Fault finding advice

[studentspark]

Lighting circuit.

L-N 243v
L-E 70v
N-E 57v

Connections at lights OK connections at CU OK

When I have come across this before it has been a snipped CPC at a junction box. Had a good look and can't find any junction boxes that connect this circuit..
By that I mean when the faulty circuit is live and the other lighting circuit are switch off at the MCB the junction boxes I have found have no power.

I have found these junction boxes and a few have snipped cables i.e.

Untitled

• studentspark
• 6 Aug 2010
• 1

It looks to me like some one has just needed the neutral.
Now since this circuit is dead when the faulty circuit is live, I have been thinking this junction box has nothing to do with the faulty circuit.
But could it be?
Impossible for you to answer I guess....
But if I come across situations such as this is it best practice to reconnect the CPC.

Also what do you think is going on in the JB
Thanks

 

[plugwash]

Connections at lights OK connections at CU OK

Do you get those screwy readings at the CU? If not then there must be some sort of fault between the CU and the light fitting you are looking at. You will need to figure out the order of the fittings in the circuit somehow then start testing at each one to narrow down which two fittings the problem is between.

Also what do you think is going on in the JB

My guess is the terminal with two reds and a black is switched live and the one with two reds and a black is permaent live. It's normal to use the red in a switch cable as permanent live and the black as switched live but it wouldn't surprise me someone doing it the other way round).

The lone black cable is probablly a neutral but not sure what it feeds, maybe an extra light that was added later or something.

 

[RF Lighting]

Have you tried a Zs reading on the circuit?

Those voltages suggest that the CPC is not connected to earth somewhere along the line.

As plugwash suggested, check at the incommer and make sure there is a good incomming earth, and that all your voltage readings are normal.

 

[sparkwright]

Also what do you think is going on in the JB

My guess is the terminal with two reds and a black is switched live and the one with two reds and a black is permaent live. It's normal to use the red in a switch cable as permanent live and the black as switched live but it wouldn't surprise me someone doing it the other way round).

Or the black is the switched live, and the the two reds go to two lights.

 

[studentspark]

Thanks.

Yes tried to do a Zs, Megger 1553 wont let me, show the polarity issue on the display - voltage between CPC and neutral.

Have tested at the CU that seems fine, other circuits test OK with good Zs, have not done a Ze yet.

Problem starts before the lights so it must be a junction box between the CU and lights. Bathroom above with tiled floor

On a earlier point if you find a connection with the earth snipped would you reconnect the earth as a matter of course? Its not going to do any harm is it!

 

[SUNRAY]

Could of course read the writing on the cables to identify them.
I suppose its too much to expect that they are all marked though