Reverse Polarity Mains Socket -- How Dangerous?

[Andrew Thelwell]

Hi,

Note: This post is talking about UK (220-240V mains supply)

My girlfriend and I recently moved into our newly-purchased house. We wanted to get the electric meter changed but when the guy came out to do so, he reported a live reading on the neutral block in our meter box. This was potentially dangerous, he said, and he called out the mains engineers. They investigated and couldn't replicate the problem. They asked if we'd experienced any problems in the house and we hadn't.

Since then we've had three occurrences of the RCDs in our fuse box tripping. I today bought a standard socket tester and tested every socket in the house. I've found the culprit to be the mains outlet in our spare bedroom, which appears to be incorrectly wired, showing a live/neutral reverse on the tester.

My questions, then...

1) Would this explain both the apparent live neutral reading in our main meter box and the tripping of the RCDs? I always thought RCDs were only concerned with earthing faults, not live/neutral reverse polarity type problems

2) I've been using the sockets in that room with no real probles apart from the trips. All equipment (TV, XBox, amplifier, phone charger) have worked OK thus far. Is the socket likely to be causing any harm?

3) Is this potentially dangerous?

4) Is getting this fixed a big (and ergo, expensive) job?

Your help would be much appreciated as I've had surprising difficult finding any info about this on the web.

Many thanks

Andy

[R D S]

Would just swaping the wires in the plug socket sort it?

[Andrew Gabriel]

Well, it's not a problem that goes away by itself. One of the engineers is wrong.

Yes - there's no fuse in the live.

It's easy, but this and and RCD trips indicate you should have the installation inspected. I don't necessarily mean a full inspection and test, but get an electrician in to give it a quick check, and advise if anything more substantial is required.

[David Hansen]

On 26 Nov 2006 10:50:10 -0800 someone who may be "Andrew Thelwell" wrote this:-

Yes.

a) switch off the main switch at the consumer unit. Check that the lights on the socket tester are now out.

b) unscrew the socket.

c) unscrew the live and neutral wires from the terminals. These are probably coloured red and black respectively, but may be coloured brown and blue respectively. There will be one, two or three wires to each terminal.

d) look at the back of the socket, where there will be terminals marked L and N for the live and neutral wires.

e) back off the screws fully and then insert the wires into the correct terminals.

f) check that all wires are secure, pushed in fully and will not pull out. That includes the earth wires, which you should not have touched.

g) screw the socket back, checking no wires are trapped. One wire to each terminal is a lot easier to get back than three and a double socket is easier than a single.

h) switch on and check the lights on the socket tester. Check the other sockets.

i) have someone who knows how to do wring look over the installation. Someone who couldn't be bothered to wire up a socket correctly will not have bothered with any other work they did, or tested it.

[dom]

1) I can't see how a single socket with live/neutral swaped can explain a "live" reading on the neutral at the electricity meter. What exactly did he mean by "live"? I doubt both phase(live) and neutral could be sitting at 240V. Nor should it be responsible for RCD trips(which happens because the current on the phase and neutral is different - i.e. leaking somewhere).

In his position I would have flicked off the main isolater and to see if the problem cleared, then switched on the MCB's one by one.

Significant voltage on the household neutral (which is what I think he meant) would suggest to me that it is poorly tied to the suppliers neutral - and I'd want that fixed urgently.

2) Yes, if a faulty piece of equipment was plugged in. Don't use that socket

3) Yes, there is something odd happening and needs professional investigation.

4) Probably not a major expense, if it's an isolated stupid mistake by a professional or an isolated case of uninformed tinkering. If however it's the tip of the iceberg of a rewire by a madman, then very urgent and somewhat expensive work may be required.

[dom]

You're assuming it's a single isolated fault. Given that there's some bizarre behaviour, I wouldn't do a quick fix until I was quite clear about other possible faults.

[John Rumm]

No, and no. Did your first engineer say what he was comparing the neutral with to delcare it "live"?

(if it appeared live compared to the live conductor, then that would suggest the polarity of your supply is incorrect (unlikely!), however if he compared it to earth, it may tell you nothing of significance).

This is true. They detect inbalance between the current flowing in the neutral and live conductors.

In the normal course of operation it will not be explicitly dangerous - many countries electrical systems do not maintain correct polarisation, and yet they remain resonably safe. The real danger comes in the event of a fault that causes the fuse to blow. Even with the fuse blown the device could still remain "live"

In itself (the reversed polarity) no, it is trivial. The RCD tripping however could be due to any number of possible causes, and it will be harder to identify the culprit.

There have been many threads on tracjing down the cause of spurious trips on RCDs in this group in the past. It may be worth googling for a few...

[David Hansen]

On 26 Nov 2006 12:28:19 -0800 someone who may be " [e-mail]" wrote this:-

Indeed. However, fixing this one fault means there is one less problem to worry about.

I don't see how fixing this one particular fault will make any others worse.

[dom]

What if phase, neutral and earth have all been rotated at both ends?

Maybe phase is connected to the neutral position at this socket and earth to the live position (and all the colours are wrong because they've been jumbled at the consumer unit end too).

I've actually seen this done on a boat. It was a corporate event, my company had hired the yacht, and the skiper proudly gave me the extension he had made up with a UK socket strip on one end and a schuko on the other. I suspect he was red/green colour blind.

[Ron]

He's a cowboy trying to rip you off. A new house should have an inspection certificate, so ask for a copy, then contact the person that arranged your survey and complain. You can't have both live and neutral on a neutral block or your main fuse would blow. What he has measured is the voltage between neutral and earth. If you have fluorescent lights this sometimes happens. It can happen if you have a faulty TV or video.

That would not cause a problem anywhere else, but should have been checked before you bought the house. The socket is not connected to anything else unless it is in a loop. Just rewire it or get the seller to correct it.

Depending how it is wired and what you use might make one circuit breaker trip.

It will not cause harm, but you will be switching the neutral instead of the live.

Not unless you take something apart and think it is switched off !

No, get a screwdriver - open the socket after witching the mains off and make sure that the L terminal is going to RED or BROWN wires and N terminal to BLACK or BLUE. If they are correct then check inside the consumer unit - then check for any junction boxes.

There are lots of cowboys about, so don't be conned in to buying new consumer units or having your home rewired. Don't bother checking for reputable trades people with trading standards either - they don't bother checking them out and are not allowed to tell the public even if a company does have a bad record!

[Adrian C]

Sigh.... Just beware that 'ron' has a poor understanding of electricity judged from previous ramblings in usenet, and is mostly trolling...

-- Adrian C

[John Rumm]

I can't see any basis for that conclusion.

He never said the house was new, only that he purchased it recently.

You know this how?

It can also happen when the sky is cloudy. There is however equally little connection between the observation and your suggested "cause"

Depends on what survey you paid for.

Which is highly likely...

What makes you say that?

[Dave Plowman (News)]

My feeling is to trust the engineers over a meter swapper. Those are simply 'trained' for the one job and usually not even employees of the electricity supplier.

First, can you be clear about this? Most houses only have the one RCD in the 'fuse box'. So do you mean MCBs (circuit breakers)? An RCD is easy to spot - it has a test button on it.

I'm afraid that isn't the 'culprit' for anything.

No and no. RCDs aren't concerned with earth faults - they work by looking for imbalance in the line and neutral current flow.

Yes. A 13 amp lug fuse is in the line for a good reason. With this socket it is effectively in the neutral.

It *should* be merely a matter of swapping the wires to the socket.

Like all things you have to ask the right questions to get sensible answers.

[<me]

The house is newly purchased and obviously the electrics are sub-standard/faulty. Take up the matter with the builder and get them to come and do a full electrical test and remedy any faults.

[chris_doran]

When I encountered this once, it transpired that some of the sockets in the house had their neutral/live terminals on left/right respectively and some, presumably a different manufacture, had them the other way round. The sparks had simply wired them all up the same way without looking. A point to check when replacing a socket.

Chris

[Andy Wade]

More than one RCD suggests that it might be a TT installation (i.e. it has its own earth rod and doesn't rely on an earth connection provided by the supply company). This could account for a variable voltage reading between the supply neutral and the local earth.

Is the house out in the sticks and fed via a long overhead line?

[Guy King]

The message from contains these words:

Newly purchased doesn't necessarily mean newly built.

[Andrew Thelwell]

Hi all, I'm the OP,

First off thanks for all the replies -- great to get so much feedback.

Let me clarify a few points... this should help with the discussion:

1) The house is newly-purchased, not a new build. It's a 1930's semi-detached

2) I may have misled when I referred to RCDs (sorry!). We have two large rotary switch type things on our consumer unit labelled "Earth-Leakage Circuit Breaker". There is apparently one on each 'loop' of the house. It is these that have tripped

3) The problem with the meter when the guy came to change it was that he was as follows: He had one of those 'mains tester' screwdrivers with the LEDs in the end that should only light up on when touched to a live terminal. However, he got it to light up on ALL FOUR terminals attached to our meter (+ in, - in, + out, - out, yes?). I don't think there was an suggestion as to the 'size' of the current though. He also got it to light up when he pressed it to the big grey block (neutral block) in the meter box, which apparently it shouldn't do. This happenend both with our consumer unit main isolator turned ON and OFF. It stopped, however, when he pulled the main fuse out of the meter box.

Two mains engineers from Central Networks came out that night and tested thoroughly for almost 2 hours in our meter box and around our roof (we have overhead supply). They found nothing out of place at all. They asked if we'd experienced any oddities inside the house, which we hadn't. However, since then we have seen our Earth-Leakage Circuit Breakers (not RCDs, sorry!) trip three times. Two of these times have been due to turning on the one socket in the spare room. One other time the other ELCB tripped when my girlfriend turned on the microwave. All sockets in the house test out fine using a normal B&Q socket tester, except the one in the spare room, which shows only one light, supposedly "L/N Reverse".

Hope this helps -- your further feedback would be much appreciated based on these additional details.

Thanks!

Andy

Guy K> The message

[meow2222]

Ohhhhhhhhhhhhhhhh. Those. In that case we know a couple of things straight away. This is oldish wiring, and uses TT earthing, meaning a local earth rod with high earth impedance.

Big house then?

ELCBs rarely trip in error. I think I know whats going on now.

OK, then we can safely ignore anything he says

And we can safely conclude nothing is out of place at all with your supply as far as the CU or fusebox.

OK, I think I see the problem clearly. An ELCB will trip when it sees large earth currents (unlike RCDs which trip on small earth currents). This means you have a major fault in whatever appliance caused it to trip when the socket was switched on. So the microwave and whatever was plugged into the spare room socket should be taken out of service now, and some checks done on them later to confirm or deny theyre the problem.

is the swapped wires behind a socket, which is a trivial matter easily fixed in 2 minutes.

Your 2 faulty appliances would not have tripped anything at your last house, which I can conclude had low impedance earthing and no RCD. However on a high impedance earth, which you have here, such faulty appliances are not safe to use at all.

NT

[meow2222]

Which brings us to the next question, how best to test your probably faulty appliances. A low cost multimeter can be used to test resistance from E to L&N pins on the plug, while its not plugged in, and any controls on the item are set to on. However if the thing has any electronic power controls, as many appliances now do, this wont test it fully. Plugging an item into an RCD into a socket would quickly detect any appliance with an earth leakage fault. Since it looks like you've got at least 2 bad appliances, I'd get an RCD plug, make it into a short extension lead, and have it to test things with.

Its a minor plus that youve got ELCBs and not an RCD, or you'd have had bigger problems.

NT