Sorry - I know nothing about electricity except it can be dangerous. My immersion heater cable ends in a plug which goes into a switched socket. It's rarely used but as far as I know works OK. I'd like to start using it more to take advantage of Economy 7 rates. Can I put a "standard" timer switch between the plug and socket? If not what is the best solution?
The problem is a 13A plug produces heat because it has a fuse and an immersion heater runs for an extended time so with the finger protectors added to 13A plug it tends to over heat this is worse if not in free air. Using a 15A plug works A1 as no fuse in plug assuming on a B16 MCB at consumer unit. The same would apply to 16A German CEE 7/4 plug however they are not permitted in UK as the lives can be swapped so neutral and line can swap over.
However using a 16A German CEE 7/4 timer would work OK as no fuse but to comply with UK rules you really need a fixed timer any fuse should be in a fused connection unit (FCU) not a plug as the FCU can get rid of the heat better.
It's not the timer but use of 13A plug which is problem. Kettle, oven, washing machine all use full 13A but not for as long as immersion heater.
I am looking at APPENDICES 15 of the BSi/IET wiring regulations BS7671:2008 [quote]The load current in any part of the circuit should be unlikely to exceed for long periods the current-carrying capacity of the cable (Regulation 433.1.5 refers). This can generally be achieved by: (ii) not supplying immersion heaters, comprehensive electric space heating or loads of a similar profile from the ring circuit. (iii) connecting cookers, ovens and hobs with a rated power exceeding 2 kW on their own dedicated radial circuit. [/quote] There is debate as to if appendices is really a part of the regulations but most consider it is.
So assuming the above has been complied with why would a 13A socket be used to supply an immersion heater? With a dedicated circuit which is required to comply with appendices 15 then likely there would be a B16 MCB or RCBO supplying the circuit so using a 15A plug and socket would be OK.
Once one breaks the requirements of regulations then one has to return to basic principles.
Thank you again ericmark. I understand what you are saying. I can't remember what the CORGI plumber did when he installed the new gas boiler and immersion heater etc years ago. I don't recall any alterations in the fuse box - so it's obviously time to get an electrician.