Cables and Accessories
What this domain tests
Cables and Accessories tests conductor and insulation selection, cable construction, current rating, derating, voltage drop, terminations and accessories such as sockets, ceiling roses, glands and lugs.
Core rules
- Copper has lower resistance than aluminium for the same size; aluminium needs suitable bimetallic lugs and antioxidant compound.
- PVC cables are common for LV; XLPE has a higher temperature rating. Armoured cables need the armour effectively earthed.
- Cable size is chosen for current-carrying capacity, voltage drop, short-circuit withstand and mechanical strength, not current alone.
- Common Indian phase identification is red, yellow and blue for three phases, black for neutral and green for earth; confirm against the installation scheme.
- Bending radius must be respected; sharp bends damage insulation and strands.
Worked example
A 20 A load is 30 m from the board. A cable has a tabulated drop of 7.3 mV/A/m. Voltage drop = 7.3 × 20 × 30 / 1000 = 4.38 V. On a 230 V supply this is 1.9%, acceptable for a typical 3% limit. The cable must still have a current rating above 20 A after derating for ambient temperature and grouping.
Traps that cost marks
Choosing cable by current only and ignoring voltage drop, undersizing the neutral for harmonic loads, leaving armour unearthed, not using a gland for armoured cable, and terminating aluminium without a bimetallic lug. A loose termination overheats even when the cable is correctly rated.