Shower tripping RCD

[davidward30]

Hello,

We have had a bathroom fitter (who is electrically qualified) replace an old (but fully functioning) 9kW shower with a new 8.5kW shower. Some additional cross-bonding work was done at the same time. I know that it involved bonding to the gas pipe and to the central heating pipes.

The old shower is and always was on its own circuit, with a 30A MCB and

30mA RCD.

It now turns out that the new shower doesn't work - the 30mA RCD is tripping. The fitter decided to upgrade to 40A.

The shower manufacturers have visited and claim there is a fault in the wiring, not the shower. Given that we had no electrical problems before, can anyone think what might be wrong?

Thanks.

David.

[Christian McArdle]

The "electrically qualified" bathroom fitter should have done a continuity and insulation test of the shower circuit, which will indicate if there are any issues. They should also use their RCD tester on the RCD to make sure the problem doesn't lie there.

Christian.

[Chipmunk]

The fitter should have left you the insulation test and earth impedance test measurement results. If so, these could provide valuable insight into the problem. Obviously there is a leak _somewhere_, and decent tests should allow you to pinpoint the nature of the leakage. Was any of the other wiring disturbed, for example in the isolator switch? [by the way, double check that switch is rated for 40+ amps, it may only have been a 30 amp switch]. A possibility if the plumbing leaked at the time of installation is that the wiring got wet. Also, there may have been a bad earth connection somewhere en route, which has now been supplimented by the new cross bonding to the pipework and is thus causing trippage.

[Ellis Greensitt]

Possibly the wiring problem has been there all along, maybe the extra bonding that has been done just brought it to the attention of the RCD.

You must get this check out by an electrician not a bathroom fitter.

[Calvin]

When is it tripping?

1. As soon as the main incomming breaker in the consumer unit (CU) is switched on?
2. As soon as the shower MCB in the CU is switched on?
3. As soon as the isolating switch in the bathroom is switched on?
4. As soon as the shower itself is switched on?
5. Only after the shower has been running for a while?

[davidward30]

I'd need to check myself. Definitely not 1,2 5.

95% sure its 4.

[Christian McArdle]

There's loads of possible explanations for that response. Some of them are:

1. Dodgy shower.
2. Earth/Neutral swapped (at any point).
3. Neutral for circuit taken from non-RCD bus bar.
4. MCB for circuit taken from non-RCD bus bar.
5. Knackered RCD.
6. Wet heating element*.

Christian.

• This happens when a mineral insulated heating element has absorbed water. This leads to earth leakage that can trip an RCD. It can be cured by warming the element to expel the water. This can be done (after eliminating all other causes and taking extra precautions) by bypassing the RCD and letting the heating element run for many minutes. Then the RCD can be reinstated.

[Calvin]

Other than a very difficult to believe double-fault I can't see any other expanation than that the shower is to blame for the tripping.

If the fault is with the live wiring external to the shower then it will trip whenever that wiring was energised, whether or not the shower itself is switched on.

If the fault is with the neutral wiring external to the shower then normally you would expect this to result in a trip even if the shower was off as an RCD is sensitive to an inbalance between the current flowing in the live and neutral and there is normally enough potential on N so that if it is shorted to earth you get a trip. (Very safe but also very annoying as the neutral is normally not switched by the MCB which can result in trips when working on circuits which are off).

The only time this would not be the case (and it's very unlikely to be this) would be if normally your N does not have enough voltage on it to cause 30mA of imbalance when shorted to earth but the resistance of the cable to the shower is such that when the shower is on the voltage rises above that threshold. In that case a fault in the neutral wire external to the shower could be to blame. If you are experienced/brave/stupid enough you can rule this possibility out in one of two ways:

1. With the shower off deliberately short N to earth. If the RCD trips you know that you do not normally have a neutral fault. or
2. Disconnect the neutral where the wire enters the shower then switch the shower on. It won't work of course but if it trips the RCD you've proved that it's the shower at fault. Unfortunately, depending on the design within the shower, if it doesn't trip you don't prove anything. PLEASE BE SAFE - only perform these experiments if you KNOW you can do them safely.

My bet is on the shower itself being faulty despite what the manufacturer says. Water heating elements are common offenders although I have to admit, not normally brand new ones. Do you know anyone with a PAT tester - the sort they use to test appliances? That would test the shower itself.

Good luck and keep safe.

[davidward30]

When the isolating switch is on, nothing trips, but the LED does not come on properly as before. Its all dim and flickery.

Any ideas?

thanks.

[davidward30]

Thanks for the reponse. I'm not experienced enought to carry out those tests; I shall leave it to the fitter and/or his electrician and failing that my electrician.

The manufacturer even installed a brand new shower and it did the same.

I found out something interesting about the isolation unit:

When the isolating switch is on, nothing trips, but the LED does not come on properly as before. Its all dim and flickery

Worrying....

[Calvin]

That LED is probably (almost certainly) a neon. It is connected across the output L and N from the isolating switch. Neons aren't something I know a lot about but it does sound to me like it might be trying to tell you something!

Anyone care to comment on what might cause a neon to flicker? High resistance contact somewhere? Intermittent contact due to a poor connection? Crossed N and E like Christian McArdle suggested?

[Chipmunk]

Likely this is not the cause, but the symptoms are so close I must relate the following experience from 10 yrs ago.

Symptom: Shed is on an RCD, which usually stays on, but when a heavy load is applied, after a few seconds the RCD would go out. Repeatedly.

The cause, after many hours of headscratching. A loose neutral had burned up in one of the connections inline, and was going to ~80 volts when the heavy load came on. That was enough to flash over to the earth next to it, tripping the RCD, the symptoms gradually became worse. As I said, it's highly unlikely that's what's happening, but the 'dim flickering' of the neon indicator in the switch points to something loose..

Such things can cause WEIRD symptoms and lead you on wonderful wild goose chases.

Hope you nail the problem before you pull too much hair out.

[Owain]

If the RCD is tripping, "upgrading" the MCB isn't going to make a blind bit of difference, and is irresponsible to do if the cable is sized for 30A.

Owain

[bigcat]

Dim and flickery is a normal condition for neons, it is not a cause for concern.

The simple answer is we dont know from your information, and can not. Leave it to your electrician.

NT

[bigcat]

There are some problems with doing this.

1. Earth leakage will occur, and in some TT systems with only the RCD as protection, this is somewhat dangerous. Not only may it make the shower water live, but may make everything earthed in the house live.
2. Heating the wet insulation will boil the water off, but as it cools it will simply suck water in again. Realise that the seal is gone, this is why its wet.

You can run immersion heaters quite happily on leaking elements, but not showers. If you have a leaking element on a shower, replace it.

NT

[Christian McArdle]

But it isn't a "seal" as such. The actual insulation itself is very slightly porous and can become saturated with water when not used for an extended period. Simply running the element "cures" it for 6 months or so.

I'm not saying that this is the problem in the OP's case, particularly as a replacement unit has been tried.

Christian.

[davidward30]

This neon wasn't dim and flickery before we had the work done though. But yes - I'll certainly leave this to an electrician.

thanks.