Overhead Lines, Poles, Insulators, Stays and Line Hardware; Pole-Climbing Safety

What to remember

• An overhead line has five parts: conductor, support (pole or tower), insulator, cross-arm with fittings, and stay (guy) with earthing. Each part has a fixed job, and exam questions test which part does which job.
• Insulator type follows voltage and use: shackle for LT lines, pin for 11 kV and 33 kV, suspension (disc) for higher voltages, strain at dead ends and angles.
• Safety order never changes: get permit, isolate, test dead, earth, then climb. Wear belt, helmet and gloves. Never work alone.

Overhead line basics

Electric power is carried by bare conductors fixed on supports above the ground. Overhead lines are cheaper and quicker to build and repair than underground cables. Faults are easy to find. The drawbacks are exposure to storms, lightning, birds, tree branches and accidental contact.

Voltage classes used in distribution

• LT (low tension): up to 1000 V. A 3-phase 4-wire supply is 415 V between lines and 240 V between line and neutral. A single-phase supply is 240 V.
• HT (high tension): 11 kV and 33 kV are the common distribution levels. Higher levels (66 kV, 110 kV, 132 kV, 220 kV and above) are used for sub-transmission and transmission.

Conductors

• AAC: all aluminium conductor. Light, used for short spans.
• ACSR: aluminium conductor, steel reinforced. Aluminium strands carry current. The steel core gives mechanical strength. It is the most common conductor for 11 kV and 33 kV lines and for longer spans.
• AAAC: all aluminium alloy conductor. Good strength and low weight.
• Copper is rarely used on new lines because it is costly.
• ABC (aerial bunched cable): insulated LT conductors twisted together on a messenger wire. It reduces theft, faults and accidental contact.
• GI wire is used for earth wire and stays.

Sag and span

• Span is the horizontal distance between two supports.
• Sag is the vertical dip of the conductor below the level of the supports.
• For a level span: sag = w × L² / (8 × T), where w is weight per metre, L is span, T is tension.
• Sag rises with a longer span and a heavier conductor. It falls when tension is higher.
• In hot weather the conductor expands and sag increases. In cold weather it shortens, tension rises and sag falls.
• Worked example: w = 2 N/m, L = 100 m, T = 2500 N. Sag = 2 × 10000 / (8 × 2500) = 20000 / 20000 = 1 m.

Supports: poles and towers

Planting depth of a pole (standard rule of thumb)

Depth = L / 6 (about one-sixth of the pole length), where L is the pole length in metres. Another common rule is L/10 + 0.6 m.

• 8 m pole: 8/6 = about 1.33 m (about 1.4 m in practice).
• 9 m pole: 9/6 = 1.5 m.
• 11 m pole: 11/6 = about 1.83 m.

The foundation is often filled with concrete or stone and well rammed. A loose foundation makes the pole lean.

Types of poles by their position in the line

• Intermediate (tangent) pole: on a straight run. It carries only the vertical load and wind load.
• Angle pole: where the line turns. It carries extra sideways pull, so it needs a stay.
• Dead-end (terminal) pole: at the end of a line. It carries the full one-sided pull of the conductors.
• Section pole: placed after a number of intermediate poles. It limits damage when a conductor breaks.
• Tee-off pole: where a branch line leaves the main line.

Insulators

An insulator holds the live conductor on the support and stops current from leaking to earth. It must have high electrical strength, high mechanical strength and must resist weather. Materials are porcelain, toughened glass and polymer (composite).

String efficiency = (voltage across the whole string) / (n × voltage across the unit nearest the conductor). Here n is the number of discs.

• The disc nearest the conductor has the highest voltage stress.
• String efficiency is always less than 100 percent. It improves with a guard ring or with grading of the discs.
• Worked example: a 4-disc string has 60 kV across it. The disc nearest the conductor has 20 kV. Efficiency = 60 / (4 × 20) = 60 / 80 = 75 percent.
• Flashover is a discharge over the surface of the insulator. Puncture is a breakdown through the body. Puncture ruins the insulator, flashover usually does not.
• Dirt, salt and moisture on the surface reduce insulation. Cleaning and washing help.

Stays (guys) and struts

A stay balances the sideways pull on a pole. It is used at angle poles, dead-end poles and tee-off poles.

• Parts: stay wire (stranded GI wire), stay rod (stay bolt), stay plate (anchor plate), turnbuckle or clamp, and stay insulator.
• The stay wire runs from the pole top to an anchor buried in the ground. A good stay angle is about 30 to 45 degrees to the vertical.
• The stay insulator (egg insulator) is fitted in the stay wire about 3 m above the ground. If a live conductor touches the upper part of the stay, the lower part stays safe for people.
• A stay must be tight, not slack. A slack stay does not help.
• Where a stay cannot be used (for example a road edge), a strut (a push member) is used instead.
• Flying stay: a stay taken across the road to a pole on the other side.

Line hardware and fittings

• Cross-arm: MS angle or channel on which insulators are fixed.
• Pole clamp, D-iron, bracket, bolts and nuts.
• Eye bolt, shackle, thimble and clevis for strain assemblies.
• Binding wire (soft aluminium) to tie the conductor to a pin insulator.
• Parallel groove (PG) clamp for jumpers. A bimetallic connector joins aluminium to copper.
• Spacers and vibration dampers on heavier conductors.
• Arcing horns and guard rings protect insulator strings from flash arcs.
• Lightning arrester at transformer and cable ends.
• Danger plate (with the words "Danger, 11000 volts" or similar) on each HT pole, and anti-climbing device (barbed wire or spikes).
• Earthing of every metal part of the support, and a guard wire where lines cross a road or a telecom line.
• Gang-operated (GO) switch and AB switch: pole-mounted isolating switches on 11 kV lines.
• Bird guards and tree cutting along the route reduce faults.

Pole-climbing and line safety

Before the work

• 1. Get a written permit to work (line clear permit) from the person in charge of the substation.
• 2. Open the breaker and isolator, and lock them. Put up a "Men at work" board.
• 3. Test that the line is dead with a voltage detector or tester.
• 4. Discharge and fix portable earths on both sides of the work place, and short-circuit the three phases.
• 5. Think about back-feed from generators, inverters and solar plants, and about induced voltage from nearby live lines.

Before climbing

• Check the pole: look at cracks, rust, lean, loose foundation and broken stays. A weak pole must be propped or stayed before climbing.
• Check the tools, belt, helmet, rubber gloves and shoes.
• Work in pairs. Never climb alone. Do not climb in rain, strong wind, lightning or poor light.

While on the pole

• Wear a safety belt or full-body harness and fasten it to the pole above the waist.
• Use a hand line (rope) to lift tools. Never throw tools.
• Keep a safe distance from live lines.
• A person on the ground must keep the area clear and watch the climber.

After the work

• Remove tools, earths and boards. Report back so that the line is charged only on the permit holder's order.

Electric shock first aid: switch off, or push the person away with dry non-conducting material. Then start artificial respiration (CPR) and call for medical help.

Exam traps

• Pin insulator vs suspension insulator: pin for lower voltage, suspension for high voltage.
• Shackle insulator is for LT lines. It is not an HT insulator.
• Strain insulator takes the pull at dead ends. A pin insulator does not.
• ACSR vs AAC: the steel core gives strength in ACSR. Current is carried by the aluminium.
• Sag falls when tension rises. Sag rises when span or weight rises.
• Stay insulator goes in the stay wire. It is not the insulator on the cross-arm.
• Flashover is on the surface. Puncture is through the body.
• String efficiency is always below 100 percent. The unit near the conductor is stressed most.

One-liners

• 1. The common HT conductor for distribution is ACSR.
• 2. Planting depth of a pole is about one-sixth of its length (L/6).
• 3. A 9 m pole is planted about 1.5 m deep.
• 4. Sag = w L² / (8 T).
• 5. Shackle insulators are used on LT lines.
• 6. Suspension insulators are used above 33 kV.
• 7. The stay insulator is fitted in the stay wire about 3 m above ground.
• 8. Angle, dead-end and tee-off poles need stays.
• 9. A danger plate and anti-climbing device are fitted on HT poles.
• 10. Test dead, then earth, before touching a line.
• 11. A hand line is used to lift tools to the pole top.
• 12. ABC cable reduces theft and accidental contact on LT lines.

Practice questions

1. Which overhead conductor has a steel core to give mechanical strength?

   1. AAC
   2. AAAC
   3. ACSR
   4. Hard-drawn copper
   Answer

   C. ACSR

   ACSR is aluminium conductor, steel reinforced. The steel core gives strength and the aluminium carries current.

2. Which insulator is normally used on LT overhead lines?

   1. Pin insulator for 33 kV
   2. Disc suspension insulator
   3. Post insulator for 66 kV
   4. Shackle (spool) insulator
   Answer

   D. Shackle (spool) insulator

   Shackle or spool insulators are used on LT lines and LT dead ends.

3. For voltages above about 33 kV, which type of insulator is generally used?

   1. Suspension (disc) insulator
   2. Spool insulator
   3. Egg insulator
   4. Shackle insulator
   Answer

   A. Suspension (disc) insulator

   Discs are joined in a string to suit higher voltages.

4. An egg insulator is fitted in which part of a line?

   1. Stay wire
   2. Cross-arm of a 33 kV line
   3. Earth pit
   4. Service wire at the meter
   Answer

   A. Stay wire

   The egg or stay insulator breaks the electrical path in the stay wire.

5. Which pole stands at the end of a line and takes the full one-sided pull of the conductors?

   1. Section pole
   2. Tangent pole
   3. Intermediate pole
   4. Dead-end pole
   Answer

   D. Dead-end pole

   A dead-end (terminal) pole has conductor pull only on one side, so it needs a stay.

6. What is the main purpose of a stay on a pole?

   1. To carry current to earth
   2. To reduce conductor sag
   3. To balance the sideways pull of the conductors
   4. To support the cross-arm weight
   Answer

   C. To balance the sideways pull of the conductors

   Stays resist the horizontal pull at angle, dead-end and tee-off poles.

7. What is a hand line used for during pole work?

   1. Pulling the conductor
   2. Lifting tools to the pole top
   3. Measuring sag
   4. Earthing the pole
   Answer

   B. Lifting tools to the pole top

   A hand line is a rope used to lift tools so that nothing is thrown or carried in the hand.

8. Sag of a conductor is the

   1. horizontal distance between supports
   2. tension at the support
   3. stretch caused by temperature
   4. vertical dip below the level of the supports
   Answer

   D. vertical dip below the level of the supports

   Sag is measured vertically from the support level to the lowest point.

9. In a 415 V 3-phase 4-wire LT supply, the voltage between a line and neutral is about

   1. 415 V
   2. 240 V
   3. 690 V
   4. 110 V
   Answer

   B. 240 V

   415/√3 is about 240 V.

10. Binding wire is used to tie a conductor to a

    1. strain insulator
    2. stay rod
    3. pin insulator
    4. earth plate
    Answer

    C. pin insulator

    Soft aluminium binding wire holds the conductor on the top groove or side groove of a pin insulator.

11. Which fitting on HT poles prevents unauthorised persons from climbing?

    1. Anti-climbing device
    2. Spacer
    3. Arcing horn
    4. Bird guard
    Answer

    A. Anti-climbing device

    Barbed wire or spikes are fitted below the live parts.

12. What must a lineman obtain before starting work on a line?

    1. A bill of materials
    2. A permit to work (line clear permit)
    3. A meter reading
    4. A tender copy
    Answer

    B. A permit to work (line clear permit)

    The permit confirms that the line is dead and earthed.

13. What is the purpose of a section pole?

    1. To limit damage if a conductor breaks
    2. To hold a transformer
    3. To feed a branch line
    4. To fix a street light
    Answer

    A. To limit damage if a conductor breaks

    A section pole is built stronger and placed after several intermediate poles.

14. A stay wire is made of

    1. PVC-coated flat wire
    2. bare copper strand
    3. soft aluminium strip
    4. stranded galvanised iron wire
    Answer

    D. stranded galvanised iron wire

    Galvanised stranded steel wire resists rust and carries the pull.

15. An aerial bunched cable (ABC) is useful on LT lines mainly because it

    1. needs no support
    2. carries 33 kV
    3. reduces theft and accidental contact
    4. has no voltage drop
    Answer

    C. reduces theft and accidental contact

    Insulated bunched conductors are hard to tap and safe against casual contact.

16. A discharge over the outer surface of an insulator is called

    1. corona loss
    2. flashover
    3. leakage reactance
    4. puncture
    Answer

    B. flashover

    Flashover is along the surface, and usually the insulator is not damaged. Puncture is through the body.

17. Which pole is made with steel wires stressed before the concrete is cast?

    1. PCC pole
    2. Rail pole
    3. RCC pole
    4. Wooden pole
    Answer

    A. PCC pole

    PCC means prestressed cement concrete.

18. Lattice steel towers are generally used for

    1. 33 kV and higher lines and long spans
    2. LT service connections
    3. indoor wiring
    4. street lights only
    Answer

    A. 33 kV and higher lines and long spans

    Towers carry heavy conductors over long spans and river crossings.

19. The standard rule for the planting depth of a pole of length L metres is

    1. L/3 + 1 m
    2. L/10 + 0.3 m
    3. L/6 + 0.6 m
    4. L/6 - 0.6 m
    Answer

    C. L/6 + 0.6 m

    This is the usual rule of thumb for poles of 8 m to 12 m.

20. What is the planting depth of an 8 m pole by the rule L/6 + 0.6 m (nearest value)?

    1. 1.33 m
    2. 2.60 m
    3. 1.60 m
    4. 1.93 m
    Answer

    D. 1.93 m

    8/6 + 0.6 = 1.33 + 0.6 = 1.93 m.

21. What is the planting depth of a 9 m pole by the rule L/6 + 0.6 m?

    1. 2.7 m
    2. 2.1 m
    3. 1.8 m
    4. 1.5 m
    Answer

    B. 2.1 m

    9/6 = 1.5, and 1.5 + 0.6 = 2.1 m.

22. What is the planting depth of an 11 m pole by the rule L/6 + 0.6 m (nearest value)?

    1. 2.00 m
    2. 3.03 m
    3. 2.43 m
    4. 1.83 m
    Answer

    C. 2.43 m

    11/6 = 1.83, and 1.83 + 0.6 = 2.43 m.

23. A level span is 100 m. The conductor weighs 2 N/m and the tension is 2500 N. The sag is

    1. 2 m
    2. 4 m
    3. 0.5 m
    4. 1 m
    Answer

    D. 1 m

    Sag = wL²/8T = 2 × 10000 / (8 × 2500) = 1 m.

24. A conductor weighs 4 N/m. The span is 80 m and the tension is 1600 N. The sag is

    1. 2 m
    2. 4 m
    3. 0.5 m
    4. 1 m
    Answer

    A. 2 m

    Sag = 4 × 6400 / (8 × 1600) = 25600 / 12800 = 2 m.

25. A span of 60 m has a conductor weight of 3 N/m and a sag of 1.5 m. The tension is

    1. 600 N
    2. 900 N
    3. 1800 N
    4. 450 N
    Answer

    B. 900 N

    T = wL²/8S = 3 × 3600 / (8 × 1.5) = 10800 / 12 = 900 N.

26. A 3-disc string has 45 kV across it. The disc nearest the conductor has 20 kV across it. The string efficiency is

    1. 90 percent
    2. 66.7 percent
    3. 100 percent
    4. 75 percent
    Answer

    D. 75 percent

    Efficiency = 45 / (3 × 20) = 45 / 60 = 75 percent.

27. A 5-disc string has 100 kV across it. The disc nearest the conductor has 25 kV across it. The string efficiency is

    1. 75 percent
    2. 95 percent
    3. 80 percent
    4. 60 percent
    Answer

    C. 80 percent

    Efficiency = 100 / (5 × 25) = 100 / 125 = 80 percent.

28. A 4-disc string has an efficiency of 80 percent with 64 kV across the string. The voltage across the disc nearest the conductor is

    1. 12.8 kV
    2. 16 kV
    3. 25.6 kV
    4. 20 kV
    Answer

    D. 20 kV

    Max disc voltage = 64 / (4 × 0.8) = 20 kV.

29. The phase voltage of a 3-phase 11 kV line is about

    1. 11 kV
    2. 3.67 kV
    3. 6.35 kV
    4. 19 kV
    Answer

    C. 6.35 kV

    11 / √3 = 11 / 1.732 = about 6.35 kV.

30. A stay at 45 degrees to the vertical must balance a horizontal pull of 2000 N on the pole. The tension in the stay wire is about

    1. 2.83 kN
    2. 2.0 kN
    3. 1.41 kN
    4. 4.0 kN
    Answer

    A. 2.83 kN

    Horizontal component = T sin 45°, so T = 2000 / 0.707 = about 2830 N.

31. A stay at 45 degrees to the vertical is fixed on a pole 6 m above ground. How far from the pole base is the stay anchor?

    1. 4.5 m
    2. 6 m
    3. 8.5 m
    4. 3 m
    Answer

    B. 6 m

    At 45 degrees the horizontal distance equals the height, so 6 m.

32. A line of 1 km has poles at 50 m spacing, with a pole at each end. How many poles are needed?

    1. 20
    2. 19
    3. 25
    4. 21
    Answer

    D. 21

    1000 / 50 = 20 spans, so 21 poles.

33. Consider these statements. 1. In ACSR, the steel core carries most of the current. 2. Shackle insulators are used on LT lines.

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

    B. 2 only

    Aluminium carries the current in ACSR, and steel gives strength. Shackle insulators suit LT lines.

34. Consider these statements. 1. A stay insulator is fitted in the stay wire above the ground. 2. Angle poles need stays.

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

    C. Both 1 and 2

    The egg insulator protects people below, and the sideways pull at an angle needs a stay.

35. Consider these statements. 1. String efficiency of a practical insulator string is 100 percent. 2. The disc nearest the conductor has the highest voltage stress.

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

    B. 2 only

    Efficiency is always below 100 percent, and the disc near the conductor is stressed most.

36. Consider these statements. 1. Flashover usually ruins an insulator permanently. 2. Puncture is a breakdown through the body of the insulator.

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

    B. 2 only

    Flashover is on the surface and usually leaves the insulator usable. Puncture damages it.

37. Consider these statements. 1. Sag increases when the span increases. 2. Sag decreases in hot weather.

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

    A. 1 only

    Sag is proportional to the square of the span. In hot weather the conductor expands and sags more.

38. Consider these statements about work on a line. 1. Earths should be fixed on both sides of the work place. 2. A line need not be tested if the breaker has been opened.

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

    A. 1 only

    A line must always be tested dead, and earthed on both sides.

39. Consider these statements about pole-top work. 1. Tools are lifted by a hand line. 2. Tools may be thrown up to the climber to save time.

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

    A. 1 only

    Tools must never be thrown. A rope hand line is used.

40. Consider these statements. 1. PCC poles use prestressed concrete. 2. Steel tubular poles are useful in cities where space is limited.

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

    C. Both 1 and 2

    Both statements are correct.

41. Consider these statements. 1. Pin insulators are mainly used above 132 kV. 2. Shackle insulators are used on 33 kV towers.

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

    D. Neither 1 nor 2

    Pin insulators are for lower voltages, and shackle insulators for LT lines.

42. Which of the following pairs is correctly matched?

    1. Pin insulator - stay wire
    2. Egg insulator - conductor cross-arm
    3. Strain insulator - dead-end pole
    4. Shackle insulator - 132 kV tower
    Answer

    C. Strain insulator - dead-end pole

    Strain insulators take the full conductor pull at dead ends and sharp angles.

43. Which pair of tool and use is correctly matched?

    1. Binding wire - joining stay rods
    2. Hand line - measuring sag
    3. Danger plate - earthing a pole
    4. Voltage detector - checking that a line is dead
    Answer

    D. Voltage detector - checking that a line is dead

    A line must be tested dead with a detector before earths are applied.

44. Consider these statements. 1. An angle pole carries extra sideways pull, so it needs a stay. 2. An intermediate pole carries the full one-sided pull of the conductors.

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

    A. 1 only

    The full one-sided pull belongs to the dead-end pole.

45. Consider these statements about safety. 1. Back-feed from a generator or solar plant can make a dead line live. 2. Portable earths are needed only on one side of the work place.

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

    A. 1 only

    Earths are fixed on both sides, and back-feed is a real danger.