Consumer Unit: why does the whole thing trip?

Sorry about the naive and simplistic subject, which (once again) reflects my knowledge of things electrical.

Our consumer unit tripped off this morning - the main trip, that is, not one of the circuit trips.

By elimination I tracked first the circuit and then the thing that tripped it, which turns out to be the pond pump. Or rather, more likely: the chain of cables and connections that connects the socket in the house with the pond pump. I suspect that water has got into one of the connection points: haven't had time to check it yet.

My question is: why does this trip the main switch on the CU, and not just the circuit breaker for that ring main?

Thanks for any replies Cheers John

Reply to
Another John
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The "main switch" *must* be a residual current device, and will be tripped by earth leakage, the secondary circuits may just be miniature circuit breakers, which will only be tripped by an overcurrent fault.

All recent consumer units should have an RCD on any supplies to exterior points, and would normally trip that first. It is recommended that if one is not fitted, you use either an RCD fitted socket or install the correct unit in the CU.

You're probably right that water has got into an exterior connection somewhere.

Reply to
John Williamson

Thanks a lot John. That explains things for me quite a bit!

The pond pump supply is in fact plugged into its socket via an RCD, but it's quite old by now: perhaps that has failed, also. I will be looking at re-making the whole shebang, from socket, up garden, to pump, as it's been going for far too long, really, in the Heath-Robinson way it was originally set up.

Cheers John

Reply to
Another John

New consumer units can be fitted with an RCBO for each circuit. These combine the functions of your two types of trip, so only one circuit will be affected whatever happens.

Reply to
Graham.

Two RCDs in series with no discrimination:-(

One in the plug for the pond and one in the CU main switch. The CU one will almost certainly trip first as there will be earth leakage from other appliances in the house going through that RCD (accumulated leakage).

You need a new CU. One with more than one RCD

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Reply to
ARW

When you have 2 RCDs in series, its a total gamble which will trip. Since the CU RCD is preloaded with leakage current on other circuits, the odds are higher that that wuold trip first.

Fix the leaky pond circuit is the solution. A more modern CU that doesnt use 1 RCD to cover the lot would cause less trouble, but I see no good reason to replace it on the basis of one trip on a fixable circuit.

NT

Reply to
meow2222

I might be missing something here, but couldn't it also be a live to neutral fault on that circuit, and that the main breaker has been faster than the one on the circuit. Especially if there is a two core flex to the pump.

Reply to
newshound

The "main breaker" as you put it is almost certainly just an RCD, and won't trip for Live/Neutral shorts. It's only interested in earth faults.

The supplier's main fuse won't blow with a fault someway down a pond pump cable - the cable couldn't pass enough current, and it would blow a downline fuse or MCB well beforehand (assuming it's got the correct protection in place).

Reply to
Andrew Gabriel

Thanks for all replies folks - very useful. Just to report back:

I have checked out the pond supply, fiddled a bit, put it back together, and all is working now, with no re-tripping (so far). This of course won't stop me from doing what I should have done years ago, namely putting proper gear in place.

WRT Newshound's point above, and also (for all I know) wrt to the other points made, and just for your info ...

The supply is a 3-core cable which is plugged into a socket in the house; this cable goes up the garden (buried) to a [95%] waterproof box (actually a clip-shut sandwich box) (I'm getting embarrassed now), which resides inside a drainpipe for extra weather proofing. Inside this junction box, as I might call it, the pump's own plug is plugged into a single socket at the end of the house cable. This temporary arrangement has worked fine for a number of years, I'm sorry to say.

Investigating the plug/socket arrangement inside my "junction [sandwich] box", I found that all was dry, although slightly damp of course, but there were many tiny creatures, which I believe were new-born woodlice. It's possible that these may have caused the short, I suppose?

Anyway: all cleaned up, and working now, but I will do the job properly this next week, assisted by the advice received here: thanks very much!

John

Reply to
Another John

It sounds like you have a RCD in place of your main switch... (it will have a test button on it) this suggests either a 15th edition style "whole house" RCD, or perhaps a system that uses a local earth spike rather than having an earth provided by the electricity supplier.

RCDs trip on imbalance between current flow in live and neutral - and that imbalance only needs to be very small (typically 30 or 100mA) to cause the trip.

MCBs on the other hand need a substantial overcurrent in excess of the value marked on the front to trip. Typically a continuous load of 1.45 x the marked range, or 5 x the marked range for an "instant" trip.

So a fault which is not a circuit overload, or a fault (i.e. a short circuit of some sort), but instead is a small leakage to earth due to water ingress etc will trip the RCD and not the MCB for the circuit.

There are various ways that this behaviour could be improved, although the need will depend on how much of a problem it is, and how frequently it happens.

Reply to
John Rumm

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