Help replacing electric shower please

[jrterror]

Hi. I need to replace our electric shower. Our old shower is a Gainsborough 850E rated at 9.5 kw. Navigate to this page for some details: http://www.comparestoreprices.co.uk/showers/gainsborough-850e.
It may be helpful to add that this shower has two power settings. If we use the shower on the highest setting it trips the mcb in the consumer unit. If used while set at its lowest setting it’s fine and does not trip. However just recently I noticed a brown mark down the side of the shower unit casing. At closer inspection it turned out that the plastic terminal block that joins the twin and earth from the consumer box to the wires inside the shower unit was overheating and burning through the outer casing.

The mcb that is currently feeding the shower is a Proteus 632/2 32A 240/415v
TYPE 2 M6.

Navigate to this page for some details:
http://www.proteusswitchgear.co.uk/product_details.asp?product=1324

It is my understanding that this 32A mcb is inadequate and that really it ought to be a 40A mcb. The reason being that a 9.5kw shower needs a 40A mcb.
Is this correct?
Was this the reason that the Gainsborough shower, while on its highest setting has been tripping and now, even while on its lowest setting, has burnt a hole through the side of the unit?

We have bought a Triton Amber 111 to replace the old one. This is rated at 8.5kw. So, s said before, I believe I need to replace this 32A mcb with a 40A. Is this correct?
I have managed to locate the part I need, I think this is it: http://www.proteusswitchgear.co.uk/product_details.asp?product=1330
My only concern is that it is labelled as a M4 not a M6 like the lower rated breakers.
Why is this and is it ok for my needs?
The twin and earth connecting everything is 10mm, so that’s fine.

Also while I’m here. Is there any reason why I can not have the shower unit standing 5-10mm from the tiles, obviously held off by spacers. I wish to do this cause otherwise I would need to get involved cutting the water pipe back and stuff…just so that I can align the showers inlet with the 15mm water feed.

Thanks D
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[GaryMo]

Another shower thread

Yes, a 9.5kW shower shouldn't have a protective device lower than 40A, also the cable supplying the shower should be capable of carrying this current.

Your new 8.5kW shower will also require a 40A MCB.

What route does the 10mm cable take to the shower and does it pass through any thermal insulation? Length?

The M6 and M4 relate to the breaking capacity of the circuit breaker - the amount of current it can safely break under fault conditions without exploding, M6 - 6000 Amps, M4 - 4000 Amps.
You would need to measure PFC at the consumer unit terminals to determine the maximum current that will flow under fault conditions and make sure that the breaking capacity of the protective devices exceeds this figure. It probably will though as it's very rare to measure PFC at more than 4kA.
There's also a safety net if the main suppliers fuse is a BS1361 type 2 and the consumer unit complies with BS 5486-13 or BS EN 60439-4. In this case the short circuit capacity may be taken to be 16kA.

 

[Spark123]

If only life was as easy as that!
Your 32A MCB at the moment is too small for a 9.5kw shower, this is why it would have been tripping. The scorching is more likely caused by a loose connection.
The bad news is that unless you can confirm that the existing cable is suitable then you cannot just replace an MCB with one of a larger size as it may cause the cable to overheat and ultimately catch fire.
M6 and M4 are both relating to the breaking capacity of the MCB, M6 = 6000A and M4 = 4000A. Unless you know the Prospective fault current at the CU you should not replace it with one of a smaller rating.
The only other option is to look at using a 7.5kw shower.
See the manufacturers instructions re mounting, I'd imagine that it will need to be mounted on a flat surface so standing it out on pillars is a bit of a no go.

 

[jrterror]

So, I can confirm that the cable is big enough. its at least 10mm
But why is it that the proteus website does not have a 40A m6 mcb. Are these not made?
And for what reason would I not be able to stand the unit off from the wall?
Thanks for the replies folks. Keep them coming.

 

[Spark123]

It depends on the length of the cable too and how it is installed as to it's current carrying capacity.
Don't know why they don't make a 40A MCB in M6 flavour - the M4 will probably be OK but without testing the EFLI and the PFC I wouldn't like to say for definate. Without a loop and PFC tester it is a job for a spark I'm afraid.
You'll have to ask the manufacturer if you can stand it off the wall - there may be a chance of water splashing in via any holes on the back or people getting their fingers into the workings etc.

 

[ban-all-sheds]

So, I can confirm that the cable is big enough. its at least 10mm

"At least" 10mm²?

Where are you measuring the cable size, and how? "10mm" refers to the cross-sectional area of the actual copper conductors (phase & neutral), not any other dimension.

But why is it that the proteus website does not have a 40A m6 mcb. Are these not made?

Not by Proteus, obviously.

Different makers pick different values to make - Proteus have not picked 40A...

And for what reason would I not be able to stand the unit off from the wall?

See the manufacturers instructions re mounting, I'd imagine that it will need to be mounted on a flat surface so standing it out on pillars is a bit of a no go.

What do the instructions say?