Uh-Oh! Polarity Reversing Again!

[securespark]

Remember this? //www.diynot.com/forums/viewtopic.php?t=69140&postdays=0&postorder=asc&start=0

Well, I've had another case of RP at a different property...this time, unfortunately, we have not got to the bottom of it, so I am unable to satisfy you with the solution!

It is a house with subterranean TT and no incoming earth. The only connection is via the gas pipe, with a Zs of around 9 ohms.

There is no RCD.

Every circuit has RP. Also, the cellar room where the CU was lit up with a volt stick on everything, pipes, walls, floors, ceilings: everything! And the meter was going like stink. I mean really motoring.

Until, I discovered, one very warm 30A socket circuit fuse was removed. When this happened, polarity returned to normal, the meter slowed to a crawl & the volt stick stopped the red-alert.

I went round and disconnected everything I could find throughout the whole house as did the customers, but it was still going like the clappers.

Obviously a problem! Out of interest, I checked the line ring continuity. There were four 2.5 conductors in that fuseway, none of which showed any connection to the others. I then thought maybe there had been a mix up with some other conductors & checked continuity between these four and two other 2.5's in a 15A fuseway (the only other 2.5's in the board). There was continuity between one of the 30A conductors and one of the 15A ones, a reading of 221 Ohms, but >1999 between the others.

There were three things I was puzzling about - when this wiring was in circuit:

Why did polarity reverse?

Why did the CU room light up with a volt stick?

Why did the meter go mad, despite everything having been disconnected?

Could there have been a fault on that circuit with such a resistance that it was acting like a high-current load?

IR for that 30A, four-wire circuit was around 40Meg to earth, method 2, so I suppose that there must've been something there, to give such a comparitively low reading, but none of us could find what it was.

[Loyd Grossman voice] "David, it's over to you..."

 

[ericmark]

Let me think aloud.
If there is an earth fault and no RCD then the earth polarity would be raised to line polarity. So it would appear to be reverse polarity.
9 ohm will allow 25 amp to flow which would not blow 32 amp fuse but would cause the meter to register a high current usage.
So what did a simple insulation test show?
Eric

 

[ericmark]

Sorry missed you did mega test. OK what about something that switches off and on like immersion heater with earth fault? Did you mega neutrals? Of course with double pole isolator off.
Eric

 

[Steve]

Did you condemn the installation?

 

[securespark]

If there is an earth fault and no RCD then the earth polarity would be raised to line polarity. So it would appear to be reverse polarity.

You mean potential?

Sorry missed you did mega test. OK what about something that switches off and on like immersion heater with earth fault? Did you mega neutrals? Of course with double pole isolator off.
Eric

Meggered L+N to E (method 2).

Did you condemn the installation?

YES!!

TT supply with no earth electrode.

No rcd protection.

Earth connection to electrical installation via gas pipe.

No PEB to water.

Gas PEB undersized (imperial eqivalent 4mm²).

No RF continuity.

Undersized conductors on 30A circuits (or oversized CPD's...)

Spurs from spurs on RF circuit.

No grommets, etc etc..........

 

[Adam_151]

I think its safe to say as others have, that nothing is actually reversed, but there is some kind of line to earth fault there, when its in place the earth gets dragged upto the potential of the line conductor, and instruments will report this as RP, as it assumes earth is at earth potential, and from this, the live conductor at the same potential must be neutral and the other conductor thats ~240v away must be the line...

if you want to get a true picture...stick a screwdriver in the lawn a reasonable distance from any electrode, pipe, etc, and measure voltage between this and live neutral and cpc in turn, this gives you a true reference save for any major fault on the DNO side

As to why it didn't show up in IR... either it wasn't there with the circuit de-engerised (perhaps a PIR floodlight spured off the ring with no fusing down, and full of water, and fault doesn't appear until contactor pulls in) or perhaps its just hiding from you! How did you do IR test, did you leave the cpc in the earth bar on the CU, or disconnect it?

Not that it really matters anyway... I assume you are putting in a quote for a re-wire?

 

[ericmark]

Why did the meter go mad, despite everything having been disconnected?

Yes I got polarity and potential mixed up. Sorry. But I would say something has to be connected! It may not be plugged in. Immersion heater? Outside light? or something else which is auto switched i.e. thermostat, PIR, timer. Even a freezer going onto defrost cycle. When it switches on it has earth fault and is connecting line to earth which with 9 ohms on earth connection would not blow fuse but would show reverse potential (Got it right this time) anything using a contactor would not show up on standard insulation test as well what about a sewage pump or the like.

Do tell us what you find. I hate those jobs, on one hand you know you should isolate but in this weather that would make most houses unusable so one really does have a problem.

We have so many conflicting reports on health and safety procedure. We are told we can override the decision of even the managing director of a company and isolate dangerous equipment then also told we are not permitted to remove power in a house! I have questioned before if one with permission one removes power to test and we find a dangerous fault can we switch back on? I would say no! If we spot a fault we may not be able to switch off but also we could not switch back on. In the past I have said I am not permitted to switch the power back on with this fault, if once I leave you switch it back on that's up to you.
How did you get around the problem? Sorry should not ask or at least if not 100% correct don't tell us.
Eric

 

[securespark]

Adam: Disconn'd.

Eric: Left the job after fault-finding for a while & discovering other electricians had been there installing stuff on the circuit that was faulty.

Customers will call them back.

Offer of a quote declined!!

 

[ericmark]

Glad you did right thing I will watch to see what it was.
Eric

 

[Adam_151]

That could be why it didn't show up on the IR then, the fault could have been to an earth on a totally different circuit, or even to to a piece of bonded metal work. If you end up going back I'd probably suggest pulling the live and neutrals out of the board, put leaving the cpcs in, and for each cable testing between L+N together and the earth block.

The 221 ohms thing between circuits sounds like a borrowed neutral, you are reading from line on one circuit, through some kind of load to a neutral of the other circuit, and then through another load (correctly connected this time), on that circuit to the line of that circuit.

if they don't want a re-wire, will they accept a quote for an electrode, main RCD (I know its frowned upon), MPB, and re-jigging into a couple of 15A/20A fuses (though it'll probably work out ceaper to re-wire the sockets than try and work out whats what!)

Did you remember to give them the certificate? http://niceic.org.uk/inc/file-get.a...icate.pdf&RURL=/en/contractor/section.asp||20

 

[securespark]

They wanted their own sparks back to sort out.

We have our own paperwork: rest assured I wrote everything down I'd found, explained it to them & they signed it.

 

[ericmark]

So we may never find out what it was. I notice similar post on DIY Doctor wonder if that's house holder from same job finding out what's going on in his house?
Eric

 

[Spark123]

Could be something as daft as the central heating system - if say the pump or boiler has a fault on it that only occurs when the system is energized and the zone valve operates.
Still waiting for my pair of socks Secure

 

[securespark]

The CH system (DP switch) was switched off at all times.

My numerical friend: I have just twigged to what you are referring!!

You can have some socks with pleasure!

However, as a service to Royal Mail & all members of the public who may come into contact with the parcel, I shall send some new, unsullied ones. Just e-mail your shoe size & address & I'll send some off by return.