Member postings for Chris Pearson 1

There is no need at all for that main equipotential bonding - the plastic pipes cannot introduce a potential into the building and its electrical installation.

My house was wired to 15th Edition and has a van's worth of supplementary equipotential bonding. Both pairs of hot and cold water pipes are joined under the double stainless steel sink and even the sink is bonded to them. This is all despite electrical continuity of the copper pipes, brass taps, and steel sink. It is mad!

Bonding is not the same as earthing even though green and yellow cables are used for both. If there is a fault within a boiler or fan-assisted radiator (where else could one occur?), the fact that the appliances are earthed will ensure ADS: the fault circuit is along the circuit protective conductor and not the pipes and bonding.

Seeking to rely upon bonding for ADS would not be safe - the copper pipes could be replaced by plastic ones or insulating joints could be installed.

Where the bonding is attached was not the point. After all, if the supply is plastic, it would be pretty pointless to put a cable clamp around such a pipe. However, Emgee is correct per 544.1.2.

We have been told that the water pipe is plastic to the stop cock, so there is nothing to bond.

However, we haven't been told about the gas supply. If it is yellow plastic, there is also nothing to bond; but if it is metal (cast iron) then it should be bonded.

If the installation is reasonably modern and has been certified, then it should be all right. If not, get an EICR -,an electrical installation condition report.

Indeed not - they will go more or less for ever at 15 A, but will go pretty promptly at 20 A.

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

Does it matter?

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.

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.

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.

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.

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!

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

18 mm DP RCBOs are widely available nowadays.

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

Martin C is correct in saying that the RCD measures the difference in flow between the line and neutral conductors, but in this instance, there is no current in the line conductor because its MCB has tripped.

As John Doe says, there must then be sufficient voltage between the neutral conductor and circuit protective conductor for a current to flow.

An RCD with a nominal current of 30 mA does not magically trip at exactly 30 mA. It must do so within 300 mS at that value, but will trip faster at a higher current and should not trip at 15 mA or less.

Edited By Chris Pearson 1 on 09/04/2023 10:32:35

Edited By Chris Pearson 1 on 09/04/2023 10:33:04

I might add that unless you are in a heavy industrial unit with its own transformer, the resistance will be measurably greater than zero Ohms.

Well, a PFC of say 1000 A would make any fuse go, "bang".

Not really, "bonded", and not always in any event.

A PME or TN-C-S supply has a combined neutral and earth supply, which splits into N and E at the service head and the nearer you are to the origin, the lower the voltage between them.

In a TN-S supply, N and E split at the transformer. In a TT supply, E finds its way back to the transformer via the general mass of the Earth and a rod at either end.

Then under certain conditions, you can have diverted neutral currents, but that seems unlikely in this event.

I'd go along with that. Fuses (including the conductors of a neon lamp) tend to go, "bang", but RCDs, etc. go, "click".

I was just about to say that the tools in the chests in the Mary Rose were stored in damp conditions for quite a while, yet it is remarkable how well they have survived.

The museum is highly recommended.

Been there, done that. Although, in my case the motor was very lumpy. Given that the VFD was built into the lathe cabinet, it was easier to change the motor, but no luck there.

So a new modern VFD was fitted. It solved the problem for a while, but then exactly the same problem recurred.

Fortunately, a replacement under warranty was fine and has continued to work for a few years now.

Really and truly, there isn't much (if anything at all) to go wrong with a 3-phase motor so the learning point was to fix the VFD first.