Trident Rapide 2 electirc shower - fuse overheating and blowing

[David P]

Hi all,

The bathroom has a Trident rapide 2 overbath electric shower unit installed and normally works fine. This morning the fuse in the consumer unit blew (30a cartridge fuse) and when I went to remove it it was hot enough to leave a burn blister on my finger.

Any thoughts, please, as to what may have caused this and what I could look at to try and identify the problem.

The consumer unit itself was replaced maybe 4 years ago and the electric shower is run from a direct supply back to a discrete fuse on the consumer unit so nothing else runs from it.

TIA

David

[fred]

In article , David P writes

It appears to be an 8.5kW shower making a load current 35 odd amps so the fuse is overloaded and can be expected to overheat and blow eventually.

If the installer can't be trusted to select the right fuse I'd worry that the circuit cable may be incorrectly rated too so I wouldn't rush out and buy a correctly rated replacement fuse without checking the cable too.

If it is wired in twin and earth cable, likely to be adequate 10mm2 cable measures 17x11mm overall and less likely to be adequate 6mm2 is approx 13.5x7mm overall.

Dims courtesy of TLC:

Next step cable size calculation: the CU was really changed only 4 years ago I am v surprised that cartridge fuses were used and not MCBs.

[Brian Gaff]

The only other question really is how long has it been in? I seem to recall that these things eventually succumb to limescale making them overheat and short out. Brian

[Bill Wright]

That means it blew due to overload over a long period, not a sudden dead short.

Bill

[meow2222]

A 35A load on for shower length times only will never blow a 30A wire fuse. They don't have anywhere near that level of discrimination.

However high current fuses in some CUs tend to make the contacts hot, which both lowers the fuse's trip current some and weakens the contact springiness, causing greater heating and worsening things. It sounds like that's happened here.

NT

[David P]

Ta all.

Fuse is of correct size (according to Trident), wiring dims are also correct. The problem is/was that the contacts in the cartridge holder had become looser/were never tight enough - pretty much what NT says above. New cartridge holder now wired in so hopefully it should be OK there is now a definite 'resistance' when the holder is closed.

David

[fred]

In article , David P writes

Glad you're sorted and thanks for feeding back, should have realised that was a likely failure mode but am surprised they say 30A fuse as it is quoted 8k5W online.

[David P]

Yes, they can be but ours is the 7kw one which needs the 30A fuse not one of the higher rated ones needing the higher fuse rating. It was a few pounds cheaper at the time we were buying and is plenty good enough for us.

David

[ARW]

But that will be 8.5kW at 240V.

And as John Rumm knows. The laws of physics and fuse discrimination go out of the window when I enter a house:-)

And fuses do seem to just age.