Conductors and Insulators
Domain 4 - Conductors and Insulators. Expect questions on conductor selection and rating, joints, and on insulator types, testing and failure modes.
Conductors
- Hard-drawn copper has the best conductivity but high cost; aluminium is light and cheap with low tensile strength; ACSR puts aluminium strands over a steel core for a high strength-to-weight ratio and longer spans; AAAC suits corrosive and coastal areas.
- Standard ACSR sizes include Rabbit (6/3.35 mm, about 52.9 mm²) and Dog (about 100 mm²). LT services use insulated ABC or 1.1 kV cable, not bare ACSR.
- Current rating depends on ambient temperature, wind, solar gain and permissible conductor temperature (65–75 °C). Derate for high ambient and for grouped circuits.
- Joints are made with compression sleeves, PG clamps or repair sleeves. Never join copper to aluminium without a bimetallic clamp, and never leave a joint in a road crossing span.
Insulators
- Pin insulators for 11 kV and 33 kV, post insulators in substations, disc suspension strings at 33 kV and above, shackle insulators on LT, stay insulators on guy wires.
- Creepage distance (about 25–31 mm per kV of line voltage in normal pollution) is the surface path; it is not the strike distance through air.
- Test with a megger or insulator tester and reject cracked, chipped or punctured units. A punctured insulator may never flash over yet fail mechanically.
Worked example
Required creepage for 11 kV at 25 mm/kV is 275 mm. A standard disc has about 292 mm creepage, so one disc is electrically sufficient - yet 11 kV lines use pin insulators and 33 kV strings use three discs, because mechanical duty and pollution, not creepage alone, set the final number.
Traps
- Confusing flashover (external) with puncture (internal).
- Fitting shackle insulators on an 11 kV line.
- Selecting a conductor by diameter instead of by current rating and voltage drop.
- Joining copper and aluminium directly, causing galvanic corrosion and a hot joint.