An electrical puzzle.

[Robert Atkinson 2Participant @robertatkinson2]

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

[Maurice TaylorParticipant @mauricetaylor82093]

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

[Chris Pearson 1Participant @chrispearson1]

Posted by Emgee on 09/04/2023 14:04:01:

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.

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

[Chris Pearson 1Participant @chrispearson1]

Posted by Robert Atkinson 2 on 09/04/2023 16:29:20:

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

18 mm DP RCBOs are widely available nowadays.

[Sonic EscapeParticipant @sonicescape38234]

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.

[Robin GrahamParticipant @robingraham42208]

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

[Stuart Smith 5Participant @stuartsmith5]

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

[Clive IndiaParticipant @cliveindia]

Posted by Stuart Smith 5 on 09/04/2023 23:49:41:

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

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

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

[Nicholas FarrParticipant @nicholasfarr14254]

Posted by Martin Cargill on 09/04/2023 08:38:44:

I think there are too many people on here trying to (wrongly) explain how an RCD works. it is nothing to do with neutral to earth voltages .To put it simply an RCD compares the current flowing in the live conductor with the current flowing in the neutral conductor. If they are not the same then there is current flowing through another route. This route is to earth. Household RCD devices are set to trip when the current comparison exceeds 30 mA of a difference.

A single RCD in a distribution board usually protects a number of circuits in the house/workshop so, because the neutral wiring is all joined together at the distribution board then earthing any one of the neutral wires anywhere in the house (even on a circuit that has its live feed isolated) will cause current to flow to earth from any of the other live circuits, thus causing the RCD to trip.

Martin

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.

[EmgeeParticipant @emgee]

Posted by Chris Pearson 1 on 09/04/2023 17:21:42:

Posted by Emgee on 09/04/2023 14:04:01:

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.

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

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

[Robin GrahamParticipant @robingraham42208]

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.

[EmgeeParticipant @emgee]

Posted by Robin Graham on 10/04/2023 23:50:48:

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

[SillyOldDufferModerator @sillyoldduffer]

Posted by Emgee on 11/04/2023 09:47:48:

Posted by Robin Graham on 10/04/2023 23:50:48:

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

[Steve Skelton 1Participant @steveskelton1]

Posted by Emgee on 11/04/2023 09:47:48:

Posted by Robin Graham on 10/04/2023 23:50:48:

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

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

[Chris Pearson 1Participant @chrispearson1]

Posted by Steve Skelton 1 on 11/04/2023 14:16:15:

Posted by Emgee on 11/04/2023 09:47:48:

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

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

[Steve Skelton 1Participant @steveskelton1]

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.

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, in this Electrical Apprentice all the systems show that metal water and gas pipes are bonded to earth without any plastic section.

Regards Nick.

[Michael GilliganParticipant @michaelgilligan61133]

I am far from expert in such matters, so am following this discussion with interest

The nearest thing I have found to a comprehensible explanation of the ‘plastic insert’ situation is here:

**LINK**

https://electrical.theiet.org/wiring-matters/years/2019/75-may-2019/to-bond-or-not-to-bond/

MichaelG.

[Chris Pearson 1Participant @chrispearson1]

In response to Steve Skelton …

The particular risk with an outside tap is that a user may be scantily dressed including no shoes and therefore be in good contact with the general mass of the earth.

Indoors, a householder is more likely to be shod and standing on reasonably insulating materials such as a suspended wooden floor, or a solid one with a damp-proof membrane. So in the event of a lost neutral AND contact with an exposed conductive part such as the case of a toaster or your favourite machine tool, the householder may well get a nasty tingle.

If the neutral is lost in the service cable to my house and I have plastic gas and water services, the leccy reaches the break and the voltage rises to 230 V, but it has no further to go. It backs up along the circuit protective conductors so they also are at 230 V. Everything is at the same potential – i.e. 230 V.

However, if I have a metal gas supply (which I do) and my neighbour also has a metal gas supply, the neutral current can jump across to my earth at the intake and thence to the main earthing terminal, which is usually a plated block by the intake, but it can be in the consumer unit. The current now goes along the main protective bonding to the gas pipe along which it can pop next door. Then it goes along their bonding to their MET and then back down their neutral back to the tranny. The only evidence of this may be warm main bonding. So, in short, there are advantages in not inserting an isolating segment.

Neutrals have not been fused for years. Should a neutral fuse blow, but not the line fuse, you are back in the realms of a lost neutral.

It is important to realise that not only will an MCB or main fuse not protect you in the event of a lost neutral, neither will an RCD of any sort.

If you live in the countryside with aerial cables and there is a storm and the telly goes off and the lights go dim, it may well be a lost neutral.Turn off the main switch and leave your lathe well alone!

[Nicholas FarrParticipant @nicholasfarr14254]

Hi, after reading MichaelG's post, I now know why there is a plastic coupler inside the house in the pipeline to the outside tap, and it does make sense. I don't have any gas pipes coming into my place, so don't have to be bothered about that one. The water main is blue plastic pipe, which is then connected to a section of copper, which is soldered into a lead pipe, which is mostly out of reach, but is bonded via the section of the copper pipe.

Regards Nick.

[noel shelleyParticipant @noelshelley55608]

or various houses are on different phases and the neutral goes ! it happened to a friend of mine – everything blew up as 415v appeared ! The supply company replaced all the electrical equipment in the house and said it was not uncommon in overhead supply ! Noel.

[Robert Atkinson 2Participant @robertatkinson2]

Chris P said "

It is important to realise that not only will an MCB or main fuse not protect you in the event of a lost neutral, neither will an RCD of any sort.

If protecting "you" means against electric shock then a RCD WILL protect against electric shock to "ground" The RCD does not care which wire you touch it only cares about a differentc between live and neutral. The only shock a RCD does not protect you against is if you touch both live and neutral with no significant connection to earth.

The change in the bonding of gas and water pipes is due to a number of issues:
Many more houses having plastic water and gas connections

The introduction of RCDs

Concerns about hazards caused by high fault currents flowing in pipework in the case of a failed nutral in the external supply. The IET are quiet on this as it was a flaw in the orgininal scheme. The house with the best earth could carry the reurn current for all houses on that phase.

Note that is is not a requirement for isolation (yet) just an option not to bond. Personally I think it is flawed as changes to the pipework or even the environment can cause a pipe that wasn't an extranious conductor when tested to become one. For example if part of a pipe is in contact with an earth subfloor or concrete it might pass a test on a dry summer day but become a hazard in wet weather.

[Steve Skelton 1Participant @steveskelton1]

Chris,

I think you are misinterpreting what I have written. I agree with all your statements up to the RCD being of no value. A current imbalance of between 15 and 30 mA will trip any final circuit that is 30mA RCD protected (and working).

Robert is right, if a 30mA RCD is supplying a Class 1 appliance and then you or an earthed object touch exposed conductive parts on the appliance and it causes a differential current of greater than 30mA between the phase and neutral legs at the RCD it will cause it to trip. Due to internal resistances of the appliance it is unlikely that there would not be a 30mA imbalance in the phase and neutral conductors back at the RCD.

[Chris Pearson 1Participant @chrispearson1]

Posted by Steve Skelton 1 on 11/04/2023 18:37:40:

Chris,

I think you are misinterpreting what I have written. I agree with all your statements up to the RCD being of no value. A current imbalance of between 15 and 30 mA will trip any final circuit that is 30mA RCD protected (and working).

Robert is right, if a 30mA RCD is supplying a Class 1 appliance and then you or an earthed object touch exposed conductive parts on the appliance and it causes a differential current of greater than 30mA between the phase and neutral legs at the RCD it will cause it to trip. Due to internal resistances of the appliance it is unlikely that there would not be a 30mA imbalance in the phase and neutral conductors back at the RCD.

I think that we are in danger of confusing things.

1a: the neutral is intact and a fault of negligible impedance develops which connects line and the casing. In this case ADS will operate before anybody has touched the faulty appliance.

1b: the neutral is intact and a fault of some measurable impedance has made the casing live, but any fault current is insufficient to activate ADS. (ADS = automatic disconnection of supply.) However, a person who touches the appliance may divert 30+ mA and additional protection in the form of an RCD or RCBO will operate.

2: there is no fault in an appliance, but the neutral is lost. Similar to 1b, the voltage of the casing is raised. However, if somebody touches it and provides a path to the general mass of the Earth (and hence the transformer), the fault current flows through the line of the RCD, through the appliance, back through the RCD along the neutral conductor, across to the circuit protective conductor to the casing and then back to earth via the hapless householder. There is no imbalance in the RCD.

[Steve Skelton 1Participant @steveskelton1]

Chris – yes I am with you on that – shall pay more attention!!

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