RCD Tripping on low load - Help much appreciated

[JesterRIP]

Hello,

Brand new to this forum - so any advice is hugely appreciated.

Plugging any power-consuming appliance (eg drill, vacuum, kettle) to any socket trips the RCD straight away - whereas a computer or radio etc has no effect and works fine. Cannot understand why this has suddenly happened as I was using an industrial carpet cleaner in the same sockets only 2 weeks ago and it was fine - and I know how much power they use (loads).

I've only disturbed 2 light switches, and I have checked them and they seem to be fine. The property is a victorian mid-terrace (red and black cables) and is empty so there are no other loads on the system. Not even a fridge-freezer. Does anybody have any ideas? Thanks.

 

[ericmark]

Neutral fault. If the neutral is connected to earth with a TN-C-S system with no load no current will flow between earth and neutral as they are at same polarity. As you load the circuit the neutral and earth voltage will start to differ and then current will flow and at around 20ma it will trip the 30ma trip.

 

[ericmark]

Just a further thought. Any heating equipment may use mineral insulation on the element. When not used it can absorb moisture and so cause an earth path. Things like cookers normally have the control switches working on only the line wire so if switched on at the main isolator can cause this problem. The same can apply to an immersion heater. So step one switch everything off.

 

[ColJack]

we worked it out once, you need about 5 amps flowing in the installation with a N-E fault 10M from the CU before the RCD would trip

 

[JesterRIP]

...so forgive my ignorance (as I am not entirely sure what a TN-C-S system is) - does this mean that somewhere in the installation a neutral cable is touching earth?

I only have a combi boiler (no immersion) and absolutely everything is unplugged and switched off.

Just as a thought - I haven't yet tried plugging one of these power-consuming appliances in upstairs yet - as I understand, there are two ring main circuits for each floor aren't there? Thanks again.

 

[ColJack]

yes.
TN-C-S means that you get 2 wires into your house, a "live" wire, and a Combined neutral / earth. ( not a problem since the neutral is earthed back at the substation. )

the earth is then split off from the neutral at the cutout to give you a Seperate earth.

there is usually only 1 ring main on each floor, so 2 rings for a 2 storey house..
the exception is that nowadays it's common to put the kitchen on it's own ringmain as it's a heavy use area..

if you switch on enough lights it may also trip the RCD..

most likely you've a screw touching a neutral cable in the light switches you removed if you have a neutral in there.. ( won't show up once you take the screws out to see if there's a trapped cable.. )

 

[DaveSinner]

usually with the last problem as described above, it tends to be when you swtich on that particular light/socket the rcd blows, so usually its pretty easy to fault find on that front. done it a few times and scratched me head as to the problem

 

[JesterRIP]

Hi, thanks again for all the advice. My light switches have red cables and black cables with a red band on them - I have since removed the facias and freed all cables from touching anything (including each other). Still the fault remains. I have not yet tried switching lots of lights on to see if the RCD goes - but will have a go at that tomorrow.
I'm thinking now... removing each socket to see if there are any neutral cables that look like they have been squeezed into earthing against the mating plates. (Sorry, not au fait with the whole sparky lingo!)
I have also had my light fitting down recently too (as a builder knocked it with one of his tools and it blew the dimmer switch somehow). Since then I have removed the dimmers and replaced them with standard light switches - which going back to the start of this thread, are the bits I have disturbed.

 

[bongos]

Does the RCD only protect the socket circuits? Have you completed any recent DIY work involving nails/screws in walls that might have gone through a cable? Any outside sockets that may have got wet or any water damage/flooding elsewhere?

These sort of faults can be an absolute pain to find. It usually requires insulation resistance tests on the dodgy circuit to try and locate the fault.

 

[ColJack]

you never mentioned the light, only the switches..

is the earth wire sleeved in the light you replaced?

with no neutral at the light switch it's unlikely that it's at the switches..

what work was the builder doing.. he may have caused the fault.. nails / screws in walls do some funny things..

 

[ColJack]

given the recent weather downpours and all, I'm surprised that it slipped the net..
do you have any outside electrics or lights? these may have got water in them which caused RCD problems..

 

[JesterRIP]

Well I took the light fitting down and similar to the switches, it had red cables and black cables with red sleeves attached. On top of that, there was an earth cable too - sleeved.
Yes I do have an outside light and the part that backs onto the wall is recessed under a gutter - but worth checking all the same.
As for the builder - yeah he just hit the fitting with something, and it blew the light and somehow also took out the dimmer. But other than that, he's not put anything into the walls or hammered anything in near an electrical conduit.

 

[ericmark]

There are three earth systems used in UK
TT this uses an earth rod and all circuits go through 100ma RCD.
TN system is split into two types.
TN-S this has a separate earth and neutral cable all the way back to the sub-station (Transformer) and is used for caravan sites where the next system is not permitted.
TN-C-S means that for part of the run both the earth and neutral share the same conductor and along this shared path many earthed electrodes are fitted but before you have access to this system it is split to separate earth and neutral.

It is often hard at the head to tell is TN-S or TN-C-S the latter also called PME.

Once the main isolator on the consumer unit is switched off on all systems there is no connection between neutral and earth.

For an electrician to find the fault he would most likely start at the consumer unit switching off all breakers and removing and testing each neutral in turn. But since even when everything is switched off in many old consumer units you can still touch line wires I do not think this is a job for a DIY man.

The RCD should be marked as to it's size is it 30ma or 100ma? That will help to guide you through.

First job turn everything off and unplug everything and remove all light bulbs then test to see if fault is still there.

If not then one by one replace things and switch them one until fault comes back.

If still there have another look around for anything missed. Immersion heater, outside light on PIR, Cooker, Oven etc. If that all fails then next must be to start removing neutrals and I would say you would be better to get someone in.

These faults can be hard to trace as a non RCD protected item can effect the RCD. This is because as the amount of current we draw increases so the volt drop increases and this allows current to flow.

There will not be an exact figure as the distance between where earth and neutral are bonded together and your consumer unit will alter the amount of volt drop between earth and neutral at the consumer unit.

I will try to draw a diagram and post it to explain.

 

[ericmark]

OK I have a program called Multisim which allows me to simulate all sorts of electrical and electronic circuits and I have used this to simulate the supply to a house with a fault.

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• ericmark
• 6 Jul 2009
• 1

This first one shows a large load and I have given the wires a resistance the resistance on line side really will make little difference but on the neutral and earth side after the second earth point the values will make a big difference to the result.
The U1 ammeter shows the leakage current. The U2 ammeter shows the current used and the U3 voltmeter shows the voltage at the load (R9) the fault is shown as R8 at 2 ohm all other resistors in milli-ohm.

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• ericmark
• 6 Jul 2009
• 1

In the second picture the only change is size of appliance and here we are just on the limit. The third

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• ericmark
• 6 Jul 2009
• 3

shows it with still smaller load and under limit.

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• ericmark
• 6 Jul 2009
• 3

the 4th one shows the same load on faulty circuit as 3 but has a second circuit with high load and you will note the milliamps leakage has increased showing that it may not be the item which is switched on when the earth leakage trip opens which is at fault and also how an item not even connected to the RCD can initiate the trip.

Once you have looked at the obvious i.e. things you have worked on then the next step is to check insulation between each neutral and earth. With a really bad fault you may find this with a multi meter which will test the circuit at between 1 and 12 volt but often you need a meter which tests at 500 volt to find the fault and these become expensive and I would think your better off with an electrician doing the job.

 

[bongos]

I have also had my light fitting down recently too (as a builder knocked it with one of his tools and it blew the dimmer switch somehow). Since then I have removed the dimmers and replaced them with standard light switches - which going back to the start of this thread, are the bits I have disturbed.

Have the problems started since the builder came to visit? Have they done any electrical work in your CU?

Does the RCD protect the socket circuits only or lighting too? Is it a dual RCD board or a split load board?

If you could post a pic of your CU and the incoming supply area it may help.