Safety Implications With Shower MCB

[gman76]

Can anyone tell me what the exact safety implications are, with having a 6mm cable protected by a 50a MCB?

I realise the cable will be overloaded before the MCB trips @ a constant overcurrent, but due to a dead short fault, the MCB would trip immediately.

Exactly what kind of fault would draw enough current to overload the cable, but not fire the trip?

Should I be worried about it?

 

[securespark]

There is a very basic rule in the regs.

The rating of the protective device must not be less than the design current of the circuit and that rating must not exceed the capacity of the cable as installed.

 

[gman76]

Thanks for your reply secure, I know it's not right, but what is the risk in reality?

 

[Dartlec]

It won't melt this week, or next week, but the problem will be over months or years as it warms up, especially if it is running in insulation anywhere. Not only is there the risk of the cable insulation melting over time and exposing a live conductor to touch (A fault to earth through a child might well trip the mcb, but not before its too late), but it could also damage surrounding material.

I've seen holes melted right through fuseboxes from badly installed shower circuits, and they aren't pretty....

The main issue is not a fault current, which would likely trip the MCB, but an overload current of larger than the cable can handle, but lower than the tripping point of the MCB. Whether you have an overload situation will (as you might expect) depend on what the load is - if you only have a 6kW load on it, you won't overload it. That doesn't stop the design being bad practise though.

Having said that, 6mm according to my OSG will take 51A should the cable be installed in a perforated cable tray or free air, though I suspect your decor is not quite that basic?

With insulation etc it drops to 32A, which is enough to run a 7kW shower/cooker/other device without overloading the cable, though there may be other factors to consider of course....

The risks may not be immediate, but they are real and certainly greater than the price of a new mcb.

 

[gman76]

Thanks for the comprehensive reply Dartlec, I understand that the easiest fix would be to replace the MCB - trouble is, the fuse box is so ancient, I don't think a 40 will be available (I'll try to post a piccie soon). I'm not 100% how the cable is run, however, it does have a 40a RCD inline. (Obviously, this offers no overcurrent protection).

The shower is 8.5kW.

 

[holmslaw]

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[sm1thson]

^read it again SS is right in what he said.

 

[holmslaw]

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[nozspark]

What OCD are you going to use if your design current is 25A then???

Design current must be less than the OCD rating which must be less than the cable rating (correction factors apply) - exactly what SS said

 

[holmslaw]

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[Spark123]

You'd normally pick the next size up.

One problem with what the OP has it the issue may lay dormant until the day someone fits a 10.5Kw shower.

 

[holmslaw]

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[RF Lighting]

Which is what securespark said.

You really are stupid arn't you.

 

[holmslaw]

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[gman76]

View media item 2186
Haven't uploaded any pics yet - hope it works.

This is the consumer unit, it's a very old Crabtree, apologies for the poor camera phone image.

Can anyone advise if there was ever a 40a MCB made for these, if so, any of you sparks got one for sale?[/img]