Different Ze readings

vwes59 · Jan 19, 2022

Hi, I've had an unusual result from taking a Ze at a clients house and wondered what everyones take on it would be:

The property is TN-C-S

A Ze reading from a disconnected Earth to Line gave a reading of 0.45 ohms
A Ze reading from the Line and Neutral gave a reading of 0.33 ohms

A TN-C-S supply should be no more than 0.35 ohms

I'm sure the Ze readings should be the same, but I could be having a senior moment. What concerns me, is that one is out of range, the other is fine. So have I got an issue to refer to the DNO or not ?

Just to add, I might have been a numpty. I disconnected the earth from the earth bar in the consumer unit, but the earth bonding came from a MET to the water and gas, which was still connected, so potential for parallel paths. I suspect I should have disconnected from the MET not the CU. Would that explain the values I've seen ?

ImpededLoop · Jan 19, 2022

For TNCS the line-neutral and line-earth Ze values should be identical as the main earth terminal is connected directly to the neutral in the cutout.

For a true Ze, disconnect the main earthing conductor and test from the MET terminal on the cutout so that you eliminate any parallel paths.

0.35 is the absolute highest value for TNCS. Are you sure it's not TNS?

westward10 · Jan 19, 2022

It is not cast in stone that the Ze for a TN-C-S system should not exceed 0.35 ohm however your line to earth and line to neutral should be nearly the same if not then you have a different earthing system or there is a problem with the existing set-up.

telectrix · Jan 20, 2022

went to a fault last year. . TNC-S earthing. Ze was 66 ohms. 2 sparks had been and given up, one had actually bonder the kitchen sink FFS. turned out to be that the link plate from N to E in the cut-out was loose at one end. tightened up with my VDE 3mm allen key and boomp. 0.18 ohms. seal fairy was booked in for xmas.

brianmoooore · Jan 20, 2022

telectrix said:
went to a fault last year. . TNC-S earthing. Ze was 66 ohms. 2 sparks had been and given up, one had actually bonder the kitchen sink FFS. turned out to be that the link plate from N to E in the cut-out was loose at one end. tightened up with my VDE 3mm allen key and boomp. 0.18 ohms. seal fairy was booked in for xmas.

How can anyone who calls themselves a spark fail not to find this one? TNC-S means, by definition, that N and E are a common point at the cut out, so any Ze at where you can measure it, will just be the resistance of the tail and main earth.

7029 dave · Jan 20, 2022

True Ze test should be carried out at the cutout, with all tails /earth leads disconnected towards the electrical installation, after all this test is an external earth loop impedance test.

brianmoooore · Jan 20, 2022

7029 dave said:
True Ze test should be carried out at the cutout, with all tails /earth leads disconnected towards the electrical installation, after all this test is an external earth loop impedance test.

And if the supply is TNC-S, then the result will be zero ohms, since your meter probes are on either end of three lumps of brass, bolted together.

7029 dave · Jan 20, 2022

brianmoooore said:
And if the supply is TNC-S, then the result will be zero ohms, since your meter probes are on either end of three lumps of brass, bolted together.

Sorry dont understand your answer, three lumps of brass bolted together? From where.
This picture admittedly is the new type of cutout combined N&E, the knock out on the side is the E connection for the main earth lead to the CU, however the older type only have one link between the N and E block.
So my question is why the Zero ohms.

brianmoooore · Jan 20, 2022

One block of brass for the type shown. Older types have a brass block for the neutral, another brass block for the earth, and if it's set up for TNC-S, rather than TNS, a brass bar bolted between them. Three lumps of brass.

Dave OCD · Jan 20, 2022

Just recently I was looking at a WPD quote for a new supply and was quite surprised to see they now quote a max Ze of 0.48 for TNCS ! [Unless it was a misprint ?!} Maybe they're acknowledging an ageing and little maintained network ?