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Can anyone help with RCDs (Residual Current Device)

shermans
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Can anyone help with RCDs (Residual Current Device)

I have a technical question about ordinary RCDs, as I do not know how they function internally, although I understand what they do.  I would be grateful if anyone has a better understanding of electrical circuitry than I do.
My problem is a bit complicated, but I will describe it as simply as possible without going into more detail than I need to !
I have recently had a wood burner with a back boiler installed, which is connected to the central heating.  It is a large capacity - 35 Kw.  My oil boiler is only 28 Kw. The wood burner is specially designed to work in tandem with an oil boiler.  It has a large box of tricks - all mechanical - which switches the heating between the two sources of heat automatically.  If the wood burner is NOT lit, then the oil boiler fires up; if the wood burner IS lit, then the oil boiler is automatically switched off.  If the wood burner does not have enough wood to heat the water sufficiently, then the oil boiler fires up to supplement it.  This is all controlled through various valves - non-return valves, over-pressure valves, anti-thermo-siphon valves and a water thermostat, all built into the manufacturer's kit.
The only non-mechanical element of the kit is a water circulation pump which is in addition to the normal central heating pump.  Basically, all the various valves on the wood burner work with this second pump (which I will call the second pump; the original central heating boiler I will refer to as the first pump).  So when the wood burner is hot enough, the thermostat on the wood burner turns off the first pump (the original central heating pump) and turns on the second pump instead.
So far, so good.  Nothing too complicated there.  But the problem arises with the electrical connection for all this.
The oil boiler is connected to the main fuse box with a dedicated circuit, beginning with a 30mA RCD. The wood burner installer has connected the second pump to the ordinary power ring main, which is served at the fuse box by its own 30mA RCD.  Therefore the first pump is served by one RCD, and the second pump is served by a separate RCD, but both RCDs are in the same fuse cabinet.
At face value, this should be alright, as the electrical circuit for the oil boiler is separate from the electrical circuit for the second pump. However, one or other of the RCDs keeps tripping, whenever the wood burner cools down and the oil boiler tries to take over, and I do not know why.  I suspect that it has something to do with the two separate sources of power for the two pumps, but the installer is adamant that that is a red herring, and that the oil boiler circuit is faulty.  However, the oil boiler has worked perfectly satisfactorily for the past five years without ever tripping the RCD before.  It is only since it has been connected to the wood burner that this has started.
The clue may lie in the way that the domestic hot water is controlled by the oil boiler.  The oil boiler actually has two pumps - one for the domestic hot water, and another for the central heating.  So that now makes three pumps including the wood burner in total !  Interestingly, the circuitry is designed so that the domestic hot water takes priority over the central heating.  In other words, if the domestic hot water is not up to temperature, the first pump (the one to the central heating) is turned off, so that the third pump circulates the hot water around the domestic hot water tank only.  When the domestic hot water is up to temperature, then its own thermostat switches on the first pump to circulate the hot water around the central heating.
So, as far as I can see, even though the second pump appears to be powered from the second RCD through the ring main, and the first pump is evidently powered through the dedicated oil-boiler circuit, there must of course be some sort of cross-over between the two sources of power, in as much as the second pump's thermostat turns the first pump on and off, and the third pump does exactly the same.  So the first pump is therefore powered through two different RCDs.
I suspect that one RCD is somehow tripping the other.  It would not be difficult for me to prove the point by simply disconnecting the second pump from the ring main and connecting it to the oil-boiler circuit to see what happens.  But I do not like to resort to "trial and error" without understanding what the cause of the problem really is first.
So, after much description, I eventually come to my question.  Can anyone tell me how an RCD works, and whether a cross-over between two RCDs could cause one of them to trip ?  If that is not the case, then clearly there must be something wrong with the circuits somewhere.
7 REPLIES
Community Veteran
Posts: 1,699
Registered: ‎30-07-2007

Re: Can anyone help with RCDs (Residual Current Device)

All sounds very complicated to me!  Undecided
If I understand it correctly, an RCD (at its simplest) works on the fact that a wire carrying current through a coil, induces a current in the coil.  By having the circuit going through the coil twice (once on its way out and once on it's way back) the current going in the opposite direction cancels the first induced current out, unless there is a leakage of current somewhere around the circuit (probably to earth, but maybe leaking across to another circuit somehow).
The fact that a higher current is going in one direction than the other means that, if a high enough current is induced (in this case 30mA), the RCD will trip.
Microfuses work quite differently and do a different job (essentially the job a fuse)
I'm sure someone who knows more than me will be along shortly, and put me right if I've got it wrong somehow.
John
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Re: Can anyone help with RCDs (Residual Current Device)

http://www.explainthatstuff.com/howrcdswork.html which seems to support your description John.
I have RCDs on the lighting circuits in my house and if a bulb fails the RCD trips plunging the house into darkness for the sake of one bulb. Roll eyes

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Re: Can anyone help with RCDs (Residual Current Device)

If everything is hunky dory then for any current flowing in the live wire there is a matching and equal current flowing in the neural wire.  RCD devices trip if that is not the case.  For example, if you touch a live wire the current may flow to earth through your body rather than returning down the neutral wire.  This imbalance between current on the live and current on the neutral should cause the RCD device to trip and break the circuit, thus preventing you from getting electrocuted.  When I moved into my house it turned out that the neutral wire from the upstairs ring main had been connected to the downstairs ring and vice versa.  The fuse box had conventional fuses which didn't care but an RCD device would not work in this configuration.
The symptoms you describe seem to indicate their is "cross talk" between your two circuits.  But thermostats usually operate at low voltage so I'm not sure those are responsible.  In principle both RCDs should be triggered by an imbalance but I suppose if one goes first that corrects the problem so the second one stays on.
shermans
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Re: Can anyone help with RCDs (Residual Current Device)

Thanks for all those replies which are really most helpful and all point in the same direction, namely that there is incompatibility between the two circuits.
Of one thing I am now certain because I got a shock last night from the boiler when it was completely turned off.  I was using a pen to trace the wiring (the pen had a metal case) and as I gently pulled a wire away to see where it connected, the pen touched the "lock-out" terminal and I got a nasty surprise !  As a result, I pulled the plug out of the boiler circuit, to completely neutralise it, and then thought "I wonder if the prongs on the plug are still live ?".  I got out a tester, and lo and behold, there was a strong current on the live prong of the disconnected plug !  ********* dangerous.
The installer is a fully qualified, certified electrician running a good size business wth 5 employees and two offices.  He is no fly-by-night and the quality of his work is very good - he installed the central heating and the all the electrical circuits new about eight years ago, and is not cheap.  So he is not a cowboy, but I do not think he has thought this through properly, as he probably has not installed a dual system like this before, although he was shown what to do personally by the manufacturer.  If anyone is interested, the attached document describes the wood burner, as it is rather unusual.
Anyway, I will now carefully study the electrical layout, work out what needs to be done and then tell the installer exactly what he has to do.
Thanks for all the advice.
VileReynard
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Re: Can anyone help with RCDs (Residual Current Device)

If he is an expert, why install two RCD's for effectively a single appliance?
If a fault occurs it should disconnect the whole thing from the mains.

wisty
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Re: Can anyone help with RCDs (Residual Current Device)

In an RCD as ReedRichards said in reply #3 there should be the same current flowing out through the neutral wire as is flowing in through the  live wire. Any imbalance >30ma and the RCD assumes that the missing current is going somewhere it shouldn't and trips.
Until I read your reply #4 I was fairly sure that the problem was that the first pump probably had two possible live connections only one of which could be in use at once, but the two neutral connections had been wired together. In that case whichever live was feeding the power in, some of it is going back out through each of the neutrals, and the RCD that is supplying the live  power is sees an imbalance  and trips. Either all the set-up needs to be on one RCD, or you need double pole switching which switches the neutral as well as the live. Unlikely in a thermostat.
However having read your reply #4 you don't have an incompatibility you have a wiring FAULT, and a very dangerous one at that. 240V AC MAINS IS POTENTIALLY LETHAL, particularly in close proximity to copper water pipes that conduct well and are earthed.
I would get the installer back. The issue may be in the boiler, but he installed it, and clearly didn't do an appropriate safety check if you are left with a plug that is live (that should never ever be possible). I would call him and tell him about the live plug - if that doesn't get him round VERY quickly then you need to start talking to trading standards or whoever. Don't start giving instructions about what to do unless you are well insured against the consequences of being wrong.
shermans
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Re: Can anyone help with RCDs (Residual Current Device)

A fox is evil
Just to answer your question about why two circuits.  There is a good reason in principle.  The heating system is controlled by a DTMF signal to turn it on and off remotely by telephone.  When the house is empty, all the mains electrics except the boiler circuit are turned off manually with one switch, leaving only the boiler and burglar alarm live and exceptionally the freezer, all of which have their own circuits which are connected to a separate RCD to those connected to the ring mains.  The reason therefore to connect the pump for the wood burner to the ring main was just in case anyone lit the woodburner (say in summer) without turning on the boiler as well, in which case the wood burner pump would not function.  So that was the logic behind his decision to connect the wood burner that way.  It was well intentioned but clearly wrong.
Wisty
Many thanks for your comments which make perfect sense.  Having looked carefully at the wiring myself without interfering with it, as far as I can see no wires have been obviously wrongly connected.  I am certain your orginal conclusions are actually correct - two separate live sources from separate RCDs but a common neutral.  That makes so much sense the way you have described it.  The cause of the live plug prongs stems from exactly the same error which is why I took a tester to the plug in the first place, because it occurred to me that if there was a common link to the separate live sources through the controls for the third (hot water) pump, then I suspected that umplugging the boiler would leave the plug still live by proxy.  I was horrified to find, after I got the shock from one of the wire terminals, not from the plug, that the plug itself, pulled out of the socket, still registered 226 volts, confirming my suspicions.  I have called the installer and he will be here in the morning, first thing.  It is going to be difficult to tell a professional what he has done wrong, but I do not want to leave anything to chance, and will have to point out what I see  as the errors but leave it to his professionalism to decide what to do.  As you so rightly say, I do not want to end up being held responsible for giving advice - only observations.
Thanks to all.  I think I would prefer to overcome the problem by having a manual two-way switch to turn on the pump when the wood burner is on and to turn the central heating pump off, but I will have to leave that to his judgement.