Back Boiler Tripping RCD

[scottfz]

Hi all,

Got a customer with a horrible fault. The system is a myson housewarmer back boiler, gravity hot water, pumped central heating. randall 3020p timeswitch no room/cylinder stat

The problem is the RCD is tripping aswell as 2 3amp fuses blowing. 1 fuse spur and another for the pump which powers a socket as the pump is on a plug top.

Ive changed the pump and checked for water around electric cables and it still tripped after 4 hrs.

Ive left only hw on and it hasnt tripped, and this is after 2 days so i think it could be the thermostat on the boiler but not sure.

Does anyone know of this fault or had it before? please help

 

[gas4you]

Ive left only hw on and it hasnt tripped, and this is after 2 days so i think it could be the thermostat on the boiler but not sure.

The boiler stat is operational during gravity HW demand, the same as it is during CH demand.

If the rcd is tripping then there is a leakage to earth, and if the 3 amp fuse is blowing then a bad one.

I once had a very similar situation with a Glow Worm Capricorn, done all what you said, but kept getting called back every week or so because the 3 amp fuse had blown.

Eventually, purely by chance, I found that the system had a cold feed blockage. Once I removed the blockage and the system was filled fully and bled, Hey Presto, the fuse never blew again

I can't really expalin it to this day, but it worked

Stranger things happen at sea, so they say

 

[scottfz]

thanks for the quick reply, i see what you mean about the thermostat. i can see this fault driving me nuts, its an elson tank too. what can cause earth leakage? fault cable's?

 

[gas4you]

Earth leakage can be caused by any current leaking to earth, be it a cut in a cable, water ingress giving a path to earth, or a faulty winding etc etc.

You need to check for a neutral to earth fault as well, not just live to earth.

The rcd monitors the current down the live and neutral and if it detects an imbalance, trips out.

If you have a 30milliamp rcd and you have eg 3000 milliamps down the live, but only 2970 down the neutral, the rcd assumes that the 30 milliamps is going somewhere it shouldn't so cuts the supply for safety.

The above is really just the basics, but hopefully might give some insight for you to test further

 

[Deleted member 105431]

if its a bbu I have had scorched cables around the bbu cause this before, when hot the insulation resistance of the insulation lowers causing an intermittent short. Worth a look especially if heat resistant cable hasn't been used.

 

[bengasman]

Had similar thing on baxi bbu last year.
turned out that the "service engineer" who "checked" the boiler every year had not noticed a heap of debris, or the the roasted cable.
Good chance you will find the problem when you remove cover and fire.

 

[scottfz]

I will have a look tmrw and keep you's posted

 

[gas4you]

Whilst the burnt cable theory is extremely valid, I have had this myself, I would have thought that this would cause the rcd to trip every time the boiler is used, not just when the ch is selected

Don't completely discount this though

 

[Agile]

Depending on exactly what is causing the leakage then its just conceivable that the different temperatures at the boiler when on HW only could be affecting the problem.

The usual expectation is that it would be associated with the pump and its wiring IF thats in the circuit which blows the fuse.

Low voltage testing with a multimeter usually does not show up this kind of fault but instaltion testing at 500v usually will.

If you still cannot find it then consider replacing the pump cable.

I once had a similar problem in a 5 y.o. house with totally hiddden cables between the kitchen Suprima and the upstairs cylinder/valves/pump. I supplied the pump independently and that reduced the tripping from every 3-4 hours to once a day.

In the end I had to give up because the wife was paranoid and wanted me to guarantee that I would fix the fault on my next visit. As it was on our fixed fee basis, although interesting, it was very time consuming. I had to explain to the husband that I found his wife too stressful and would not be able to continue working for them. He understood and apologised for her. They were both Chinese doctors!

Tony

 

[bengasman]

Whilst the burnt cable theory is extremely valid, I have had this myself, I would have thought that this would cause the rcd to trip every time the boiler is used, not just when the ch is selected

Don't completely discount this though

I know what you are saying, but with systems that have been in place for decades, often neglected for long periods of time and almost guaranteed to have a couple of cowboys/numpties work on it over the years, my motto is: forget all the logic and trace all those things that are often done wrong.
wiring is at the top of my list when it comes to that.

 

[gas4you]

I can only aree with you, but unfortunately even though I think it, it usually the last thing I think of to check everytime

 

[bengasman]

Ah, that is because I used to be a politician, so I am used to looking at everyone I can blame the problems on rather than solving them.

 

[scottfz]

bingo, got the fault

think its called a supressor, thats why only tripping on the ch. anyone else had this?

 

[ajstoneservices]

its called a supresso

.

That would be this labour government, they're stopping everyone/thing working

 

[Deleted member 105431]

bingo, got the fault

think its called a supressor, thats why only tripping on the ch. anyone else had this?

the supressor is wired into the switched live and it used to stop tv/radio interference. It would not explain why it's only tripping on ch.