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Paul.M
It's all very well looking at pretty diagrams in college and in books but its different when your out in the field for the first time or doing a simulated exam. Hope this thread helps you regarding 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.
TN-C-S (PME)
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.
TN-S (separate or Sheathed)
This is where the main earth cable from the met is clamped or soldered 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.
TT
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. But the earth electrode (rod) needs to be accessible at all times for Inspection and Testing so you should be able to locate it.
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?
First of all we have 3 types of earthing arrangements, TN-S, TN-C-S and TT.
TN-C-S (PME)
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.
TN-S (separate or Sheathed)
This is where the main earth cable from the met is clamped or soldered 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.
TT
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. But the earth electrode (rod) needs to be accessible at all times for Inspection and Testing so you should be able to locate it.
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?
Post up your answers and the reason why you have come to that decision. Just to help I will tell you that not all MET's (main earth terminal) are in the c/u, some are connected by a Henley block outside of the c/u as above.
