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Old 30th Jul 2014, 12:40 am   #1
ben
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Default RCD trip mode question

This thread is in a similar vein to this one from two years back.

In our flat, power is supplied to a fairly usual 30mA RCD, downstream of which are four MCBs.
For some months, the RCD has been tripping whenever the cooker, and also recently other high-wattage appliances such as the washer or air con have been in use for roughly 5-8 mins. None of the circuit breakers have ever tripped.
The main question is this: why, when the RCD trips, does it do so 'internally' - i.e. power is cut but the switch remains in the upper (on) position?

Today I bought and fitted a new RCD module, but this has not solved the problem as it cut out in exactly the same way (power off, switch up) after I tried using the oven for a celebratory pizza! I had no idea that this could happen under normal operating conditions but that is now two devices exhibiting the problem.

As to causes, I have a few ideas but will leave it for now. Any thoughts welcomed.
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Old 30th Jul 2014, 1:22 am   #2
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Default Re: RCD trip mode question

Metal clad heating elements in grills, cookers and immersion heaters go leaky and trip RCDs so that's not unusual.

You can also have some earth to neutral voltage such that a lower impedance neutral to earth fault can trip RCDs even when 'power' is off at a single pole switch like an MCB.

All my RCDs flip the toggle to the off position when triggered.

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Old 30th Jul 2014, 8:16 am   #3
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Default Re: RCD trip mode question

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Originally Posted by Radio Wrangler View Post
All my RCDs flip the toggle to the off position when triggered.
Mine (a newish MK one) doesn't. Its actuator goes to an intermediate position, requiring pulling fully down to the OFF position before it can be pushed up to its ON position to reset it.

I think the theory is that you can tell at a glance whether it was switched off manually or tripped automatically, but it can be confusing (my wife certainly had no luck resetting it when it tripped after someone overfilled the kettle).

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Old 30th Jul 2014, 11:04 am   #4
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Default Re: RCD trip mode question

Some types of RCD also incorporate overload protection, these being known as RCBOs.
Some types revert to the off position if tripped on overcurrent, and to an intermediate position if tripped on earth leakage.
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Old 30th Jul 2014, 1:06 pm   #5
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Default Re: RCD trip mode question

Here's a snippet from the MK RCD instructions:
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Old 30th Jul 2014, 3:26 pm   #6
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Default Re: RCD trip mode question

There are some which indicate whether they have tripped or have been just been switched off by a coloured indicator. I think the ones we use show white for off and red for tripped. That mid position on the MK ones causes all sorts of confusion for the elderly. I'm on call for electrical problems. When asked if all the switches are in the on position they always say yes, on visiting it's obvious that it's in the mid position, especially when compared to the second one fitted in 17th edition boards. Not such a useful feature if your eyes are failing.

I would be carrying out insulation tests on the wiring and appliances next. Don't forget to test all the circuits protected by the RCD as a neutral to earth fault on a lighting circuit may not trip the RCD until current is drawn, from the cooker for example. It's probably something simple like a failing cooker element though, but only an insulation test will show this. Don't forget to test when the elements are hot too.
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Old 30th Jul 2014, 5:40 pm   #7
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Default Re: RCD trip mode question

Yes, I would think you have one or two leaky appliances..
One tool which is useful for this is a PAT tester which can quickly show leakage. But of course you probably don't have one of those. So you will be left in going round all the candidates (cooker/washer/kettle - check whether water has got spilt into the base unit of the kettle, very common).
For what it is worth, my 30 year old MK RCD has only ever tripped if there has been a real fault.
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Old 30th Jul 2014, 5:49 pm   #8
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Default Re: RCD trip mode question

A Megger is of course helpful too.
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Old 30th Jul 2014, 6:15 pm   #9
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Default Re: RCD trip mode question

Hi,

I was speaking to a electrician yesterday about exactly the same problem. On this circuit the RCD would trip if nothing at all was connected. He advised to use a 500v test between Neutral to Ground. In the majority of cases this will show the breakdown. I've had quite a few combi-microwaves that have had insulation breakdown of the heater element also.

Regards. SJM.
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Old 30th Jul 2014, 6:58 pm   #10
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Default Re: RCD trip mode question

An RCD can certainly be tripped by a neutral to earth short. I found this out when replacing a light fitting in my shed. The shed is supplied via a 4 sq. mm feed that supplies the 13A sockets, and the lights are run as a fused spur protected by a 5A cartridge fuse. I had removed the 5A fuse before removing the old fitting, and when disconnecting the cable, the neutral momentarily touched the earthed metal housing and the RCD immediately tripped.
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Old 30th Jul 2014, 9:09 pm   #11
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Default Re: RCD trip mode question

RCDs can be brought close to the trip-point by modern switch-mode power supplies that have proper EMC filtering/RFI-proofing - a centre-tapped pair of capacitors [ends connected to L and N; centre tap to E].

If you've got a number of such supplies dotted around the place [TVs, radios, microwave-ovens, laundry-appliances] providing 'background' L-N imbalance it then only needs a smidgin of extra leakage to trip a properly-sensitive RCD. Cookers are a favourite trigger, as are infrequently-used convector-heaters kettles irons laundry-appliances and immersion-heaters, all of which can develop excess leakage by way of dampness.
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Old 30th Jul 2014, 9:20 pm   #12
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Default Re: RCD trip mode question

That is one of the reasons we mostly fit consumer units with RCBOs now. Much better at reducing nuisance tripping and a fault can instantly be located to one circuit rather than having to narrow it down from several.
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Old 30th Jul 2014, 9:24 pm   #13
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Default Re: RCD trip mode question

Here in the south we have a fairly sensitive board but not to the extent of being a problemn [in fact I saw this as a protective advantage]. The switches are up when on and down for off! The first time it tripped though I was confused until an electrician friend demonstrated the need to return all the switches off and then on again to reset the RCD.

I presume ben is talking about a dry Spanish eviroment so this may not be exactly pertinent but a couple of years ago I put on the washing machine which lives in an old style wash house extenal to the main building. This tripped the supply feeding both the kitchen [freezer oven etc] and the external wash house-most inconvenient on a cold, damp, winter Saturday morning.

I quickly established a problem with the washer [not the dryer] which tripped things whenever operated. I reset the board and temporarily isolated the washer on an extension lead running from another part of the house... no problem. Subsequently I discovered that supplying minimal background heating in the wash house sent the problem away-some sort of moisture related leakage. Couldn't find anything else wrong with the washer. No problems since! Sounds like the oven elements are prime suspect.
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Old 30th Jul 2014, 9:30 pm   #14
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Default Re: RCD trip mode question

if you've got a modernish cooker with one of those circular small oven elements, these are forever going leaky. If the oven's off by the knob it's usually only switched by a single pole in the live so the leaky element still exists from earth to neutral..and as other have said, this can then trip an RCD if another appliance is switched on. To prove a point, use the cooker isolator switch then try other appliances. I assume Spanish cookers are supplied from double pole isolators like in the UK so that would isolate the neutral too.
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Old 30th Jul 2014, 9:34 pm   #15
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Default Re: RCD trip mode question

I had a washing machine which would routinely trip the RCD during the spin cycle. Megger tests showed everything was OK.

Eventually I took the panels off and inspected the wiring. This was secured using bin bag ties with a steel wire insert rather than plastic cable ties. The steel wire of the tie had cut through the plastic insulation of a wire serving, I think, a temperature or level sensor attached to the drum. During the high speed spin cycle, movement of the drum was stretching or flexing the wiring and earthing the chafed wire to the chassis and tripping the RCD.

A bit of plastic tape over the chafe and fitting proper cable ties gave the machine many more years of life. Eventually the telescopic shock absorber ripped its mounting off the drum holing it, rendering the machine BER.
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Old 30th Jul 2014, 9:38 pm   #16
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Default Re: RCD trip mode question

Quote:
Originally Posted by matthewhouse View Post
That is one of the reasons we mostly fit consumer units with RCBOs now. Much better at reducing nuisance tripping and a fault can instantly be located to one circuit rather than having to narrow it down from several.
I'm sort-of doing the same.. Then if something trips it only takes out itself.

And I'm seeking to do away with 'ring mains' too - each room's power will become its own radial with one or two 16A circuits feeding appropriate BS1363 13A outlets and maybe a classic BS546 15A socket or a 16A "Commando" blue outlet. That way if anything goes wrong in a particular room I can easily isolate it without disrupting power to other rooms/functions.

Personally, I consider 'ring mains' to be the spawn-of-Satan!!
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Old 30th Jul 2014, 10:11 pm   #17
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Default Re: RCD trip mode question

Trouble with radial circuits is the regs permit only a small floor area to be covered by a 16A radial whereas a ringmain can cover up to 100M squared. A 15A socket should have it's own radial circuit. That's an awfully large consumer unit, and an awful lot of cable.
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Old 30th Jul 2014, 10:55 pm   #18
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Default Re: RCD trip mode question

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Originally Posted by matthewhouse View Post
Trouble with radial circuits is the regs permit only a small floor area to be covered by a 16A radial whereas a ringmain can cover up to 100M squared. A 15A socket should have it's own radial circuit. That's an awfully large consumer unit, and an awful lot of cable.
There's no area-limit on the number of physical outlets served by a radial fed by a single 16A trip, provided it's wired in 4mm-or-greater T&E (thereby placing the limiting factor on the trip rather than the cabling). And to be honest it's not really more cable to drop a couple of feeds into each room when compared with doing 3 or 4 rings each with a 32A trip.

I like the ability to isolate each room separately - and using room-specific radials reduces the degree of noise-coupling between rooms [it's the best way I've found of stopping 100-watts of 80/40-metre RF in the radio-room coupling back to the ADSL modem in the rumpus-room and causing said modem to do a hard-reset].
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Old 31st Jul 2014, 8:55 am   #19
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Smile Re: RCD trip mode question

Hi,
Just as an aside. There's no requirement in France for ovens or cookers to have a local two pole isolator. It seems to be a peculiarly British thing. I imagine it's the same across most of Europe.
But, they must have their own feed of 6mm cable and a 32amp mcb.
Cheers, Pete.
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Old 31st Jul 2014, 9:57 am   #20
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Default Re: RCD trip mode question

I hope this isn't too far off-topic, but last year I moved my hobby room from the UK, with 32A ring mains, to Poland, with 16A radial circuits. What I learned was that the UK ring main arrangement is very tolerant of big, difficult loads and high inrush currents, and the radial arrangement...well...isn't.

I can think of three pieces of equipment in regular use in my workshop - isolating transformer, variac and a big battery charger - which almost always caused the 16A breaker to trip when switched on. I never had that problem in the UK with 32A breakers. I've therefore had to go round and add soft-start circuits to the affected gear.

This is something worth considering when planning changes to wiring!

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