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

 

[Wilko]

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

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

 

[westward10]

Could be because it's a brand new socket.

Could be the mft, needs a proper tester.

 

[The Ghost]

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.

 

[Stueyb]

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.

 

[Deleted member 26818]

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

Actually it can.

 

[Toneyz]

What was your RFC end to end and R1+R2? is there anything plugged in/spur with parallel earth path/supplementary bonding between circuits. Could be dirty contacts see what the readings are on the terminals of existing sockets.

 

[TheCodboy]

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.

ze

 

[TheCodboy]

Well,tested it first thing when I got here..... 0.28. Tested about 5 times,virtually the same numbers. Unsolved mystery!!

 

[loplug]

What r1 rn r2 R1+R2 you got? Have you checked correct connection of the socket ?

 

[telectrix]

Well,tested it first thing when I got here..... 0.28. Tested about 5 times,virtually the same numbers. Unsolved mystery!!

i stand by my post#3.

 

[Lucien Nunes]

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.

 

[TheCodboy]

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.

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.

 

[Octopus]

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.

So what were your dead test results ......

 

[TheCodboy]

So what were your dead test results ......

r1 0.35 rn 0.35 r2 0.79 end to end
R1+R2 0.29 Zs at that socket 0.25-0.29
Zs at Db is 0.21,Which is lower than Ze all of these are tested numbers,not calculated.

 

[Deleted account]

Try doing the external loop test with main earthing in the MET.

 

[davesparks]

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

Why can’t ypu have a Zs lower than Ze?

 

[davesparks]

Could be the mft, needs a proper tester.

What do you mean? Are you saying that an MFT is not a proper tester?

 

[davesparks]

Was this test carried out with a low current/anti trip test by any chance?

 

[Adam W]

ze

Eh?

 

[TheCodboy]

Was this test carried out with a low current/anti trip test by any chance?

No trip setting

 

[TheCodboy]

Eh?

Fat finger syndrome

 

[Wilko]

Why can’t ypu have a Zs lower than Ze?

Yes, thanks for that
Unfortunately my response was not written in detail but post 26 might help with my thinking at the time.

 

[Adam W]

Fat finger syndrome

Ah.

 

[westward10]

What do you mean? Are you saying that an MFT is not a proper tester?

I was being derisive that these mfts seem to need a bit of pampering for them to function.

 

[davesparks]

I was being derisive that these mfts seem to need a bit of pampering for them to function.

I only have personal experience of 5 MFTs, only one of them had any issue which was a fluke with a dodgy 4mm banana socket. Apart from that they’ve not had any need for pampering