Domestic and Industrial Wiring, Earthing, Cables and Conduit

What to remember

• Earthing connects the metal body of equipment to the general mass of earth through a low-resistance path so that a fault current flows to earth, the protective device trips and nobody gets a shock. Fuses, switches and MCBs always go in the live (phase) wire, never in the neutral.
• Domestic wiring is parallel wiring. Common systems are conduit (concealed or surface), batten, casing-capping and cleat. Concealed PVC conduit with copper wire is the most common in new houses.
• MCB protects against overload and short circuit. RCCB/ELCB protects people against leakage current (a 30 mA sensitivity is used for human protection).

Conductors, wire sizes and cables

Conductor materials: copper (better conductivity, easy to join, higher cost) and aluminium (lighter, cheaper, lower conductivity, oxidises, needs larger size for the same current). Wire size in India is given as nominal cross-section in square millimetres, such as 1.5, 2.5, 4 and 6 mm². The older Standard Wire Gauge (SWG) system is still asked in exams: the higher the SWG number, the thinner the wire.

Resistance of a conductor: R = ρ l / A. Copper resistivity is about 1.7 × 10⁻⁸ ohm-metre.

Worked example 1: A 100 m length of 2 mm² copper wire has R = 1.7 × 10⁻⁸ × 100 / (2 × 10⁻⁶) = 0.85 Ω. The one-way voltage drop at 10 A is 8.5 V, so longer runs need thicker wire.

Insulation:

• PVC (polyvinyl chloride): common for house wires and low-voltage cables; normal maximum conductor temperature about 70 °C.
• XLPE (cross-linked polyethylene): better thermal and electrical properties; maximum conductor temperature about 90 °C; used in LT and HT power cables.
• VIR (vulcanised India rubber) and PILC (paper-insulated lead-covered) cables are older types. Paper insulation must be protected from moisture.

Parts of a power cable: conductor (stranded copper or aluminium), insulation, bedding, armour (galvanised steel wire or strip, to protect against mechanical damage), and outer sheath. Armoured cables can be laid directly in the ground. Cables may be single-core or multi-core (twin, three-core, four-core).

Cable laying: direct burial in a trench with sand bedding and a warning tile or brick cover; in ducts; or on trays and racks. Bends must not be too sharp, or the insulation will crack.

Wiring systems

Advantages of conduit wiring: protection from mechanical damage, dust, moisture and fire; safety. Metal conduit must be earthed. PVC conduit does not rust. Fill the conduit with cable to a limited share of its area (about 40%) to allow heat to escape and to make drawing in easy. Avoid sharp bends. Joints in the conduit must be made only in junction boxes. Never join wires inside the conduit.

Wire joints: married joint, Britannia joint, western union joint and tee joint. Joints are soldered and taped. A good joint has the same strength and conductivity as the wire.

Domestic wiring practice

• The supply goes through the energy meter, then the main switch and fuse (or MCB) in the distribution board, then to sub-circuits.
• Light and fan sub-circuits use 5 A circuits (1.5 mm² copper); power sub-circuits for heavy loads like geysers, ACs and sockets of 15 A use 15 A circuits (2.5 mm² and above). Very heavy loads like an air conditioner have a dedicated circuit.
• A lighting sub-circuit is normally limited to about 800 W and about ten points.
• Switches are placed in the live wire. Lamp holders, ceiling roses and sockets must be connected as per the standard polarity.
• Staircase (two-way) wiring: two two-way switches control one lamp from two places. For control from three places, an intermediate switch is used between two two-way switches.
• A three-pin socket has line, neutral and earth pins. The earth pin is the longest and thickest so that earth is connected first and disconnected last.

Worked example 2: A 1500 W heater on 230 V takes I = 1500 / 230 = 6.5 A, so a 10 A MCB or higher is needed (the exact rating is chosen from the standard).

Worked example 3: A 5 A circuit on 230 V can carry about 1150 VA, so at most 11 lamps of 100 W.

Protection devices

An old voltage-operated ELCB is not used much now. RCCB is current-operated.

Earthing

Purpose: safety of persons, protection of equipment, and stable voltage. Without earthing, the body of equipment can become live when insulation fails.

Types of earth connection:

• Equipment earthing: metal parts of machines, switch boxes, motor frames and enclosures.
• System (neutral) earthing: the neutral point of the transformer or generator is earthed.

Methods of earth electrode:

• Plate earthing: a GI or copper plate (about 60 cm × 60 cm) buried vertically in a pit, usually 3 m or more deep, packed with alternate layers of charcoal and salt. A funnel and pipe allow water to be poured in.
• Pipe earthing: a GI pipe (about 38 to 40 mm diameter and 2.5 m long) with holes is driven or buried vertically, with charcoal and salt around it. This is the commonest method for houses and shops.
• Rod, strip and mat (grid) earthing: used for substations and towers; counterpoise wire for transmission lines.

Earth resistance must be as low as possible. It is measured by an earth tester (Megger earth tester) using the three-terminal or four-terminal method. It reduces in wet soil and increases in dry or rocky soil, so the pit is kept moist. Earth wire colour: green, or green with yellow stripes. The earth wire must be of copper or GI strip of sufficient size and must have no switch or fuse.

Lightning protection uses a pointed rod (air terminal), a down conductor and a separate earth electrode.

Insulation resistance of a wiring installation is measured with an insulation tester (megger), commonly of 500 V for low-voltage wiring.

Faults and testing in wiring

Before any repair, switch off, remove the fuse or open the MCB and test with a tester that the line is dead. Use a test lamp or continuity tester to find breaks. Insulation resistance between conductors and between conductors and earth should be high (in megohms) when tested with a megger. Never touch a conductor just because it looks dead.

Good practice: use the correct size of wire for the load; use the correct rating of fuse or MCB (it protects the wire, not only the load); keep joints tight; use switches of the right rating; keep wiring away from gas pipes and water pipes; and never replace a fuse with a thicker wire.

Exam traps

• Fuse or switch in live wire only; neutral is never fused or switched alone in single-phase domestic wiring.
• The earth pin is the longest and thickest of the three plug pins.
• The higher the SWG number, the thinner the wire.
• XLPE (about 90 °C) takes a higher temperature than PVC (about 70 °C).
• MCB gives overload and short-circuit protection; RCCB gives shock protection. They are not the same.
• Charcoal and salt are used in earth pits to keep the resistance low, not to carry current themselves.
• A joint is made inside a junction box, not inside the conduit.
• Aluminium needs a larger size than copper for the same current.

One-liners

• 1. Domestic loads are connected in parallel.
• 2. The live wire is the one with the switch and fuse.
• 3. Green or green-yellow is the earth wire colour.
• 4. A stair-case light uses two two-way switches.
• 5. HRC fuses are filled with sand.
• 6. RCCB protects persons from electric shock.
• 7. The standard 30 mA RCCB gives protection to persons.
• 8. Pipe earthing uses a GI pipe, charcoal and salt.
• 9. An earth tester measures earth resistance.
• 10. A megger measures insulation resistance.
• 11. XLPE is cross-linked polyethylene.
• 12. Armouring of a cable protects it against mechanical damage.

Practice questions

1. Which of the statements is/are correct? 1. A fuse or switch in single-phase domestic wiring must be connected in the live wire. 2. A fuse should be connected in the earth wire.

   1. 1 only
   2. 2 only
   3. Both 1 and 2
   4. Neither 1 nor 2
   Answer

   A. 1 only

   The earth wire carries no switch or fuse so that earth continuity is never broken.

2. Which of the statements is/are correct? 1. An MCB gives protection against electric shock from leakage. 2. An RCCB protects a person against earth leakage current.

   1. 1 only
   2. 2 only
   3. Both 1 and 2
   4. Neither 1 nor 2
   Answer

   B. 2 only

   An MCB protects against overload and short circuit; shock protection is by an RCCB.

3. Which of the statements is/are correct? 1. The earth pin of a three-pin plug is longer than the other two pins. 2. The earth pin of a three-pin plug is the shortest pin.

   1. 1 only
   2. 2 only
   3. Both 1 and 2
   4. Neither 1 nor 2
   Answer

   A. 1 only

   It is the longest and thickest, so earth is made first and broken last.

4. Which of the statements is/are correct? 1. PVC insulation is better than XLPE for high-voltage power cables. 2. XLPE cables can work at a higher conductor temperature than PVC cables.

   1. 1 only
   2. 2 only
   3. Both 1 and 2
   4. Neither 1 nor 2
   Answer

   B. 2 only

   XLPE has better thermal and electrical properties and is used in HT cables.

5. Which of the statements is/are correct? 1. In SWG, a higher gauge number means a thinner wire. 2. In SWG, a higher gauge number means a thicker wire.

   1. 1 only
   2. 2 only
   3. Both 1 and 2
   4. Neither 1 nor 2
   Answer

   A. 1 only

   The two statements contradict each other; thinner wire has the larger gauge number.

6. Which of the statements is/are correct? 1. Joints should be made inside the conduit pipe for neatness. 2. Conduit wiring provides protection against mechanical damage and moisture.

   1. 1 only
   2. 2 only
   3. Both 1 and 2
   4. Neither 1 nor 2
   Answer

   B. 2 only

   Joints are made only in junction boxes, not inside the pipe.

7. Which of the statements is/are correct? 1. A metal conduit system must be properly earthed. 2. PVC conduit is easily corroded by moisture.

   1. 1 only
   2. 2 only
   3. Both 1 and 2
   4. Neither 1 nor 2
   Answer

   A. 1 only

   PVC conduit does not rust; it is used where moisture and corrosion are a problem.

8. Which of the statements is/are correct? 1. The earth resistance should be as high as possible. 2. Charcoal and salt in an earth pit help to keep the earth resistance low.

   1. 1 only
   2. 2 only
   3. Both 1 and 2
   4. Neither 1 nor 2
   Answer

   B. 2 only

   Low earth resistance allows fault current to flow and trip the protection.

9. Which of the statements is/are correct? 1. A staircase lamp controlled from two places needs two two-way switches. 2. A lamp controlled from three places needs only two two-way switches and no other switch.

   1. 1 only
   2. 2 only
   3. Both 1 and 2
   4. Neither 1 nor 2
   Answer

   A. 1 only

   A third control point needs an intermediate switch.

10. Which of the statements is/are correct? 1. HRC fuses cannot interrupt short-circuit current. 2. HRC fuses are filled with sand or a similar filler to quench the arc.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    HRC means high rupturing capacity, meant for short-circuit duty.

11. Which of the statements is/are correct? 1. An earth tester measures the earth electrode resistance. 2. A megger of 500 V measures earth electrode resistance directly.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    A megger measures insulation resistance; an earth tester measures electrode resistance.

12. Which of the statements is/are correct? 1. Aluminium has a better conductivity than copper. 2. Aluminium conductors need a bigger cross-section than copper for the same current.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Copper conducts better than aluminium for equal size.

13. Domestic electrical loads are connected in

    1. series
    2. series-parallel only with fuses
    3. parallel
    4. star
    Answer

    C. parallel

    Parallel connection gives each load the full supply voltage.

14. A cable that is to be buried directly in the ground should be

    1. cleat-mounted
    2. twin flexible
    3. unsheathed single-core
    4. armoured
    Answer

    D. armoured

    Armour protects the cable against mechanical damage.

15. In house wiring a switch is fitted in the

    1. earth wire
    2. neutral wire
    3. live wire
    4. both neutral and earth
    Answer

    C. live wire

    Opening the live wire makes the load dead.

16. A 1500 W heater is used on a 230 V supply. The current is nearly

    1. 65 A
    2. 3.5 A
    3. 0.15 A
    4. 6.5 A
    Answer

    D. 6.5 A

    I = 1500/230 = 6.52 A.

17. The resistance of 100 m of 2 mm² copper wire (resistivity 1.7 × 10⁻⁸ Ω m) is

    1. 8.5 Ω
    2. 1.7 Ω
    3. 0.085 Ω
    4. 0.85 Ω
    Answer

    D. 0.85 Ω

    R = ρl/A = 1.7e-8 × 100/2e-6 = 0.85 Ω.

18. A wire has a resistance of 0.5 Ω and carries 10 A. The voltage drop and the power lost in the wire are

    1. 20 V and 200 W
    2. 0.05 V and 0.5 W
    3. 5 V and 50 W
    4. 5 V and 5 W
    Answer

    C. 5 V and 50 W

    V = IR = 5 V; P = I²R = 100 × 0.5 = 50 W.

19. The maximum number of 100 W lamps that can be fed from a 5 A, 230 V circuit is

    1. 23
    2. 2
    3. 11
    4. 5
    Answer

    C. 11

    5 × 230 = 1150 W; 1150/100 = 11.5, so at most 11 lamps.

20. The earth wire of an installation is coloured

    1. red
    2. blue
    3. black
    4. green or green with yellow stripes
    Answer

    D. green or green with yellow stripes

    Green (or green-yellow) is the standard earth colour.

21. The commonest earth electrode for houses and small installations is

    1. a lead sheet only
    2. an iron rod without any filling
    3. a GI pipe with charcoal and salt
    4. a nylon cord
    Answer

    C. a GI pipe with charcoal and salt

    Pipe earthing is simple and cheap and keeps low resistance.

22. The size of a GI plate used for plate earthing is commonly about

    1. 10 cm × 10 cm
    2. 5 m × 5 m
    3. 1 m × 5 m
    4. 60 cm × 60 cm
    Answer

    D. 60 cm × 60 cm

    The standard GI plate is about 60 cm square.

23. An earth tester is used to measure

    1. current in a motor
    2. earth electrode resistance
    3. insulation of a cable
    4. power factor
    Answer

    B. earth electrode resistance

    Earth resistance is measured by an earth tester.

24. A megger is used to measure

    1. frequency
    2. earth tester current
    3. phase sequence
    4. insulation resistance
    Answer

    D. insulation resistance

    A megger generates a high DC voltage for insulation tests.

25. The thermal tripping of an MCB protects against

    1. overload
    2. low voltage
    3. earth leakage only
    4. lightning
    Answer

    A. overload

    A bimetal strip bends on a prolonged overload; the magnetic trip handles short circuits.

26. An RCCB trips when

    1. temperature of the room rises
    2. frequency changes
    3. the line and neutral currents differ
    4. voltage exceeds the rated value
    Answer

    C. the line and neutral currents differ

    Leakage to earth makes the two currents unequal.

27. The RCCB sensitivity usually specified for protection of persons is

    1. 30 A
    2. 30 mA
    3. 3 A
    4. 300 A
    Answer

    B. 30 mA

    A 30 mA rating protects against dangerous shock.

28. A rewirable fuse wire is made of

    1. pure copper
    2. silver-plated steel
    3. tin-lead alloy
    4. iron
    Answer

    C. tin-lead alloy

    A low melting point alloy melts at a small overcurrent.

29. Which wiring system is the safest and most commonly used for new buildings?

    1. Twisted bare wires
    2. Open wires on poles
    3. Cleat wiring
    4. Concealed conduit wiring
    Answer

    D. Concealed conduit wiring

    Conduit protects wires against damage, dust and fire.

30. Cleat wiring is generally used for

    1. hospital wiring
    2. temporary installations
    3. permanent house wiring
    4. underground cables
    Answer

    B. temporary installations

    It is cheap and quick but offers poor protection.

31. A conduit should be filled with cables up to about

    1. 40% of its cross-section
    2. 90% of its cross-section
    3. 100% of its cross-section
    4. 5% of its cross-section
    Answer

    A. 40% of its cross-section

    Spare area lets the cables cool and permits easy drawing in.

32. The lowest-resistance joint of two copper wires that also carries the strain is made by

    1. twisting without tape
    2. taping only
    3. tying with thread
    4. a properly soldered joint
    Answer

    D. a properly soldered joint

    A soldered joint keeps low contact resistance and strength.

33. Which joint is used to take a branch from the main wire?

    1. Western union joint
    2. Britannia joint
    3. Tee joint
    4. Married joint
    Answer

    C. Tee joint

    A tee joint is a branch joint.

34. In armoured cables the metal armour is made of

    1. lead foil only
    2. galvanised steel wire or strip
    3. plastic tape
    4. bare copper
    Answer

    B. galvanised steel wire or strip

    Armour gives mechanical strength.

35. The insulation XLPE stands for

    1. exterior laminated PE
    2. extra low power energy
    3. crossed lead polyvinyl ether
    4. cross-linked polyethylene
    Answer

    D. cross-linked polyethylene

    XLPE is cross-linked polyethylene.

36. Why is the earth pin of a plug thicker and longer than the line and neutral pins?

    1. To save material
    2. To fit only one way
    3. To carry more load current
    4. So that the earth contact is made first and broken last
    Answer

    D. So that the earth contact is made first and broken last

    It gives protection from the moment the plug is inserted.

37. Staircase wiring of a lamp from two places uses

    1. two two-way switches
    2. one one-way and one two-way switch
    3. two one-way switches in series
    4. one intermediate switch only
    Answer

    A. two two-way switches

    Two two-way switches give control from both landings.

38. Which of the following is NOT a method of earthing?

    1. Rod earthing
    2. Pipe earthing
    3. Cleat earthing
    4. Plate earthing
    Answer

    C. Cleat earthing

    Cleat is a wiring system, not an earthing method.

39. A Standard Wire Gauge of 14 compared with 18 is

    1. unrelated to thickness
    2. thicker
    3. thinner
    4. same thickness
    Answer

    B. thicker

    Lower SWG number means thicker wire.

40. The main purpose of a grounding (earthing) system is to

    1. increase supply voltage
    2. reduce frequency
    3. give fault current a low-resistance path to earth
    4. raise load power factor
    Answer

    C. give fault current a low-resistance path to earth

    It makes the protective device trip and keeps touch voltages low.

41. A total load of 4600 W at unity power factor is fed from a 230 V single-phase supply. The current is

    1. 20 A
    2. 10 A
    3. 46 A
    4. 2 A
    Answer

    A. 20 A

    I = 4600/230 = 20 A.

42. Ten lamps of 60 W each burn for 5 hours. The energy used is

    1. 3 units
    2. 0.3 units
    3. 30 units
    4. 300 units
    Answer

    A. 3 units

    10 × 60 = 600 W = 0.6 kW; 0.6 × 5 = 3 kWh.

43. In PILC cables the letters stand for

    1. plastic-insulated low-current
    2. paper-insulated lead-covered
    3. paper-insulated laminated copper
    4. polyvinyl insulated lead-cored
    Answer

    B. paper-insulated lead-covered

    PILC cables use paper insulation with a lead sheath.

44. Earthing the neutral point of a transformer is called

    1. equipment earthing
    2. floating earth
    3. lightning earthing only
    4. system (neutral) earthing
    Answer

    D. system (neutral) earthing

    Equipment earthing refers to metal parts; the neutral is the system earth.

45. Which fuse is meant for protection against short circuit in industry and has a sand-filled cartridge?

    1. Rewirable fuse
    2. Thermal fuse of a lamp
    3. HRC fuse
    4. Fuse of the kit-kat type
    Answer

    C. HRC fuse

    HRC means high rupturing capacity.