Trippin’

[Dinny991]

I have an boiler on a separate circuit. Every now and again it trips at the CU ONLY when it is initially turned on. When I flip the switch back on at the CU the boiler is OK. It gets turned off every night, and it is usually OK for 2-3 weeks, but then sometimes when I turn it on again it will trip at the CU. But again, when I flip the switch back, and try again, it’s OK. It seems odd, because I would have thought if it was the appliance at fault, it wouldn’t let me switch it back on again so quickly. As it only happens when I turn it on, not when it’s in use, I feel
like it’s a power surge? But the 20A breaker should be ample for the appliance! I’ve taken a look at the wiring from the appliance to the CU - it’s modern wiring resting below floorboards with no apparent breaks/cracks etc. Any ideas would be greatly appreciated.

 

[Dinny991]

… and RCD trips. MCB is fine. All connections at the appliance switch are tight.

 

[DetlefSchmitz]

So this is an immersion heater? It sounds like it has leakage to earth, and probably needs replacing. Difficult to be sure without the correct electrical test equipment though.

 

[Dinny991]

Thank you. So get a sparky to do a PAT test you mean?

 

[Dinny991]

Yes - immersion heater.

 

[EFLImpudence]

Thank you. So get a sparky to do a PAT test you mean?

Not a Portable Appliance Test; various tests to determine if and where there is any current leakage.

Likely the element when it heats and warps.
Might be difficult or laborious if it does not trip to order.

 

[Dinny991]

Ok thank you. Yes, I was really thinking it might well be difficult to sort out as it was so intermittent. Just so confused why it only happens when I initially turn I it on a and never when operating or hot. Hoping that someone would say faulty RCD, but I guess I’ll call out a local sparky and see what they say. Thank you.

 

[EFLImpudence]

Ah, I missed that it only happened on switching on.

Sometimes RCDs do just trip when a switch is turned on or off. I don't think we really know why.

It might sound daft but maybe you could swap the Ls and Ns in the switch - assuming it is double poled - or just try a new switch before an electrician is employed.

 

[Dinny991]

Thank you. Ah I see - yes it is double poled - that’s interesting. I could try that. Just so I know how could that help?

 

[Dinny991]

Definitely will get another switch though to see if that helps.

 

[EFLImpudence]

Just so I know how could that help?

As I said, I don't think we really know.

Sometimes, it is said that one pole connects before the other but that shouldn't cause an RCD to trip. It just does.

 

[Harry Bloomfield]

So this is an immersion heater? It sounds like it has leakage to earth, and probably needs replacing. Difficult to be sure without the correct electrical test equipment though.

If you turn off the isolator (assuming double pole) next to the immersion heater, does it still ever trip? No trip, rather confirms the immersion heater element is faulty.

 

[Dinny991]

If you turn off the isolator (assuming double pole) next to the immersion heater, does it still ever trip? No trip, rather confirms the immersion heater element is faulty.

So we have heater —-> double pole switch —-> CU. I can only turn switch on and off.

 

[Dinny991]

As I said, I don't think we really know.

Sometimes, it is said that one pole connects before the other but that shouldn't cause an RCD to trip. It just does.

Hmm - so interesting - I’ll definitely change the switch and then let you know what happens. Will be some time until I know as it’s often up to 20 days before anything happens. Thank you. At least worth a try before anything else.

 

[EFLImpudence]