MCB trips 3 mins AFTER switching off shower?

[ComplicatedJones]

I hope somebody can point me in the right direction please.
We have a Triton Opal II shower installed by proper electrician about three years ago. It's in a new extension, and I am confident that the job was done properly. There is a dedicated MCB on our CU and the whole power side of the CU is RCD protected. There is an isolator pull cord in the bathroom.
Now, the shower operates as normal but about 3-4 mins AFTER pushing the STOP button on the front, and without touching the isolator switch, the CB is tripped. Not the whole RCD protection, just that circuit.
Resetting the CB allows everything to operate again but the same happens next time, some time after switching off.
All has worked fine for three years, (except when the pcb failed in the shower about a year ago, but that was replaced by Triton Service engineer, under warranty).
Can anybody suggest what's happening please?

 

[martinxxxxxx]

It needs checking.

If you turn it on for only a very short duration (ie 10 seconds or less) does it still happen?

 

[ComplicatedJones]

Hi Martin
Thanks for your very quick reply.
The short answer to your question is that no, it does not happen if I only operate the shower for about 10 secs.
However, although I had searched DIYnot for this topic prior to posting and did not find anything similar, as soon as I had submitted then several suggestions were automatically made and one was an almost identical problem.
From the replies to that post it seems that the MCB which is rated at 40A should have been a 45A one instead because the shower is in the 9.6-10.5kW range. This is quite clear in the Triton manual. Although, as I said, it has worked OK for about three years.

It appears that my confidence in the electrician was misplaced, though it was he who insisted on upgrading the cable because of the higher rated shower that we had selected.
Thanks once again and I shall close this post now, if I can only work out how to do that.
Regards

 

[JohnD]

Might there be a water leak inside the casing? Run it on "cold" setting for ten mins and see if it still trips. If you stand next to the shower after finishing, can you hear any fizzing?

BTW aren't you using the isolating switch on the ceiling after use? Feel that too to see if it is hot.

 

[JohnW2]

There's something odd about this story. If it were an RCD which was tripping, one could posulate all sorts of possible explanations. However,to trip a 40A MCB as quickly as the OP indicates would probably take about 70A (haven't got my books close to hand!). If the MCB is tripping when the shower is off, that ~70A must be going somewhere, and I would have thought that would result in some fairly dramatic indication of where the problem was!

That being the case, I cannot help but wonder whether it is a malfunctioning MCB?

Kind Regards, John.

 

[xr4x4]

i guess the only way to find out it to get a clamp meter in the CU after the shower is turned off (as well as on i guess) ....and see what happens...!

 

[JohnW2]

i guess the only way to find out it to get a clamp meter in the CU after the shower is turned off (as well as on i guess) ....and see what happens...!

I suppose so - but, as I said, if ~70A is flowing in the circuit when the shower is off, I'd be very surprised if that 70A (for a few minutes) did not 'show itself' is some fashion.

As I've said before, I suspect that the common belief that RCDs are less reliable than MCBs may well result from the fact that there is no practical way to test the latter, so we haven't really got a clue as to how many faulty MCBs are actually in service.

It's a bit ironic that (with TN systems) we take very seriously the matter of ensuring that Zs figures are adequately low to achieve the required 'disconnection times' for L-E faults without actually being able to check whether the OPD in question is capable of disconnecting at all! There's something to be said for fuses, since the laws of physics (temperature rise and melting points) can't really be affected by faults!

Kind Regards, John.

 

[EFLImpudence]

I suspect the mcb is damaged because of three years of over-heating and now, for whatever reason, trips on cooling down.

Plausible ?

What about leaving the shower on and seeing if it trips after a bit longer than a normal shower?

I would replace it anyway.

 

[HERTS P&D]

Advice:
Don't use it until it is fixed. Water and electric don't go well together.

Andy

 

[xr4x4]

I would replace it anyway.

good advice.

 

[JohnW2]

I suspect the mcb is damaged because of three years of over-heating and now, for whatever reason, trips on cooling down. Plausible ?

Very credible - or maybe MCB malfunction for some other reason. As I said, I find it very hard to believe that the OPs problem can be due to anything other than a malfunctioning MCB, whatever the cause of that malfunction.

Kind Regards, John.

 

[securespark]

Which board have you got?

What size cable is it?

What is the exact rating of the shower?

 

[bernardgreen]

It is possible that the MCB trips on thermal over load some time after the over load has been removed where the heating "element" and the bi-metal strip are not the same thing.

It might be that while the heating "element" has become hot during the overload the separate bi-metal strip is slow to heat to the trip point. This indirect heating of the bi-metal would be needed to create the acceptable time delay before tripping on transient overloads.

After the over load current has been removed the element remains hot enough to continue heating the bi-metal strip for several minutes and long enough for the trip to operate.

 

[bernardgreen]

deleted copy

 

[JohnW2]

It is possible that the MCB trips on thermal over load some time after the over load has been removed where the heating "element" and the bi-metal strip are not the same thing.

Yes, that's a possibility. As EFLI has suggested, the test would be to see if it operates eventually if the shower is left switched on for longer than usual.

Having said that, a 40A MCB should not really operate as a result of the operational current of the shower in question, even though it is under-rated. A 40A Type B MCB should carry 45.2A 940*1.13) indefinitely without operation. I suspect that a “9.6-10.5kW shower” means 9.6kw at 230V and 10.5 kW at 240V, which would equate to less than 45.2 A for any voltage up to about 248V.

Kind Regards, John.