Nuisance MCB tripping-solution?

[tarquinlinbin]

Hello, i have had a very long term problem with nuisance RCD tripping.

Have a look at

is my older style consumer unit which at sometime in the past has had rewireable ccarts' replaced with MCB's. I suspect that it might actually have been originally installed like this from new build maybe

20 years ago.

Anyway for the last few years we have had nuisance trip of the main MCB. It can go for months and months without tripping and then suddenly its off. This week i came back from holiday and found that the main RCD had trippede,leaving my fridage/freezer nicely defrosted. Luckily there wasnt much in it.

We had had an electrician test the installation in the past and found nothing.

I am pretty certain that it is a low level leakage and not an instand fault which is tripping it but it is so intermittent that it is hard to trace.

I also suspect that it might be something on the ring main.

My latest thought is to by a new mcb for the ring main supply and fit this in place of the older mcb. Hopefully it will have a faster more reliable trip as at present i suspect that the main RCD has a faster trip time and greater sensitivty than the MCB's and therefore the MCB never trips but the main one does thus turning the lot ff instead of just the affected circuit.

I did think about fitting a new consumer unit but i think i'd just be stuck with the same problem but with a new consumer unit instead!!

any thoughts?

joe

[ARWadsworth]

Change the box for a split load CU. It should set you back about £100 and you can put the things that do not need rcd protection on the non rcd side (IE immersion, lights and maybe cooker)

Adam

[Dave Liquorice]

Can you be more precise in your usage of "main MCB" and "main RCD". They perform very different functions. I think I know what you are getting at from the rest of your message but there is nothing quite like accurate information at the start to avoid the garbage in/garbage out scenario. B-)

[Christian McArdle]

Installations aren't done with whole house RCDs any more for two reasons.

1. It makes an RCD trip more likely, as it is measuring the earth leakage for the entire house, the sums of which can get perilously close to the tripping current.
2. It makes an RCD trip more serious, as it cuts power to the lights and the fridge/freezer.

Today, installations either use a split load unit, so that RCD protection is only on socket circuits, or use RCBOs which give independent RCD protection to each circuit that needs it. Fridges, freezers and lighting are generally wired to their own independent circuits without RCD protection, so don't suffer from nuisance trips.

If you have TT earthing, you are required to have RCD protection on every circuit. This is usually done by using a low sensitivity time delay one that guarantees not to blow until the socket circuit RCD/RCBOs have had a chance.

If you install a split load or non-RCD unit with RCBOs, the problem will be cured. You will also need to run a separate circuit for the fridge/freezer.

Christian.

[Mike Harrison]

Faults in the feed cable in the street can also cause RCD trips (not totally sure I understand how...), but a while ago my RCD tripped a few times a few hours before the cable finally blew up and smoke came pouring out of a crack in the pavement....!