mcb question

jim23 · 27 Jul 2006

can anyone explain the operation of triple pole mcb's to me.

for a 32A tp does that mean 32A in each phase before the breaker trips or is it 32A total?

JohnD · 27 Jul 2006

Neither.

Its 32A x the breaking factor x time ON ANY ONE OR MORE PHASES

So an immediate trip would be about 45 amps on one or more poles. Smaller overloads take longer.

jim23 · 29 Jul 2006

was wonderiing because at our work we have three pole circuit breakers rated at 60A that protect switch disconnectors & 63A socket outlets. i believe the wiring to be in 6mm singles in trunking and a sparky friend of mine told me the 6mm singles were only rated to about 50A.

is this set up ok?

Steve · 29 Jul 2006

jim23 said:
was wonderiing because at our work we have three pole circuit breakers rated at 60A that protect switch disconnectors & 63A socket outlets. i believe the wiring to be in 6mm singles in trunking and a sparky friend of mine told me the 6mm singles were only rated to about 50A.

is this set up ok?

in a word, no. The singles should be 10mm²

jim23 · 29 Jul 2006

strange because they had a periodic inspection done few months back and this hasnt been mentioned in the report as far as i know.

shouldnt the inspector have picked up on this?

Spark123 · 29 Jul 2006

Surprisingly 6mm² single-core 70deg C cable installed in a 3 phase configuration to method 3 (in conduit/trunking) from table 4D1A is only rated at 36A. 10mm² is rated 50A and 16mm² is rated 68A, all these are before correction factors are applied.

jim23 · 30 Jul 2006

right so im guessin that means the set up isnt ok?

cant believe the inspector wouldnt pick up on this? is there any way this could be alright or should this have been commented on in the inspection?

if the load is balanced then presumably the cables should be ok but if unbalanced then the protection isnt there surelly?

Spark123 · 30 Jul 2006

No, if you have a 63A mcb on a single phase then 63A is available to that phase regardless of ballancing, just think of the 3phase mcb as three single phase mcbs, only mechanically joined together so they all trip at the same time. As it is feeding 63A socket outlets then all 3 phase and the neutral conductor need to be able to supply the full 63A.

jim23 · 30 Jul 2006

ok thanks for that. maybe they need to get that inspector back in

.....or maybe a different one!

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mcb question

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