An electrical puzzle.

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.

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.

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!

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.

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.

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.

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.

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

In the article linked to by Michael one paragraph sums up 1 reason why the insulating insert is fitted:

It is a common misconception that bonding such items won’t cause any harm even if it is not required by BS 7671:2018 so a “better to be safe than sorry” attitude is taken. However, it is important to remember that by connecting to the main earthing terminal, in some circumstances, fault currents can be exported throughout the installation which would not be there if protective bonding had not been applied. This can cause an electric shock risk for persons outside of the installation in contact with the general mass of Earth and earthed equipment such as pipework, for example an outside tap or metal (class 1) electrical equipment.

The older electricians will remember the fault voltages from ELCB (Earth leakage circuit breaker) devices passing via the ground to the neighbours ELCB and tripping that also, not easy to get good separation distance for the earth stake in a row of small cottages.

Perhaps someone here has the latest BS7671 for reference to the reg.

Emgee

This is quite fascinating. Much more complicated than I thought. Would I be right in thinking that if it could all be designed from scratch it would be possible to simplify? My house is maybe 200 years old and has seen many changes - the plumbing is crazy, lead water pipes in some places, dead legs all over the place and still after 7 years in here coming across lead gas lighting pipes. The electrics, though obviously more more recent, have clearly 'evolved'. Some black and red, some blue and brown. I suspect that it's a bit of a mess, though when I had the cellars wired up to make workshops the sparks tested the whole house and gave me a piece of paper saying it was OK.

In the picture below is the bulging out bit before the meter a conventional (ie it melts) 100A fuse? The leccy comes in through the connection labelled L1. Strangely the bulgy bit isn't secured with a seal - I can wiggle it and suspect I could pull it out.

Robin.

Edited By Robin Graham on 12/04/2023 00:11:16

Edited By Robin Graham on 12/04/2023 00:12:23

Edited By Robin Graham on 12/04/2023 00:16:12

Edited By Robin Graham on 12/04/2023 00:31:53

Hi Robin, yes that is a fuse, and they do normally have a seal.

Regards Nick.

I don't think so: the systems used are designed to meet requirements, which vary with circumstances. For example, the PME system common in modern homes is unsuitable for a long list of industrial needs. The house being 200 years old is a bit of a red-herring: although it may be wired as a patchwork, it's probably compliant with one of the legal alternatives.

In the UK, there are (I believe), 4 basic single-phase variants, each with pros and cons:

These days PME is preferred in UK homes, option 3 above, because it saves the cost of separate earth wires running back to the transformer and avoids the problems caused by dodgy consumer provided earths. Home earths suffer a multitude of practical difficulties - corrosion, difficult to fit in terraced houses with no gardens, water-pipes later converted to plastic, unsuitable geology, and DIY bodges galore. But there are circumstances where a consumer provided earth is essential, like a chemical plant manufacturing TNT. These earths have to be properly engineered, not just a spike banged into the ground.

The last option, with no earth, isn't general purpose. An electrical engineer would recommend it for needs not met by more conventional solutions.

I'm dimly aware what goes on abroad electrically may be different. For example, although rings are beloved in the UK (reduced cable costs), the rest of the world prefer spurs. Differences occur because the engineers who consider risks and requirements before setting standards respond to local needs. In the UK we were happy to risk 250V in the home (reduced cable costs again), whilst the US wimped out and went for 110V. The rest of the world decided we were both wrong and standardised on 220V. Similar local considerations led 50Hz vs 60Hz.

The engineering logic isn't obvious, not least because standards rarely explain the background thinking or history. In truth most practical earthing systems are a compromise between safety, cost, and convenience. There isn't a single simple right answer that meets all requirements.

Dave

If I had to be on an all radial system I would need a room just to house the dis boards, having 6 ring mains to cover all the outlets. I do not believe in long trailing leads and adaptors so all rooms have ample double sockets within easy reach of where they are needed.

On the subject of the US (and Canada) their practise of using screwits, small cup shaped internally taper threaded to hold twisted wire joints is seriously old fashoined.

The seal fairy must have stopped off and removed it.

The seals are the property of the DNO and must not be removed by unauthorised persons. Some DNOs will permit registered electricians to remove them subject to certain conditions.

Whatever you do, do not be tempted to remove the fuse. First, if you do it under load, you may suffer an injury from an arc. Second, you would feel pretty foolish if removal of the fuse carrier pulled the service head off the wall, or even worse, displaced one of the cables.

Where do you look for a regulation which does not exist?

Perhaps confusion has arisen in the light of 542.2.6, which prohibits the use of a metallic pipe of a water utility supply as an earth electrode.

In response to SillyOldDuffer, the nominal voltage in UK has been 230 V for some time.

The type of supplier's earth depends largely upon the type of cable. PME (TN-C-S) is standard but a factory, for example, with its own transformer may well prefer TN-S.

It is safest to assume that all domestic supplier's earths are PME.

In the countryside, there may be no alternative to TT because of the distance from the transformer.

So can anyone tell me which of SOD's options I have? House built 1930, rewired since. Used to have 3 phase but take out when meter moved. I've measured the earth neutral potential at a socket near the consumer unit at 160mV with hardly anything switched on.

Neither the gas nor water pipes are earthed, water is in plastic pipe up to my stop tap. It's complicated by the unit on the right, which monitors my consumption for some research project (and pays me a small sum each year). The fat green/yellow wire is my system earth, connected to a brass block on the side of the fuse carrier. Next question will be should they be earthed, I'm confused by what's gone before on this thread

Forgot to mention, I have RCD and MBCs in the consumer unit

PME/TN-C-S. You can tell because the earthing conductor comes off the side of the service head.

Does it matter?