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 :doh

[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]

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 ?

[macabethiel]

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 :doh

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.

[jims-terrano]

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]

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?

[solarman216]

sounds to me like you have two lives, one on a time switch and the other on a manual switch to override for permanently on, can you locate these controls? if so operating them in turn will identify the two lives, what sort of sign is it? does it have two separate lights (one bright, one dim for instance) Rick

[jims-terrano]

But if there are multiple lives, the possibility of old sun faded cables. Do these facts alone ring alarm bells and ask for the cable to be replaced. I understand what you've said about three phase, I'm not a sparky and never had anything to do with three phase. If the cables are so unknown I still think renew and replace.

As I said with my first reply if you touch the cable you are responsible for it as far as authorities or insurance companies are concerned.

Just think part p and if youre not treat with extreme caution as you could get the book thrown at you if something went wrong even in the future.

[solarman216]

sorry Clive your last line was off the bottom of the screen when I read your post, Rick

[macabethiel]

sounds to me like you have two lives, one on a time switch and the other on a manual switch to override for permanently on, can you locate these controls? if so operating them in turn will identify the two lives, what sort of sign is it? does it have two separate lights (one bright, one dim for instance) Rick

I am wondering if the reading between earth and black could just be an inducted voltage as the cable run from building to sign is about 300 feet long. That said could it be that the Neutral showing some voltage is not unusual if there is a phase difference in the generation area?

I am definitely not connecting anything until I am 100% sure which is Neutral.

When its a bit warmer I am going to have to spend some time checking the outer wall of the building. Definitely only one Live and that is when the timer is in the on position.

There is only one cable that feeds the lights they are normal economy screw in bulbs. I cant rule out that at some time in the past there was a different sign or signs and that might explain the extra cable.

[rayf3262]

Your reading of ~50V between earth and black suggests that you have found the neutral black conductor. Your measurements are consistent with a TNS supply. However I would advise that you check.

This voltage will vary with the type of supply you have:
TNS where the supplier (DNO) provides a separate earth connected at the substation.

TNCS where the DNO only provides only phase and neatral.
The local installation earth being bonded to the incoming neatral by the DNO.
This system used to be known as PME or protective multiple earth, reffering to the fact that the neutral conductor is connected to earth at multiple points on route to the sub.

TT where the DNO provides no earth, and a local earth connection (spike in ground) must be established and maintained. Protection is frequently provided by a 100mA RCD for these supply types.

If the sign had flourecsent lights, it is quite common for " UV from these to cuase colour fade.

[macabethiel]

Your reading of ~50V between earth and black suggests that you have found the neutral black conductor. Your measurements are consistent with a TNS supply. However I would advise that you check.

This voltage will vary with the type of supply you have:
TNS where the supplier (DNO) provides a separate earth connected at the substation.

TNCS where the DNO only provides only phase and neatral.
The local installation earth being bonded to the incoming neatral by the DNO.
This system used to be known as PME or protective multiple earth, reffering to the fact that the neutral conductor is connected to earth at multiple points on route to the sub.

TT where the DNO provides no earth, and a local earth connection (spike in ground) must be established and maintained. Protection is frequently provided by a 100mA RCD for these supply types.

If the sign had flourecsent lights, it is quite common for " UV from these to cuase colour fade.

I am slightly re-assured by your thoughts on Black being the Neutral as its in line with what I thought was the case. The sign was flourescent some years ago so would explain trace of Red colour on the Yellow live terminal consistent with fading over many years.

I checked the voltages a couple of times over a gap of about half an hour and there was some drop in the voltage between Black and Earth but it seemed to be in the 40-50 volt range. Likewise the Live cable showed a voltage of between 230-245 Volts.

Your terms TNS, DNO,TNCS etc are a bit greek to me but it is what I know as PME does that sound right ?

I will persist with trying to find where the cable enters the building and is connected to the supply.

[rayf3262]

Your terms TNS, DNO,TNCS etc are a bit greek to me but it is what I know as PME does that sound right ?

I will persist with trying to find where the cable enters the building and is connected to the supply.
TNS = DNO separate (S) supplied earth from sub.
TNCS = PME, earth Combined via nuetral
DNO = Supplier eg. Eastern power networks

[macabethiel]

TNS = DNO separate (S) supplied earth from sub.
TNCS = PME, earth Combined via nuetral
DNO = Supplier eg. Eastern power networks

So if its PME and that's now called TNCS does that point to black being Neutral as it has a lowish voltage reading ?

[rayf3262]

So if its PME and that's now called TNCS does that point to black being Neutral as it has a lowish voltage reading ?
In all probability the black is neutral, I would suggest checking continuity of this with a known neutral elsewhere in the installation.

On a TN-CS (PME) supply the local installation earth is directly connected to the incoming supply neutral, as such you should normally expect a very small voltage differential between N & E, with a maximum loop impedance of 0.35R

On a TN-S supply, the earth goes all the way back to the sub as a separate conductor, so the N & E voltage differential here is likely to be greater.
This type of supply can often be identified by an lead sheathed incoming cable with the local installation main earth connected to the lead sheath.
The loop max. loop impedance here can be as high as 0.85R.
A reading between N & E of ~50V would suggest this type of supply.

[ian's terrano]

remember the old sparky saying red to red, black to black , blue to bits !!! 300 feet is a long length of cable they might have doubled up on the cable to stop volt drop depending on the size of cable and what load was on it.

ian