Advice and suggestions requested on random cable Crystal ball needed

[Sadsyndrome]

Hello All
Strange question for you, I know you don’t have crystal balls but can anyone give me any clue on the following…

I have mid eighties house (since new) and extended early 1990s. I’m just checking out the wiring (visually) prior to CU change. The original board was Wylex RCD protecting 1 Ring, 2 Lights, 1 Immersion,1 Cooker.
I extended ring from CU at the time of extension. It did have CH gas boiler that was moved from kitchen during extension. I was just looking at CU to see where wires come in the back so I can choose the right height of the new junction box that I will be using when moving CU.

I can see the the cables for circuits mentioned above but also another 2.5mm (that has been cut off? I can’t remember doing that and can’t remember what it was connected to if it was me. The cables go into capping and plastered over in cupboard under stairs going vertically up I.e they were there from new.

I don’t have a tester, other than multi meter. I just checked continuity between the 2 ring main conductors that are currently connected to mcb (just to check they are in fact the ring main cables. Thankfully that was OK. But the strange random cable has around 8v volts on the red wire to earth with all mcbs on, it increased by about 1v for each mcb I turn on. The meter also shows 14v from the neutral wire to earth.
Wtf could it be and why would it have volts on? I’m guessing it may have been a spur from ring main at cu but why would they do that from new?? As I said this cable was in house from new so nowt to do with me

Any ideas from houses you have come across from that era what this extra cable may have been for? I don’t want to hack plaster off wall to trace it

 

[snowhead]

If the induced voltage, 8v, is increasing as mcb's are turned on then it would suggest the mystery cables may run close to those cables.
Re the Neutral, that may suggest it's still connected to the main neutral bar in the C.U.

Is the mystery cable cut off in the consumer unit?

Count the number of neutrals used on circuits verses the number connected to neutral, maybe someone has only cut off the live.

 

[littlespark]

Worked on plenty old houses and always get surprised.

So ring mains for sockets, (rfc nowadays) is a pair of 2.5mm
Lights would be a single 1.5mm
Cooker, probably a 6mm

The immersion would be a 2.5. Usually went to kitchen first, then to the tank location. Would this random cable be the old hot water tank?

As above, it seems to be only partially disconnected.
I’d make it safe. (Connectors in a box) and deal with it when the cu is changed. The electrician needs to check all the outgoing circuits beforehand, so it will be found and disconnected fully then.

 

[Sadsyndrome]

Thanks for quick replies much appreciated. Yes, the cable is disconnnected in CU… both red and black wires, must admit didn’t check earth (sorry, cpc). The 6 cables going vertically are in 3 pieces of plastic capping. (LightsUp + 1 ring main leg), (Immersion + random), (LightsDown + 1 ring main leg) with approx 30mm gap between each piece of capping.

Actually that point about Immersion heater makes me wonder. On ours I assume the cable goes directly to fused spur in airing cupboard next to HW tank. I just wonder if those houses without CH had a cable going to a switch in the kitchen first (like littlespark said),
to give homeowners option to turn off HW without going upstairs? Houses at the time were either, no CH or an option to do it, or in our case I think the builder just decided to do it. Maybe the earlier version of our house was an option hence the additional cable.

I hope that’s right because it would mean I didn’t cut it off and forget

 

[littlespark]

I’m just thinking back to my childhood home, which was built in the 70’s, just like me…
Originally it had a hot blown air heating system from a big cupboard downstairs. Before it was later fitted with storage heaters, then later again gas CH.
It would have needed some form of electrical supply for the heating and fan.

 

[Lucien Nunes]

There are ways to locate hidden cables but involving test gear you don't have, such as tone injectors and tracers, or involving Heath-Robinson methods that I can't recommend on a forum. With the system completely de-energised it is sometimes possible to roughly guesstimate the length using a multimeter capacitance range, if yours has one.

 

[Sadsyndrome]

There are ways to locate hidden cables but involving test gear you don't have, such as tone injectors and tracers, or involving Heath-Robinson methods that I can't recommend on a forum. With the system completely de-energised it is sometimes possible to roughly guesstimate the length using a multimeter capacitance range, if yours has one.

I like heath Robinson methods.

 

[newfutile]

I am wondering what they could be as well now

 

[Lucien Nunes]

I did once post on here about hacking a VGA cable and using a laptop and monitor as a time-domain reflectometer to measure a cable length. It worked really well. Lucien's Heath-Robinson cable break location trick - https://www.electriciansforums.net/threads/luciens-heath-robinson-cable-break-location-trick.185239/ but unfortunately the picture links in that thread are broken.

If you protest that it would be easier to get the proper test equipment, I used this as an example of using available resources to solve problems. The first time I did this, IIRC I was in an office building during the night. I had a laptop and there was a monitor available and was able to get an answer in 15 minutes when I couldn't just call a friend with a TDR set. Two other examples:

Find a hidden open-circuit cable by injecting wideband noise and searching with a detuned AM pocket radio. A noise source could be a dimmer or universal motor (small appliance like food mixer) with its suppression chokes bypassed. I have a Victorian medical induction coil that works very well for the purpose. Or, if the cable is earthed at the far end e.g. one of a number of SWA runs between cabinets, inject AF tone from an audio amp via a stepdown transformer and search with a pair of headphones and coil (I used the field coil out of a motor.) and was able to identify the correct cable mid-duct when it couldn't be traced physically.

Obviously the more random electrical junk lying around, the more likely that there will be the means to conjure up a piece of redneck test-gear to solve the problem at hand.