Voltage drop

[defadog]

I have a pumphouse about 80m from the house with a 10mm supply cable although no longer used. Nearby is a shed which I believe shares the supply. However, suddenly getting massive no load voltage drop from 238 to 220 in the shed next to it, and with only a tiny load drop to 198. Bizarely, when I turn the board off in the shed it then drops to 125v. The cable must be split underground, becuase the supply to the shed is much smaller guage. Would failure of an underground junction box / corroded contacts cause this? Have only experienced problems over the last couple of weeks.
My garage comes off the same supply, but is only 30m away from the house - no voltage drop at the garage board at all. My assumption is a fault downstream of garage, most likely an underground junction box fault. Any ideas how I prove it before I dig the garden up?

 

[JohnW2]

... However, suddenly getting massive no load voltage drop from 238 to 220 in the shed next to it, and with only a tiny load drop to 198.

It's not really possible to get a 'no-load voltage drop', since the drop can only be due to current flowing through it.

More generally, I would suggest that before you contemplate doing any digging that you probably ought to get an electrician to make some measurements and try to work out what is going on.

Kind Regards, John

 

[JohnD]

wait for frosty weather, and look for the melted patch.

You must of course turn off the supply to the suspect cable until it is repaired.

 

[defadog]

Thanks john..that would be my intent but will probably hire a cable detector to work out where everything is first. One question...small distribution unit, supply two sockets and a light on two breakers. With a meter across live and neutral at the main switch, meter reads 220v with the main switch "on" but no load on the sockets or lights. Without moving meter probes, switch board to "off" and the meter drops to 125v. Cannot figure that out at all. Is there an obvious fault?

 

[JohnW2]

wait for frosty weather, and look for the melted patch. You must of course turn off the supply to the suspect cable until it is repaired.

The patch of frost won't melt if the supply is turned off! As I said, I think the OP probably needs an electrician.

Kind Regards, John

 

[JohnW2]

With a meter across live and neutral at the main switch, meter reads 220v with the main switch "on" but no load on the sockets or lights. Without moving meter probes, switch board to "off" and the meter drops to 125v. Cannot figure that out at all. Is there an obvious fault?

That would seem to make little/no sense! What side of the switch did you have your meter probes on - the 'supply' side or the 'load' (switched) side?

Kind Regards, John

 

[defadog]

That will be a long wait! Supply is isolated. Cannot figure out what would cause fully armoured cable to fail...hence my assumption on a junction box somewhere.

 

[defadog]

John, had probes on supply side..hence my confusion. To be sure I disconnectedthe supply entirely and re measured on open cables...same result (125v) Simply cannot see how thatvwould happen...for a near 110v drop I would expect significant current leakage..but none is apparent. And I was running 2.5Kw log splitters from this board just 6 weeks ago. Bloody powerful mole if the cable itself is damaged!

 

[JohnW2]

John, had probes on supply side..hence my confusion. To be sure I disconnectedthe supply entirely and re measured on open cables...same result (125v) Simply cannot see how thatvwould happen...for a near 110v drop I would expect significant current leakage..but none is apparent. And I was running 2.5Kw log splitters from this board just 6 weeks ago. Bloody powerful mole if the cable itself is damaged!

Hmmm! Obviously without intending any disrespect to you, that sounds ridiculous - something crazy is going on

Even if some massive current were producing a massive voltage drop (which you would almost certainly 'know about' by now!), it would make no sense for that voltage drop to only appear when you turned a switch 'off'! Are you really saying that you have 125V at the end of the (disconnected) supply cable, but that that rises to 220V when you connect it to the switch and switch the switch on? It almost sounds as if the electricity (well, most of it!) is 'coming from' the load side of the switch!

Have you recently successfully run any significant load from the socket?

I'm totally lost - and I can but repeat my suggestion that you 'need an electrician'!

Kind Regards, John

 

[zebedee2001]

I suspect capacitive coupling is going on here. Can you measure the live relative to earth as well as neutral....

 

[DetlefSchmitz]

A standard multimeter has a very high impedance so measurements on switched off circuits will just be picking up stray voltages. You need to measure with something having a deliberately lower resistance. A 10k resistor for example, across the meter will enable much more useful readings. However be careful as this will potentially expose dangerous voltages.

 

[JohnW2]

I suspect capacitive coupling is going on here. Can you measure the live relative to earth as well as neutral....

Yes, if the incoming 'supply' cable is actually 'dead' that could explain where the 125V measurement is coming from, but it still makes little sense that the voltage rises to 220V when connected to an unloaded CU (unless, as I said, the CU is getting a supply from somewhere else). That's partially why I asked whether 'useful electricity' is currently available at the socket.

Anyway, what you suggest would certainly be interesting - as would measuring the apparent voltage across the load side of the switch (when it is 'off').

Kind Regards, John

 

[SUNRAY]

It may sound stupid but did you measure the voltage on the load side of the main switch with it on and off?


EDIT; Ignore, I hadn't realised John had asked.

 

[defadog]

Update. Could not make head nor tail of the problem. Chased back to underground joint. Resin filled strsight through...which looks like it had been diy adjusted to permit a three way!

 

[defadog]

Plastic cover smashed off, and wrapped up in a pair of waterproof trousers! Resin appears to have cracked bottom RH side, and the small earth cable is completely clear of the resin bottom left hand side.