MCB TRIPPING FROM SHED

kieranlaverty

Hi guys

It's my first post but I'm puzzled.

My father in law has a shed which gets its supply from the main DB in the house. It goes into a JB then out to his shed which has its own DB which has a 63amp main breaker a 6amp MCB for lights a 16amp MCB for sockets. The shed DB is run off a 20amp MCB from the house DB.

It's been working for years with no problems then recently it's been tripping the MCB in the house with nothing even on in the shed. Bypassing the DB in the shed.

Could it be something in the shed or something else.

Thanks

2011

So the 20A MCB in the main DB is tripping. The most likely cause of this is an over current condition on a downstream cable. This is unlikely to be the 6A circuit for the lights in the shed as a fault on this should cause this 6A MCB to trip (not the 20A MCB in the board)

So what are you left with?
1) The cable between the main DB and the shed
2) Wiring within the shed DB.
3) The socket circuit in the shed. As this MCB is rated at 16A discrimination is not a guarantee.

So what do you look for??
I would start by carrying out an insulation resistance test on the cable supplying the shed. I would also have a good look at the JB. This is really work for a qualified electrician, not a DIYer.

Risteard81

2011 wrote: »

As this MCB is rated at 16A discrimination is not a guarantee.

I agree with the rest of your post, but would just add that as well as a 16A MCB not discriminating with a 20A MCB, in fact you will find that no two MCBs of any rating in series will reliably discriminate with each other when it comes to fault currents. They may, however, on overload.

2011

Risteard81 wrote: »

I agree with the rest of your post, but would just add that as well as a 16A MCB not discriminating with a 20A MCB, in fact you will find that no two MCBs of any rating in series will reliably discriminate with each other when it comes to fault currents. They may, however, on overload.

Let’s not get too technical here. A question about something very fundamental should not be answered by something too involved.

Risteard81

2011 wrote: »

Let’s not get too technical here. A question about something very fundamental should not be answered by something too involved.

Fair enough.

Steve

Is it tripping an RCD on the house board?

If so, could be insulation is compromised in the cable.

TomOnBoard

kieranlaverty wrote: »

Hi guys

It's my first post but I'm puzzled.

My father in law has a shed which gets its supply from the main DB in the house. It goes into a JB then out to his shed which has its own DB which has a 63amp main breaker a 6amp MCB for lights a 16amp MCB for sockets. The shed DB is run off a 20amp MCB from the house DB.

It's been working for years with no problems then recently it's been tripping the MCB in the house with nothing even on in the shed. Bypassing the DB in the shed.

Could it be something in the shed or something else.

Thanks

I'm puzzled also....

If I read you correctly, he is taking a circuit through a 20A MCB in the house and feeding that to the shed which has a 63A main breaker? If there's any issue at all, other than a total catastrophe, the house 20A would trip before the shed main breaker would trip out. Any problems between the house and the 2 MCBs in the shed are likely to be picked up by tripping the MCB in the house long before the shed main breaker would trip.c

How are ELCB/RCD functions being handled?

Steve

63A in the shed is just an isolator. 20A on the house board is protecting the cable.

Normally discrimination dictates that the relevant breaker in the shed will trip first.

The breaker in the house would be non RCD, this would be handled be the sub board.

2011

Steve wrote: »

Is it tripping an RCD on the house board?

Not according to the OP. I doubt that the shed is fed through an RCD in the house.

If so, could be insulation is compromised in the cable.

I suspect that there is an issue with the insulation this is why I recommended an IR test.

Bruthal

2011 wrote: »

Not according to the OP. I doubt that the shed is fed through an RCD in the house.

I wouldnt doubt anything in these type circumstances.

Thomyokk

From the description it sounds like the shed is fed from the common RCD in the house

Never a good idea of course

2011

Thomyokk wrote: »

From the description it sounds like the shed is fed from the common RCD in the house

Please explain why you suspect that is the case.

2011

I think that if the “main RCD” was tripping this would have resulted in a loss of sockets throughout the house and the OP would have been far more concerned about this. As this issue was not raised by the OP I think it is not unlikely that the shed is fed from the “main RCD”.

TomOnBoard

Sure, we're all simply guessing anyway. OP flew in 3 days ago with this one and only post on Boards, laid this egg that had lots of relevant details missing and flew off again, and hasn't been seen since. Complete waste of all our time! Grrrr!

Thomyokk

2011 wrote: »

Please explain why you suspect that is the case.

You may be right I probably misread the op

It is a little unusual for an mcb to be tripping on a distribution circuit if installed properly

2011

Thomyokk wrote: »

You may be right I probably misread the op

I have misread a few myself

Thomyokk

2011 wrote: »

I have misread a few myself

There was no mention of RCD in the sub board which led me to that conclusion