No Ze through main earth?...

[Baddegg]

Testing Ze (TNs) prior to fitting a small 2 way unit for a kitchen....disconnected main earth and my tester is flashing telling me polarity is wrong ( it’s not polarity as I can confirm that visually) so I reconnect the main earth and test straight to the external MET and get a reading.....so obviously parallel path giving me a reading, out of curiosity I also clamped sheath of DNO incoming with main earth connected to it and MET and again get a reading (still parallel path) but as soon as test with main earth only I get no reading?...customer contacting supplier tomorrow but am I missing something or have the DNO lost the earth??

 

[Ian1981]

When you say no reading what do you mean?
Does it go over scale eg greater than 1999 ohms or just read 0 on the display?
Or does the meter not allow the test to be carried out due to a high voltage( 50 v touch voltage exceeded) on the earth?

 

[ipf]

When you say no reading what do you mean?
Does it go over scale eg greater than 1999 ohms or just read 0 on the display?
Or does the meter not allow the test to be carried out due to a high voltage( 50 v touch voltage exceeded) on the earth?

He's testing external impedance.....live.
Seems like a supply fault.

 

[Baddegg]

thanks for replying Ian, my meter flashes 0 v and the reverse polarity light flashes, so I can’t perform the high loop test function but as soon as I move the clamp from the disconnected main earth and put it on the MET (all other bonding in place) the reverse polarity light goes green the voltage reads correctly and I get a reading of 0.14,so reading is fine it’s just not through the main earth (I think the polarity light in this instance indicates that all three probes are not attached ie when on the disconnected main earth it’s not going anywhere) never experienced it before

 

[Ian1981]

He's testing external impedance.....live.
Seems like a supply fault.

Yes I get that..
I was asking the reading displayed on the test instrument
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thanks for replying Ian, my meter flashes 0 v and the reverse polarity light flashes, but as soon as I move the clamp from the disconnected main earth and put it on the MET (all other bonding in place) the reverse polarity light goes green the voltage reads correctly and I get a reading of 0.14,so reading is fine it’s just not through the main earth (I think the polarity light in this instance indicates that all three probes are not attached ie when on the disconnected main earth it’s not going anywhere) never experienced it before

Sounds like a fault with the means of earthing into the installation
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You mentioned testing the sheath of the dno supply but with parallel paths in place,have you tried testing it on its own, parallel paths removed?

 

[Baddegg]

Yes tried that as well and again reverse polarity/no reading which points to supply issue I assumed
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I’ll update once I be spoken to customer/DNO...

 

[DefyG]

I also clamped sheath of DNO incoming with main earth connected to it and MET and again get a reading

The sheath (do you mean lead?) is the DNO 'main earth' conductor if TN-S (Max Ze 0.8ohms)

 

[Baddegg]

The sheath (do you mean lead?) is the DNO 'main earth' conductor if TN-S (Max Ze 0.8ohms)

Yes mate the lead sheath but initially I disconnected the 16mm main earth cable from the met and clamped on to it to get my readings and nothing so as discussed above I also clamped on to the sheath/DNO earth and still nothing unless I left the main earth connected to the Met and the sheath then I’d get a reading but obviously not true as it’s reading parallel paths..

 

[DefyG]

Sounds like a supply fault, had a few myself locally but usually high Ze readings.
Recorded 137ohms on one installation for a TN-s (Max 0.8)
DNO sorted it.
DNO your best bet.

 

[Baddegg]

Sounds like a supply fault, had a few myself locally but usually high Ze readings.
Recorded 137ohms on one installation for a TN-s (Max 0.8)
DNO sorted it.
DNO your best bet.

Thanks mate yep customers onto DNO tomorrow, strange my readings were so low then? Always consistent at 0.14-.015...

 

[Pretty Mouth]

Had similar recently when went to fit a loft light. Ze > 1999 ohms, but approx 5 ohms with earthing conductor connected to MET. In my case the clamp (which looked very much like a BS951) connecting to the incoming sheath has visibly corroded.

 

[Baddegg]

Update on this......customer rang DNO this morning and they were out within the hour confirmed fault and are sorting this afternoon...