Shower MCB

hansPulesh · 16 Sep 2014

Good morning all,

Over the past week, the MCB for my electric shower has tripped once. On switching it on again it stays on and the shower is usable again.

It happened again this morning and now on trying to switch the MCB on again there is no switch like feeling coming from it and it won't stay on anymore (The MCB that is)?

Could this be a problem with the MCB or the shpwer itself? Is it worth having the MCB changed to begin with as it'd be a lot cheaper than a new shower unit?

Just to add, on swithing off the main switch in the consumer unit, I still cannot switch the shower MCB on, it doesn't seem to stick in the on position and just goes back into the off position.

What could have caused this?

Many thanks.

ericmark · 16 Sep 2014

When a MCB trips it has heated up a component inside the unit and will not reset until cooled down.

Many showers are supplied from a MCB which is in theroy too small but since the shower is only used for a short time people get away with it. However over time the MCB degrades due to overload and starts to trip.

Often the MCB can't be upgraded because the cable is too small. Years ago 7.5 kW was used for showers now many are 10.5 kW and in the main the problem arises where the shower has been swapped for a larger one.

So step one is look at the MCB and see what size it is likely something like B32 written on it. Next look at shower and see what size that is.

7.5 kW = 32.6 amp
8 kW = 34.8 amp
8.5 kW = 37 amp
9 kW = 39 amp
9.5 kW = 41 amp
10 kW = 43 amp
10.5 kW = 46 amp

The shower can be slightly bigger than MCB but not too much as the MCB will be damaged so 8 kW on 32 amp MCB would likely be OK but not a 8.5 kW.

JohnW2 · 16 Sep 2014

hansPulesh said:
Over the past week, the MCB for my electric shower has tripped once. On switching it on again it stays on and the shower is usable again. It happened again this morning and now on trying to switch the MCB on again there is no switch like feeling coming from it and it won't stay on anymore (The MCB that is)?

For how long has it been working satisfactorily prior to this last week (days, months, years or what?).

Are we definitely talking about an MCB (rather than RCD, with a test button?) - if so, as eric has asked, what is it's rating (B32, B40 or whatever), and what power/rating is the shower?

hansPulesh said:
Could this be a problem with the MCB or the shpwer itself?

It could be either (or even something to do with the wiring/ isolators/ whatever). From what you describe, it is probably more likely that it would be the MCB (particularly since shower worked OK for a while after the trip last week), but there is obviously no certainty about that. As eric has implied, if the MCB (and possibly also cable) is undersized for the power of the shower, it's possible that it has been getting hot, eventually resulting in its failure.

hansPulesh said:
Just to add, on swithing off the main switch in the consumer unit, I still cannot switch the shower MCB on, it doesn't seem to stick in the on position and just goes back into the off position.

With some MCBs (and RCDs), once they have tripped, one has to push the lever right down ('off' direction) before they will reset (and feel as if they are resetting) when you push the lever back up.

Kind Regards, John

hansPulesh · 16 Sep 2014

Many thanks for the replies, it is deffo the MCB and not the RCD.

It is a 40W MCB, the cable that it services is a 10mm twin and earth which is a max of 12m long. The shower is 10.5kW's. It has been working fine with no trips or problems since it was fitted (3 years).

Many thanks

JohnW2 · 16 Sep 2014

hansPulesh said:
It is a 40W MCB, the cable that it services is a 10mm twin and earth which is a max of 12m long. The shower is 10.5kW's. It has been working fine with no trips or problems since it was fitted (3 years).

In that case, I think my money would probably be on the MCB. As eric has intimated, a 40A MCB is strictly too small for a 10.5 kW shower (which would draw about 42A at 230V, even if the 10.5 kW were quoted at 240V), so it may well have been getting quite hot during those 3 years.

Kind Regards, John

ericmark · 16 Sep 2014

I would agree over time the 6A overload had caused the heater in the MCB to degrade to a point where it now trips on a regular basis.

JohnW2 · 16 Sep 2014

ericmark said:
I would agree over time the 6A overload had caused the heater in the MCB to degrade to a point where it now trips on a regular basis.

As I said, it might well not be a 6A overload - if (as is usually the case) the 10.5 kW relates to 240V, then the current would be under 44A if the OP's supply was 240V, and just under 42A at the 'nominal' 230V. However, even a slight degree of overload (or, I imagine, even currents close to, but not over, the rating of the MCB) is presumably likely to reduce life expectancy of the MCB to at least some extent.

Assuming that the OP's electrician can confirm that the 10mm² cabled is installed in a manner that can support it, it would presumably make sense to replace the MCB with a 45A or 50A one.

Kind Regards, John

hansPulesh · 21 Sep 2014

Thanks for the help all, is a 40amp MCB dangewrous in this situation? or just not the right one to use?

I think that the 40amp MCB was used as the cut off switch for the shower is rated at 40amps. Are larger cut out switches than 40amps now available?

EFLImpudence · 21 Sep 2014

hansPulesh said:
Thanks for the help all, is a 40amp MCB dangerous in this situation? or just not the right one to use?

Not dangerous as such.
The shower is 10500W @ 240V = 43.8A
and 9660W @ 230V = 42A.
So, as has been said, could have gradually caused the MCB to deteriorate.

I think that the 40amp MCB was used as the cut off switch for the shower is rated at 40amps.

It would make no difference in that respect.

Are larger cut out switches than 40amps now available?

There are 50A switches.

JohnW2 · 21 Sep 2014

hansPulesh said:
Thanks for the help all, is a 40amp MCB dangewrous in this situation? or just not the right one to use?

As EFLI has said, not really dangerous, but possibly the cause of gradual deterioration over time. Certainly not 'the right one' (or ideal one) to use. As I said, assuming the cable is up to it (which is almost certainly the case), it would make sense to have the 40A MCB replaced with a 45A (or possibly 50A) one.

hansPulesh said:
I think that the 40amp MCB was used as the cut off switch for the shower is rated at 40amps. Are larger cut out switches than 40amps now available?

As you've been told, there are 50A switches, and probably 45A ones. Whatever, IMO it's far more important to have an MCB big enough for the shower than to worry about a slight exceeding of the rating of the switch.

Kind Regards, John

hansPulesh · 21 Sep 2014

Many thanks again - could you post any links for trusted sites which sell 45 or 50 amp triple pole isolator switches?

EFLImpudence · 21 Sep 2014

They're just double pole. That's all you need.

http://www.screwfix.com/p/crabtree-50a-pull-cord-switch-neon/90597

hansPulesh · 21 Sep 2014

EFLImpudence said:
They're just double pole. That's all you need.

http://www.screwfix.com/p/crabtree-50a-pull-cord-switch-neon/90597[/QUOTE]

I think that this may be part of the problem - the isolator switch that I have is not of the pull cord type. It's a wall mounted switch with a neon light in it. I think the max rating that was available when I had the house re-wired was 40Amps and that's why the MCB was rated at a lower 40amps as well.

Are larger wall mounted isolators now available? Or will it need to be kept as it is?

EFLImpudence · 22 Sep 2014

There are 50A wall switches but some/most of them are the size of a double socket but vertical.

There are plenty of 45A single socket size.
A 45A cooker switch will be the same.
http://www.toolstation.com/shop/Cooker+Switch+DP+45A/p12273

EFLImpudence · 22 Sep 2014

This one is better quality.

http://www.screwfix.com/p/crabtree-cooker-switch-1-gang-45a/42524
http://www.toolstation.com/shop/p82363

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Shower MCB

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