Thought it was the deredded borrowed neutral

[bulmer]

2 lighting circuits, 2 RCBOs. Checked for the borrowed neutral, insulation resistance between neutral of each circuit and proved OK. So though to myself, grand job, crack on with the rest of the tests, connect up, paperwork, then off to the pub for a couple and watch the footie.

However, when testing insulation resistance between L-N, L-E and N-E, I get a dead short 0Mohms on both circuits. I don't understand, surely a dead short would have been an issue a long time before on the old fuse board, which was a BS3036 jobbie.

What stumped me though was if both circuits are put into just 1 RCBO, it resets and if they are put into their own RCBOs, either one will not reset. Why is this, especially if i'm seeing a dead short whilst insualation testing?

 

[ianmacd]

All 3 (ln le ne) gave dead shorts? Is it possible something is left in? I guess you are saying if they are in their own rcbo's they do not reset but if you stick them in both they are fine? It could still be bollowed neutral as they do not need to be interconnected.

 

[sellers]

could try all insulation tests with both circuits disconnected from the board?

 

[bulmer]

could try all insulation tests with both circuits disconnected from the board?

My thoughts too, so I tried it and still no joy.

 

[bulmer]

All 3 (ln le ne) gave dead shorts? Is it possible something is left in? I guess you are saying if they are in their own rcbo's they do not reset but if you stick them in both they are fine? It could still be bollowed neutral as they do not need to be interconnected.

All 3 ian. RCBO will only stay on if both circuit are in the same one. There is one fitting that has bulbs in that are blown, but have made sure its isolated whilst testing. (Had to leave them in as the fitting is about 22 feet up in the middle of a landing.)

 

[springcrocus]

Aerial booster in the loft? Smokes or alarm system? Light on somewhere? Shaver point? Quite a few possibilities, I would think.

 

[ADS]

2 lighting circuits, 2 RCBOs. Checked for the borrowed neutral, insulation resistance between neutral of each circuit and proved OK. So though to myself, grand job, crack on with the rest of the tests, connect up, paperwork, then off to the pub for a couple and watch the footie.However, when testing insulation resistance between L-N, L-E and N-E, I get a dead short 0Mohms on both circuits. I don't understand, surely a dead short would have been an issue a long time before on the old fuse board, which was a BS3036 jobbie.

What stumped me though was if both circuits are put into just 1 RCBO, it resets and if they are put into their own RCBOs, either one will not reset. Why is this, especially if i'm seeing a dead short whilst insualation testing?

This wouldn't show up a borrowed neural

 

[bulmer]

There was an alarm, but wasn't used. So this was removed totaly from the house whilst on my quest to sort the isssue.

Would any of these items cause bad ins res to earth though?

 

[Lurch]

2 lighting circuits, 2 RCBOs. Checked for the borrowed neutral, insulation resistance between neutral of each circuit and proved OK. So though to myself, grand job, crack on with the rest of the tests, connect up, paperwork, then off to the pub for a couple and watch the footie.

That wouldn;t have neccesarily shown up a borrowed neutral as the light with the offending borrowed neutral could have been turned off and even if it was on that's not how you'd test for it anyway, or unless you had some other items on the one circuit on as well you still wouldn;t get continuity between both neutrals. Unless you happened to know where the borrowed neutral was it's very difficult to actually test for it.

However, when testing insulation resistance between L-N, L-E and N-E, I get a dead short 0Mohms on both circuits. I don't understand, surely a dead short would have been an issue a long time before on the old fuse board, which was a BS3036 jobbie.What stumped me though was if both circuits are put into just 1 RCBO, it resets and if they are put into their own RCBOs, either one will not reset. Why is this, especially if i'm seeing a dead short whilst insualation testing?

I think these are 2 issues, the low IR and RCBO tripping issues. I think the fact the RCBO's trip when one or the other is turned on points towards there being a borrowed neutral. A borrowed neutral wouldn;t affect IR reradings of N-E or L-E though.

 

[SPECIAL LOCATION]

2 lighting circuits, 2 RCBOs. Checked for the borrowed neutral, insulation resistance between neutral of each circuit and proved OK. So though to myself, grand job, crack on with the rest of the tests, connect up, paperwork, then off to the pub for a couple and watch the footie.However, when testing insulation resistance between L-N, L-E and N-E, I get a dead short 0Mohms on both circuits. I don't understand, surely a dead short would have been an issue a long time before on the old fuse board, which was a BS3036 jobbie.

What stumped me though was if both circuits are put into just 1 RCBO, it resets and if they are put into their own RCBOs, either one will not reset. Why is this, especially if i'm seeing a dead short whilst insualation testing?

Hello Bulmer.. three quick observations m8;

1/

Testing between N of two circuit does NOT necessarily prove or disprove a Shared neutral problem.

Did you have ALL light switches ON when you tested?

Borrowed (shared / bridged) neutral does NOT mean there is a direct continuity between two neutrals!

It means that one section of neutral conductor can be energised from two circuit breakers.

(Therefore BOTH must be off to safely work on the circuit which the neutral is part of.)

Typically it is a load is bridged between two circuits.. e.g. Live from one circuit, through load to neutral of another circuit.

So you will NOT get a direct continuity between the neutrals during testing.

Paths can be made through loads that are switched on.. e.g. continuity through one or more bulbs on the circuit that the Live is derived from.

2/

0Mohms Is NOT a dead short.

e.g. if you were getting 0.0Meg...

one more decimal place could give 0.01Meg which is 10,000Kohms

10K is not a dead short in any electrical sense

230v across 10K = 0.023amps,

23ma, which would have had negligible effect on a non RCD fuse box?

3/

99.99999999% You HAVE got a shared neutral here.

You need to go back and test a few more bits IMHO..

Try looking at this little pickture wot I dun with the coloured pens..

(Don

 

[bulmer]

Cheers guys, all very helpfull. Going back tommorrow now to look at it again, who knows a fresh pair of eyes may see the issue right away.

Specs, I see what you say regarding a dead short, thanks. Looking at your nice drawing, I'm now beginning to think I may have been looking at the problem all day.

Roll on tommorrow.

 

[ianmacd]

Put the meter in low ohms range as if you were doing an R1+R2 test and see what the difference is. It could be very bad insulation somewhere or a neon. I would suspect the low form L&N is due to the accessories still being connected so maybe try testing L&N combined to E too.

 

[ktguk]

2 lighting circuits, 2 RCBOs. Checked for the borrowed neutral, insulation resistance between neutral of each circuit and proved OK. So though to myself, grand job, crack on with the rest of the tests, connect up, paperwork, then off to the pub for a couple and watch the footie.However, when testing insulation resistance between L-N, L-E and N-E, I get a dead short 0Mohms on both circuits. I don't understand, surely a dead short would have been an issue a long time before on the old fuse board, which was a BS3036 jobbie.

What stumped me though was if both circuits are put into just 1 RCBO, it resets and if they are put into their own RCBOs, either one will not reset. Why is this, especially if i'm seeing a dead short whilst insualation testing?

double check the commons on the 2way lighting

 

[sellers]

put your meter on ohms scale, test between N and L of opposing circuits, operate all switches, at no point should you have continuity, if you do, you have a borrowed neutral at that point.