Electric Shower and burnt 30amp fuse

[N. Thornton]

that might be all it is - and it might not. Personally I'd want to know before reapplying power and getting in the shower :)

[Andy Wade]

The fuse rating's a bit irrelevant here, provided that it still provides short circuit fault and earth fault protection. It's the _load_ current in relation to the cable rating that matters. Increasing the fuse rating from 30 A to 40 A won't increase the current flowing through the cable and therefore won't increase the conductor temperature.

The higher rated fuse will result in a higher final conductor temperature after fault clearance, but such heating is essentially adiabatic and isn't significantly affected by thermal insulation round the cable.

So it's the earth fault loop impedance you need to check, not what's happening in the loft.

[Andy Wade]

Not in this case: the 0.725 cable derating factor for circuits with rewireable fuses only applies to circuits liable to be overloaded. A shower circuit isn't liable to overload because you have a fixed load resistance. Overload protection can be omitted [473-01-04(ii)] and the

433 group of regulations then doesn't apply. See also appendix 4, section 6.3.

I'd say it can certainly contribute though. 35 / 30 is 17% over the In of the fuse, leading to 36% excess power dissipation in the fuse assembly (more in fact because the resistance of the fuse wire will increase when it's hot). The temperature rise of the fuseholder parts could then be, say, 40% more than was intended - probably enough to contribute to the slow cooking.

I beg to disagree. As I said in reply to Grunff, increasing the fuse rating won't affect how hot the cable gets. Yes, the cable installation conditions should be checked as part of the process of putting this right - but in relation to the actual load current (the design current of the circuit Ib, modified by any grouping and/or ambient temperature correction factors that apply) not the fuse rating. The important thing is to check the earth fault loop impedance to ensure that, with the larger fuse, < 5 s disconnection time is still met, and for adiabatic compliance of the CPC.

I'd swap the MCB anyway, but again, in a circuit not liable to overload, the cable rating required can be determined by the load current, not the protective device rating. There may be no need to replace the 6mm^2, provided you're prepared to do some sums.

[Andy Wade]

Sorry but it isn't. A fault (whether phase-earth or phase-neutral) would blow the fuse quickly -- or if it didn't, due to gross miscalculation of the circuit design, it would set fire to the cable instead. Either way, the shower would be rendered inoperative.

OTOH the symptoms described here by the OP seem to me to be those of a 'slow cooking' of the fuse, probably over a lengthy period. This is entirely consistent with a diagnosis of some combination of an under-rated fuse and loose connection(s).

[Stefek Zaba]

Amen to that! I think we need to get some T-shirts printed with that on the front, and the full wording of 473-01-04(ii) which you mention, and ship them out to the "uprate everything" I-can't-believe-I'm-NICEIC-registered gold-braid brigade ;-)

Stefek

[Andy Wade]

:~)

[N. Thornton]

I can be quite clear that that is not correct. Fuses require large overloads for long times to blow. An L to E leak that took a significant time to blow might be present. Fitting a new fuse and getting under such a shower would likely kill.

They are, but that is not the only scenario with which the description is consistent. It would be different with a portable appliance, but we're talking an electric shower here. If it were mine I would insulation test it first.

Regards, NT

[Andy Wade]

You've moved your own goalposts. Now you're postulating a 'leak' that would overload the circuit enough to cook the fuseholder, but not blow the fuse quickly. Such a leak would be passing several amps and would dissipate lots of heat, thus drawing attention to itself - unless the leakage was purely reactive, but what could cause that? I suppose that what could happen is an earth fault part-way along the shower's heating element, turning the 8.5 kW shower into a 10 kW one - pretty unlikely though.

I would too, as a precaution, and also check the load current (easy if you've got a clamp-meter). But, IMHO, checking the tightness of connections, the earth fault loop impedance and the supplementary bonding are all more important if you are worried about being electrocuted under the shower.

[N. Thornton]

I think youll find such a fault is normal and common. Its how millions of immersion heaters fail, they split open but keep going for a while, with current flowing through the water, and thus total load higher than it should be. The supply wiring gets hot but keeps going. It can take a very long time for a wire fuse to blow - in some cases like a whole year.

Nothing is obviously wrong, it continues to function, and if earth impedance is low, as is usually the case, no-one is any the wiser.

If the install is modern with rcd and bonding, this will not be a real safety problem, but if it has neither of those, the OP might possibly be in for a very nasty surprise.

I agree, either approach would ensure safety.

Regards, NT