UK Old cable in dwelling light circuit

[RealNetworks]

We are currently renovating an old house after recently moving in. Whilst purchasing the property we received written assurances that the original 1920/30s wiring had been fully upgraded and subsequently tested.

After removing all of the downstairs ceilings, it is clear than new electric 2.5m2 and 1.5m2 cable/wiring has been installed, however there would appear to be an old and rather thick set brown fabric cable routing upstairs. Using a ‘non touch’ volt tester I was able to identify that the cable was live and part of the house light circuit. (The full house is on a single ring light circuit is, not ideal but it works). I noticed that this old cable routes into a round, brown junction box with the lid missing. The cable would seem to be an old 2 core cable with no earth present. Both cores are large ‘silver’ cables with several strands twisted together. I’m no electrician and therefore will refrain from touching it or proding around, however I would be grateful if someone would kindly advise if this cable should be changed, or indeed what it is. Everything works as it should, I’m just concerned that if we install new ceilings and it then needs to be replaced. The house is an old property and uses a TT system setup which I believe was upgraded with the installation of the EV charging unit. Any advice would be very much appreciated.

 

[telectrix]

Indeed you would think. However I am assuming it has been wired as a circuit as there are two wires in to MCB. Surly you wouldn’t except 2 radial circuits to be wired in to the same MCB? Nevertheless I am pressuring but will have a better idea tomorrow. But as said above, we’re just going to have it replaced before having new ceilings installed. Better to be safe than sorry. I’m just curious to know what type of wire it is as I’ve never seen its like before. It’s using a 6am MCB with an RCD so I hope the electrician will only need to replace this wire with a new run of 1.5mm2 cable but in all truth I won’t know until I have the EICR completed.

not unusual to have 2 lighting circuits in 1 MCB, but then, by definition, they both become 1 circuit.

 

[RealNetworks]

This suggests there may be further issues.

I suspect there may be what's called a borrowed neutral between what was originally two lighting circuits and instead of fixing it they have put both cables into the one MCB to make it safe.

I very much think you are right. I would rather 2 separate circuits, 1 for upstairs and 1 downstairs to allow lighting in the event one area goes down. This wouldn't be an option if both wires are using the same MCB. The only correct way to do this is to replace the entire circuit with new cable. As we’re eliminating the circuit we wouldn’t need to use 3 core with cpc and therefore only need normal 2 core with cpc. Having looked at some of the 2 way switches, there are some single 1mm cables with 2core and cpc. I’m suspecting tge borrowed neutral is going to be in one of these switch/connections.

 

[Lucien Nunes]

That does look like concentric cable, rather than twin with all the red insulation missing. Inner conductor, plain rubber insulation, outer conductor, cambric tape and compounded braid sheath. Very rare stuff, I've been collecting cables for 30 years and never seen any of that before.

Interestingly it appears to be 7/.029, going by the size relative to the JB terminals and the fact that .029" was the smallest strand generally used in non-flexibles. This is a larger cable than needed for any normal lighting circuit and, allied with the fact that this type of cable was rarely used in domestic work, I wonder whether it was installed separately to the original installation perhaps as a repair, by someone who had some surplus on hand.

Concentric cable achieves its compactness and enhanced rating in part by having reduced or no insulation over the outer. This imposes the requirement that the neutral conductor is reliably earthed which was not always done until after the war and the amendments to the 11th edition, and sometimes required permission from the supply company. Use of concentric wiring would tend to be a deliberate choice, very often following a proprietary system, not one made on a whim by the installer. It might have been done to the specifications of an engineer who was personally keen on the idea, more likely in the 20's than 30's.

I would be very grateful for an opportunity to save whatever of it you can get out intact, for the museum. Old rubber cables don't always withstand removal without crumbling, but the longer the pieces you can get, the better. I will happily pay for you to pack it in a box and send it down. Even if you can only get short stubs as samples, they are still interesting.

In the meantime when I am next in the museum store I will try to find a reference to it in a catalogue.

 

[pirate]

Thank you Lucien...really interested in such things, and love your knowledge.

 

[Aaron b]

That looks like a bit of VIR which is in surprisingly good condition if it is. As far as I know it shouldn't ever have been installed outside of conduit if it is VIR as its the equivalent of modern singles.

I didn't realize it was supposed to be in conduit in floors. I've seen it through conduit in the walls but usually fed through small holes in the joists around here.

 

[RealNetworks]

That does look like concentric cable, rather than twin with all the red insulation missing. Inner conductor, plain rubber insulation, outer conductor, cambric tape and compounded braid sheath. Very rare stuff, I've been collecting cables for 30 years and never seen any of that before.

Interestingly it appears to be 7/.029, going by the size relative to the JB terminals and the fact that .029" was the smallest strand generally used in non-flexibles. This is a larger cable than needed for any normal lighting circuit and, allied with the fact that this type of cable was rarely used in domestic work, I wonder whether it was installed separately to the original installation perhaps as a repair, by someone who had some surplus on hand.

Concentric cable achieves its compactness and enhanced rating in part by having reduced or no insulation over the outer. This imposes the requirement that the neutral conductor is reliably earthed which was not always done until after the war and the amendments to the 11th edition, and sometimes required permission from the supply company. Use of concentric wiring would tend to be a deliberate choice, very often following a proprietary system, not one made on a whim by the installer. It might have been done to the specifications of an engineer who was personally keen on the idea, more likely in the 20's than 30's.

I would be very grateful for an opportunity to save whatever of it you can get out intact, for the museum. Old rubber cables don't always withstand removal without crumbling, but the longer the pieces you can get, the better. I will happily pay for you to pack it in a box and send it down. Even if you can only get short stubs as samples, they are still interesting.

In the meantime when I am next in the museum store I will try to find a reference to it in a catalogue.

Fascinating and extremely interesting. Thank you for taking the time provided all of tris information. And of course, it would be my pleasure. From the research I’ve done regarding the property, it originally had gas lights fed through led pipes. However in 1929 it was wired with electricity. We are located in a rural location. Obviously in the 2000s it was then once again rewired. The wire is in generally very good condition and I’m sure I will be able to retrieve a good 4-5 meters if that was of any use to you. I would be happy to provide you with some information documenting the property and the chaps who built it, a rather sad story. I will pm you once it has been removed.

 

[davesparks]

I very much think you are right. I would rather 2 separate circuits, 1 for upstairs and 1 downstairs to allow lighting in the event one area goes down. This wouldn't be an option if both wires are using the same MCB. The only correct way to do this is to replace the entire circuit with new cable. As we’re eliminating the circuit we wouldn’t need to use 3 core with cpc and therefore only need normal 2 core with cpc. Having looked at some of the 2 way switches, there are some single 1mm cables with 2core and cpc. I’m suspecting tge borrowed neutral is going to be in one of these switch/connections.

The most common borrowed neutral situation is a single core live taken from the downstairs lighting circuit to the 2 way switch at the bottom of the stairs. Then a twin is taken from downstairs switch to upstairs switch and another single, the switched live, taken from their to the light above the stairs. This light would then have its neutral connected to the upstairs lighting circuit. Thus the two circuits become linked, and quite dangerous for anyone working on it.

 

[Lucien Nunes]

I will be able to retrieve a good 4-5 meters if that was of any use to you. I would be happy to provide you with some information documenting the property and the chaps who built it, a rather sad story. I will pm you once it has been removed

Thanks!

The fact that it is rural adds another possible dimension, which from your research you might be able to dismiss or not, and that is whether the original wiring was powered from a generating plant rather than a connection to the public supply. If the property had its own lighting plant, the voltage might have been well below modern mains voltage. 50V and 110V were popular choices, and especially in the case of 50V, larger conductors might have been used hence the 7/.029 for lighting.

I didn't realize it was supposed to be in conduit in floors. I've seen it through conduit in the walls but usually fed through small holes in the joists around here.

We need to be careful and clear with the terminology here. 'VIR' (Vulcanised India Rubber) normally referred to single-core insulated cables without sheath, intended for installation in conduit just like modern singles e.g. 6491X. The external surface of VIR is the braid, not the rubber itself.

'TRS' (Tough Rubber Sheath) normally referred to rubber-insulated and sheathed cables made like modern flat twin and earth, with two or three insulated cores, optionally an uninsulated earth, and an overall extruded rubber sheath. This is not intended to be run in conduit. The term TRS is still used with the same meaning today, mostly in reference to flexibles e.g. H07RN-F

The OP's cable, if it is what it appears to be, is something of an oddity. The outer conductor doesn't need insulating so there is no rubber around it, only the braid. This gives it the appearance of single-core VIR when it actually serves the purpose of 2-core TRS. There were flat twin rubber-insulated cables with braid overall but these are also rare. I do however have samples of a couple of varieties.

 

[Aaron b]

Was the lead covered flat cable used at the same time as vir singles? I've never seen it terminated.

 

[Mike Johnson]

The volt stick pro with the shielded tip is very sensitive and will not indicate on the neutral side of T&E its an advance on the normal volt sticks: Volt Stick instant non-contact voltage testers - https://voltstick.com/products/selective

 

[Lucien Nunes]

Was the lead covered flat cable used at the same time as vir singles? I've never seen it terminated.

Generally yes, but lead fell out of use earlier. Where cost was the main consideration, TRS was cheaper than lead and increasingly found acceptance for domestic work. Where durability was the main consideration, VIR in conduit had always been and remained the system of choice. TRS was superseded by PE/PVC T+E, VIR singles by PVC singles. Lead sheaths kind-of fell through the gap in the middle for building installations, although continued to offer advantages for underground cables.

Lead twin was normally jointed using pressed metal JBs with screw-down clamping bars at the cable entries to connect the sheath to the box. At fittings such as roses and switch pattresses, continuity could be maintained and/or a connection could be picked up off the sheath using screw-on clips, or by pairs of flat bars that could be clamped around multiple cables. These were called 'bonding bars' because at that time 'bonding' meant maintaining earth continuity generally, rather than specifically connecting extraneous conductors to the equipotential. For ironclad fittings and switchgear there were saddle-type clamps that screwed into the conduit entries.

Because lead flows easily, these various clamping methods did not remain very tight. Although continuity was normally maintained, it was recognised as a weak spot and for high-class lead work even by the early '30s there was the option of lead twin with a bare tinned copper CPC inside, in contact with the sheath. This was terminated in the JB in the usual way offering a reliable CPC connection to both the sheath and the next point.

 

[telectrix]

The volt stick pro with the shielded tip is very sensitive and will not indicate on the neutral side of T&E its an advance on the normal volt sticks: Volt Stick instant non-contact voltage testers - https://voltstick.com/products/selective

got one. ideal for when you have multiple cables in back of a switch. bloody expensive though, was >£30.

 

[RealNetworks]

The most common borrowed neutral situation is a single core live taken from the downstairs lighting circuit to the 2 way switch at the bottom of the stairs. Then a twin is taken from downstairs switch to upstairs switch and another single, the switched live, taken from their to the light above the stairs. This light would then have its neutral connected to the upstairs lighting circuit. Thus the two circuits become linked, and quite dangerous for anyone working on it.

This is precisely what has been done, although it is initiates im the downstairs hallway which is also controlled by a 2 way switch. It is such a mess. I’ve also noticed that at every junction box the installer has simply twisted the individual CPC together at the back of each box. Whilst it’s not necessary dangerous as such, it is bad/lazy practice. Indeed at 2 junction box on the final ring, the CPC has separated. I’ve replaced each junction box with new Wago box and connected the CPC securely. It really is quite a mess.

 

[Lucien Nunes]

@RealNetworks, Hope your renovation is going OK. Just curious if you managed to liberate this cable yet?

 

[RealNetworks]

@RealNetworks, Hope your renovation is going OK. Just curious if you managed to liberate this cable yet?

Unfortunately very slow!!! It will hopefully be removed over the next 4 weeks and I will make sure that I inform you. All the best