Main breaker trip

[tank]

Hi guys, why might a fault on a final circuit in an apartment trip the dp c63 submain circuit breaker in the plantroom but not any mcbs on the consumer unit in the apartment?

 

[Pete999]

Have you carried out any tests on the cable?

 

[tank]

A kitchen fitter did something which took out the breaker in the plantroom but not any mcbs on the consumer unit in the apartment. I insulation resistance tested the submain and it was fine.

 

[telectrix]

if kevin kitchen has been around, anything is possible.

 

[snowhead]

Probably he drilled through the tails.
Ask him to remove all his units so you can investigate.
He may find the Live screw he's created and save you some time searching.

 

[D Skelton]

If the fault current was high enough to achieve instantaneous tripping on both devices, it's a coin flip which one goes first.

MCBs are notoriously poor for achieving effective discrimination/selectivity and are often installed in-line as a product of poor design and nothing more.

 

[telectrix]

same thing with this 3A fuse for bathroom fans crap, when they are on a 6A MCB protected circuit. the MCB is likely to trip instead of (or as well as) the 3A fuse.

 

[westward10]

The 3A fuse is for fire protection apparently not circuit protection as such.

 

[telectrix]

The 3A fuse is for fire protection apparently not circuit protection as such.

you mean to replace a safety device that the manufacturers should have fitted integral to the fan?

 

[pc1966]

Typically you need the fancier electronic type of MCCB that allows a short delay before firing (at least up to its own high instant magnetic trigger point) to allow any chance of good selectivity - i.e. if a down stream fault occurs then it only trips the downstream breaker.

MCBs in series rarely have much selectivity. Often it is barely more than the instantaneous trip of the upstream one, so if you have a 63A C-curve breaker that is 315-630A. Hence if the fault impedance is below 0.73-0.36 ohms then the upstream one probably goes as well (since it is already underway to trigger before the downstream MCB contacts have opened).

Or you use a fuse up-stream. Then you typically need a far bigger PFC to blow it, and might even get total selectivity.

 

[pc1966]

Re-reading the OP's post, it would be very unusual for the downstream one not to fire. It would have to be seized or contacts welded shut (which is very bad news in any circumstances).

Personally I would suspect the cable-drilling culprit rest the breaker hoping all would be well and then found out the upstream one had gone.

But just in case I would check all MCBs toggle normally, and check if there is any chance of the supply tails being drilled, and look carfully in the CU in case some absolute moron has taken a cable from the busbar side of the MCBs.

 

[pc1966]

Very unlikely (more a USA thing), but they are not Federal Pacific / Federal Electric ones by any chance?

I have heard various horror stories of them failing to fire on faults, thankfully never met one myself.