CH 3A fuse repeatedly blows even when connected to boiler

[altough]

Hi, we have a Glow-Worm hideaway boiler connected for CH and HW, fully pumped system. Recently the 3A fuse blows very shortly after the boiler fires up. We have identified a fairly small leak above the Wilo pump so very likely that is the cause. Have contacted a plumber to investigate.

What is puzzling is that the 3A fuse blows even when disconnected from the boiler. That is, when the mains electric is disconnected by removing the white terminator from the boiler. We have had all electrics tested OK by an electrician with meter, and in fact have replaced the very old Microgyr switch which was playing up with a newer Siemens RWB2E. The electrician has found no shorts on the electric supply so why would the fuse blow when not connected to anything ?

Thanks in anticipation, Alex

 

[jackthom]

I'm assuming there is something like an electric lead with a normal mains plug on one end and a white connector on the other which plugs into the boiler.

If this is blowing the 3A fuse in the plug, even when not connected to the boiler, there must be a short somewhere in that cable or it's connector. Do you have a multimeter?

 

[altough]

Thanks for getting back so quickly.

Yes a white connector at the boiler end, connected to timer backplate and then to 3A fuse spur. This lead behind a cabling conduit so difficult to get at, but may be required !

I guess we could disconnect the connection from that cable since electric connections downstream away from boiler have already tested OK.
Then retest without cable.

Yes, I do have a fairly basic multimeter, not at home right now though.

thanks, Alex

 

[jackthom]

Does anything else come out of that fused spur or is it just the one cable going to the boiler control unit plug?

If it's all by itself then it must be the problem. Maybe heat has damaged the cable insulation or the plug at the boiler end could be falling apart inside.

 

[altough]

Fused spur all by itself. No connections to anything else but the boiler control unit.

When I get home tonight I plan to retest with boiler cable live/neutral disconnected from timer backplate. If it is the cable to the boiler then the fuse should not blow.

To confirm, I think I should be able to do some tests with my basic multimeter (which does not have a continuity test).

Probes connected at either end of the cable should show 0 ohms (or very small) to indicate continuity. Infinity would indicate lack of continuity.

Probes connected at either exposed live/neutral wires or at boiler connector shold show some reesistance. A value of 0 would indicate a short circuit.

Either no continuity or short would indicate a problem with the cable.

Does that sound correct ?

I just want to know either way its an electric problem before we confirm the callout with the plumber.

Many thanks, Alex

thanks, Alex

 

[jackthom]

I think I've misunderstood what you were saying earlier and actually this fused spur is also directly supplying power to the timer and pump etc. which is the normal setup.

In that case the pump itself is a likely candidate for a short circuit and should probably be the first thing to look at.

Can you post a picture of the wiring box for these ancillary components?

 

[altough]

Thanks, will attach a photo a bit later today when I am at home.....

 

[altough]

Thanks, plumber visited us over the weekend and we have a faulty motorised valve causing a short, we have a leak that needs fixing !

Hopefully he can fix before the cold weather sets in.

thanks again for your help.....