Earthing system question please???

[bartkusal]

Hello, I wonder if there is anyone here to help me out. Thanks in advance for the help.
Ok. I am doing electrics on 6 metal containers in community fields( I know, I don’t like them metal boxes too), I got it all under control, but there is little thing what buggers me. I kind of finding trouble working out the earthing arrangements. I know, stupid me, forgot to take the pictures, but the picture in words is:
From the first view it seems like TN-S (it has the earth cable coming out from the sheathing of the incoming supply cable, but on closer look I found that it has another earthing cable coming to MET, and when traced,I found it attached to the rod, so maybe TT then.
Measured Ze from the sheath only, it came up as 0.25 ohms (lovely),measured from electrode only, it came as 525 ohms ( up’s), measured them both together, it came as 0.27 ohms. Lookslike rod is worsening the Ze for me. I thought just to get away with the rod,but something not right here, because I Have the 30 mA RCD before the whole installation too, which is strange too.(100 mA would be sufficient and then down to 30 mA on the final circuits)

So my questions are:
· What earthing arrangement I have:
· TN-S (earthing from sheath)?
· TN-C-S( some sort of converted and because is so low reading)
· TT ( as per earth rod and RCD )

Thanks again for your input guys, and I need it sort of ASAP, started today and planning to finish on Friday, so realy need to work this out.

 

[Marvo]

....measured them both together, it came as 0.27 ohms. Lookslike rod is worsening the Ze for me.

How do you figure a rod could worsen the Ze? I'm confused and I don't think your test figures make sense.

Sounds like you have TN-C-S and sounds like you might want to whack another couple of rods on top of the existing one to get a more stable Ra.

Would this installation come under the same regs for caravan sites?

 

[Geordie Spark]

I'm waiting for E54 to arrive ........ pass the popcorn over bonny lad.

 

[bartkusal]

Why you think its TN-C-S? Its just I never came across that TN-C-S would come from the sheath, maybe not seen a lot just jet.

 

[bartkusal]

I know that, if that is TN-C-S,( bearing in mind lost neutral and stuff)then I need to bang more rods in, and make the TT better then it is , but I want to be 100% sure that it is, thanks.

 

[Marvo]

TN-C earthing is the problematic one when the neutral is lost.

 

[bartkusal]

TN-C earthing is the problematic one when the neutral is lost.

Thanks Marvo, I understand that, its just I want someone to tell me that TN-C-S earthing comes from sheathing too sometimes,cos I never come acros, normaly it is split at the head, which is not the case here. thanks

 

[Geoffsd]

How can you have TN-C-S from the sheathing?

What's the problem when Ze is 0.25?

Have you measured Zn/PSCC?

 

[bartkusal]

How can you have TN-C-S from the sheathing?
Thats what I trying to work it out,its just somene pointed out to it stright away
What's the problem when Ze is 0.25?
I know, looks good to me.
Have you measured Zn/PSCC?
Will do first think this morning,was too busy yeasterday digging trenches,could not think other than to finish digging that day.

thanks
Read more: http://www.electriciansforums.net/e...ing-system-question-please.html#ixzz2Ck526skk

 

[diditrain]

Don`t forget your pictures

 

[bartkusal]

Tested today again. Ze(E to L)=0.26 ohms, Ze(L to N)=0.16 ohms. Loks TNS to me...??? Sorry no pics again, cant get used to the new phone, too bussy.....

 

[Engineer54]

Where are you seeing the earthing coming off the incoming cable sheath (and what type of cable are we talking about??), at these metal containers or from some other point of supply??

There are plenty of TN-S supplies that can provide a 0.25 Zealand back in the 50's it was common practice to provide a local rod at the point of domestic/commercial TN-S supply. Though i doubt if they would be at the 500+ Ra level you mention...lol!!!

 

[SYKRAPS]

I come across quite often when working in the Leeds area that the earth is taken from the sheath and appears to be a TN-S supply but with gas/water earths disconnected th Ze is way too high, when this occurs I request a PME earth terminal is provided and the DNO always provide a PME terminal and attach a link from the terminal to the cable sheath.

This is so I am told (before the PME terminal is installed) a sysyem called Leeds Block, thus called because all of the cable sheaths at the sub-station are made off into a wooden block setup with obviously no electrical connection or continuity properties.

There are loads of these in Leeds that are relying on the gas and water services for their earth, so as the supply authorities are going around replacing lead and cast water and gas incoming supplies with alcothene then the property loses its earth.

This was told to me by a DNO engineer so I guess it must be the case.

 

[bartkusal]

Where are you seeing the earthing coming off the incoming cable sheath (and what type of cable are we talking about??), at these metal containers or from some other point of supply??

There are plenty of TN-S supplies that can provide a 0.25 Zealand back in the 50's it was common practice to provide a local rod at the point of domestic/commercial TN-S supply. Though i doubt if they would be at the 500+ Ra level you mention...lol!!!

I am seeing it coming from the sheath at the intake, which is a supply for them containers only, nothing else. It comes in the made for that purppose pillar/box 15 m away from the main container where the main board will be. Not sure about the incoming cable, the only thing I good see that it has a fine copper strands where the earthing is attached, and I think it is 3P supply too, it has 3 cutouts at the had 100A each, dont want to cut seals on other 2 to prove it by mesuring it. It not like i realy care, I only use one cutout as per previous instalation. Basicly if you look at the head it looks like dead standard TNCS 3P. But then if it is TNCS the EFLI and Ze(n to L) should be the same...???

 

[Engineer54]

I am seeing it coming from the sheath at the intake, which is a supply for them containers only, nothing else. It comes in the made for that purpose pillar/box 15 m away from the main container where the main board will be. Not sure about the incoming cable, the only thing I good see that it has a fine copper strands where the earthing is attached, and I think it is 3P supply too, it has 3 cutouts at the had 100A each, don't want to cut seals on other 2 to prove it by measuring it. It not like i really care, I only use one cutout as per previous installation. Basically if you look at the head it looks like dead standard TNCS 3P. But then if it is TNCS the EFLI and Zen to L) should be the same...???

Well the only DNO cable i know where the earth comes directly off the sheath, is the older PILC cable. These days the DNO's's will use Split concentric cables in there legs up to the TN-S service heads. I doubt these pillars have been in place since the time of PILC's, so i'm a little confused now. Shame you can't post a photo or two. A picture can explain many questions... lol!!

 

[bartkusal]

Well the only DNO cable i know where the earth comes directly off the sheath, is the older PILC cable. These days the DNO's's will use Split concentric cables in there legs up to the TN-S service heads. I doubt these pillars have been in place since the time of PILC's, so i'm a little confused now. Shame you can't post a photo or two. A picture can explain many questions... lol!!

thanks for reply, i dont think it is PILC, there is no lead sheath, probably it is more like split concentric as you say becouse of the fine copper strands visible on the sheath, I promise to do pics tomorrow.