Electric shower tripping rcd/mcb

[Brookhouse]

Hi I have a 10.8 shower which was fitted approx 12 months ago, new cable 10mm I would say, fitted to new consumer box and connected to a wylex nsb40 unit ( not sure if this is an RCD or an MCB.
Shower runs for approx 4/5 mins then trips?
I assumed the shower was faulty so bought a new one but on replacing it I got the same issue
Any ideas?
Sorry forgot to say that for the last 6 months its worked fine.

 

[EFLImpudence]

The nsb40 is a Miniature Circuit Breaker which trips on overcurrent or short circuit.

As it takes a little time it would appear to be overcurrent causing it.

40A is a little low for a 45A shower but it shouldn't trip.
It could be just faulty.

 

[JohnW2]

40A is a little low for a 45A shower but it shouldn't trip. It could be just faulty.

Agreed, it definitely should not trip at 45A. Since that is fractionally below its I1 (45.2A), it ought to be able to carry 45A 'indefinitely' without tripping. As you say, it could well be a 'slightly faulty' (not quite in spec) MCB.

Kind Regards, John

 

[riveralt]

Hi I have a 10.8 shower which was fitted approx 12 months ago, new cable 10mm I would say, fitted to new consumer box and connected to a wylex nsb40 unit ( not sure if this is an RCD or an MCB.
Shower runs for approx 4/5 mins then trips?
I assumed the shower was faulty so bought a new one but on replacing it I got the same issue
Any ideas?
Sorry forgot to say that for the last 6 months its worked fine.

So a new shower and consumer unit was fitted 12 months ago and no RCD was included in the circuit - not by a registered electrician I hope?

And you bought a new shower 6 months ago to replace it and it was working fine but not now?

Why didn't you get a replacement for the shower that wasn't working from the electrician who fitted it or the manufacturers who supplied it. It is well within the 24 months guarantee that most showers have.
Also if there is a problem you should have called the electrician back - all those who are registered with a competent persons scheme will have an insurance backed guarantee.

Something doesn't smell right in the house of Brook?
I take it you have checked all the potential loose connection points in particularly the pull cord or wall isolation switch?

 

[JohnW2]

So a new shower and consumer unit was fitted 12 months ago and no RCD was included in the circuit - not by a registered electrician I hope?

Your guess may be right but, AFAICS, no-one has told us that the circuit, with its B40, is not RCD protected.

Kind Regards, John

 

[JohnW2]

I take it you have checked all the potential loose connection points in particularly the pull cord or wall isolation switch?

By what mechanism could a loose connection cause an MCB to operate after a few minutes of loading?

Kind Regards, John

 

[ban-all-sheds]

Overheating

 

[JohnW2]

Overheating

Why would overheating at a loose connection in a switch lead to operation of a physically remote MCB?

If anything, a poor connection in the circuit would reduce the current through the MCB, wouldn't it?

Kind Regards, John

 

[DetlefSchmitz]

Now if Riveralt had said

'I take it you have checked all the potential loose connection points in particular the mcb?'

then it would have been a likely candidate.

I still think we know too little about the cable size and the protection device(s) to be able to offer wider advice. We know what 'think it's 10mm' usually means.

 

[JohnW2]

Now if Riveralt had said ... 'I take it you have checked all the potential loose connection points in particular the mcb?' ... then it would have been a likely candidate.

Indeed it would - but, as you know, that is not what he wrote!

I still think we know too little about the cable size and the protection device(s) to be able to offer wider advice. We know what 'think it's 10mm' usually means.

Well, we know there is a B40 MCB (which is tripping), and I think the cable size is largely a red herring. It's most unlikley that it is larger than 10mm², and, if it's smaller, if anything that would slightly reduce the current through the MCB. I frankly doubt that the amount of local heating due to using only 6mm² (or even 4mm²) cable would appreciably influence the functioning of the MCB.

As is always the case, we can but guess from our remote positions. From what we know, either the MCB itself or the connections to it would seem (to me) to be the most likley culprits - but that is simply an 'intelligent guess'.

Kind Regards, John

 

[DetlefSchmitz]

In some ways, John, that was a pretty negative approach to my post.


Let me try again.

1) To the OP: Try tightening any connections to the MCB. The problem may be overheating caused by a loose connection. (This has to involve safely disconnecting power first - only do if you know what this means - see the wiki for safe isolation methods)

2) To the OP: Was the installation fitted by a qualified electrician and did (s)he give you a certificate for it. It may be that you need further advice to make your installation safer.

 

[riveralt]

Agreed, it definitely should not trip at 45A. Since that is fractionally below its I1 (45.2A), it ought to be able to carry 45A 'indefinitely' without tripping. As you say, it could well be a 'slightly faulty' (not quite in spec) MCB.

It is a 40Amp mcb isn't it - not a 45A?
Shouldn't 45.2A actually be 46.95A?
JohnW2 - negative intelligent guesser

I wouldn't suggest an OP who doesn't know the difference between an RCD and MCB go anywhere near a CU nor MCB/RCD terminals with a screwdriver - why do you think it is 'intelligent'?

 

[JohnW2]

In some ways, John, that was a pretty negative approach to my post.

It may have seemed like that, but such was not really my intention, even though I probably could have said the same of your response to my prior post!

Let me try again. ... 1) To the OP: Try tightening any connections to the MCB. The problem may be overheating caused by a loose connection. (This has to involve safely disconnecting power first - only do if you know what this means - see the wiki for safe isolation methods)

There currently is no information on these matters in the wiki - just a 'placeholder' page I created to house the new (AFAIAA the first ever) page on such matters in the wiki. If I ever receive enough agreement from others based on the suggested drafts I've been producing, I will upload something to that page. In the meantime, the only advice on such matters in this forum is that in the 'ELECTRICS SAFETY' sticky.

Kind Regards, John

 

[JohnW2]

Agreed, it definitely should not trip at 45A. Since that is fractionally below its I1 (45.2A), it ought to be able to carry 45A 'indefinitely' without tripping. As you say, it could well be a 'slightly faulty' (not quite in spec) MCB.

It is a 40Amp mcb isn't it - not a 45A? Shouldn't 45.2A actually be 46.95A? JohnW2 - negative intelligent guesser

I'm lost. The I1 of a B40 MCB is 1.13*40 = 45.2A ... where does your 46.95A come from? (I'm assuming {yes, I know!!} that you understand what I1 is)

I wouldn't suggest an OP who doesn't know the difference between an RCD and MCB go anywhere near a CU nor MCB/RCD terminals with a screwdriver

I couldn't agree more.

- why do you think it is 'intelligent'

I don't, and I didn't say that. What I said was an 'intelligent guess' (agreeing with others who have posted) was that the likely culprit was either the MCB or its connections. It was DetlefShmitz, not me, who suggested that the OP should go inside the CU and check the MCB connections.

Kind Regards, John

 

[EFLImpudence]

46.95 is 10800/230 which is not what the manufacturer would have quoted.

10800/240 = exactly 45A - coincidence?