Continuity between neutral and earth

[leenowell]

Hi All

I have broken into a ring main to extend it to another room. Essentially cu tting the return from one socket and then connecting each side of the exten ded ring to one of the cut ends.

I test the extended ring and it was sound (ie only continuity between the c orresponding pairs of wire). I have just connected it to the main ring and did a quick continuity check. Live is ok but it seems that neutral (blue) a nd earth now have continuity. Given I tested the extended ring it would app ear that the original ring suffered this problem. However if this was the c ase wouldn't the RCd at the MCU have tripped before.

Any ideas?

Thanks

Lee

[T i m]

I'm not sure this is the case in every installation but I think the neutral and earth are connected together somewhere (CU?) and further, the earth is actually supplied via the neutral on the incoming supply sometimes?

So, as long as you don't have any N to E [1] or L to E leakage current on your side of the RCD you should be ok?

Or something like that .. I'm sure someone will be along shortly to give it to you more accurately. ;-)

Cheers, T i m

[1] Often seen when cutting though a cable even with that cct MCB switched off.

[Andy Burns]

Depending on your earthing arrangement, this is probably normal, do you know type of supply you have?

The type determines whether or where your neutral meets earth ...

[leenowell]

Thanks both. Looking at the wiki it looks like it is tn C's.

The incoming wire goes to a fused black box and outbof that comes over neutral and earth. Earth goes to cu and the live and neutral go to the meter and then to the cu via a new isolation switch.

[Andy Burns]

You're *probably* correct, though depending what connections are made inside the black box (cutout) it could be TNC-S or TN-S, as some of the photos show (those with/without links from N to E inside the box).

[leenowell]

The black box has the electric company fuse in it so don't want to take the cover off to check :(. The incoming cable comes in from the bottom earth c omes out to the right and live and neutral out to the too where the fuse is . In coming is a single black cable.

Does that help in any way. Would both give the symptoms I describe?

[leenowell]

Hi

Yes I think it is TN CS. This picture looks very much like my installation

So sounds like neutral and earth continuity is correct for my installation.

Thanks for your help

Lee

[Brian Gaff]

I was always told that Neutral was supposed to be earth, or almost so. Otherwise we would have two lives. Brian

[harry]

cutting the return from one socket and then connecting each side of the ext ended ring to one of the cut ends.

corresponding pairs of wire). I have just connected it to the main ring an d did a quick continuity check. Live is ok but it seems that neutral (blue) and earth now have continuity. Given I tested the extended ring it would a ppear that the original ring suffered this problem. However if this was the case wouldn't the RCd at the MCU have tripped before.

Neutral and earth are bonded together back at the substation. It's important that this is the only place they are bonded.

[John Rumm]

N&E continuity (of some sort) is correct for the vast majority of installations (assuming you are doing the test with the circuit connected to the CU[1]). With TN-C-S the link is in your property. With TN-S its back at a substation. With TT its via the general mass of the earth, and may have a higher resistance, but is still good enough to make a continuity test "pass" in many cases.

[1] If you want to test your "new" combined circuit in isolation, then you need to disconnect it at the CU end and test there. That way you will be free of the influences of the supply itself and any EQ bonding connections.

[John Rumm]

That is true.

That is certainly not true for TN-C-S (PME) supplies which are the default "standard" for installs these days.

(the clue is in the "M"!)

[Huge]

In our last house, there was an earth at every pole on the overhead supply.

[charles]

It's earth at the substaion, but its volage wrt earth at your home will depend on how evenly the phases are loaded. 10v wrt earth isn't uncommon.

[Dave Plowman (News)]

Assuming you are testing with the main switch off (double pole) you have a fault. If testing with only the MCB etc (single pole) off, normal.

[damduck-egg]

One of which would then be a negative wouldn't it?

In the past I think some people referred to the black , later blue wire in domestic wiring as the negative which was wrong but possibly influenced with tinkering with cars where batteries have a positive and negative pole, or maybe for old timers it was a hangover from the days of DC mains.

The yellow 110 volt plugs intended for use with transformers in the UK to give a 55-0-55 volt supply usaully have both pins identified with a moulded L rather than an L and an N but I reckon few people using or even wiring one have noticed.

G.Harman

[dr.s.lartius]

cutting the return from one socket and then connecting each side of the ext ended ring to one of the cut ends.

corresponding pairs of wire). I have just connected it to the main ring an d did a quick continuity check. Live is ok but it seems that neutral (blue) and earth now have continuity. Given I tested the extended ring it would a ppear that the original ring suffered this problem. However if this was the case wouldn't the RCd at the MCU have tripped before.

ISTM that a person who does not know that neutral and earth MUST be connect ed together in at least one place is a person who is not qualified to do su ch work and the resulting installation is at risk of being in breach of the Building Regulations. See

182-h.htm, VI.

[tabbypurr]

Dc mains used the 2 phase 3 wire system, so half of end users got + and half got -ve dc supplied.

NT

[Huge]

And in a PME installation, at lots of other places.

[leenowell]

Yes only the MCB was switched off. Rest of the CU was on

[John Rumm]

Yup, that only isolates the live. N is still connected to the neutral bus bar and hence the supply neutral via the main switch. E would be connected to the supply neutral either at the PME head end (and multiple other places) or at the transformer if your supply was TN-S. (it will also be connected to an independent earth in the case of TT or any installation with main equipotential bonds to incoming metallic services).