Mains Voltage fluctations

[John Geddes]

A few weeks ago, our consumer unit tripped out at 5am - and it turned out that a neighbour had had the same happen to them.

Had I been away for a couple of days or longer, I would have come back to a fridge-freezer full of rotted food.

The Credit Card machine in the village shop helpfully reported three power interruptions that night, and Central Networks (who distribute electricity here in Derbyshire) having initially pretended to know of nothing odd, have now belatedly accepted that something did happen - they say that a neighbouring area's supply had tripped out and reset on three occasions (timings match exactly), which might well have caused variations in supply to our village - but all within their allowed operating limits (of course!).

They are suggesting that I should spend money on getting my 7-year-old consumer unit tested as it must be hyper-sensitive to have tripped out on such minor provocation.

Should I believe them? What can I do?

John Geddes Derbyshire

[Ian Stirling]

Msnip>

Maybe. I assume that your CU has a earth leakage detector, and that's what's tripping, rather than anything else.

Depending on the design of this, and the way the network is setup, large currents flowing in the ground (the large flattish thing the house is built on, not electrical) can cause the leakage detector to trip.

This is not a fault in the CU - it's working as designed. It _may_ be a fault with the installation.

Also, current flowing in the ground wire, from the electricity company, can cause similar problems.

[Chris Cowley]

I don't know if you should believe them or not, but if the problem is in their distribution network then you're fairly powerless I reckon. We have about 2 power-cuts a year on average at the sub-station up the road, each lasting for about 45 minutes to an hour and affecting the street-lights, and (at a guess) 150-200 houses. And this is on the outskirts of London, not out in the sticks somewhere and has been going on for about 8 years.

The 'leccy board clearly aren't interested in fixing whatever the real root of the problem is, they just sent a man out to the substation each time it happens to fix the effect rather than the cause. 2 hours outage per year is probably within whatever service levels they're allowed to get away with. I've given up complaining, and bought a couple of UPS's.

[meow2222]

no

nothing. This will happen with a whole house RCD, does everything go off or just some things?

If you're willing, replacing single RCD with RCBOs would help. Cost around =A3100 in bits. In principle replacing rcd with a v-elcb would also fix it, but theyre no longer regs accepted.

Wiring, appliances and filters all have capacitance. Capacitance causes low leakage at 50Hz. A glitch is much higher frequency, so leakage much higher. Hence the RCD trips.

There is a chance youve got an oversensitive rcd, but thats probably not it, more likely just a single 30mA rcd, which is poor circuit design.

NT

[Colin Wilson]

If you see dips on a regular basis, ask them to fit a voltage recorder. There are guaranteed standard service level agreements that cover this (although I forget the timescale involved) - I think they should have one in within 7-10 days IIRC (I should know this :-} )

[Colin Wilson]

Correct - the current time period before you can start to claim compensation for an outage is 18 hours.

[Chris Cowley]

Ah, I suspected as much - you'd have thought it'd work out cheaper for them in the long run to diagnose and fix it properly, but I guess they have bigger fish to fry.

I keep trying to convince SWMBO that buying a small-ish generator for the back garden would be a good idea (mainly so I could tinker with it), but she doesn't seem impressed with the idea.

[Colin Wilson]

I`m not sure whether multiple outages of a shorter period may class as claimable under the guaranteed standards for loss of supply - IIRC any more than 4 outages in a year bags you a payment, but you have to claim it yourself - its not an automatic payment.

[John Geddes]

It's not the outages that are the problem. We get a couple per year and you cope with this (learning not to buy mains-only telephone answering machines for example). The point is that a normal outage sees the power restored to everything in the house when the main supply is restored.

It is having the consumer unit trip off that is the bigger worry. Even if the main supply comes back after 30 seconds outage, if the consumer unit has tripped, then there is no power to anything in the house until someone realises there is a problem and manually resets the consumer unit.

And if there is nobody at home when the unit trips, the house stays unpowered until someone comes back - to find everything in the freezer wrecked if they have been away for more than about 24 hours.

John Geddes

[nightjar]

Years ago, when industrial disputes were producing regular four hour power cuts, the electronics magazines were full of designs for standby power supplies, usually run off a car battery, to keep freezers and gas or oil boilers running. It sounds as though you need something similar, perhaps a really big UPS?

Colin Bignell

[Tony Williams]

[snip]

Do you by any chance have one of those filter plus surge protection devices, plugged in anywhere? The sort of thing that is built into an enlarged 13A plug top and used to power things like computers or modems, etc.

The one I have has the normal filter components and then a 270V VDR directly across Line-Earth. This VDR appears to do nothing until there is an overvoltage surge, at which time it conducts and pulls an unbalance current through the RCD, tripping it OFF to disconnect the surge.

Do you have anything like this, that may have responded to an overvoltage surge as the power wobbled from being connected/disconnected?

[The Natural Philosopher]

It happens here every time the wind blows or we get a thunderstorm..the power overheads get shorted by tree branches and strikes, and the automatic trips go out, then attempt automatic reset, and if the branch is still there, trip again,and reset again.

The voltage surges are quite injurious and I have learnt NOT to set the computer to 'autoboot' on restoration of power..the quickest way to head crash a hard disc is to remove power when its loading up the operating system doing lots of reads..

And a massive switch on transient is almost guaranteed to flip a 30mA 'whole house'RCD...which is why I stuck a 100mA in mine...

[Christian McArdle]

Can you tell us exactly what is written on the switch that tripped.

I'm looking for anything like:

RCD/MCB/RCBO/ELCB/16A/32A/100A/63A/30mA/100mA/0.03A,0.1A

Christian.

[Broadback]

A bit off subject perhaps. We also frequently get power cuts here. However yesterday the voltage dropped for about 4hours such that lamps were just glowing, the microwave thought it needed setting up, the 'fridge was grumbling, and the burglar alarm became confused! So I switched everything off. Now my question is could low voltage cause any harm, like damage motors that are trying to run but failing?

[Dave Liquorice]

We get the same occasionally, the auto recloser tries three times within a minute to re-enstate the power before locking out. Once locked out it requires a man to come out, isolate/reroute power around the fault (there are two ways of feeding every where around here), find and fix the fault and reset the recloser. This normally takes 6 to 8 hours... Fortunately it doesn't do it every time the wind blows, very few trees along the line routes. B-)

We did have one spate, which was a tree, that would trip the recloser but not often frequently enough for it to lock out. off 2s, on 60s, off 2s, on 300s, off 2s, on 90s, off 2s, on 600s, off 2s, etc it did eventually clear (ie burn the branch back on the tree) but only after about 30 mins of this cycling...

I have a small UPS that will supply power long enough to shut down the PCs gracefully it then powers the iPBX until flat (up to 12hrs or so, depending on how quick I can switch the PCS off). The UPS doesn't restart on power return until it has got back at least 15% battery capacity.

A "whole house" RCD should be a 100mA time delayed one, so that you maintain descrimination between the whole house 100mA one (there for circuit/fault protection due to poor earth provision) and 30mA non-delayed ones for shock protection.

[Christian McArdle]

Yes, although most modern things should be designed for it. Indeed, many (not all) switched mode supplies won't even care and will work normally down below even 100V.

Christian.

[meow2222]

fridge & freezer compressors.

NT

[The Natural Philosopher]

I once worked on a unit that was specified as 48-400V DC or AC up to

400HZ :-)

[Chris Cowley]

All the power 'round here is underground, but there was a big underground fire a few years ago (way after the sub-station first began tripping). I rather optimistically hoped that the work carried out after the fire would solve the problem, but it didn't.

There's a small company up the road who manufacture, refurbish and test electricity meters and I harbour vague (and probably completely unfounded) suspicions that this problem is being caused by them in some way.

Oh yes, I've experienced the same thing. It used to be that whenever the supply was first restored, it would go off again after 10 seconds or so before coming back on permanently (or what passes for permanently around here). Just long enough for my hard disks to spin back up before having the rug pulled again...

[Chris Cowley]

Ta. I reckon there have been times in the past where we'd have qualified for that. Sounds like more trouble than it's worth though. I'd rather they just traced and fixed the problem!