Mains Electric Cable puzzle.

[macabethiel]

I am helping a friend repair an illuminated sign that was vandalised by either kids or possibly cable thieves.

The armoured cable has been ripped out of the junction box leaving it live and exposed to the elements. I have fitted a new junction box but due to the lamp wiring having been ripped out I am having trouble identifying the correct connections.

The armoured cable is about 20 years old and consists of two yellow cables, a black cable and a blue cable. One of the yellow cables is sleeved as earth, the second yellow cable is clearly live and has signs of faded red marking on it. This yellow cable lights up my Neon Screwdriver and tests at mains voltage on my Multimeter.

I am trying to work out if the neutral is black or blue and up to now have been unable to trace where the wiring enters the building.

Is there any reliable way of establishing which is the neutral from the junction box end or is it essential to find the other end of the cable where it enters the building ?

[jonela]

Unfortunate that you cant trace it back, It sound like someone has just used a piece of three phase cable they had lying about. safety is obviously utmost;
Do you have a multi meter or a voltage tester yet ?, If you put the red + to the wire you think is live,and then you then put the other terminal- in turn to the other wires, if you get a reading or a light it is ether a neutral or an earth if there is no light it is another live or not connected to anything at the other end. to check test this against one that you now know is neutral or earth if it lights its another live if it dosen't its dead. but remember to check the switching also,if somone has used somsort of two way switching , whats dead when you thinck the switch is on could come live when you think it is off

[jims-terrano]

I know armoured cable is costly but I'd be wanting to replace it and know it's safe. At the moment you got wiring that someone in the past has not done correctly and it is supplying power to the outside world. If anything went wrong you'd be held responsible if not by your friend then the authorities.

[macabethiel]

Originally Posted by jims-terrano

I know armoured cable is costly but I'd be wanting to replace it and know it's safe. At the moment you got wiring that someone in the past has not done correctly and it is supplying power to the outside world. If anything went wrong you'd be held responsible if not by your friend then the authorities.

So am I wrong in thinking its just old cable with superceded colours ?
Is this consistent with someone using the completely wrong cable in the first instance ?
Are the colours consistent with having used old 3 phase cable ?

[jims-terrano]

Originally Posted by macabethiel

So am I wrong in thinking its just old cable with superceded colours ?
Is this consistent with someone using the completely wrong cable in the first instance ?
Are the colours consistent with having used old 3 phase cable ?

I'm not a qualified electrician but as far as I'm aware yellow hasn't been used for live.

[jonela]

It all sounds a bit Heath Robinson to me!. If I was you I would try to trace it back to the time switch and if that's not possible just run a new supply to it.

[Lazy-Ferret]

Originally Posted by jims-terrano

I'm not a qualified electrician but as far as I'm aware yellow hasn't been used for live.

3 phase used to use Red, Yellow, Blue, and Black, with Green/Yellow as Earth, but on Armoured cable, often the screen was the earth, so no colours.

Just as a guess, has the Red faded from the light, and gone a Yellow(ish) colour?

4 coloured wires were also used for 2 way change over circuits, like stair landing lights, where you have 2 switches controlling 1 light. This was Red, Yellow, blue, and Green/Yellow.

Either way, the 4 colours are all different.

Does the sign have 2 sets of lights, maybe brighter when open, and dim when closed, that would account for why it has 4 wires?

[macabethiel]

Originally Posted by jonela

Unfortunate that you cant trace it back, It sound like someone has just used a piece of three phase cable they had lying about. safety is obviously utmost;
Do you have a multi meter or a voltage tester yet ?, If you put the red + to the wire you think is live,and then you then put the other terminal- in turn to the other wires, if you get a reading or a light it is ether a neutral or an earth if there is no light it is another live or not connected to anything at the other end. to check test this against one that you now know is neutral or earth if it lights its another live if it dosen't its dead. but remember to check the switching also,if somone has used somsort of two way switching , whats dead when you thinck the switch is on could come live when you think it is off

I have put the Red probe of my Multimeter on the Live Yellow and if I put the Black probe onto either the Blue or the Black cable I get a 230/240 volt reading likewise if I put the probe onto the Earth connection (Sleeved Yellow as earth) I get 230/240 volts. There is a slight reading between Earth and the Black cable it was about 50 volts with nothing between the Earth and the Blue cable. I am inclined to think that Black was used as the Neutral due to the age of the building and wiring but am not prepared to connect the supply back until I am 100% sure.

Looks like I am going to have to look harder for where the armoured cable enters the building. I know the identity of the trip fuse as its marked on the fuseboard but obviously the armoured cable junction box is elsewhere. There is a timeclock for the sign but again no obvious sign of where the wiring runs as its all plastered in.