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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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I was under the impression that we were to assume a value of 16ka(but I could be wrong)
According to the OSG the maximum fault current likely to be found in domestic properties in the UK (unless in heavily built up areas) is 16kA.

However this is probably stated because annex Za of BS60439 provides an additional test, for domestic consumer units that are protected by a BS1361 fuse at the supply, to ensure that a BS60439 consumer unit can take a 16kA fault current without danger.
This permits the use of protective devices in the consumer unit that may not have the short circuit rating to cope with the prospective fault current since the assembly as a whole will cope.
This is part of why the manufactures do not permit different makes of breaker in their consumer units, they say they are not tested for those units.
Here's our overhead supply for comparison and comment. Our transformer is served by two high voltage conductors, which in turn come from a string of poles carrying three conductors - ie two of the three branch off to our property. I assumed these were two phases from three phase, but comments here now make me unsure.


One green/yellow sheathed cable comes from the case of the transformer into the ground on the N side of the pole.


On the other side of the pole the two cables out of the transformer join the underground service cable which goes to our meter cabinet. One connects via the isolator or fuse or whatever it is (the grey item), the other is clamped directly. After the join to the service cable, there's another earth that goes underground at the foot of the pole on the S side. The DNO guy who traced and marked the cable routes for us said that this would run some distance horizontally underground while still insulated, then further distance uninsulated.


Our earthing arrangement was described as "CEW" and also "TN-S" by the DNO engineer, after he opened part of the cut-out to check.

Tony S
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If you have a neutral connected to an installations earth terminal (eg PME/TNC-S) then the incoming supply cables metallic sheath connection must be disconnected. Especially important where a supply cable is suppling both TN-S and TNC-S earthing arrangements!! eg, ....where the DNO cable is in the process of being PME'd....
Well Dan,on your recommendation ,I decided to read this thread (all 12 pages of it). Very interesting.Feels a lot older than 10 years.It also raised a question for me ,based on the comment above from "Engineer54". He suggests that the metallic sheath of a TNS cable must be electrically isolated as soon as the TNS supply is converted to TNCS.Why does this have to be done?
This thread hasn't been replied to for 14 days, so replying to this one may not get a response. Post a new thread instead.

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