Discuss TNC Property & filling out EICR in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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Hi. New to the forum. I hope I've chosen the correct category to place my post.
I tested a landlords property recently, turned out to be TNC. I haven't much experience encountering TNC systems. So I've signed up here to pick a few brains.
I'm filling out the report now and keep stumbling on parts due to this type of earthing system.

Bit of info on the property:
Cable enters property into cut-out (60amp) 16mm^2 tails from cut-out to meter, 16mm^2 tails from meter to CU. (No earth as TNC. Do I record the Earthing conductor as 16mm^2?)
Old Wylex board BS5486. Once used re-wireable fuses but now has BS3871 in place.
I then have a 6mm^2 bonding conductor from the Earth-bar in the CU to the Water/Gas

When testing I performed a Zs at the CU., My result 16.3 ohms
I also performed a Zline test at the CU. My result 0.22 ohms & 1.04Ka
Obviously my readings for the Zs are not compliant. I'm questioning myself if I've done it right and how you would test & record the results from a TNC? or would you take the Z Line result? but that's a different test measuring the PFC.

The property also had no supplementary bonding in place, would I be right in thinking due to the system type that it's quite significant? (eg. Code C2)

I'm sure I have more questions to come...
Thanks
 
OP
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At first i thought it was. Couldn't locate an electrode, opened the cover to the CU and only CPC's for circuits + Bonding conductor. The property is a row of council houses, some ex council houses, back to back with each other.
 
OP
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Who would be able to help determine what system has been used? DNO?
Definitely no signs of an Electrode. Each property should have its own electrode right?
Thinking about it, the cut-out they've used would be one you'd find with a TNC-S. Not sure if they use what they've got or use specific types for specific systems. I did perform a test Line to MET and that still returned a reading of 18.8ohms.
 

westward10

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It isn't TN-C as you have separate cpcs and neutrals for your circuits.
It isn't TN-C-S or TN-S as you have no evidence of an earthing conductor from the service head or supply cable however TN-C-S could probably be enabled.
This leaves TT or no earthing system. Where does the "bond" go from the consumer unit.
 
OP
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It isn't TN-C as you have separate cpcs and neutrals for your circuits.
It isn't TN-C-S or TN-S as you have no evidence of an earthing conductor from the service head or supply cable however TN-C-S could probably be enabled.
This leaves TT or no earthing system. Where does the "bond" go from the consumer unit
Would there be anyone (DNO for example) who would be able to confirm what Earthing system has been used?
The bond, annoyingly enough goes under the floor and is attached onto the pipework under the floor. Even though the gas meter is in the house I was able to stick my head under a little hatch and see. However the hatch wasn't big enough for me to get in and get a proper look.

Have you measured Ze at the cut out or CU?
I measured Ze from the CU. Main fuse was tagged, I know it's a bit of a grey area cutting tags & removing main fuses. However it would give me a better insight of what's going on inside the cut out I suppose.
 

DefyG

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Its possible that you would still get an earth reading (Zs) through parallel earth paths even though the earth electrode is missing.
As #8, would determine if TT or TN-C-S is possible.

Do I record the Earthing conductor as 16mm^2?)
What earthing conductor, I thought you said there wasn't one?
I measured Ze from the CU. Main fuse was tagged, I know it's a bit of a grey area cutting tags & removing main fuses. However it would give me a better insight of what's going on inside the cut out I suppose.

You dont need to remove the main cutout fuse to test Ze
 

timhoward

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The bond, annoyingly enough goes under the floor and is attached onto the pipework under the floor. Even though the gas meter is in the house I was able to stick my head under a little hatch and see. However the hatch wasn't big enough for me to get in and get a proper look.
Get your phone down the hole and take a photo!

A few random questions...
Are you sure there isn't a main earth going straight through the wall behind the CU and down outside in an alleyway to an electrode - I had one like this once and it was quite well hidden.

I have had a couple of rare houses where the water pipe was still used as the earth, which hasn't been allowed for longer than anyone can seem to remember (I asked on here once), but they did have an up front RCD or VOELCB indicating it was designed as TT.

If you have any photo's feel free to post them by the way.
 
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Its possible that you would still get an earth reading (Zs) through parallel earth paths even though the earth electrode is missing.
As #8, would determine if TT or TN-C-S is possible.


What earthing conductor, I thought you said there wasn't one?


You dont need to remove the main cutout fuse to test Ze

No earthing conductor. If TNC was utilised. Would you take the conductor size of the Neutral? (For future reference. Like I say I haven't dealt with any TNC systems)

I did the test at the CU. Didn't remove the main earth (Assumed missing) Incoming Line to Earth bar of CU. Would it be ideal to run the test with the main bond removed?
 

westward10

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If you understand earthing systems you would realise it is not TN-C as this requires a PEN conductor through your circuits, in other words a line and neutral whereby the neutral is the PEN (Protective Earth Neutral). There is no separate cpc and the earth is connected from the neutral through your system. If you cannot verify an earthing conductor either from the head, service cable or electrode you have no valid earthing system.
 
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DefyG

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I did the test at the CU. Didn't remove the main earth (Assumed missing) Incoming Line to Earth bar of CU. Would it be ideal to run the test with the main bond removed?
All earth bonding conductors and cpc's are removed from the earthing conductor (of which you dont have) to test for Ze, so you would have to install an earth electrode to test and/or test to N to determine if TN-C-S is possible. Currently it looks like there isnt't an effective earth (C1)!
 

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