Earthing Arrangements Explained + Photo's

[Paul.M]

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?

 

[Hohmage]

Great, thank you Archy. Did you find much then? It seems that this system is new to some of you - I'd hoped you might find it a bit challenging!

 

[Silly Sausage]

Hahaha....Well I wouldn't have know what it was if it came up and kicked me in the nuts and told me!
I'm still non the wiser.
I'll keep an eye on the IET forums in case you ask over there.

 

[Hohmage]

I found this comment on the IET forum (thanks Archy S): "Up in Scotland as one of the DNOs, CEW or (SEN) is a Continuos earth wire earthing system given to consumers who have an earth loop higher than 0.35 ohms. ie 3 wire single phase system consisting of a phase, neutral and a separate earth. If PME they'd just get a phase and a combined earth/neutral."

This doesn't sound as if it's much better than a TT system, does it? Can anyone say whether I ought to be having my sheds TT'd, based on this comment? I am about to have a RCD-protected split CU installed. 30mA RCDs sufficient? Having seen some shoddy workmanship up here, I want to know exactly what an electrician should be suggesting!

 

[aesmith]

How's the earth actually made, and where does it join neutral (if at all).

I'm comparing with our supply, which is the same type of pole mounted 11kV transformer but in our case we're connected by underground split concentric to provide TN-S (DNO called this CEW as well). As explained by the DNO our transformer's neutral on the LV side is connected to an earth cable/rod/spike or something at the foot of the pole.

A previous house, served by overhead 240V, only had L and N overhead with a locally provided earth.

In this case, are they really providing Earth on an overhead cable?

 

[Silly Sausage]

I found this comment on the IET forum (thanks Archy S): "Up in Scotland as one of the DNOs, CEW or (SEN) is a Continuos earth wire earthing system given to consumers who have an earth loop higher than 0.35 ohms. ie 3 wire single phase system consisting of a phase, neutral and a separate earth. If PME they'd just get a phase and a combined earth/neutral."

That'd be TNS then. (?)
Further research needed, when I'm totally C[SUB]2[/SUB]H[SUB]5[/SUB]OH free!

 

[aesmith]

"Up in Scotland as one of the DNOs, CEW or (SEN) is a Continuos earth wire earthing system given to consumers who have an earth loop higher than 0.35 ohms."

I'm not sure if it's the same thing he was meaning, but SSE publishes a target of max 0.35 ohms for TNCS, but up to 0.80 for TNS. It doesn't necessarily mean that TNS will be over 0.35 though, ours is 0.29.

 

[Hohmage]

How's the earth actually made, and where does it join neutral (if at all).

I'm comparing with our supply, which is the same type of pole mounted 11kV transformer but in our case we're connected by underground split concentric to provide TN-S (DNO called this CEW as well). As explained by the DNO our transformer's neutral on the LV side is connected to an earth cable/rod/spike or something at the foot of the pole.

A previous house, served by overhead 240V, only had L and N overhead with a locally provided earth.

In this case, are they really providing Earth on an overhead cable?

Hi aesmith, thank you for replies. I am only versed in basic electrics. Can you tell me were I should b looking to answer your question "how's earth made, and whre does it join neutral". I have a basic understanding of the more obvious systems TN-S etc. I could take more pics.

I have been told that this overhead earth is still common up here. I had a response on another forum about split concentric: "It looks like an old TN-S to me. Separate overhead earth (no longer done). You could report lack of earthing and see if they will replace your aged supply with a SPLIT concentric drop (TN-S)". I decided to post on this thread because it was specifically targeting earthing systems. I did try to get the chap from SSE to explain more to me, but he seemed unwilling.

 

[Hohmage]

That'd be TNS then. (?)
Further research needed, when I'm totally C[SUB]2[/SUB]H[SUB]5[/SUB]OH free!

I am going to join you on that stuff right now! The electrics in this house are doing my head in!

 

[Hohmage]

"Up in Scotland as one of the DNOs, CEW or (SEN) is a Continuos earth wire earthing system given to consumers who have an earth loop higher than 0.35 ohms."

I'm not sure if it's the same thing he was meaning, but SSE publishes a target of max 0.35 ohms for TNCS, but up to 0.80 for TNS. It doesn't necessarily mean that TNS will be over 0.35 though, ours is 0.29.

This is interesting, thanks. I was hasty in my response about the resistance and the thought of TT, best to be quick whilst kind folk are in the process of helping you out!

 

[aesmith]

Hi aesmith, thank you for replies. I am only versed in basic electrics. Can you tell me were I should b looking to answer your question "how's earth made, and whre does it join neutral". I have a basic understanding of the more obvious systems TN-S etc. I could take more pics.

You may be misunderstanding me, I'm not an electrician but I have taken a keen interest in our earthing arrangements because of the need to take supplies out to some outbuildings with a lot of extraneous metalwork so I made it my business to get a definitive answer.

I see you're supplied by SSE as well, but I guess there are regional differences. I contacted them ([email protected]) to ask what sort of earth they provided. Their first response was that I should get it checked by an electrician, but they also said they were installing PME and could check whether we could be "upgraded". When I asked to follow that up they arranged for their guy to come to site, who clarified the system and also took the Ze reading. I think you have to be a bit persistent, you're probably dealing with someone non technical in the first instance, so you need to give them a reason to pass your question onto the technical team. Are you concerned about the safety of your supply from an earthing point of view?

 

[Hohmage]

Hi aesmith, yes, my main concern is with the safety of the earthing. I was originally concerned that the system was TT or similar, and so the CU required a 100mA RCD. At present it has no RCDs at all. As someone else said, the earthing arrangement that I do have here could deteriorate underground. Similarly with the outbuildings, I wondered if earthing rods were required and am still unclear on that one, because some folk say you can never have too much earthing. If he system here is TN-S, then I should worry less. SSE is installing a new substation couple of miles away from here right now, so I will ask again for someone more technical to visit. Thank you for your help.

 

[Knobhead]

Is it a posh TT? Earthing cable comes down a big pole?

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

 

[Hohmage]

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

Hi Tony, thank you for this. However, being a layman (but really wanting to know more about the subject) I don't understand where the HV would be at the house, let alone the earths. Could you point me in the right direction of a simple explanation please? Many thanks.

 

[Knobhead]

Look for voltage gradients.

 

[GLENNSPARK]

That'd be TNS then. (?)
Further research needed, when I'm totally C[SUB]2[/SUB]H[SUB]5[/SUB]OH free!

for more reading :

http://kisdwebs.katyisd.org/campuses/THS/teacherweb/chemapgt/Teacher Documents/AP reactions.pdf