are noted.
4. The readings are recorded and the test is repeated if required.

Calculations:

• Since the LV side is open, the no-load current I₀ is very small (2–5% of full-load current) and flows only through the exciting branch. So copper loss is negligible and the wattmeter reading gives practically the core (iron) loss: $W_0 = P_i \text{ (core loss)}$
• No-load power factor: $\cos\phi_0 = \frac{W_0}{V_1 I_0}$
• Magnetizing component: $I_m = I_0\sin\phi_0$
• Working (loss) component: $I_w = I_0\cos\phi_0$
• Core-loss resistance: $R_0 = \frac{V_1}{I_w}$
• Magnetizing reactance: $X_0 = \frac{V_1}{I_m}$

Main purpose: The main purpose of the no-load test is to determine the core (iron) loss and the no-load equivalent circuit parameters (R₀ and X₀) of the transformer. Since the core loss remains constant at all loads, this test gives the constant loss of the transformer, which is needed to calculate its efficiency at any load.

(a) Single line diagram of a typical distribution system (4 marks)

 Generating station (11 kV)
          │
 [Step-up transformer 11/132 kV]
          │
 ═════ Transmission line (132 kV) ═════
          │
 [Step-down transformer 132/33 kV]
          │
 ┌────────┴────────┐
 │  33 kV bus-bar  │   Receiving / primary substation
 └────────┬────────┘
          │
 ═════ Sub-transmission line (33 kV) ═════
          │
 [Step-down transformer 33/11 kV]
          │
 ┌────────┴────────┐
 │  11 kV bus-bar  │   Distribution substation
 └───┬─────────┬───┘
     │         │
  Feeder 1   Feeder 2        Primary distribution (11 kV)
     │         │
 [DT 11/0.4 kV] [DT 11/0.4 kV]   Distribution transformers
     │         │
 ──┬─┴──┬──   ──┬──┴──┬──     Distributors, secondary distribution
   │    │       │     │       (400 V, 3-phase 4-wire; 230 V single-phase)
 [Load][Load] [Load] [Load]    Service mains to consumers

Power is generated at 11 kV and stepped up to 132 kV so that current and $I^2R$ loss are low during bulk transmission. It is stepped down to 33 kV at the primary substation and to 11 kV at the distribution substation. Feeders carry 11 kV to the load area. There, distribution transformers reduce it to 400 V (230 V per phase), and distributors and service mains supply consumers.

(b) Ring main distribution system, circuit diagram (1 mark)

              11 kV bus-bar (source substation)
        ┌──────────────┴──────────────┐
      [CB1]                         [CB2]
        │                             │
        ├────────[Isolator]───────────┤
        │                             │
   ┌────┴──── ring main feeder (closed loop) ────┐
   │          │            │           │         │
   │       [Iso]        [Iso]       [Iso]       │
   │          │            │           │         │
   │       [DT1]        [DT2]       [DT3]      [DT4]
   │          │            │           │         │
   │       Load 1       Load 2      Load 3    Load 4
   │                                             │
   └─────────────────────────────────────────────┘

The feeder starts at the bus-bar, runs through the load area passing near every distribution transformer (DT), and returns to the same bus-bar (or to a second source). Each DT is tapped from the ring through an isolator or ring main unit. The loop is closed, so every DT can be supplied from either direction. If a section faults, the two isolators on each side of it are opened and the rest of the ring continues to supply all DTs from the other side. This gives better continuity of supply and voltage regulation than a radial feeder, but costs more. Some utilities run the ring with one section normally open to limit fault current.

Comparison of Fuse and MCB (5 marks)

Conclusion: The MCB is superior in accuracy, speed, reusability and safety, so it has largely replaced the fuse in modern installations; the fuse still survives where cheap, simple, one-shot protection is enough.

1. Necessity of Earthing in Power System (4 marks)

Earthing (grounding) means connecting the non-current-carrying metallic parts of equipment and the neutral point of the system to the general mass of earth through a low-resistance path. Its necessity:

1. Safety of human life: If insulation fails, the metal body of a machine or appliance becomes live. With proper earthing, the fault current flows to earth and the body is kept at earth potential, so a person touching it does not receive a shock.
2. Protection of equipment: It provides a low-resistance path for fault current so that the protective device (fuse, MCB, relay) operates quickly and the equipment is disconnected before damage.
3. Voltage stabilization: Earthing the neutral fixes the potential of the neutral point with respect to earth, so the phase voltages remain balanced and limited to a safe value during faults.
4. Prevention of fire and explosion: Leakage currents and static charges are discharged to earth, avoiding sparks that could ignite inflammable material.
5. Lightning and surge protection: Earthing of surge arresters, lightning arresters and shields diverts lightning and switching surges safely to earth.
6. Reliable operation of protection system: A good earthing system gives a definite return path for fault current, so protective relays sense the fault correctly and isolate only the faulty section.

2. Working of a Lightning Arrester (6 marks)

A lightning arrester is a protective device installed between the line and earth at substations and on transmission lines. It provides a low-resistance path to earth for surge voltages but behaves as an insulator at normal system voltage. The most common type is the valve-type (non-linear resistor) arrester, consisting of:

• Spark gap (air gap): a series of small gaps between the line terminal and the stack of non-linear resistor discs.
• Non-linear resistor discs (valve elements): made of silicon carbide or zinc oxide, whose resistance falls sharply as voltage rises.
• Series gap and terminals: the top terminal is connected to the line, the bottom to the earth electrode.

Working:

1. Normal condition: At normal system voltage (say 11 kV or 132 kV), the voltage across the gap is less than the breakdown value of the spark gap, so the gap remains non-conducting. The arrester draws only a negligible leakage current and acts as an open circuit; the line is effectively isolated from earth.
2. Surge condition: When a lightning surge or switching surge travels along the line, the voltage across the arrester rises steeply. The spark gap breaks down and becomes conducting, connecting the line to earth through the resistor discs.
3. Discharge: The non-linear discs offer very low resistance at high surge voltage, so the heavy surge current (thousands of amperes) is discharged directly to earth. The voltage across the arrester is thus clamped to a safe value called the residual (discharge) voltage, protecting the transformer or equipment insulation.
4. Sealing (resealing): As soon as the surge is over, the system voltage returns to normal. The discs immediately regain their high resistance (non-linear property), the current through the gap falls to a small value, and the arc at the spark gap is extinguished. The gap thus reseals and the arrester is ready for the next surge.
5. Zinc oxide (metal oxide) arrester: In modern gapless arresters, the ZnO discs alone provide the non-linear characteristic; at normal voltage the leakage current is only a few milliamperes, and on surge the discs conduct heavily and clamp the voltage, then return to the insulating state automatically.

Thus the lightning arrester acts as a "safety valve" — it conducts only during overvoltage and remains an insulator at normal voltage, protecting the power system from lightning surges.

Different methods of starting of an induction motor:

1. Direct-on-line (DOL) starting
2. Star-delta starting
3. Autotransformer starting
4. Rotor resistance starting (for slip-ring/ wound rotor motors)
5. Stator resistance/reactance starting
6. Soft starter (electronic) starting

(3 marks)

---

Brief explanation of each method: (7 marks)

(i) Direct-on-line (DOL) starting The motor is connected directly to the full supply voltage through a contactor and overload relay. Full voltage is applied at start, so the starting current is very high (about 5 to 8 times full-load current). It is simple, cheap and used for small motors (generally up to about 5 kW) where the supply can tolerate the inrush current.

(ii) Star-delta starting The motor is started in star connection (each winding gets $V_L/\sqrt{3}$, i.e. about 58% of line voltage) and then changed over to delta connection when it reaches near full speed. Starting current and starting torque are reduced to one-third of the DOL values. It needs a motor designed for delta running with six terminals brought out. Used for large motors started on light load.

(iii) Autotransformer starting A tap on an autotransformer applies a reduced voltage (say 50%, 65% or 80% of line voltage) to the stator at start. Tapping is then changed to full voltage as the motor accelerates. Starting current is reduced in proportion to the tap, and starting torque reduces as the square of the tap voltage. It is suitable for large motors where a smooth reduction of current is required.

(iv) Rotor resistance starting (slip-ring motor) External resistance is inserted in the rotor circuit through slip rings at start and gradually cut out as the motor speeds up. This reduces rotor current and hence stator current, while giving a high starting torque. It is the best method for motors that must start under heavy load.

(v) Stator resistance/reactance starting A resistance or reactance is connected in series with the stator winding at start and short-circuited in steps as the motor accelerates. The voltage across the motor terminals is reduced, so the starting current falls. Power loss in the resistance makes it less efficient than autotransformer starting.

(vi) Soft starter (electronic) starting Thyristors (SCRs) in the supply lines are fired with a gradually increasing firing angle so that the applied voltage rises smoothly from a low value to full value. This gives a stepless, jerk-free start with controlled current. It is widely used now for pumps, fans and compressors.

Voltmeter is connected in parallel with the component whose potential difference is to be measured.

Reasons:

1. To measure potential difference: A voltmeter measures the potential difference (voltage) between two points. For this, the two terminals of the voltmeter must be connected across those two points, i.e. in parallel with the component.

2. High resistance: A voltmeter has a very high resistance (ideally infinite). When connected in parallel, it draws a negligible current from the circuit, so it does not disturb the current flowing through the component and the reading obtained is nearly the true potential difference.

3. If connected in series: Since its resistance is very high, it would greatly reduce the current in the circuit (almost to zero), and the circuit would not work properly. Also, the voltmeter would then measure only the small voltage drop across itself, not the voltage across the component. Hence it is never connected in series.

Connection and Measuring Procedure of an Insulation Tester (Megger)

1. Purpose An insulation tester (commonly called a Megger) is used to measure the insulation resistance of cables, windings, motors, transformers and other electrical equipment, to check whether the insulation is healthy or leaking to earth. It applies a high DC voltage (500 V, 1000 V, 2500 V, etc.) and measures resistance in megohms (MΩ) or gigohms (GΩ).

2. Connections An insulation tester has two or three terminals:

• Line terminal (L) — connected to the conductor whose insulation is to be tested.
• Earth terminal (E) — connected to the earth/ground or to the metal body (frame, sheath, armour) of the equipment.
• Guard terminal (G) — used only when surface leakage (e.g., moisture or dirt on the terminal end) may give a false low reading. It bypasses surface leakage current so that only the true leakage through the insulation is measured.

Basic connection steps:

1. Isolate and de-energize the equipment completely; switch off supply, remove fuses, and discharge the circuit to earth.
2. Disconnect the equipment from the supply and from other parallel circuits so that the reading is not affected.
3. Connect the E terminal to the earth/body of the equipment.
4. Connect the L terminal to the conductor under test.
5. If required, connect the G terminal to the surface (e.g., the insulation surface near the terminal) to eliminate surface leakage.
6. Ensure all connections are clean, tight and dry.

3. Measuring Procedure

1. Safety check: Confirm the equipment is isolated, locked out and discharged. Warn others before testing.
2. Zero and infinity check: Short L and E together — the meter should read zero; keep L and E open — it should read infinity. This confirms the tester is working.
3. Select voltage range as per the equipment rating (e.g., 500 V for LV motors, 1000 V for cables).
4. Apply test voltage by pressing the test button or turning the hand crank at rated speed (for manual Megger, ~160 rpm).
5. Hold for the specified time (usually 60 seconds) so that the reading stabilizes; note the value.
6. Record the reading in MΩ. Also note the temperature because insulation resistance falls as temperature rises.
7. Discharge the equipment to earth after the test (insulation stores charge), using the discharge rod or by keeping the terminals shorted.
8. Interpret the result: Compare with the minimum acceptable value. As a rule of thumb, insulation resistance should be at least 1 MΩ per kV of operating voltage + 1 MΩ, or as specified by the manufacturer. A very low reading indicates moisture, dirt, damage or aged insulation.

4. Precautions

• Never test a live circuit.
• Always discharge before and after testing.
• Do not touch the terminals during the test — high voltage is present.
• Keep the equipment clean and dry to avoid surface leakage errors.

Causes of Electric Shock (3 marks)

1. Direct contact with live parts — touching bare conductors, exposed wires, or terminals that are still energised, usually because insulation has worn out or been damaged.
2. Faulty or damaged equipment — broken plugs, cracked insulation, loose connections, or appliances with defective wiring pass current to the metal body, which a person then touches.
3. Lack of earthing (grounding) — if equipment is not properly earthed, leakage current cannot flow safely to the ground and instead flows through the body of the user.
4. Wet conditions — water and damp floors lower body resistance sharply, so even a small voltage becomes dangerous.
5. Carelessness and unsafe practice — working on live circuits without switching off, using tools with damaged cords, overloading sockets, or ignoring warning signs.
6. Contact with overhead lines — using long metallic tools, ladders, or pipes near overhead conductors.

Measures to Minimize Electric Shock (2 marks)

1. Switch off and isolate the supply before working; use lock-out/tag-out and test for dead.
2. Proper earthing of all equipment and use of fuses, MCBs, and ELCBs/RCCBs to cut off supply on leakage or fault.
3. Good insulation — use double-insulated tools, undamaged cords, and insulated gloves, mats, and shoes.
4. Avoid wet conditions — keep hands and floors dry; never touch switches with wet hands.
5. Use safety devices — residual current devices, correct-rated fuses, and three-pin plugs.
6. Training and awareness — follow safety rules, display warning signs, and never work alone on electrical jobs.

Points to be Considered for Preventive Maintenance of Generator

Preventive maintenance is the scheduled, routine servicing of a generator carried out before a breakdown occurs, so as to keep it reliable and extend its life. The main points to be considered are:

1. Maintenance schedule (periodicity)

• Prepare a maintenance calendar — daily, weekly, monthly, quarterly and annual checks — as per the manufacturer's manual.
• Keep a log book / history card for each generator recording every service, repair and reading.

2. Fuel system

• Check fuel level daily and keep the tank sufficiently filled to prevent condensation.
• Drain water and sediment from the fuel tank and water separator.
• Replace fuel filters at the recommended interval; check for leaks in pipes, joints and the fuel pump.
• Use clean, correct-grade diesel and keep fuel stored properly.

3. Lubrication system

• Check engine oil level daily; top up with the correct grade of oil.
• Change engine oil and oil filter as per the service interval (usually by running hours).
• Check for oil leaks and inspect the oil pressure gauge reading.

4. Cooling system

• Check coolant level and concentration of antifreeze/antirust.
• Inspect the radiator for dust, leaves and scale; clean the fins and core.
• Check the fan belt tension and condition, water pump, hoses and hose clamps for leaks.
• Replace coolant periodically and ensure the radiator cap holds correct pressure.

5. Air intake and exhaust system

• Clean or replace the air filter element at proper intervals.
• Check the exhaust pipe, silencer and joints for leakage, cracks and carbon deposit.
• Ensure the air intake is free from dust and obstruction.

6. Battery and electrical system

• Check battery electrolyte level and top up with distilled water.
• Clean and tighten terminals; apply petroleum jelly to prevent corrosion.
• Check battery voltage / specific gravity and charging rate of the alternator.
• Inspect wiring, cables, fuses, circuit breakers, control panel and AVR connections.

7. Starting system

• Test the starter motor, starting battery and manual/auto start switch.
• Check the fuel solenoid, safety shut-down devices (low oil pressure, high water temperature, over-speed) for correct operation.

8. Engine and mechanical parts

• Check and adjust valve clearance, injector timing and injector spray pattern as required.
• Inspect mounting foundation bolts, anti-vibration pads and coupling alignment.
• Check for abnormal noise, vibration, smoke or overheating during trial run.

9. Alternator (generator end)

• Clean windings and check insulation resistance with a megger.
• Check slip rings, brushes, bearings and their lubrication.
• Verify voltage, frequency and load output at rated values.

10. Safety and general care

• Keep the generator room clean, dry, well ventilated and free from combustible materials.
• Ensure proper earthing, fire extinguisher, and safe clearance around the set.
• Run the generator on load periodically (at least once a month) to keep it in working condition.
• Allow only trained personnel to carry out maintenance, using proper tools and PPE.

Conclusion: A well-planned preventive maintenance programme reduces breakdowns, lowers repair cost, ensures uninterrupted power supply and increases the service life of the generator.

Single-Phase Residential Wiring Diagram (with Protection Scheme)

Neat Sketch

        SERVICE ENTRANCE (from utility pole)
                    │
        ┌───────────┴───────────┐
        │   Energy Meter (kWh)  │
        └───────────┬───────────┘
                    │  Phase (L)  ───────────────┐
                    │  Neutral (N) ────────┐     │
                    │  Earth (E) ────┐     │     │
                    │               │     │     │
        ┌───────────▼───────────────▼─────▼─────▼───────────┐
        │        MAIN DISTRIBUTION BOARD (MDB)              │
        │                                                   │
        │  [Main Switch / MCB 32 A DP]                      │
        │        │                                          │
        │  [RCCB 30 mA DP]  ── protects all circuits        │
        │        │                                          │
        │  ┌─────┴─────┬──────────┬──────────┬─────────┐    │
        │  │           │          │          │         │    │
        │ MCB 6A     MCB 6A     MCB 16A   MCB 20A   MCB 16A │
        │ (Light)    (Fan)      (5A Socket)(AC/Geyser)(Power)│
        └──┬──────────┬──────────┬──────────┬─────────┬─────┘
           │          │          │          │         │
           │          │          │          │         │
      ┌────▼───┐ ┌────▼───┐ ┌────▼───┐ ┌────▼───┐ ┌───▼────┐
      │ Light  │ │  Fan   │ │ 5A     │ │ 15/16A │ │ Power  │
      │ Points │ │ Points │ │ Socket │ │ Socket │ │ Points │
      │ (L+N+E)│ │ (L+N+E)│ │ (L+N+E)│ │ (L+N+E)│ │(L+N+E) │
      └────────┘ └────────┘ └────────┘ └────────┘ └────────┘
           │          │          │          │         │
           └──────────┴──────────┴──────────┴─────────┘
                              │
                    ┌─────────▼─────────┐
                    │  EARTH PIT (GI    │
                    │  pipe + salt +    │
                    │  charcoal), R<1Ω  │
                    └───────────────────┘

Protection Scheme Used

Key Points

• Supply: 230 V, 50 Hz, single phase — Phase (L), Neutral (N), Earth (E).
• All sub-circuits are connected in parallel across L and N.
• Light and fan circuits use 6 A MCB; 5 A sockets use 6–16 A; 15/16 A sockets (geyser, AC, heater) use 16–20 A MCB.
• RCCB (30 mA) is mandatory for shock protection; MCB handles overload and short circuit.
• Earthing is essential — all metal enclosures, socket earth pins, and appliance bodies must be earthed.
• Wiring: PVC conduit / casing-capping with 1.5 mm² for lighting, 2.5 mm² for sockets, 4 mm² for power circuits.

Runway lights and taxiway lights used in airports

1. Runway edge lights White lights along both edges of the runway, marking its full length and width. On instrument runways the last 600 m (or one-third of the runway length, whichever is less) shows yellow lights to warn pilots of the approaching end. They are normally elevated, omni-directional, and spaced about 60 m apart.

2. Runway threshold lights Green lights in a row across the beginning of the runway (the landing threshold), showing the start of the usable landing surface. The corresponding row at the far end is red, showing the runway end. They are unidirectional — green facing the approaching aircraft, red facing the opposite direction.

3. Runway centre line lights White lights embedded along the centre line, mainly on precision approach runways. From 900 m to 300 m before the end they alternate white and red, and in the last 300 m they are red, giving the pilot a clear picture of the remaining runway length.

4. Runway touchdown zone lights Short rows of white bar lights on both sides of the centre line in the touchdown zone of precision approach runways. They help the pilot judge the touchdown point and flare height.

5. Approach lights A system of white lights extending up to 900 m before the threshold (with a crossbar and sequenced flashing "rabbit" lights on precision systems), guiding the aircraft to the runway in poor visibility. They are treated as part of the runway lighting system.

6. Runway threshold identification lights Two or more synchronized white flashing lights placed at the threshold of a non-precision runway to make the threshold conspicuous.

7. Taxiway edge lights Blue lights along both edges of taxiways, marking their limits so that aircraft and vehicles stay on the paved surface. They are usually elevated and spaced about 60 m apart.

8. Taxiway centre line lights Green lights embedded along the taxiway centre line, used on runways meant for low-visibility operations. They give continuous guidance from the runway to the apron. Where a taxiway crosses a runway, the centre line lights change to alternate green and yellow, and stop bars (red lights) are provided at runway holding positions.

9. Stop bars and runway guard lights Red lights across a taxiway at the holding position before entering a runway. Runway guard lights (alternately flashing yellow) warn that the aircraft is about to enter an active runway.

10. Apron and other lights Apron edge lights, floodlights and illuminated guidance signs direct aircraft on the apron and at parking stands.

In short, runway lights (white, yellow, green, red) define the landing and take-off surface and its limits, while taxiway lights (blue edges, green centre lines, red stop bars) guide the aircraft safely between the runway and the apron.

Importances of Earthing (4 marks)

Earthing (grounding) is the deliberate connection of the non-current-carrying metallic parts of an electrical installation to the general mass of earth through a low-resistance path. Its importances are:

1. Safety of human life: If insulation fails and a metal body (motor casing, refrigerator body, panel board) becomes live, the earthing wire provides a low-resistance path. The heavy fault current then blows the fuse or trips the MCB quickly, so the body does not remain at a dangerous potential. A person touching it is thus saved from electric shock.

2. Protection of equipment and property: By quickly clearing the fault, earthing prevents sustained arcing, overheating and fire in the installation. It also protects costly equipment such as transformers, generators and motors from damage due to lightning surges and voltage spikes.

3. Voltage stabilisation and reference: Earth provides a common zero-potential reference for the system. It keeps the potential of the neutral and of non-current-carrying parts near zero, so that voltage does not "float" and cause erratic operation of instruments and protective relays.

4. Discharge of lightning and static charge: Earthing rods and lightning arresters conduct lightning strokes and accumulated static electricity safely to the ground, preventing damage to buildings and to aircraft/communication equipment.

5. Proper operation of protective devices: Fuses, MCBs, ELCBs/RCCBs and relays can operate correctly only when a low-resistance earth path exists. Without earthing, faults remain undetected and the protection system fails.

Types of Earthing (6 marks)

Earthing is broadly classified into natural earthing and artificial earthing.

A. Natural Earthing

Ready-made earth connections that already exist, such as underground metal water pipes, the steel frame of a building, or the metal sheath of cables. These are used only when their resistance is low and reliable; they are cheap but not dependable, so they are rarely used alone.

B. Artificial Earthing

Specially installed earthing systems. The main types are:

1. Plate Earthing

• A copper plate (60 cm × 60 cm × 3 mm) or GI plate (60 cm × 60 cm × 6 mm) is buried vertically in the ground at a depth of about 3 m.
• The pit is filled alternately with layers of charcoal and salt to reduce soil resistance and retain moisture.
• A funnel pipe is provided at the top to pour water in summer.
• Use: Substations, large installations. Merit: low resistance, good for heavy fault current. Demerit: costly, needs large excavation.

2. Pipe Earthing

• A GI pipe of 38 mm diameter and 2–3 m length (length depends on soil moisture) is buried vertically.
• A second pipe with a funnel is placed at the top for watering.
• Charcoal and salt surround the pipe.
• Use: Most common for domestic and small industrial installations. Merit: cheap, easy to install, easy to maintain. Demerit: resistance is higher than plate earthing.

3. Rod Earthing

• A copper or GI rod of 12–25 mm diameter is driven vertically into the ground by hammering, up to 3 m or more.
• Use: Where soil is soft and the water table is high. Merit: simple and quick. Demerit: not suitable for dry, rocky soil; not effective for large fault currents.

4. Strip or Wire Earthing

• A copper strip (25 mm × 1.6 mm) or GI strip (25 mm × 4 mm) is laid horizontally in trenches about 0.5 m deep, in a ring or radial pattern around the building.
• Use: Where a large area is available and a single deep pit is not possible. Merit: distributes the fault current over a wide area. Demerit: needs long trenching, more strip material.

5. Mesh Earthing

• A grid or mesh of horizontal conductors is buried under the substation/switchyard, often combined with vertical rods at the corners.
• Use: High-voltage substations. Merit: keeps step and touch voltages within safe limits over the whole yard.

Comparison at a glance

Conclusion: Earthing is essential for safety of life and equipment. The choice of type depends on soil condition, fault current level, space available and cost.

Factors that determine cable size for electrical wiring (7 marks)

1. Load current / current-carrying capacity
   The cable must carry the full-load current continuously without overheating. Total connected load, diversity factor and power factor decide the design current.

2. Permissible voltage drop
   Voltage drop from the supply point to the farthest point must remain within the standard limit (usually about 3–5%). Longer runs require a larger cable size.

3. Type of load and starting current
   Motors, air conditioners, compressors and similar inductive loads draw high starting/inrush current, so they need a larger cable than lighting or resistive loads of the same rating.

4. Ambient temperature and installation condition
   High ambient temperature, cables bunched together, enclosed in conduit or buried underground reduce heat dissipation. Derating factors are applied, which may require a larger size.

5. Type of insulation and conductor material
   Copper has higher conductivity than aluminium. PVC, XLPE or rubber insulation has different temperature withstand limits. Higher temperature-rated insulation can allow a smaller size for the same current.

6. Short-circuit withstand capacity
   The cable must withstand the prospective short-circuit current until the protective device operates. Fault level and clearing time determine the minimum cross-sectional area.

7. Mechanical strength and future expansion
   A minimum size is required to avoid mechanical damage during installation. Some spare capacity is also kept for future load expansion and for long routes with many bends.

Standard cable sizes used in wiring (3 marks)

Cable size is normally expressed as nominal cross-sectional area in $mm^2$ (sq. mm). The standard series is:
$1,\ 1.5,\ 2.5,\ 4,\ 6,\ 10,\ 16,\ 25,\ 35,\ 50,\ 70,\ 95,\ 120,\ 150,\ 185,\ 240,\ 300,\ 400,\ 500,\ 630\ mm^2$ and above.

Thus, cable size is selected by balancing current capacity, voltage drop, fault level, installation condition and future load, using the standard $mm^2$ sizes given above.