combi trips circuit breaker when firing up

niman · 15 Jan 2010

Dear all
have a combi that trips the circuit breaker when the hot tap is turned on to demand hot water or when the central heating switches on.

the boiler has a fan (always on) which speeds up, as it should, just before breaker trips.
Also the pilot light igniter works as normal.

The boiler in question is a ravenheat rsf 820/20T fanned combi

hope someone can help throw some light on the problem

thanks

jameskeane · 15 Jan 2010

I believe a short circuit check will need to be done here, but by a competent electrical safe person. It will probably be the pump or fan causing this, but may be a pinched wire somewhere inside the boiler

niman · 16 Jan 2010

Thanks for the suggestion.
Using my limited knowledge I tried
1. with fan disconnected
2. with pump disconnected
but still circuit breaker tripped, so have called for a professional.

As a matter of interest I used a multimeter to measure ohms between each of the three terminals on the fan to earth expected the reading to be infinity but to my surprise measured something like 3000 ohms. I expected the motor windings would be completely insulated from the motor body. Can anyone enlighten me?

slapper · 16 Jan 2010

Had this problem a few years ago, turned out to be the gas valve.Let us know how you get on.

Agile · 16 Jan 2010

If you disconnected the fan connectors it should be at least 1 Meg from winding to earth.

I assume its an RCCD tripping and not a current overload MCB.

Its rare for the fan to be the problem, usually the pump and sometimes the gas valve.

Very rarely its caused when the boiler actually sparks but thats usually when there is already some earth leakage from something else in the house. You need an earth leakage clamp on meter to trace that.

Sometimes the faulty component will not show the fault when tested with a low voltage meter and only shows when tested at mains or 500v with a Megger®.

Tony

niman · 13 Mar 2010

Central heating engineer tested the same things again and drew a blank.
He called in an electrician who diagnosed the fan as being at fault.

He was correct, a new fan solved the problem.

I don't know how I or the central heating engineer missed this - we both disconnected the fan completely (or so I believe) and still the RCD tripped.
Must be somekind of voodoo

Agile · 14 Mar 2010

I cannot answer that either but your reading of 3000 olms to chassis indicated a serious fault.

In these cases powering the suspect part from an independent supply will usually adequately test it.

Tony

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combi trips circuit breaker when firing up

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