Simplified Earthing Arrangements

A simplified description of commonly used Earthing systems in the United Kingdom,

along with a basic diagram of the most common,

** note, :- extending the PME system outside the equipotential zone has been OMITTED from the diagrams as it is considered IMHO to be outside the remit of a simplified description of earthing systems and is dependant on a number of varying factors that could only be established via a site visit and survey, and would also be dependant on the electrician concerned being wholly conversant with the inherent risks etc thereof and understanding the principles of earthing systems and how they work and intermix with each other.

TNC-S and PME

The subtle differences.

TN earthing systems in the UK are always earthed at the supply transformer, a TN-S system utilizes a separated PE conductor throughout its length, The TNC-S system utilizes the neutral conductor, PEN , which is earthed at the supply transformer, and then separated at the consumers point of supply.

A PME system, utilizes the neutral just as the TNC-S does, but is earthed at intervals throughout its length, and more importantly, to be fully PME, should be earthed at point of, or just before the consumers point of supply.

PME earthing is still an ongoing upgrade to the older TNC-S earthing system, the supply companies still have some time to bring all TNC-S systems in line (2012).

The debate would be, if no earth electrode is supplied at the end of the distributors cable, does it still comply with PME? To understand this we would have to know why this last earth electrode is important to the integrity of the true PME.

Broken neutral on TNC-S systems.

The fact is, whilst possible, this is still a rare occurrence, however it should not be excluded, indeed the supply companies take the possibility very seriously, otherwise why would they be converting to PME.

In the event of a broken neutral the fault paths will have no defined route, and will migrate to and from all users on the same system, this can give rise to dangerous voltages on the neutral conductor.

One of the main protective conditions of this type of fault is the correct bonding within the installation, however this is becoming near on impossible due to the utility companies now supplying their services with plastic pipes. There will always be a leakage to earth, to some extent, but not enough to clear most dramatic faults of this nature.

PME systems are designed to lower this possible fault, and to provide at least a connection with earth, at one, or multiple points. For this reason it is important to have an earth electrode close to, and not shared with another installation, if designed this way, any neutral faults will always have a path to earth, and whilst the fault can still be shared, the voltages on the neutral will not be allowed to rise as high ,with multiple earth electrodes at each installation point.

Using PME to extend the equipotential zone.

It is possible to extend the equipotential zone of a true PME system, with correct design, and can sometimes be confused for a TT system. Remember PME stands for Protective Multiple Earthing.

The neutral conductor is still connected throughout the system, however earth electrodes could be introduced a various points within the installation, and connected to the main earthing terminal at origin, or separated to provide protection in those circuits, providing multiple points of reference to earth. This type of arrangement must be fully compliant with all regulations, and the earthing within the installation should be verified to ensure continuity throughout.

Excellent post Steptoe, I have made this Sticky, so we can all find it again quickly when required.

Doc H.

Nice one Steps

This will be a great help to those thinking about doing their 2391-10..Great post Steps..

not all my own work

I was given some assistance with the wording, :Blushing

I am sure it was all your thoughts though, and thats all that counts, how it is put into words, makes no difference as long as it says what you meant to say. I think its a very good post, and well worth the effort of writing. Good job, well done I say.

Here here nice one steps thanks scoobed

Thanks this is ace.

thanks = little star bottom left,

thanks = little star bottom left,

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I did - This morning, along with a lovely comment.

Have just been reading some old threads, so apologies if this has been answered elsewhere!

This is a brilliant explanation and I am now enlightened to the 'correct' difference between TN-C-S and PME ... however, the BGB shows the two systems as 'one' on page 44 to quote "TN-C-S (PME) system"

This can really confuse electricians into thinking that the converted ones that have been labelled are as described in the book but may not actually be correct within the cut-out.

I would like to know if anyone can describe to me a definitive method that can confirm the system IS actually connected correctly at the cut-out without being able to access inside the cut-out unit - as this is illegal to open unless you are the DNO.

Can it be done by measurement at all or can it only be seen by looking at the link inside the cut-out/supply head.

My reasons for asking is that I suggest to customers that have TT systems to get them converted telling them that it would give a better Earthing arrangement and therefore a greater degree of compliance to meet disconnection times (in layman

I have also noticed that the diagram (Fig 3.9) in the BGB on page 44 shows a TT earth arrangement at the consumers end (albeit an 'additional' one) This could be translated into a requirement for having an earth rod in the ground local to the MET and earth that goes to the PME terminal in the cut-out, on every TN-C-S / PME installation.

Click to expand...

that would be in my mind how a proper PME would be,

however, looking inside the head would likely tell you nothing , the supply would most likely be rodded at the underground joint where it Ts to come into the property, that is, if it is rodded at all,

to measure it to find out if indeed you actually have PME as opposed to TNCS you could drive a rod in and measure Ze at the rod, it should be almost identical to Ze on the DNO supplied earth if your supply is proper PME, as the DNO rod should pick up your measurement from your rod and give it a good path back to star.

Thanks Steptoe,

I agree with your method to check PME with a local earth rod. Although it may be a waste of a good piece of copper. After i posted my thought I realised that it wouldn't tell me much about the what is inside the cut-out anyway except to see that there is a link to the neutral!

I shall try that out to see what reading i get - after the local DNO and Electricity supplier sort out between themselves who is going to make the final connection !

Will post an update when it happens.

Locked, author and most others no longer post, start a new thread if you have a question