Further investigation required

[fattony]

Doing an EICR in a commercial building, taking on the advice from last post I was doing the IR individually rather than across the entire board, circuits wired in singles so had to locate the correct neutrals when I found this on 2 circuits, wondered if anyone had come across this before:

With Neutral disconnected I measure 400v between N & L1/L2, 0v between N & L3 so I presume its a L3 conductor, I also measure 230v between N & E

Reconnect the neutral and I measure 230v between L1/L2/L3

Need to look further into it but any thoughts appreciated

 

[Marvo]

Maybe that neutral is borrowed somewhere along the final circuit to provide a return for another final circuit.

 

[fattony]

You may be right, its such a simple lighting circuit I'd be surprised but who knows who last worked on it, I made a typo its actually 3 out of 5 circuits like it.

The fluorescent lights are plugged into a galvonised track, there are 3 switches and 5 tracks.

 

[Marvo]

Hang on, I just re-read you first post. Did you disconnect the neutral on a 3-phase final circuit that was still live? If you did it would have assumed a potential approaching that of the phase with the highest load which fits the test results you gave.

 

[fattony]

its a 3p board but the lighting is obviously single phase, switched off the mcb and measured 230v between N-L1/L2, switched the mcb on and measured 400v N-L1/L2 which made me think it was a phase conductor as I measured 0v between N-L3

It was definitely the correct neutral as the lights didn't switch on without it

 

[tony mc]

The results you are getting correct once you disconnect the neutral from the neutral terminal its at the same potential as L3!
What are you expecting between N and L3 then!

 

[fattony]

The results you are getting correct once you disconnect the neutral from the neutral terminal its at the same potential as L3!
What are you expecting between N and L3 then!

Then why on only 3 out of 5 circuits did I measure this?

I wasnt expecting 400v between N & L1/L2 and 230v between N-E

Thinking about it could it be the live returning through the emergency fittings..... hence the 400v N-P and N-E

 

[tony mc]

Have a think about it the Neutral is on the L3 circuit so you disconnect the N and if the cct is energised it is the same potential as L3 so N E 230 volts
N L1 /L2 400 or am I missing the obvious here?

 

[fattony]

Perhaps its me missing the obvious??

With 1L3 switched off and the N disconnected I measure 230v N-L1/L2 and 0v N-E, energise 1L3 and I get 400v N-L1/L2 and 230v N-E which must be the return path from the emergency fittings hence only on 3/5 circuits.....

 

[tony mc]

I would be IR testing the circuit you might find a Neutral to Earth fault on that circuit IMO !

 

[fattony]

IR N-E was fine, but I think something is adrift, further investigation required

 

[rapparee]

you would have been better doing dead continuity tests to identify conductors

 

[infinity]

Perhaps its me missing the obvious??

With 1L3 switched off and the N disconnected I measure 230v N-L1/L2 and 0v N-E, energise 1L3 and I get 400v N-L1/L2 and 230v N-E which must be the return path from the emergency fittings hence only on 3/5 circuits.....

You just answered your own question again, with 1l3 OFF N -E 0 V
When energised N-E 230V! Because the neutral is now live!
What are you trying to prove by even doing this? It's not even a test you should be doing!

 

[fattony]

You just answered your own question again, with 1l3 OFF N -E 0 V
When energised N-E 230V! Because the neutral is now live!
What are you trying to prove by even doing this? It's not even a test you should be doing!

Tested it because I had a strange reading??? if 230v to e was normal it would be on all circuits not just a select few

 

[PEG]

Hi dude,What device are you using to provide those voltage readings?

 

[davesparks]

I hope I'm reading this wrong as I've read this as you having disconnected the neutral of a circuit and then switched the circuit on?
If I'd even suggested that as a testing procedure where I used to work I think I would have been putting my job in jeopardy!

 

[fattony]

Hi dude,What device are you using to provide those voltage readings?

Calibrated Fluke 1651

 

[fattony]

I hope I'm reading this wrong as I've read this as you having disconnected the neutral of a circuit and then switched the circuit on?
If I'd even suggested that as a testing procedure where I used to work I think I would have been putting my job in jeopardy!

In an empty building with that being the only circuit energised, yes I did

 

[PEG]

Calibrated Fluke 1651

...Then this building may be haunted...:icon12:

 

[davesparks]

In an empty building with that being the only circuit energised, yes I did

I can honestly say if you were working for me you'd be lucky to still have a job after doing that.

Maybe I'm being harsh, but it is one of those things which was drummed in to me as an apprentice, never energise a circuit with either the cpc or neutral (if it has one) disconnected!

The only time I've ever really lost my rag with an apprentice is when they did this.

 

[Gavin A]

I suspect you have a shared neutral somewhere on those circuits that's effectively connecting all the phases together when a load is switched on on the circuit, have disconnected all the neutrals from the neutral bar, have a load on L1 and L2, very low load on L3, have created an unbalanced delta system where L3 is acting as the neutral for L1 and L2.

Something along those lines anyway.

 

[fattony]

...Then this building may be haunted...:icon12:

:ack2:

 

[Silly Sausage]

Hi dude,What device are you using to provide those voltage readings?

Calibrated Fluke 1651

Uncalibrated brain :smilielol5:

 

[Knobhead]

Unbelievable.

I despair, I really do.

 

[fattony]

Unbelievable.

I despair, I really do.

I get that it's a live but why not on all circuits

 

[Marvo]

...........What are you trying to prove by even doing this? It's not even a test you should be doing!

I hope I'm reading this wrong as I've read this as you having disconnected the neutral of a circuit and then switched the circuit on?.......

I also didn't understand exactly what tests were being made and why.
Spurious and confusing tests are going to produce spurious and confusing readings I guess. Disconnected neutrals will float, their voltage with respect to other parts of the circuit will be fluid, just stick to the recognised testing regime in GN3. Floating neutrals on a live circuit are highly likely to damage equipment that's connected to the circuit, if you do this on the wrong circuit the damage could run into many thousands of pounds.

 

[fattony]

I also didn't understand exactly what tests were being made and why.
Spurious and confusing tests are going to produce spurious and confusing readings I guess. Disconnected neutrals will float, their voltage with respect to other parts of the circuit will be fluid, just stick to the recognised testing regime in GN3. Floating neutrals on a live circuit are highly likely to damage equipment that's connected to the circuit, if you do this on the wrong circuit the damage could run into many thousands of pounds.

Thank you for the constructive post