H.A. Jarvis asks about short circuit (SC) protection for lighting flexes. His question has been answered by the NICEIC (Theme – Fire, Safety & Security):
Question: An electrical installation has 0.75mm three-core HR flexible cords with the circuit protected by a 10A Type C MCB. The fixed wiring is 2.5mm LSF in steel conduit - Zs for Is in the range 0.2 to 1.2 ohm with a prospective current of 8-13kA at the lighting distribution boards.
We know that Table 4F3A rates 0.75 at 6A and 1.0mm at 10A, so are these flexes too small?
Could you clarify the position with regard to flex sizes for the drops into luminaires, and their short circuit protection?
Can flexes with a lesser rating than the MCB be used, and an SC calculation be carried out to justify the reduced flex size?
Answer: In terms of overload protection, this can be omitted in accordance with 433.3.1 (ii), where the nature of the load is such as to make overload unlikely. This is normally the case with a flex feeding 100W lamps.
Zs values will need to be such as to operate in appropriate times. They should also allow the protective device to trip.
To see many more Q & A in Voltimum UK's Experts Area, please click on the link:
www.voltimum.co.uk/consult.php?universe=consult.index.questions