Help! RCD trips on Volex consumer unit no matter what I do!

[scottbrewster]

I've just rewired my house and was using the old consumer unit with some aftermarket MCBs. I'm pleased to say everything has been working fine. I decided I should go for a new consumer unit to give me more control over my circuits and safety. I've bought an already populated Volex CU from Screwfix. This has a 63A RCD (VR63) which trips at 30mA. I installed the unit and wired everything up (no instructions came with it), with the busbar not cut so (presumably) everything is protected. With everything connected I can switch on really low current devices like lamps plugged in to sockets or mobile phone chargers absolutely fine. The second I try and switch on something more hungry, like a TV, fridge, freezer DVD player or computer the RCD trips out. This doesn't happen EVERY time, just most of the time. I've tried this with all the lighting circuits off and every other ring circuit off and it does it on all three (kitchen, downstairs and upstairs). I can't understand why this happens, so I've been reading up on circuit capacitance and TNC / TNCS supplies, but as I'm not a sparky by trade I've just got confused. On top of this, with the mains on and some lamps switched on or even off, when I press the "T" button to test the RCD, nothing happens. Shouldn't it trip itself? I'm thinking (hopefully) that I haven't done anything stupid, and that the RCD is in fact faulty out of the box. Can anybody help me with this? I've gone back to the old consumer unit for now, and I'm not happy! (though at least I can watch TV and have a cup of tea). Thanks, Skote

[bigcat]

But I would also rewire the new CU differently next time. Running a whole house off an RCD is just asking for ongoing trouble.

If you can spring another =A3100 or so, your best bet is to:

1=2E use a split CU 2=2E put each power socket circuit on its own RCBO, 3=2E have the lights with no RCD anywhere 4=2E Use type C mcb for lights, not type b.

And if you have the opportunity, put the fridge freezer and boiler on a circuit with no RCD.

I'm assuming you dont have TT system, if you do you'd need to add a

100mA time delay RCD in front of that CU configuration.

If you cant afford another penny, at least use a split CU with RCD only covering things that need the RCD. Lights should not be on RCD, nor immersion heater and cooker.

NT

[Richard]

Is it unwise or prohibited to run heavy loads such as immersion heater and oven via an RCD?

This is how things were installed in my place when we bought it. NB the previous owners did no DIY! FWIW

TIA Richard.

[scottbrewster]

Thanks for the information. I had a feeling I was right about the RCD being faulty. I'm going to split the circuits for now then and run the upstairs and downstairs sockets on a new RCD and run the kitchen (boiler, fridge, freezer; I don't have an electric cooker or an immersion heater) and lights without. I don't have any spare cash at the moment as I'm doing up a house that time forgot. Before I put the new wiring in, I had one spur circuit for lights and one spur circuit for sockets. There was one surface mouted socket in each room, and a few round pin type bakelite sockets too, to go with the bakelite light switches. Nasty. Once it's all in I'm going to get it certified. On top of this, there wasn't any earth at all until I put one in. The mains supply is labelled as being PME but there was no connection for earth. At the moment I've earthed the CU via the cold water inlet, which is cast iron and the gas inlet (which I have just had replaced) which is now copper. Presumably then, my system is acting as a TT system? The regs say a 30mA RCD though and don't mention a time delay for a TT system.....? Thanks for the advice. Much appreciated.

[James Salisbury]

1. That is SO bad I would like to point out today is NOT the 1st of april.
2. TURN IT OFF at the mains, you need a tested earth spike if you are going down the TT route.
3. You can't install a consumer unit unless you inform building control or otherwise comply with part P, in other words get a propper sparky in

[tony sayer]

In article , [e-mail] writes

Bet U as much as U like you've got a neutral/earth short somewhere on the "consumer" side of the RCD, sounds just typical of what would happen when you have that. Small loads cause no tripping until the in-balance gets too much for it to bear!.

It may well be that this has been there for somewhile and the RCD has started to show it up!.

Get a test meter and disconnect your wiring from the incoming supply and check for any shorts or low ohms between the earth and neutral conductors, and then disconnect each circuit in turn till you have found it. In fact it should read totally open circuit any resistance here shouldn't be there! .

The object of the RCD is to monitor the current "into" your system on the live and match it going "out" on the neutral any "leaking" away to earth will cause an in-balance and hence the tripping. Check also things like immersion heaters they can and do cause this.....

[scottbrewster]

Hmm..... Does that mean that I could have a good RCD and just a bad connection somewhere? The devices I was connecting, like TV, DVD etc. don't have any earth wiring, so surely they wouldn't cause this short?

[Andrew Gabriel]

It's unnecessary and slightly unwise. It's not that they're heavy loads, it's that they are stationary or fixed appliances which don't represent any significant electrocution risk, and both these particular appliances are known to occasionally trip RCDs which will be inconventient in that it will stop them working when they would otherwise have done so fine, and knock out other things.

It has been quite common to RCD protect a whole installation, even though it is not correct to do so. I suspect this is because a 30mA RCD was often used to substitute an older Voltage Operated earth leakage circuit breaker. These do different things, and whilst it was correct to protect a whole installation with a single Voltage Operated earth leakage circuit breaker, it is not correct to do this with a single 30mA RCD.

[John Rumm]

A TT system requires all circuits to be protected by a RCD. Typically a

100mA trip one is used for this. There is a also a requirement for sockets likely to feed portable appliances used outside to be further protected by a 30mA trip RCD.

This creates a potential discrimination problem, in that with a typical split load CU (with the 100mA RCD as the incomer) the 30mA RCD is downstream of the first RCD. Should you get a fault occur on a socket circuit you can't predict which RCD will trip and may end up killing the lights as well as the socket power.

The solution to this is either to use a 100mA RCD with a built in time delay on the incomer. That ensures that in a fault situation on a socket circuit, the main RCD will not trip instead of, or as well as, the socket circuit one. Alternatively, use two separate CUs in parallel with a 100mA RCD in one and the 30mA one in the other.

Sounds like you need to get the supply company to come and instate the PME earth connection PDQ however.

[scottbrewster]

Quote:

"Sounds like you need to get the supply company to come and instate the

PME earth connection PDQ however."

Yep, and that is my intention. get it installed, get it certified, and then get the supply to connect a decent earth. What I have now is ALL new, as opposed to the bakelite and spur earthless nightmare that was in before.......

[Jeff]

But I would also rewire the new CU differently next time. Running a whole house off an RCD is just asking for ongoing trouble.

If you can spring another £100 or so, your best bet is to:

1. use a split CU
2. put each power socket circuit on its own RCBO,
3. have the lights with no RCD anywhere
4. Use type C mcb for lights, not type b.

And if you have the opportunity, put the fridge freezer and boiler on a circuit with no RCD.

I'm assuming you dont have TT system, if you do you'd need to add a

100mA time delay RCD in front of that CU configuration.

If you cant afford another penny, at least use a split CU with RCD only covering things that need the RCD. Lights should not be on RCD, nor immersion heater and cooker.

It must be a split CU as the RCD is 63 amp

Regards Jeff

[bigcat]

no, that was mistaken advice. If your RCD doesnt trip when the test button is pressed, its dead.

NT

[bigcat]

ok, just one thing, you need to ensure that a socket likely to be used by outdoor equipemnt is on an RCD to pass inspection.

OK. If you want you can do it in a way that costs you nothing extra now, but leaves room to upgrade the RCD aspect later if you ever need to. The way to do this is just to have a fewe spare ways on the CU so that MCBs can be replaced with the double wide RCBOs later. You may never need to do this upgrade, but a significant number of single RCD protected installs do have ongoing problems.

If youve got time, running a separate wire in for fridge etc would also enable you to put that on a separate non RCD conection as well. if not thermally insulated, a 2.5mm radial needs 20A breaker, or 4mm can be one a 32A breaker.

Just in case theres any unclarity, radial means one wire rather than a ring.

If youre replastering over the wire runs, now is a good time to put some low v cable in as well. Cat5e is ideal, but even 4p/metre bell wire can be quite useful later, for phones, internet, etc. If OTOH youre running wiring thru floor cavities, including string makes it possible to pull more wiring through any time later.

I guess you mean radials, with each item spurred off the previous. Even round pin sockets shouldnt be wired like that.

ohhh.

The neutral feed will be the PME earth. PME is a method of using the neutral as earth as well.

ok, that should work ok, but will not pass inspection. You need a pukka earth rod with inspection pit, and earthy things need to be connected up with the right size wire.

Bathroom equipotential bonding is also required for a pass.

yep. Running it as PME would be better really, and cheaper as theres then no need for a whole house 100mA time delayed RCD.

it is required regs wise - and it matters, as TT earths are often inadequate to blow the MCBs. With a TT earth and less than complete RCd cover, you could have a fault running to earth and the entire earthed system sit there live, waiting to bite everyone, and gobbling power.

Hope you dont get prosecuted for dramatically improving the safety of you and your family :)

NT

[Richard]

Thanks Andrew,

When we moved in (genuinely 5 years ago) the entire installation apart from the freezer spur was fed from an RCD.

I fitted two CUs - one fed from the RCD and the other (lighting, garage

- with own RCD/CU - and freezer) not fed from the RCD.

In doing so I found that the upstairs ring had not been opened and expanded to include the attic - as I presumed because there were only two rings shown in the CU - rather a loop had been taken from the upstairs ring into the attic and back to the same point on the upstairs ring. But the return was not connected - just a live end tucked into the floor void!!!!! And this was done by a building firm when they did the attic conversion.

Maybe this sort of thing is what Part P will stop?

I now have a separate ring for each floor and one for the kitchen.

Should I relocate the oven and immersion feeds? The wiring is (IMHO) beutifully dressed into the (RCD) CU.

Cheers

Richard

[Dave Stanton]

I would not bet on it !!

Dave

[tony sayer]

In article , [e-mail] writes

As I said it seems very likely that you have a short or low ohms between the EARTH conductor and the NEUTRAL, any devices like DVD players or TV's or bedside lights don't even come into this, with respect to earth wiring.

What is happening is the RCD monitors the current into your house on the LIVE line, and compares it with what goes OUT on the neutral line and provided these are the SAME the AMOUNT does not matter (as long as its within the current capacity of the unit) all will be well.

i.e. as long as 100 milliamps are coming in and 100 going out all is well, and if say 50 amps are coming in and 50 are going out all is well to.

But as soon as a fault causes 100 ma in but 70 ma out then this is tripping time!, as is 50 amps in and 49.970 amps out trip time as well!.

Now suppose you or yours grab hold of a live wire somewhere on the consumer side of the install, then the current in on the live is diverted away, to earth, via you or yours!.

There now exists a current unbalance in that less current is now going out via the neutral conductor in the RCD and provided that this current is greater than its rated tripping current, usually around 15 to 20 ma for a 30 ma device, then the trip will disconnect and remove the current so you don't get a shock!!.

Now conversely if the earth and neutral are shorted !together! somewhere the same thing in effect happens. Instead of current flowing back through the neutral conductor of the RCD, it can go directly to earth and thus is "shunted" away from the RCD neutral current going OUT.

Of course if nothing is switched on then no current will be coming in the RCD and nothing will be flowing out either so no current UN_BALANCE can occur.

Now when something is switched on the current that is coming in is supposed to match the current going out. But because some of this is going down the earth line and some through the neutral line via the RCD then with a light load there isn't, as yet, enough current to cause the trip level to be reached. As the current increases then there will come a point when this reaches the trip level, and then the trip fires!.

Try to regard this as the live and neutral conductors in your instal must NOT be connected to earth anywhere on the consumer side of the RCD they can of course connect together on the supply side as with PME systems, and it doesn't even matter whatever earthing system you use but on the CONSUMER side of the RCD, no connections from live or neutral to Earth FULL STOP!!!.....

[Richard]

Ditto!

But it seems that it will, or attempts to, put paid to competant DIY-ers who have the time and inclination to do own-use wiring.

Richard

[Owain]

Except that the separation between Neutral and Earth is carried out within the service intake and must be kept separate from that point on. It is not permitted to use a combined earth and neutral conductor on a customer's wiring.[1]

What I think has happened is the supply network has at some point been upgraded to PME but the service intake has not. The supply authority will have to be asked to provide the PME earth terminal.

Owain

[1] Except for privately generated supplies using earthed concentric wiring, IIRC. Oh, joys.

[scottbrewster]

Good explanation thanks. I have had a good look at my work this morning, and couldn't find any short. I reinstalled everything, this time with the bus bar on the CU cut and up- and down- sockets on the RCD with lights and kitchen not. Everythig works. No trips, and the RCD tests OK. Bizarre. Thankyou all for your help.