Electric Shower blowing fuse

[Taylortwocities]

Thats not the OPs fuseboard, thats a picture linked from wikipedia....

I can't help him if he cannot help us....

We need a picture of the ACTUAL things involved..

 

[EFLImpudence]

The re-wire-able fuse today is not permitted when the installation is in the charge of an ordinary person, it has to be a cartridge fuse or MCB because it is so hard to be sure the wire used is the correct size, but cartridge fuses are expensive.

Is that true?

An electrician will have a clamp on meter and can easy test what power the shower is using, so he will know if fault is shower selection or faulty fuse wire, but then the next problem is although there is nothing to say you can't continue to use the existing consumer unit, nothing new can now be added and comply with regulations.

Does that mean it cannot be done?

I have not seen a shower in years which does not say must be RCD protected, so if wrong shower then unlikely a new one can be officially fitted powered from that fuse box,

It should say "the circuit must be RCD protected", therefore a new shower may be fitted if the circuit is not being altered.

so likely your looking at changing the fuse box for a type tested distribution unit called a consumer unit,

Is that true?

 

[winston1]

True, but if one can find the relevant small print, one discovers that the power consumption of these things are nearly always quoted at 240V, in which case the current drawn by a "8.5 kW" shower at 230V would be roughly ..

8500 / 240 x (230 / 240) = 33.94 A

... but still over 30A. However, although slightly over 30A, that current should not blow a 30A fuse (if that is what we are talking about) - although, as you suggest, it could reduce the lifespan of the fuse.

Kind Regards, John

And a lot over that if his supply is around 245v as most are.

 

[JohnW2]

Is that true? [the first one!]

I wondered that, and suspect that it probably isn't (true). I also wondered whether it is really true that it is appreciably harder "... to be sure the [fuse] wire used is the correct size" that it is "... to be sure that a cartridge fuse of the correct rating has been used".

Kind Regards, John

 

[JohnW2]

And a lot over that if his supply is around 245v as most are.

As I (and others) keep telling you, for better or for worse, and silly though it may be, the regs require one to calculate what the current would be at 230V, not what the current actually will be with the supply voltage the installation has.

As I also recently wrote, one would expect that sensible safety-related regs would require the calculation be be undertaken at 253V - not 230V, 240V, 245V or any other figure ... but they don't.

Kind Regards, John

 

[winston1]

As I (and others) keep telling you, for better or for worse, and silly though it may be, the regs require one to calculate what the current would be at 230V, not what the current actually will be with the supply voltage the installation has.

As I also recently wrote, one would expect that sensible safety-related regs would require the calculation be be undertaken at 253V - not 230V, 240V, 245V or any other figure ... but they don't.

Kind Regards, John

That may be so but the fuse does not know that. If you overload the fuse by haveing a higher voltage than designed for it is more likely to blow. The power (and thus heating effect) is a square law of voltage.

 

[JohnD]

but the regulations require cables, for example, to be oversized, and fuses to have which would accomodate the extra current due to voltages at the top of the range.

In this case, the OP might have got cables and a fuse that would be undersized at nominal voltage and current, so don't have the leeway for a bit of excess.

 

[EFLImpudence]

It still won't blow the 30A fuse wire, though.


If installed correctly in the first place, it will have 6mm² for the 30A BS3036 so the cable will be safe.

The shower will obviously draw the current that it draws regardless.

 

[EFLImpudence]

Surely the problem must be a short-circuit; the shower can't draw the 55+A required to blow the fuse - eventually.

 

[JohnW2]

That may be so but the fuse does not know that.

You know that, and I know that, and I'm pretty sure that most/all of the authors of BS7671 will also know it.

However, if you have a problem with their not writing the regs to take into account those things "which they know", then you should complain to them, not repeatedly moan about it here.

Kind Regards, John

 

[JohnW2]

Surely the problem must be a short-circuit; the shower can't draw the 55+A required to blow the fuse - eventually.

That would seem to be a logical conclusion - unless I've missed it, I don't think that the OP has yet told us whether the fuse blows immediately on switch-on of the shower (which it obviously would with a ';short circuit'), or what.

Kind Regards, John

 

[JohnD]

...How soon does the fuse blow after you turn on the shower?

Do any other sockets or appliances stop working when it blows...?

 

[ericmark]

Sorry may have mislead, what I am saying you can do what ever you like, even if it does break the rules, your unlikely to get caught, however employ an electrician and he has to follow the rule book, if he is a scheme member and he does not follow the rules he can get kicked out of the scheme so he can't afford to take a chance.

So although you can likely fit a new shower with lower current rating, or a larger fuse or swap to MCB, and some way get around the problem without too much expense, a scheme member electrician will need to dot all i's and cross all t's so will likely mean new consumer unit.

 

[EFLImpudence]

No it won't.

A replacement shower does not mean you have to alter anything else.

RCDs are not to protect the shower - nor the circuit for that matter. They are for the protection of people.

 

[chivers67]

Some of the old Wylex fuse boxes can take a larger fuse with first fuse carrier only, I don't know what part numbers change there is between the two types, I can only tell once the fuse carrier base is removed, however it would also likely require a heavier cable and that would be a major problem.

You can fit both the old style BS3871 "Push Button" MCBs into those old Wylex boards and the small 60898 MCBs with a white back insert. Though a 40 Amp with an Orange base insert actually fits I don't think any of the old boards are designed for anything over 30/32 Amp