Earth trip testing

[usenetnospam]

Hi, I don't have the proper test equipment but tested my socket circuits earth trip by connecting a 15w bulb to live and earth (15w is the lowest I could find that draws > 30ma). This test worked and I found a few sockets that did not have an earth and I corrected this.

Next I tested my lighting circuits which don't have any earth wires connected. I am replacing some existing lighting circuit wiring as I have the ceiling pulled down during renovations. I have linked the lighting earth to one of the existing socket earths. I found that connecting the bulb from live to earth did not cause a trip. I also tried with a 60w and 100w bulb. Just to ensure earth continuity I connected the lamp from live on a socket junction to earth on the lighting junction, this did not trip. Is there some wiring that may need to be changed at the consumer panel to get this to work? I wouldn't touch the consumer panel myself but I'd appreciate any advice so I can get someone in if required.

Note that I ensured nobody else in the house would touch any metal objects that might be earth bonded.

Thanks, Declan.

[usenetnospam]

Hi, I don't have the proper test equipment but tested my socket circuits earth trip by connecting a 15w bulb to live and earth (15w is the lowest I could find that draws > 30ma). This test worked and I found a few sockets that did not have an earth and I corrected this. All sockets now cause an earth trip when live goes to earth.

I am replacing some existing lighting circuit wiring as I have the ceiling pulled down during renovations. The old lighting wiring does not have any earth so I am replacing the old twin single wires with

1.5mm twin and earth. I connected the lighting earth to an existing socket junction earth terminal. I found that connecting the bulb from live to earth on the lighting junction did not cause a trip. I also tried with a 60w and 100w bulb and the bulb lit rather than causing an earth trip. Just to ensure earth continuity I connected the lamp from live on a lighting junction to earth on the socket junction, this did not trip. Is there some wiring that may need to be changed at the consumer panel to get this to work? I wouldn't touch the consumer panel myself but I'd appreciate any advice so I can get someone in if required.

Note that I ensured nobody else in the house would touch any metal objects that might be earth bonded during testing.

Thanks, Declan.

[Christian McArdle]

It is normal to not run lighting circuits through an RCD, as the risk of injury from failing lighting during a "situation" (i.e. fire or chopping through a cable with a power tool) is actually much greater than that posed by a circuit without supplementary earth fault protection.

Indeed, it is close to not permissible these days to wire a consumer unit so that the lighting shares an RCD with the socket circuits. There are exceptions.

Christian.

[Vass]

"Christian McArdle" wrote >

Really? I've just had a consumer unit fitted and the sparky has put everything on the RCD side of the unit, including downstairs and upstairs lights is this wrong then ?

[Christian McArdle]

Yes it is wrong, unless it is a TT system and it is all off a 100mA RCD.

The regs aren't particularly clear on the point though, so you could argue the toss on it.

Christian.

[usenetnospam]

Thanks for the reply Christian, In that case how should I provide earth protection on my metal light switches and fittings (other than replacing them with plastic ones) ? Is simply linking the lighting earth to the socket earth sufficient? I want to make the lighting fittings and switches safe but prefer the look of metal switches for the kitchen. If this is the case I suppose I should just test for continuity between the lighting earths and a known good socket earth.

Declan.

[Vass]

Come to think of it, he did say I might want to move the lights over to the non RCD side if they keep tripping the RCD ???? no mention of regs though

[usenetnospam]

** Just repeating this post as I accidentally replied to my own post the last time.**

Thanks for the reply Christian, In that case how should I provide earth protection on my metal light switches and fittings (other than replacing them with plastic ones) ? Is simply linking the lighting earth to the socket earth sufficient? I want to make the lighting fittings and switches safe but prefer the look of metal switches for the kitchen. If this is the case I suppose I should just test for continuity between the lighting earths and a known good socket earth.

Declan.

[David Hansen]

On 5 Jan 2006 06:30:52 -0800 someone who may be [e-mail] wrote this:-

What you need to do is run the new T&E cable back to the consumer unit, where you can connect the protective conductor in the T&E to the earth bar.

Using the protective conductor of another circuit is a no-no. For testing it must be possible to disconnect the whole circuit at the consumer unit.

As you are renovating things, now is a good time to replace all the lighting circuit wiring. You may well be able to use the existing wiring to pull through the new wiring.

[usenetnospam]

Thanks David, I was thinking it wasn't safe to leave it as it is anyway because I suppose an earth fault in the lighting circuit could potentially make other bonded metal live without tripping anything. Perhaps time to call in the professionals as I don't really understand why it doesn't cause the socket circuit to trip when I have power going to earth from the lighting circuit.

Many thanks, Declan.

[Christian McArdle]

Well this is not entirely true. Whilst the earth should indeed be taken back to the consumer unit, there is no requirement to be able to reliably disconnect the earth at the consumer unit. This would be very difficult in some cases, such as a bathroom supplementary bond between lighting circuits and socket circuit for example.

New T&E back to the consumer unit is the way to go (or singles if the original is in conduit).

Christian.

[usenetnospam]

I'm curious, if you do have an earth fault on the lighting circuit (lets say the earth goes back to the consumer unit earth bar). If nothing trips how will anyone know that the fault has occurred. Surely it can't run like that indefinitely?

If I can't run a new cable back to the CU is it better to have no earth at all on the lighting circuit or to take it from the socket circuit? I suppose I could take an earth off the lighting circuit now and connect it to the CU whenever I renovate that area of the house (may be years from now). I suppose plastic switches would be required until I get it properly earthed.

[Christian McArdle]

In the real world, an earth fault will be a dead short, not via a light bulb. Therefore, there will be a dead short between the lighting circuit live and the earth. When designing the installation, the earth loop impedence is calculated, which gives the expected earth fault current. This must be more than 5 times the MCB rating for the circuit. An MCB will disconnect instantaneously at this multiple.

All circuits should be designed like this, even socket circuits. The RCD then provides supplementary earth leakage protection to the socket circuits, as it is more likely to have a non-short circuit earth fault, such as a child sticking a screwdriver into the live whilst touching a radiator. However, in the event of a dead earth short, both the MCB AND the RCD should have a chance of cutting the power.

Yes, you may have to wait. Alternatively, I heard a rumour that some ranges of metal light switches are available double insulated and do not require earthing.

Christian.

[usenetnospam]

That makes it much clearer for me, I was forgetting the difference between my bulb test and a real world short !

Just to satisfy my remaining curiosity, I take it an RCD only trips when power from it's own circuit goes to earth (as opposed to my test which was live from the lighting circuit to socket earth).

Thanks a lot for the advice. I'll sleep much better at night with a clear picture of how it all works :-)

Declan.

[David Hansen]

On Thu, 5 Jan 2006 15:26:22 -0000 someone who may be "Christian McArdle" wrote this:-

It was simplified. However, we both said to take the T&E back to the consumer unit, which is the correct answer.

[Dave Plowman (News)]

An RCD looks for an unbalanced current between neutral and line - which of course may mean a 'short' to earth. But it will also protect on a two wire circuit with no earth.

However, if you have a split load CU and the lighting circuit is on the non RCD side running a lighting load to an RCD protected circuit earth won't cause it to trip.

[usenetnospam]

Thanks again Christian/Dave/David. I now fully understand what's going on and what I need to do (or get done).

Cheers, Declan.