High Ze

[Donkey_Dave]

Hello,

Doing an EICR for a storage company who share an old aircraft hanger with an engineering firm.The mains are shared and split at the main intake position by means of busbar and switch fuses which date from the 50's. The earthing system is TNS, with the mains armoured into a cast iron spreader box and the main earth off of this.
Not able to isolate the mains,I found an unused switch fuse at the mains position and carried out the Ze test.
It was performed with a Fluke multi meter (calibrated,check boxed,new leads) Earth on the earth bar, Neutral on the neutral bar and the Live across each of the phases. L1-1.10 ohms L2-1.41 ohms and L3- 1.09 ohms
Hmmm, thinks I, may have an earth fault. Contacted SSE who turn out and put a two wire test on the Ze. Neutral and earth of there meter to the earth bar and Live to each of the phases. 0.68 was there result. 'nothing wrong with that,mate'.
OK, I can replicate there two wire meter test and get the same results , however, when I try and record a Zs at a socket or light I am back with the higher values.
Why the discrepancy between the three wire and two wire Ze results?
Why does connecting the meter to neutral make such a difference?

Thanks
Dave

 

[Simonslimline]

Are you set on no trip or high loop setting?

We used the megger 1553 at my old job and when taking Ze it would be a 2 lead high loop test. If taking Zs with RCD's in circuit then a 3 lead no trip.

 

[Donkey_Dave]

Are you set on no trip or high loop setting?

We used the megger 1553 at my old job and when taking Ze it would be a 2 lead high loop test. If taking Zs with RCD's in circuit then a 3 lead no trip.

But would the results not be the same wether using the 2 lead or 3 lead?

 

[Plonker 3]

How have you taken Ze if you haven't isolated the supply and disconnected all parallel paths?

 

[Donkey_Dave]

As I posted,not able to isolate supply as factory in use. Would parallel paths cause this difference between the two test results? I would have thought I would be getting a better earth with the parallel paths.As it happens nothing is bonded as such.

 

[Plonker 3]

No, but you need to verify weather the bad reading is caused internally or externally on the DNO cable first.

 

[Donkey_Dave]

Yes that is a good point. I may well have a dodgy neutral connection throwing every thing off.
what I do need to know is if the 3wire Zs test get the same results as a 2wire test.I'll try it tomorrow on a supply somewhere
thanks
Dave

 

[davesparks]

Why were you using a 3 wire test at the intake?
Always test at a higher current whenever possible. The 3 wire test is designed as a work around for carrying out loop impedance tests on RCD protected ccts, not an across the board replacement for a proper test.

 

[Simonslimline]

But would the results not be the same wether using the 2 lead or 3 lead?

Have a look at Guidance note 3 on earth loop impedance testing. It explains this a bit.

 

[Donkey_Dave]

What's baffling me is I have taken a Ze and Zs from the same switch fuse and arrived at significantly different results-something must be wrong

 

[telectrix]

iif you use the no-trip setting, the test current is only 15mA. any joint/termination with a small resistance will give you spurious results. the 20A current on hi setting will overcome such a resistance and give you a sensible result.

 

[Fox]

Yep, a 2 or 3 wire high current loop test will eradicate any electrical noise or interference on the circuit, whereas a lower current test will send multiple pulses of around 10-15mA into a circuit and average out the multiple impedance results.


Where an RCD is not installed, it's always best to use a high current loop test to obtain the most accurate results. So many variables can effect a loop test result

 

[Engineer54]

What's baffling me is I have taken a Ze and Zs from the same switch fuse and arrived at significantly different results-something must be wrong

That's because the test for Ze and Zs are different tests and therefore can/will give totally different values!!
If you're trying to conduct a Ze test with a 15mA test load what do you expect?? Ze is is only conducted on the DNO's supply line (on the high current setting of your test equipment)...eg phase to the DNO's provided earth!! Which from what i can understand from your OP post, is exactly what the DNO electrician did to measure the installations Ze value, and hey presto,.... a measured Ze value of 0.68 ohms!

 

[tony mc]

This may be of interest to you re No trip loop testing !
View attachment Why-do-non-trip-loop-tester-results-differ.pdf