Low Zs at socket, Any ideas??

T

TheCodboy

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Arms
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
 
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operator error. with a Ze of 0.26, it's impossible the get a Zs that low.
 
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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.
 
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Gavin John Hyde said:
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.
Click to expand...
Its the first time ive used it since its been calibrated, just changed the batteries this morning. Ill try everything again tomorrow!
 
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Gavin John Hyde said:
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.
Click to expand...
Its the first time ive used it since its been calibrated, just changed the batteries this morning. Ill try everything again tomorrow!
 
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Gavin John Hyde said:
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.
Click to expand...
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.
 
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telectrix said:
operator error. with a Ze of 0.26, it's impossible the get a Ze that low.
Click to expand...

Wilko said:
Hi - can't have Zs lower than Ze, so likely a measurement error and I'd just redo it :) .
Click to expand...

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 :)
 
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happyhippydad said:
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 :)
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agreed, but why does OP have the low reading at just 1 socket ?
 
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i've got it. the contrary socket is made of anti-matter. it's probably blown your MFT into space.
 
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i've got it. the contrary socket is made of anti-matter. your MFT is now useless on this planet.
 
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happyhippydad said:
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 :)
Click to expand...
Yes, I've rather stepped into it here, haven't I? I'm thinking as Ze was 0.26 the bonding in/out isn't going to cause this. I have measured bonding as low as 2 Ohms (ish) which would've given 0.23 Ohms for example.
Whole different story with TT :)
 
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So this socket is definitely part of the ring final circuit? Not a radial or anything odd. Were you measuring Zs on L-PE or L-N was ATT on or off or accidentally switched off for it? Was the right type of MCB selected on the MFT? That would bug the hell out of me 'til I got the bottom of it.
 
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Harmonics/ highly inductive cable run/ EMI.
Try plugging something like a 3kW resistive heater in to the other socket if it's a double to reduce the effect of the mft loading the circuit.
 
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Zsocket can be lower than Ze due to bonding, and even lower than Zdb if there is a local earth path to the socket that is very low indeed. Unlikely though, I agree.

One oddity that has given me nonsensical low readings is incorrect nulling of the test leads. Some testers cannot or could not in the past display a negative reading for impedance. So if you null the leads with say 0.4Ω additional resistance due to poor contact, measuring 0.2Ω will still read 0.2Ω but measuring 0.39Ω will read 0.01Ω.
 
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Lucien Nunes said:
Zsocket can be lower than Ze due to bonding, and even lower than Zdb if there is a local earth path to the socket that is very low indeed. Unlikely though, I agree.

One oddity that has given me nonsensical low readings is incorrect nulling of the test leads. Some testers cannot or could not in the past display a negative reading for impedance. So if you null the leads with say 0.4Ω additional resistance due to poor contact, measuring 0.2Ω will still read 0.2Ω but measuring 0.39Ω will read 0.01Ω.
Click to expand...
All tested fine today so I’m happy enough now. Giving the expected results. May have been something squiffy with the mft or something silly like that.
 
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TheCodboy

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Low Zs at socket, Any ideas??
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