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Electromagnetic Effects

Shaun Humm

Hi guys, after peoples opinions here, I’ve encountered 6 63A single phase sub main distribution circuits protected by 1361 fuses from a distribution board passing through 3 32mm brass bushes cut virtually on top of each other through the side of two pieces of metallic trunking. The bushes are bridging a change in direction from horizontally to vertically and creating a reduction in trunking size from 9-4 inch. The installation is 3 TP&N and there’s two circuits Allocated per phase. Each two circuits per phase, the line and neutral conductors all pass through one bush together and then like wise for the remaining two phases. There is no slot between the the bushes, and steel locknuts used. 

Although an effort has been made to take all line and neutral conductors for each phase through one hole would this be classed as a non compliance giving its a three phase installation and the amount of current capable of passing through the sub main circuits collectively creating some overheating in the trunking?

Parents
  • 0

    regs to one side, brass bushes are not affected by magnetic field, so the hole mayy be bigger than you think

    (and in some quarters, when cutting the dog bone slits between holes in steel plates, the slot is then brazed shut for reasons of blast containment or ingress of contamination.)

    Now  It is not too clear from your description, how the cables are paired - it would be OK if it was  L1A L1B N1     L2A L2B N2  L3A L3B N3 where N1 carries the return current from L1A and L1B etc.

    But even if it is worst case, for thin sheet steel spaced off from the wire by several mm because of the brass bush, you need hundreds of amps of unbalanced current to give noticeable heating. If you had unbalanced cables in long runs of steel conduit it is a different story.

    not compliant perhaps. but assuming not obvious heating, either a C3 or no action needed.

    Mike.

Reply
  • 0

    regs to one side, brass bushes are not affected by magnetic field, so the hole mayy be bigger than you think

    (and in some quarters, when cutting the dog bone slits between holes in steel plates, the slot is then brazed shut for reasons of blast containment or ingress of contamination.)

    Now  It is not too clear from your description, how the cables are paired - it would be OK if it was  L1A L1B N1     L2A L2B N2  L3A L3B N3 where N1 carries the return current from L1A and L1B etc.

    But even if it is worst case, for thin sheet steel spaced off from the wire by several mm because of the brass bush, you need hundreds of amps of unbalanced current to give noticeable heating. If you had unbalanced cables in long runs of steel conduit it is a different story.

    not compliant perhaps. but assuming not obvious heating, either a C3 or no action needed.

    Mike.

Children
  • 0 in reply to mapj1

    Thanks for your reply! That’s exactly how they are paired. Both circuits on L1 all pass through one bush together, live and neutral. Then likewise for L2 and L3

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