Burnt out fuse on immersion water heater

[tudge56]

Have a fault where the ecomony 7 (or whatever it's called) overnight water heater element keeps causing heat damage to the fuse in the local fused switch.

The element seems to be 15A rated so the first time I was called out I told the landlord it was too high for the 13A fused switch and needed to be replaced with a 3kW element.

What puzzles me is that the day time element is exactly the same set up and has never had the same problem....anyone any suggestions why?

I've tested both elements and resistance on each is around 18ohms. Both circuits are giving similar continuity readings making me think it's not a loose/high resistance connection issue. (The heat damage was only around the fuse and not at any of the connections within the fused switch which also backs this up).

Is it possible that the night time voltage could be higher causing the element to draw a higher current?

And could I just replace the fused switch with a isolator and rely on the 16A re-wireable fuse at the consumer unit (run in 2.5 single cables in PVC trunking).....or should I put my foot down and tell the landlord he definitely needs to downgrade the element to a 13A??

Any advice much appreciated

 

[Ian1981]

The off peak element is used the most so over time it has caused thermal damage to the 13amp fused spur.
I would change spur and fit a 20 amp double pole switch.
The immersion heater is a fixed load so wouldn't be subject to overload protection however the cable supplying the circuit and the heat resistant flex connecting to the element would require fault protection.
1.5 heat resistant flex off the top of my head is rated to 16amps so can handle a 3kw load 13amps at 230 volts no problem.
If it's 2.5 installed in trunking protected by a bs 3036 fuse 15 amp then Iz x 0.725 = 17.4 amps so cable is protected tho as I said overload protection wouldn't be required.

 

[telectrix]

FCUs do break down regularly when running high loads. asIan suggests, fit a 20A D/P switch in place of FCU.

 

[Wilko]

Just out of interest - the fcu that fails vs the one that doesn't - are they the same make?

 

[Ian1981]

I'm guessing the on peak boost switch is hardly used and when it is it's on for a small amount of time say an hour or 2 so the 13amp fuse won't be subject to an extended amount of use unlike the off peak spur which maybe subject to 7 or 8hrs of use give or take when the stat reaches desired temperature.

 

[PEG]

Hi bud,is the off peak supply,via a separate meter?