Lo Volts

[Dave Plowman (News)]

Got a call from a mate tonight - said his lights had gone dim. ;-) And so it proved - the volts in the house read 173. 25 year old town house in a terrace of four in South London. Everybody out in that block - but got a neighbour on the other side in another block in and checked it there

- 233v. The services and CU etc were in an 'outhouse' at the front door with no light - but everything seemed ok. Nothing hot or smoking. Switching on the heaviest load - the cooker - made only about a volt or so difference. Tried a fan heater too - the fan only just turned.

They'd been having probs with their boiler recently - and tonight it doesn't work at all. Wonder if it was low volts causing the original problem there? Not something you'd expect the average service engineer to check.

So what's the likely cause? I'd have expected a sub station to feed the whole area.

[Dave Liquorice]

The phase feeding that small terrance has a problem. One of the other phases feeding the other block is OK. With 173 incoming I'd switch off at the main breaker and call the REC and report it. Some kit may object by letting out the magic smoke with such a supply. Other kit will start just draw more current and your cooker/fan heater experiment indicates that there is plenty of current available, just that the voltage is low.

[fred]

In article , Dave Plowman (News) writes

It's three phase in the street and they rotate the phases through the street 1-2-3 to balance the load so the neighbour is likely on a different phase.

The neighbour 3 doors down will likely have the same fault.

Don't know the likely cause but as it's not load dependent it's not likely to by a high resistance joint anywhere near him. Maybe one of the ex-distribution network regulars will have a better idea but it's def one to report to the local regional supplier tomorrow.

[Andy Wade]

AIUI the phasing usually goes L1-L2-L3-L3-L2-L1-etc., so the pattern isn't as simple as you suggest. And that's for new estates where all the services are single-phase and have been connected up at the same time. In older urban areas you're more likely to have a mixture of 1-ph and 3-ph connections with different properties having been connected at different times, so the phasing might best be described as pseudo-random, notionally balanced...

Don't wait that long, it's a potentially dangerous situation - report it right away.

[John Rumm]

I would second the report ASAP and isolate vulnerable kit advice...

Filament lamps will be ok if very dim, but things like fridge freezers (especially older ones) will be vulnerable to damage, boilers may malfunction etc.

[The Wanderer]

Report it straight away. Dunno if it's still the case, but I have the idea in the back of my mind LEB used to run lots of their lv systems all interconnected, no open points, so it could be a fuse gone in the closest s/s with a backfeed coming from a much greater distance.

Why they did it I have no idea, I know fault levels were always a problem in the London area.

[Tim Watts]

The Wanderer wibbled on Friday 12 March 2010 07:18

Yes - this is good enough reason to call the LEB on their emergency number which is on the bill, or here:

area).

Just say lights are remaining dim and you suspect low supply voltage. They'll be out pretty fast.

[Dave Plowman (News)]

Got sorted last night. Fault was only reported at about 21.00.

Interestingly, the call centre bloke said a low volts report wasn't an emergency in the same way as no power at all. Even with the volts being low enough to stop near everything but lights working. I'm sure they make these things up as they go along...

[The Wanderer]

Yebbut. Trained monkeys an' all that.....

[Andrew Mawson]

The danger issue with low voltage is that fuses will not blow within the specified times - with low volts and another fault fire is a distinct possiblity.

[Mitch]

It sounds like it is a TNCS supply (based on 25 yr age) in which case if the N is about to go open circuit this is dangerous. Might be worth testing if anything more than 10v N to earth with load on; if so I would isolate supply until fixed. Danger is that if N is failing/broken on DNO side you just have a L and dependant on how good your PME is you might end up with live metal surfaces.

Alternatively you might just have a drugs farm next door!

S

[Harry Bloomfield]

Dave Liquorice has brought this to us :

I would not expect an entire terrace to be on one single phase. The other block might well be on a completely different supply. Whilst they do feed single phase premises one phase then the next phase, then the last one - there is no absolute rule to be relied upon. They might well have fed two adjacent premises of one phase, then the next two of the next phase and so on.

Low voltage (a brown out) could simply mean they have had to load shed in a certain area due to a cable fault, or it could be the supply cable itself has developed a fault. Another possibility (especially if the voltage is varying) is that there is a break in the neutral So the load is simply balancing itself across the phases - which means if you have lower than spec voltage, someone else has higher than spec voltage.

[Harry Bloomfield]

It is often wired similar to a ring circuit, where one leg helps support the other and should one leg fail or need some work, there is always a second one able to provide for some load capability.

[Colin Stamp]

[snip]

That doesn't sound right. How can the fuse know anything about the supply voltage?

Cheers,

Colin.

[Andrew Mawson]

within

A fault generates less fault current at the lower voltage resulting in a fuse taking longer to blow.

AWEM

[Colin Stamp]

Ah, so I guess you're talking specifically about earth faults in systems where fuses are the only protection.

Actually, I wonder how low voltage would affect the operation of an RCD. Would a 30mA RCD become a 60mA one at 120V?

Cheers,

Colin.

[The Natural Philosopher]

as the wiring its designed tp protect takes longer to catch fire..

More stoopthink from the half knowledgeable..

[The Natural Philosopher]

No.

the average RCD works at under a volt: It's a current sensor only.

He's talking bollocks.

[Tim Watts]

The Natural Philosopher wibbled on Friday 12 March 2010 20:35

There's also the factor of disconnect time to prevent an excessive touch voltage remaining for more than the specified time - to do with the potential for administering a lethal shock.

But and RCD if present will sort that out.

[Dave Liquorice]

Mr Plowman wrote "25 year old town house in a terrace of four ..." not a whole street of two up two downs. B-)