Can I confirm how to test for and how to find a borrowed neutral?

[HappyHippyDad]

What does everyone think about the actual figure of 0.97ohms? Could this be a borrowed neutral (In which case the figure 0.97 ohms must be going through the load somewhere) or with a figure that low does it have to be 2 neutrals actually connected?

 

[timhoward]

...deleted (brain asleep)

 

[Pretty Mouth]

From your description, I'd say neutrals are directly connected together somewhere. IME not many loads have very low continuity across them like that (I think perhaps a compressor might be one example).

If they were connected through a load, it would likely have to be through 2 loads. Eg N1 - load1 - L - load2 - N2. But then you're measuring continuity across 2 loads, unlikely to be very low.

Or if N1 - load - N2, then the load wouldn't work as no potential difference across it. Again, unlikely.

 

[SJD]

Would seem to be directly connection somewhere

An interesting one I had a few years ago, the L&N from a PIR on a socket circuit was connected to a L&N from a light circuit at an outdoor light. Intent was the light switch overrode the PIR when required. The N was always interconnected, while the L connections only got interconnected when the PIR operated and the light switch was on at the same time!

 

[HappyHippyDad]

Well this is looking like an odd one then.

What possible reason could there be for an actual neutral connection between lights and sockets! I suppose the DIY homeowner could have just got confused with a few junction box and put all the black conductors together.

I shall update when I find it.

 

[timhoward]

What possible reason could there be for an actual neutral connection between lights and sockets!

I think this one is highly likely to be two sources of supply to something.

I like the idea mentioned of using a wander lead and checking sockets neutral to lighting points, and finding lowest reading, then checking lighting neutral to sockets neutrals and finding lowest reading. In theory you are then have the two joined points.
Otherwise, breaking down the sockets circuit won't take long.
Good luck!

 

[Dave OCD]

With the two circuit CPCs separated [removed from the earth bar] do you still have the very low reading between the neutrals ?

 

[HappyHippyDad]

With the two circuit CPCs separated [removed from the earth bar] do you still have the very low reading between the neutrals ?

That's an interesting point Dave. I'm afraid the cpcs were still in the MET. I should have tried with them out. I got a minor ticking off from @westward10 some years ago for IR testing without the cpcs in the earth bar and now its second nature to leave them in. Of course, during fault finding it would be fine, even necessary to remove, so that you get more information.

That was a pretty long winded way of saying...I didn't do it.

 

[Lucien Nunes]

The test with the CPCs disconnected would be mainly to find out whether the N-N fault is actually a manifestation of two N-E shorts, one on each circuit, linking the neutrals together via the CPCs. However, we already know that there are no solid N-E shorts, only that 0.1MΩ that drew your attention in the first place. So while a test with disconnected CPCs might help trace the low IR, it won't shed any light on the N-N interconnection.

Returning to the original title and the following comment:

In your example. If the line had been taken from a switch on the lighting circuit and the neutral from a nearby socket, how would that show continuity between neutrals? I realise there will be a resistance figure showing up between L and N as not all loads will be disconnected, but are there any appliances that would be as low as 0.97ohms?

This observation is bang on the money. An N-N resistance of an ohm or two involving a lighting circuit is not going to be a borrowed neutral because no load will have such a low resistance, and a simple borrowing puts two loads in series between N-N as per @Pretty Mouth. The lowest load resistance one is going to encounter in a domestic environment is likely to be a large motor or transformer of a few ohms - saw bench, welder etc. - or a shower at half a dozen, but these are the exceptions.

An accidental interconnection of two lighting neutrals is quite easy to visualise in a system with assorted junction boxes but it's harder to see why a socket-outlet circuit would be involved if only because the meeting of 2.5mm² and 1.0 or 1.5 is likely to draw attention. Lights fed from a fused spur tangled up with the normal lighting circuit? Central heating controls? Extractor fan? Automation? Floorboard nail through two cables?

 

[timhoward]

Mad idea - as there are two N paths back, two resistances in parallel, would putting mA clamp meter on the socket neutral and turning each light on in turn result in the highest reading when the lowest resistance path is increasingly in use?
The low power consumption of modern LED bulbs might make this idea a non starter….
There also may not be a material difference between the ‘wrong route’ and the higher resistance path of the smaller cable that is the correct route.