An electrical puzzle.

One of the points I was making is that even though the MCB for this particular circuit had tripped, The RCD, because the neutral network will allow current to flow to earth from other live circuits, will still trip.

Its one of the problems with RCD circuits - you can isolate any circuit to work on it but inadvertently touching the neutral of the isolated circuit to earth will trip the RCD and will plunge the whole house into darkness.

Looks like it might be a good job our house has separate RCBOs in the consumer unit for each circuit.

Edited By V8Eng on 09/04/2023 13:52:36

Only good if the RCBO's are double pole type, many fitted are only SP devices which still leaves the problem of neutral to earth tripping of the main RCCB if fitted.

Emgee

The Electrician who did the work said they are all double pole and a main switch in there.

Edited By V8Eng on 09/04/2023 15:18:02

Double pole RCBOs are not normally used. Apart from cost they take upe twice the space.

Hi Dave,

Have you measured earth to neutral voltage at your consumer unit and between earth to neutral at furthest away socket in your property ? .I can’t at moment as on holiday.

Maurice

Edited By Maurice Taylor on 09/04/2023 16:46:54

You wouldn't have a main RCCB unless it is a TT installation and even then if all the circuits are protected by RCBOs, you do not need one.

Edited By Chris Pearson 1 on 09/04/2023 17:26:59

18 mm DP RCBOs are widely available nowadays.

About measuring the neutral voltage, to get the real value it helps to load the neutral with a 10kΩ resistor. Unless you don't have a multimeter designed for mains measurements that can switch to low impedance mode. Otherwise like SillyOldDuffer mentioned before you might measure a ghost voltage.

As often happens here, a question is asked, an answer is given, then a more general conversation starts. Which I like!

I'm now wading my way through the acronyms (yeah, I know, initialisms if you prefer) in an attempt to understand how my house is wired.

The earthing is like this:

Ah, that's a bit truncated on the right, but gas meter goes to water main which goes to the CU.

The CU seems to be locally earthed via the gas/water pipes, but if I've interpreted the label (Earthing Terminal P.M.E. System) on the box connected to the supply correctly my local earth is bonded to the grid neutral at this point. Is that right?

Robin.

Edited By Robin Graham on 09/04/2023 23:46:54

Robin

Your main earth is still provided by the earth terminal at the main fuse. 

 With a PME (protective multiple earth) system the neutral is connected to earth at the substation and the mains cable and individual service cables to each house have a combined neural/earth conductor rather than separate neutral and earth. At the meter position, the main fuse unit (cutout) has a fused live and the combined neutral/earth is connected to two terminals - neutral and earth to give the separate neutral and earth for the internal wiring. The neutral is also earthed at the end of the mains cable.

With a traditional system, the neutral is only connected to earth at the substation ( although it may also be connected to earth at other points if the cable has more recent alterations using combined neutral/earth cables or if PME type services have been connected to the main).

The earth bonding cables to the water and gas services are to ensure that the electrical earth and any other metalwork in the house are kept at the same potential. The downside of a PME system is that if there is a break in the neutral conductor , the voltage on the earth terminal in the properties past this point will be floating relative to true earth. The additional earth at the end of the main is to provide an alternative path and restrict this voltage. 

Stuart

Edited By Stuart Smith 5 on 09/04/2023 23:55:53

Edited By Stuart Smith 5 on 10/04/2023 00:00:03

Exactly Stuart. Agree. Spot on. Thank you very much for the clarity.

Edited By Clive India on 10/04/2023 13:42:04

Hi, I found out what Martin is saying about the RCD tripping, during the time I was wiring my garage (this was before all the current rules about who can install what/where etc.) I had put in my consumer unit and laid out all the cables to where I wanted sockets etc. The first thing was to get some lights working. As I progressed and the cables were connected into their places around the garage, but none of the fuses/trips were in place, however when making of one end of a cable ready to go into one of the sockets, I dropped the cable on the steel floor and the bare end of the neutral wire was the only one that touched the floor and duly tripped the RCD, the only things drawing any current were the lights.

Regards Nick.

That's the reason I said "if fitted", many TT installations still exist and most will be protected by an RCCB on the incoming supply.

Emgee

Thanks for your detailed explanation Stuart - very useful. I had assumed that the connections to the gas/water mains constituted a local earth in that the pipes are (or were) metal and buried in damp earth.

Robin.

If the supply type is a PME system the incoming water main if metal must have an insulated insert fitted to break the path to earth.

Emgee

I believe that's true, but why?

Not having studied the subject, I'd have guessed that an extra earth through a metal water pipe would only add to the safety of the system.

Is there a way a dangerous potential can develop between two earths in a PME system? Or is the ban in recognition that domestic supply pipes make rotten earths, and should always be avoided. Gas and water pipes are both likely to corrode, be insulated, have poorly conductive junctions including at the meter, and then be connected to a plastic mains in the road. Gas pipes are banned outright as safety earths because gas leaks and electrical faults don't mix.

Not an academic question. Radio amateurs living in PME houses often need multiple earths, for example: to stop static electricity building up on antenna; to ground electrical noise on the exterior of feeder cables before they enter the building; and again where cables connect to equipment. And what a sparky thinks is a good earth (between 25 and 3 ohms at 50Hz) is inadequate by radio standards! For example, a 3 metre whip antenna at 1.8MHz has a radiation resistance of about 0.2 ohms; connected to a 10 ohm earth, it can't be more than 0.2% efficient. This means a radio earth might be considerably lower resistance than the supply earth, but not engineered to take a heavy mains fault current.

Dave

Can you please quote a source for this Emgee i.e. the regulation in BS7671- It is not something of which I was aware.

Nor I.

Any metal water supply is likely to be quite old or a private supply.

If the electricity board changes its supply to PME, would the water board have to come along and insert an insulating section?

What is quite sensible with a PME supply is to put an insulating segment in the pipe to an outside tap just in case of a lost neutral.

Edited By Chris Pearson 1 on 11/04/2023 14:54:46

Edited By Chris Pearson 1 on 11/04/2023 14:55:11

Chris, yes it certainly would do no harm to put a plastic section downstream of any outside tap supply.

The fitting of an insulating section to an incoming water pipe would, I have thought, be counter-productive on the failure of the neutral supply to a PME property. If any supply ie lighting, is switched on then the neutral and hence the earth cabling would become live. This would then cause the metal casing of all appliances and other earthed metallic class 1 fittings to rise to phase potential with the obvious result if touched whilst standing on a potentially earthed floor.

If the water pipe is connected to the earth (via bonding) then the fault current is likely to pass down the water pipe through the class 1 items on the water/heating circuit and would likely cause disconnection of the over-current protection device on that lighting circuit – this would (if the only circuit switched on was lighting) very likely disconnect the phase from earth I.e cause the fuse/mcb to blow.

Admittedly if a number of higher amperage circuits were still connected (through appliances that were turned on when the neutral was lost) then the resistance of the water pipe may not be adequate to disconnect all of the circuits.

The solution to all of this is to fit RCD’s (or RCBO’s) to all circuits. I wonder how many people regularly test their RCD/RCBO’s????

As a matter of interest, a company I have dealings with that test RCBO’s and RCD’s say that up to 10% of the units they test fail – even some new uninstalled ones have been known to fail.