25 ohms Zdb + same reading for all circuits

[nik0s95]

Hi everyone,

I have a particular job visited this morning, that on all circuits, at each point, i got a reading off around 25ohms for a earth fault loop impedance test(zs).

I then thought to do the same at the DB for those circuits. Again, same reading was obtained.

I then turned off the whole installation, to obtain a Ze with the incoming earth disconnected. Same value obtained.

Can anyone shed any light to what may be going on here, I suspect an issue on the DNO side of things?

Thanks in advance.

Nick Sage

 

[westward10]

What is your earthing system.

 

[nik0s95]

What is your earthing system.

Hi Westward,

Earthing system appeared to be TN-S.

Nick

 

[davesparks]

That's a DNO issue by the sounds of it

 

[GBDamo]

A slightly embarrassing admission but after a long day of fault chasing, tripping sockets, not Ze related, I reinstated the circuit and on my way out did a quick Zs and got a similar double digit Zs.

Chased it all the way to the MET and had the same result from the bus bar camber to the MET.

As I could physically see the main earth all the way to the MET I just assumed it was a poor incoming earth (TNS)and called the DNO.

Hung round and they turned up within the hour, popped the head off the service head and showed a Ze of 0.12 Ohms.

Put the main earth back on the MET bar and back up to double digits, turned out to be a corroded MET bar. Not green and crusty just a dull brown colour.

Unbolted all the various earth's and bondings and took some emery cloth to the bar, coupled it all back up and Zs at the bus bar chamber was 0.12Ohms.

Never seen it before or since, very weird.

 

[timhoward]

If you're right about TN-S then this needs urgently looking at. Don't be afraid to call the DNO, better safe than sorry.

How much (if any) of the installation has RCD protection?
A fault to earth would give a fault current of 9 amps, and without RCD protection the only thing that might trip is lighting, and then only after around 50 minutes! So this is potentially quite a dangerous situation.

Do you have any photos?

 

[HappyHippyDad]

A slightly embarrassing admission but after a long day of fault chasing, tripping sockets, not Ze related, I reinstated the circuit and on my way out did a quick Zs and got a similar double digit Zs.

Chased it all the way to the MET and had the same result from the bus bar camber to the MET.

As I could physically see the main earth all the way to the MET I just assumed it was a poor incoming earth (TNS)and called the DNO.

Hung round and they turned up within the hour, popped the head off the service head and showed a Ze of 0.12 Ohms.

Put the main earth back on the MET bar and back up to double digits, turned out to be a corroded MET bar. Not green and crusty just a dull brown colour.

Unbolted all the various earth's and bondings and took some emery cloth to the bar, coupled it all back up and Zs at the bus bar chamber was 0.12Ohms.

Never seen it before or since, very weird.

Bit confused. Were you working alongside the OP, or is this a separate thread to the OP's?

 

[GBDamo]

Bit confused. Were you working alongside the OP, or is this a separate thread to the OP's?

No, just a related story from my past.

A long winded way of saying check the MET.

 

[cireland]

Other possible scenario is a fault with your meter. Do you have access to another?

 

[HappyHippyDad]

No, just a related story from my past.

A long winded way of saying check the MET.

Oh I see, I was confused! Cheers

 

[westward10]

Is it earthed by way of the cable sheath if so can you direct Ze to the sheath.

 

[Aaron b]

I had some silly Zs readings once and I just hadn't zeroed the meter properly. Were the R1+R2 readings done with the same probes?

 

[7029 dave]

When you took Ze did you disconnect the main earth lead away from the installation. We dont want parallel paths.

 

[pc1966]

I had some silly Zs readings once and I just hadn't zeroed the meter properly. Were the R1+R2 readings done with the same probes?

They could do "Zs" with N & E probes both to the N and should get the same for PFC and PSCC and it should be in keeping with the supply rating.

So you would expect, say, no more than 10% DNO side drop at the fuse rating, so 600A PSSC or above, hence "Zs" measured that way should be around 0.4 ohm max.

 

[pc1966]

When you took Ze did you disconnect the main earth lead away from the installation. We dont want parallel paths.

Agreed, but high Zs means either nothing useful in terms of parallel paths to the true Earth vis service pipes, etc. Or the meter (or its probes) is borked!