I had the RCD trip out this afternoon, so went to investigate under the stairs, the RCD wouldn't reset, so one by one I switched off the MCBs on the RCD protected half of the consumer unit, and tried to reset the MCB, the first two made no difference (I switched them each back on before trying the next) when I got to the third MCB, the RCD did reset, so I switched the third MCB back on expecting it to trip the RCD again, but it didn't.
Can anyone suggest a reason for that behaviour (the circuit in question is the kitchen ring)?
RCD's can trip simply due to various filters on the input to equipment each having their own contribution to the overall leakage. Microwaves have them, as do PC's, some washing machines and etc.. Added together these small leakages can be enough to cause the trip.
An alternative problem might be leakage from a cooker element or similar.
Thanks, I know about the filters each adding a small amount of leakage; on the kitchen circuit are the washer/drier, the gas hob (for the igniter and presumably the flame failure device) the microwave, extractor hood, breadmaker and toaster, also an unused socket in the garage.
Fridge/freezer, underfloor heating and boiler are on a different circuit.
Oven is on a different circuit with the kettle on the 13A socket of the
45A cooker switch.
But my question was really why turning the relevant MCB off/on allowed the RCD to reset? I mean the circuit in question (along with all the other RCB protected ones) was "off" due to the RCD tripping.
Slight changes in temperature making the RCD less sensitive or the leakage less as the filter cools slightly. Possible moisture which evaporated as you were messing with the RCD / MCB's.
All you can really do is wait and see if it happens again and take note of what fixes the problem, until you can pin it down to a single MCB circuit - then progress it to an actual appliance causing the trip.
These intermitant things can be notoriously difficult to pin down and the best person to pin it down is the one reseting it, by applying logic to the problem.
Cooker, and socket in the garage would be worth checking.
Different or no RCD I take it?
Well its not off once you reset (or try to) the RCD. Note also that something like a neutral earth short can be enough to trip a RCD. You will probably only then see the problem under high load conditions - and the load in question can include that of your neighbours as well as yours.
(any circumstance that tends to pull the neutral away from earth potential, can allow a neutral to earth current flow. More likely to be an issue on TN-S installs)
My initial though was that it did rain overnight, not that the garage leaks but just that it is more damp than it has been for a while, it has only tripped once before like this, about 18 months ago, and I *think* it was also damp then ...
At the moment they are on the same RCD. They were deliberately not put onto the newer kitchen/garage ring circuit, the intention being to eventually move the fridge/freezer and boiler from the main house ring circuit to a non-RCD radial circuit.
Certainly nothing connected to the kitchen ring (whose MCB seemed to clear it) was taking any significant current when the RCD tripped, just appliances left on standby.
My brother used to live in the middle of nowhere with overhead supply. If there was a storm in the area it would trip their RCD out if the kettle was plugged in - didn't have to be on. kids putting 'bent' bread that touches the elements in your toaster will do it as well. Happy hunting for the source.
as will kids burning silver foil on the exposed elements of old fashioned bar fires, I know I was that child :-) never diiiiiiid mmmmme annny harrrmmmmmmmmmm Might have stopped me growing old gracefully possibly yea ha
Trips from power transients (lightening, bulbs blowing etc) tend to indicate a sensitized RCD - i.e. one where the cumulative leakage is getting close to the tripping point.
Yup, the moisture in the bread may be enough to conduct a few mA.
I do now. Having lived here for 18 years, when doing up the kitchen I discovered someone had chopped out the bonding to the water pipe while using a disc-cutter to make the opening in the wall!
What can cause a lot of head and arse scratching is when there is a live to neutral short which only shows up when there is sufficient power flowing to raise the neutral above the trip threshold whereas you would normally be looking for a Live to Earth fault;!...
I have never tracked it down. It seems I have a high static leakage due to probably a load of electronic filters everywhere. Once the trip has gone, its remarkably reluctant to be reset under load.
I THINK, but cannot be sure, that the mechanism is more or less that the total wiring and RFI filters in house represents a fair bit of capacitance between live and earth. This is not in itself a problem, BUT if there is any voltage transient on the house - or indeed incoming - mains., its enough to push the setup over the edge. When I lost mains supply for a week and EDF put a generator outside, it was markedly worse, with the most apparently offensive ring being the one that runs all the computer equipment.
So anything hat represents a bloody great spike on the mains, will trip it. Yea even unto simply switching the house on.
Its bloody irritating, and one day I will get a clamp meter and measure all the earth currents and see if there is a prime offender. Or fit RCBO's everywhere.and get rid of the whole house trip altogether. . To fully load a 30mA trip at 250v is about 8K ohms. I make that about
380nF at 50hz.
I have not been able to identify what most phase-to-earth capacitors in RFI filters applied to domestic equipment is, but 10nF would seem to be what I would expect. That puts 40 such units as well able to trip a 30mA trip. I am certain I have more. Which is why I am on a 100mA trip and it still goes sometimes on surges.
In addition, inter-conductor capacitance on T & E is about 100pF/meter allegedly, so a couple of hundred neters will get a further 20nF on that load.
Add in CFL bulbs with RFI filters..and the whole way in which houses are protected starts to look incredibly shaky.
I suspect that in due course the regulations will catch up, and RCBO's bet the way to go on a per circuit basis.
FWIW I calculate 8.3Kohms or 380 NF between live and earth to be enough to trip a 30mA trip.
The former is unlikely in the absence of some pretty drastic insulation breakdown. The latter is all too easy to arrange, and as I said in the previous post, thats at 50Hz. Switching a switch ON at anything other than a zero crossing point of the mains can be infinitely worse.
When my board trips, I nearly always have to flip every circuit off and bring them up one by one after resetting the RCD.: A pretty clear indication that my problem is capacitative and not resistive.
I was thinking of gong masiveley RCBO with a 180mA main RCD..but how do you isolate a board to refit a main switch?
I'm not an expert in this - and I'd agree if all cross bonding was brought back to a mecca close to the earth/neutral bond at the cut-out
- it would lower the impedance of both neutral an earth near equally.
However if a wiring earth is bonded close to a fixed appliance (e.g. from an appliance chassis to a water pipe) and away from the mecca - wouldn't that lower the impedance of earth to possibly a significantly greater extent than neutral?
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