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Ahhhh..... - oldbloke's mutterings
June 24th, 2011
02:00 pm
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Ahhhh.....
If the boiler contents is at 90+, and you turn the thermostat down to Min, then switch off at the wall, the thermostat is generating a voltage that has to go somewhere, and there's nowhere left but Earth - trip goes the breaker.
So, don't turn the thermostat down, or don't switch off at the wall.
Simples.

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From:g8bur
Date:June 24th, 2011 02:04 pm (UTC)
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I suspect there's an intermittent earth fault - perhaps even neutral-to-earth - which is tripping the breaker. Once the thermostat - which, to the current, is just another switch - is off, then switching a second switch off at the wall should make no difference, as the current has already stopped flowing through the element.
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From:oldbloke
Date:June 24th, 2011 03:01 pm (UTC)
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Could that make it only trip /after/ you switch it off?
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From:g8bur
Date:June 24th, 2011 03:44 pm (UTC)
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Just possible, if whatever's breaking down remains connected to the mains after the thermostat has opened due to being set to Min. The thermostat may be electronic, with some of its components remaining live when it's in its 'off' state; in that case, switching off at the wall might just cause a transient pulse high enough in peak voltage to flash through an iffy component - possibly a mains interference filter capacitor connected between live and earth or between neutral and earth.

It might be worth checking what happens if you pull out the plug, rather than using the switch on the socket.
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From:oldbloke
Date:June 24th, 2011 03:00 pm (UTC)
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Well, thing is, when it trips, it's /after/ it's switched off.
If it was just a wiring fault of some sort I'd expect it to happen anytime.
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From:drdoug
Date:June 24th, 2011 06:42 pm (UTC)
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Get a sparky in, is my advice.

That sounds all sorts of wrong. The "don't do that then" approach is normally sound, and is probably good in the short term, but that it can happen at all suggests a potentially serious problem. Better waste a couple of ponies on a needless non-emergency call out than a chance of having your house burn down due to an undiagnosed electrical fault.
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From:oldbloke
Date:June 24th, 2011 07:54 pm (UTC)
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but this ring is only 2 yrs old and certified!
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From:g8bur
Date:June 24th, 2011 08:14 pm (UTC)
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Couple of thoughts:-

.Does it only happen when the heater's plugged into one socket, or into any socket?

.Do any other appliances make the breaker trip when plugged into that socket?
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From:oldbloke
Date:June 24th, 2011 08:46 pm (UTC)
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Only tried it in that one socket. It's a double, the kettle's in the other half, never given trouble. Not sure we've ever used it for anything else!
Somebody elsewhere suggested putting something in it that draws a lot, like an iron, and seeing what that does. Simple enough. Tried the kettle in it and it's fine, but that won't pull much current.
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From:g8bur
Date:June 24th, 2011 08:52 pm (UTC)
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I'd say it's still worthwhile trying the boiler heater in another (completely separate) socket; that'll at least give an indication as to whether it's the heater or the mains wiring. My money's on the boiler heater, or its lead or plug, being leaky to earth.
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From:oldbloke
Date:June 24th, 2011 08:59 pm (UTC)
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So definitely not the Peltier effect generating charge that can only find a way out via the earth, then?
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From:g8bur
Date:June 24th, 2011 09:23 pm (UTC)
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I'd not have thought so; if it's a Peltier device, and it is experiencing a Seebeck effect, ICBW but I'd be most surprised if it could create anything like the potential that the peak value of the AC mains rises to (~340 volts peak for a nominal 240v RMS AC supply). Any charge would have to break down the insulation to reach the earth - and to trip the breaker, it would have to pass through either the live or the neutral side of the breaker, but not both.

In any case, any device that generated any kind of charge that was not sustainable by the device's insulation, and/or significantly higher than the peak mains voltage, would be unsafe for sale and use and would probably be plagued by failures.

I'd still say there's an intermittent insulation breakdown or similar fault in the device or its lead or plug, causing the live and neutral currents through the breaker to differ by more than the trip rating for long enough to trip it. I've just described the situtation to my brother Geoff, who's done an enormous amount of 'sparks' work over the years, and has come across similar leakage problems, and he also reckons that this is the most likely cause.
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From:femsc
Date:June 25th, 2011 12:10 pm (UTC)
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Speaking from experience, and having seen the results of Geoff's leccytrickery work, if Geoff says something is so, he's probably right. If he doesn't know for certain, he says he doesn't know. And then he goes off and works it out until he does know.
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From:g8bur
Date:June 25th, 2011 09:12 pm (UTC)
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Absolutely; he knows of what he speaks, either already, or after the appropriate research.
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