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Earthing Arrangements Explained + Photo's

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This is for the people still in or has just left their electrical education or for those of us that need a refresher in domestic earthing arrangements. It's all very well looking at prity diagrams in college and in books but its different when your out in the field for the first time. Hope this thread helps you and I hope that other members will post up their pictures of main incomers/earthing so others can learn and understand this subject better. I wish I could refer to something like this when I was in college.

First of all we have 3 types of earthing arrangements, TN-S, TN-C-S and TT.


This is where the main earth cable from the main earth terminal (met) is connected to the neutral at the suppliers main fuse. A good way to remember the name of this arrangement is to think of the C meaning COMBINED.


This is where the main earth cable from the met is clamped or solderd to the steel of the SWA or the led outer sheath of the incoming supply cable. Again a good way to remember this is to think that the S stands for SHEATH.


The main earth cable from the met is connect to an earth electrode (aka earth rod). This is because not all properties are supplied with a TN system by the supplier so we have to insert a rod into the ground.

Notice how the earth cable on the TN systems go back to the main incomer, one goes to the main fuse (TN-C-S) and the other goes to the incoming cable (TN-S). If the main earth cable doesn't go back to the fuse or incoming cable it will be a TT. This is the simplest way I can put it without going into extended detail.

Now that we've seen some prity diagrams (am I starting to sound like a teacher lol) we will now look at real world photos that are not as straight forward as the diagrams.

First example, is it a TN-S or a TN-C-S or both?

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Air conditioner and Tel, these are the things that I'm on about. It's great learning from books to pass an exam but its a different world out there once you pick up your tool box and go to your first job.

Has any other members got main incomers photos? Post them up.
Hi Paul, sorry if i sounded flippant with my relies but it did have a serious message. If the first two diagrams have every detail on them, the third should also.
Hi Paul, sorry if i sounded flippant with my relies but it did have a serious message. If the first two diagrams have every detail on them, the third should also.

No offence taken mate. If I could find the osg diagram of a TT system on Google images I would have posted that instead. Seen as I do all my posting from a mobile phone (yes, over 3,800 atm) and I've spent over an hour today on this thread to give basic learning to others. If I can put the effort in, why can't the people that are learning the trade? Let's encourage them to better their knowledge. Well I'm still learning and I'm not a half bad sparky lol.
Is it not the case that if a TNCS system has been given this is the primary source of earthing.

Just a thought
Would the DNO then leave in the TNS connection or connect the cable sheath to the TNCS terminal if the sheath earth is suspect in the area.

One word of caution in the first photo the TNS earth connection was made via an earth strap around the lead sheath. Beware of tightening these up if they become loose as the effect is to squeeze the lead sheath into the line and neutral conductors. End result BANG.
This also used to be common on SWA cables (not sure why). The outer sheath was striped back around 100mm from the gand and a earth strap fitted around the armour. While carrying out maintenancee or periodics the straps were found to be loose so were tightened up. After several rounds of this the armours were squeezed into the conductors BANG.

Obvious I know but beware.
Paul - It may be a way to remembers the 'S' as connected to Sheath (which of course it is) - but the important point is that it actually stand for 'Separated'.
i.e. Separate Neutral and Protective conductors throughout the system.

I know that you and experienced guys know that, but I think that guys learning the ropes ought to understand the importance of that. And how it differs from TN-C-S. (PEN conductors)
T = Terra (earth)
S = Separate
C = Combined

Apologies if I've opened a can of worms. But good thread. I think that Earthing is one of the most misunderstood subjects in the trade.
Thanks anyway Paul.
Terminator that was tomorrows lesson/debate lol.

Terminology, especially for those taking exams are key.

Step by step, day by day this thread will grow with everyone adding to it just like you have, thank you, keep it coming.

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