Low Zs at socket, Any ideas??

[TheCodboy]

Hi all, while testing Zs on a ring final circuit i found one socket to have a particularly low Zs. All others were reading between 0.25- 0.30 but this one was reading 0.08-0.09. Its got a brand new Crabtree double socket installed. A full RFC test has been carried out, and this is the only socket with a reading this low. Obviously i know its better than high, but was just wondering if anyone could shed some light.
Its a TNS system, Ze 0.26

 

[Leesparkykent]

Damp wall?

 

[telectrix]

operator error. with a Ze of 0.26, it's impossible the get a Zs that low.

 

[TheCodboy]

Damp wall?

Seem's to be dry,no signs of damp that i can see.

 

[TheCodboy]

operator error. with a Ze of 0.26, it's impossible the get a Ze that low.

Its the only socket thats giving such a low reading, new batteries in the MFT, tested a few times and its still reading that.

 

[Octopus]

What was the r1 + r2 for this socket?

 

[westward10]

Is your quoted Ze with the main protective bonding in place.

 

[TheCodboy]

What was the r1 + r2 for this socket?

I'll have to go back to my notes and double check but I've left my notes at the job. Ill be going back tomorrow.

 

[TheCodboy]

Is your quoted Ze with the main protective bonding in place.

No, thats just the 16mm earth taken out of earth bar.

 

[loplug]

Is the circuit rcd protected? Have you done insulation reseistance testing. Could a neutral earth fault in the socket or box be giving a false low reading?

 

[Octopus]

I would be checking the ring again - all dead tests ..... and the Ze

 

[TheCodboy]

Is the circuit rcd protected? Have you done insulation reseistance testing. Could a neutral earth fault in the socket or box be giving a false low reading?

Its on a RCBO, IR tested it before connecting it. Tested 500M ohm

 

[TheCodboy]

I would be checking the ring again - all dead tests ..... and the Ze

I'll test it again tomorrow. Just strange why its just this one socket.

 

[Wilko]

Hi - can't have Zs lower than Ze, so likely a measurement error and I'd just redo it .

 

[Gavin John Hyde]

check the leads, batteries and the fuse in back of your mft is nice and tight. then zero/null your leads. retest some of the others to check consistency then re test the problematic one. should give you an indication of whether its the mft or not.
I often find that if i dont use the kewtech kt64dl mft for a few days - say over a bank holiday weekend then on the tuesday the results can be fluctuating a bit, so i now do the things i said above each morning. and my results are now much more stable. i dont seem to get weird anomalies like before.

 

[TheCodboy]

check the leads, batteries and the fuse in back of your mft is nice and tight. then zero/null your leads. retest some of the others to check consistency then re test the problematic one. should give you an indication of whether its the mft or not.
I often find that if i dont use the kewtech kt64dl mft for a few days - say over a bank holiday weekend then on the tuesday the results can be fluctuating a bit, so i now do the things i said above each morning. and my results are now much more stable. i dont seem to get weird anomalies like before.

Its the first time ive used it since its been calibrated, just changed the batteries this morning. Ill try everything again tomorrow!

 

[TheCodboy]

check the leads, batteries and the fuse in back of your mft is nice and tight. then zero/null your leads. retest some of the others to check consistency then re test the problematic one. should give you an indication of whether its the mft or not.
I often find that if i dont use the kewtech kt64dl mft for a few days - say over a bank holiday weekend then on the tuesday the results can be fluctuating a bit, so i now do the things i said above each morning. and my results are now much more stable. i dont seem to get weird anomalies like before.

Its the first time ive used it since its been calibrated, just changed the batteries this morning. Ill try everything again tomorrow!

 

[Adam W]

Hi - can't have Zs lower than Ze, so likely a measurement error and I'd just redo it .

Theoretically, but in reality you should measure the Ze with the main earth out, and the Zs with all bonding, ie parallel paths in place.

 

[Adam W]

I often find that if i dont use the kewtech kt64dl mft for a few days - say over a bank holiday weekend then on the tuesday the results can be fluctuating a bit, so i now do the things i said above each morning. and my results are now much more stable. i dont seem to get weird anomalies like before.

I had one of those at my last place; it's a really easy tester to use but I found almost bang on the day it was due for calibration I started getting really high readings when I measured Zs using the probes. For some reason it was fine using the plug adaptor, it was just off using the probes. It was fine again after calibration.

 

[HappyHippyDad]

operator error. with a Ze of 0.26, it's impossible the get a Ze that low.

Hi - can't have Zs lower than Ze, so likely a measurement error and I'd just redo it .

I've always thought that, but what about the following scenarios..

1. Property is TNCS or TNS. The metal pipework on the bonding is deep underground and aids in the Zs (but is not included in the Ze as the main earth has been removed from the MET). If the additional earth pathway created by the bonding outweighs the R1+R2 couldn't the Zs<Ze?

2. Also, what if you have an Ra = 200Ω (TT) and the bonding of the property is also attached to the bonding of the adjacent property which is TNCS? Wouldn't the Zs of the property be influenced by the adjacent properties TNCS?

3. Ra = 200Ω (TT). Pipework on gas/water deep underground. Would this not influence the Zs and perhaps make it <200Ω

Just having a sit in the garden and pondering these deep matters