tripping problem

[markwohlgenannt]

Hello, wonder whether anyone can help. One day out of the blue, when my hotwater system was programmed to come on, it tripped out the main power switch to the whole house. This started to become randomly more frequent. I replaced the programmer which I believed to be at fault, but this made no difference. I tried isolating the pump/valve and cylinder stat each in turn to identify the problem and this seemed to point to the cylinder stat. I replaced this but this still hasn't fixed it. Now I can turn the hw or ch heating on without it tripping,by turning the power on when the cylinder stat is on low, and then by turning the stat up, the boiler will fire up.

Does anyone know how i can solve this strange problem? A heating engineer came round a while ago when the system was behaving itself and he couldn't recreate the problem, but i still had to pay a call out charge !

[Roger Mills]

Is it tripping an over-current device or an earth leakage device?

What type of control system do you have? [Does it correspond with one of the 'plans' shown in

? If so, which one?]

If it's an earth leakage trip (RCD), and if it's a fully pumped system (S-Plan or Y-Plan), my first port of call would be the pump. It is not uncommon for pumps to develop intermittent earth faults before failing completely.

[meow2222]

RCD tripping means something on the CH is leaking current from live to earth, or less likely from N to E. Its a lot cheaper to get a multimeter and find out which item is faulty (some conduction from L&N to E) than it is to replace everything randomly.

NT

[markwohlgenannt]

Thanks Roger and NT. Yes randomly replacing parts is getting expensive ! Looks like I am a Y plan, and it is the RCD which is tripping. I think I shall invest in a multimeter and see if that helps. Many thanks.

[markwohlgenannt]

Thanks Roger and NT. Yes randomly replacing parts is getting expensive ! Looks like I am a Y plan, and it is the RCD which is tripping. I think I shall invest in a multimeter and see if that helps. Many thanks.

[Roger Mills]

In that case, my money's definitely on the pump.

[Roger Mills]

One multimeter should be enough - you don't need two!

[noelogara]

Roger Mills wrote: >

you ought to tell him what to do with it. A multimeter in a novices hands is like JCB in the hands of a drunken man.

[Roger Mills]

Actually, I'm not sure that a multimeter is going to help.

For an earth leak (most likely in the pump) to trip a 30mA RCD, there would have to be an impedance of less than about 8k Ohms between live and earth. A multi meter will measure the DC resistance, but I'm not sure that that is the same thing - and the fault seems to be intermittent anyway.

[noelogara]

Roger Mills wrote: > Actually, I'm not sure that a multimeter is going to help.

I agree its tricky to find the fault. I had an mcb trip out last week and after a lot of searching I found the problem was a squirrel had bitten a cable that crossed a tree plus it was a wet day only when it tripped. It a bit of detective work to find.

[noelogara]

Roger Mills wrote: > Actually, I'm not sure that a multimeter is going to help.

I agree its tricky to find the fault. I had an mcb on a line to a pump trip out last week and after a lot of searching I found the problem was a squirrel had bitten a cable that crossed a tree plus it was a wet day only when it tripped. It took a bit of detective work before I found the fault.

I think he should go over every bit of wiring he has first and look carefully at the connections and check screws are tight and cables look in the right place etc etc before he starts anything else. Then check the board for any flaws. It could be a bevy of spiders webs or a dead mouse in the back of the board so a good inspection of everything is elementary first.

[The Natural Philosopher]

It most certainly will measure that, and the DC resistance is usually what you get in a fault condition.

Intermittent is less easy to spot.

[Roger Mills]

I'm not so sure! If the fault is caused by the presence of water in the wrong place - which it may well be - my impression is that water behaves differently at mains AC voltage from what it does at low DC voltage.

[The Natural Philosopher]

No. Its about the same. You can stick meter probes in a glass of water and see the resistance OK.

Only insulation breakdown by arcing/sparking needs a high voltage to make it happen...but that CAN be DC as well.

Only capacitors e.g. in RF filters behave differently under AC to DC..but they seldom fail by increasing capacity..usually they short and go BANG instead.

[Martin Bonner]

To be pedantic, inductors (eg in shaver socket transformers) also behave differently under AC to DC ... but that doesn't alter the main point that a) the difference between low-voltage and high-voltage is more significant than between AC and DC; b) domestic water behaves pretty much according to Ohms law so a multimeter should be fine. (I'm not convinced high purity distilled water with low disolved gasses would behave ohmically, but that's getting /really/ pedantic).

[The Natural Philosopher]

Agreed.But indictors generally get better at not leaking under AC conditions ;-)

[markwohlgenannt]

thanks for all your help. The problem is indeed intemittant when using the programmer to turn the heating on at the set times, but when i manually start up the hot water by turning on the power with the cylinder stat on its lowest setting and then turning the stat temp setting up, it NEVER trips. Surely this must rule out an intemittant wiring fault ?

[markwohlgenannt]

thanks for all your help. The problem is indeed intemittant when using the programmer to turn the

heating on at the set times, but when i manually start up the hot water

by turning on the power with the cylinder stat on its lowest setting and then turning the stat temp setting up, it NEVER trips. Surely this must rule out an intemittant wiring fault ?

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