Currently reading:Earthing Arrangements Explained + Photo's

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Paul.M

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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.

TN-C-S

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

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.

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.

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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The three wire is an old way of N→E connection. It moves the LV earth rod away from the MV earth rod.

Come on Tony, please explain what CEW is in simple terms, maybe with one of your infamous drawings! (If you're feeling in a generous mood! )
I just can't visualise it.

Cheers Cock! I've sussed it.
Cerium Tungsten Electrodes.

That link takes me back to school chemistry, nightmare!
Who let you back in?

How it was explained to me at the EMEB collage.

Around each of the earth rods there will under earth fault conditions be a voltage gradient in the ground. In order for the MV and LV to be interconnected the LV earth electrode must be <2Ώ. Higher and the site is referred to by the DNO as hot and no direct MV→LV earth connection is allowed.
Therefore the LV earth has to be moved away from the MV equipment earth. This is to prevent a MV earth fault migrating to the LV system.

My drawing for OH supply

DNO drawing for ground mounted

How it was explained to me at the EMEB collage.

Around each of the earth rods there will under earth fault conditions be a voltage gradient in the ground. In order for the MV and LV to be interconnected the LV earth electrode must be <2Ώ. Higher and the site is referred to by the DNO as hot and no direct MV→LV earth connection is allowed.
Therefore the LV earth has to be moved away from the MV equipment earth. This is to prevent a MV earth fault migrating to the LV system.

My drawing for OH supply

DNO drawing for ground mounted
and when you think about it Tony...its just common sense isn`t it...

differing potentials n all that lot...

Ah so when a lot of people install earth rods they are potentially increasing the chance of nuisance tripping

Ah so when a lot of people install earth rods they are potentially increasing the chance of nuisance tripping

Only if they link N→E. Which as we all know they shouldn't.

Ta Tony, I'll have to get back later on this one.

and i`ll say this as well Tony..

one of the neighbours up until recently had issues with extranious in his house with voltage sitting on it..

then he said one day the DNO came out and were doing something...
ever since then the extranious in his house has been clear....

hmm.....

The DNO’s also have another crafty trick with MV OH earthing. You may have seen section switches on OH lines. If you look there will be an earth running from the switch metalwork to the ground. It isn’t a rod, there will be a mat.
The idea is that the ground the operator is standing on is at the same potential as the switch metalwork. Where it gets tricky is approaching the switch, I was taught “the shuffle”. You don’t go striding up to the switch as there could be a ground voltage gradient, you shuffle towards it not putting one foot fully in front of the other.

PS I don’t like MV OH lines.

Cows are more sensitive to the gradient effect because of the distance between their legs (this applies to some TV 'celebrities' as well). Luckily most cows don't need to operate MV switchgear - they normally stick the the LV stuff. Daz

PS I don’t like MV OH lines.[/QUOTE]

Oh boy, this is all filling me with confidence, not! So, as a poor soul who has to live with an overhead supply, is there anything I can do/have done to make it safer? Apart from avoid crawling about on all fours! The DNO is clearly not concerned until some poor sod discovers a fault!

The advantage of having sheep and cattle in the field is you know there’s a ground voltage gradient around the switch. Dead sheep and cows tend to give it away.

Oh boy, this is all filling me with confidence, not! So, as a poor soul who has to live with an overhead supply, is there anything I can do/have done to make it safer? Apart from avoid crawling about on all fours! The DNO is clearly not concerned until some poor sod discovers a fault!

If you know there's a fault, supplies gone off etc. Don't go nosing around the MV gear, simple as. If you're that woried, talk to the DNO. If you're on a farm then it's worth a phone call.

Remember I’m talking of fault conditions, not the everyday life of country folk.

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