Electric shower tripping rcd/mcb

[riveralt]

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)

No your not - yes I do - sorry
Mixing my bits up by focusing on the max load for a shower for that capacity 46.95A with a MI requirement of 45A MCB.
BTW - reading my response again - I don't think you're negative (devils advocate on occasions yes but) - I was laughing at the thought that someone would think so.
Too busy watching my team playing Sunderland!!!

 

[JohnW2]

46.95 is 10800/230 which is not what the manufacturer would have quoted. 10800/240 = exactly 45A - coincidence?

Quite - and. assuming that (as I had assumed) the 10.8kW is quoted at 240V (which is nearly always the case) and that resistance remains unchanged at the lower voltage, then the current at the usual design voltage of 230V would be (10,800/240) x (230/240), which I make 43.125A - definitely under a B40's I1 of 45.2A.

Of course, if the OP's actual supply voltage is appreciably above 'nominal', that could perhaps explain what he's experiencing - and that would illustrate the folly of undertaking design on the basis of nominal (rather than 'worst case) supply voltages!

Kind Regards, John

 

[Brookhouse]

Hi gents. Appreciate your feed back.
The new consumer unit is RCD protected 63 amp and the shower is connected to a 40amp MCB.
Wylex don't seem to do a 45A MCB.
Have ran the shower just now on the Eco setting so low power and it didn't trip even after 10min.
I checked the terminal connections in the shower and the pull switch all tight.
Will power down the main and check the MCB.

Do MCB,s often fail. I can get a new one for £5 so not a big out lay to check it.

 

[JohnW2]

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)

No your not - yes I do - sorry Mixing my bits up by focusing on the max load for a shower for that capacity 46.95A with a MI requirement of 45A MCB.

Ah, I see - no problem. I'm at least as guilty of 'mix-ups' as anyone else!

[BTW - reading my response again - I don't think you're negative (devils advocate on occasions yes but) - I was laughing at the thought that someone would think so.

Oh, I see - yes, as you realise, I misunderstood!

As I've just written, if the OP's normal supply voltage is appreciably above nominal, that could possibly explain what he's experiencing, given the very iffy (in fact, no-compliant, even at nominal voltage) combination of MCB and shower he has - asuming the cable was adequate to take it, a 45/50A MCB would be 'correct', and very probably would alleviate the tripping issue.

Hope your team wins!

Kind Regards, John

 

[Brookhouse]

Gents if I put a 50A MCB in place of the present 40A is there any risk?

 

[riveralt]

You and I don't have the equipment to test whether an MCB is broken or 'breaking.' So replacement would be the only case.
The question is can you totally isolate the new CU from the main supply - and do you have a method of identifying that the circuit(s) are dead.

If there isn't a means of totally isolating the supply then I would leave the replacement to an electrician.

If you can isolate the supply then the question of your confidence and competence to work with this type of equipment has to be assessed - and from a distance only you can do that.

One other, rather annoying thing - is that some of the MCB manufacturers and I think Wylex is one are asking those fitting their MCB's to their make of CU's is to use a torque screwdriver for both the connection to the buzz bar and the cable.

 

[JohnW2]

Wylex don't seem to do a 45A MCB.

True, but they do a 50A one. IF it really is 10mm² cable and IF the method of installation of that cable is suitable, then it might be OK with a 50A MCB. You'd need to be sure about the cable size and installation method before deciding whether that was possible. As we've been discussing, no matter how one looks at the figures, a 40A MCB for a 10.8 kW shower is not actually compliant with the regs

Kind Regards, John

 

[JohnW2]

One other, rather annoying thing - is that some of the MCB manufacturers and I think Wylex is one are asking those fitting their MCB's to their make of CU's is to use a torque screwdriver for both the connection to the buzz bar and the cable.

As you say, I think that Wylex are one of those manufacturers - but, frankly, it worries me (a long time ago, I think I started a thread about this). If I tighten an MCB to the manufacturer-specified torque (or even main swtrch/RCD, which have higher specified torques), then I can often get at least quarter of a turn more by hand without much effort. OK, I might be 'overtightening', but I'm used to regarding an MCB screw which I can easily 'tighten further' by hand as being too loose!

Kind Regards, John

 

[EFLImpudence]

Is the required torque setting to ensure we don't leave it too loose or to ensure we don't do it too tight?

 

[Brookhouse]

Would yo say that for a 10.8kw shower using 10mm cable with approximate 18/20 meter run that a 40A MCB would be the normal choice.
Or would you go for a 45/50A MCB

 

[riveralt]

Would yo say that for a 10.8kw shower using 10mm cable with approximate 18/20 meter run that a 40A MCB would be the normal choice.
Or would you go for a 45/50A MCB

I'm sure the manufacturers instructions will say 45A on a 10mm ² T&E cable.
What is the make of your shower - all Mira 10.8w are 45A for example.

 

[Brookhouse]

It's a Mira Vie 10.8kw.

 

[EFLImpudence]

No. It is the wrong choice.

However, it may have been fitted to cover up another failing which, while not dangerous, has resulted in the present inconvenient situation.

Of course, it may just be a mistake.

 

[Brookhouse]

Can I fit another manufacturers MCB in the Wylex CB or should I just go for a Wylex 50A

 

[EFLImpudence]

IF it really is 10mm² cable and IF the method of installation of that cable is suitable, then it might be OK with a 50A MCB. You'd need to be sure about the cable size and installation method before deciding whether that was possible.