Cable size less than protective device rating

[Pat H]

Noted today an interesting comment on page 43 of christopher kitchers Practical guide to inspection, testing and certification of Electrical Installations ( a great book )

Under Shower curcuits he states:
For this type of circuit it is possible to have a cable which has a current rating of less than the rating of the protective device. This is because we do not have to provide overload protection to circuits which cannot overload.

Is this accuate and safe? Isn't it possible for a shower heater element fail in a way that draws extra current?

 

[Lucien Nunes]

It's almost impossible for a sheathed heating element to short in such a way that the resistance between L & N is reduced. Element failures usually involve a short to the metallic sheath which will trip an RCD and/or blow itself to pieces due to the heat / arcing concentrated at the fault. Other faults would be hard shorts in the internal wiring or controls, which require only short circuit protection, not overload. If, somehow, a fault occurred to earth near the neutral end of the element on a non-RCD-protected circuit, the load could theoretically increase, but if it were sufficient increase to risk cable damage the thermal fuse or limit stat would probably open anyway.

 

[Pat H]

Good points. Yes most shower or immersion faults I've fixed have been shorts to the metal sheath. Normally due to limescale build up causing local overheating.
As every water heating device over 15ltr capacity needs to be on its own circuit I can't see why it would have a protective device over the cable rating?
When could that occur?

 

[Wilko]

Hi Pat, yes @darkwood has covered this nicely in post #1 of
Omission of overload protection for fixed loads. - http://www.electriciansforums.co.uk/threads/omission-of-overload-protection-for-fixed-loads.76708/
Cheers.

Edit - sorry took me a while to find and may have been answered already

 

[Pat H]

Thanks Wilco. Interesting thread and 20mins of my life used up