TN-S High Ze measured 3 wire no trip.

[Bruno Branson]

Hi knowledgeable collective.

To the chase. TN-S Megger 1553

2 wire Ze 0.09 but was getting Zs on cooker cable 1.5 metre long using No trip 3 wire got 33.4ohms So went back through system right to isolator post meter and was still getting 33 ohm reading. So 2 wire uses higher current so might overcome a flakey connection but to this extent? Chucked meter on home circuit as was doubting meter measured normal. Back tomorrow to try to reveal top of incomer then prob onto DNO but would like to know what might be occuring.

 

[timhoward]

Did you try 2 wire Zs@DB or at isolator?
I'd be very surprised if this was anything to do with 2/3 wire test.
First question is are you sure you reconnected the earth properly after Ze?
It sounds like a loose live tail at isolator, meter or cut-out, or loose main earth conductor.

 

[Bruno Branson]

Did you try 2 wire Zs@DB or at isolator?
I'd be very surprised if this was anything to do with 2/3 wire test.
First question is are you sure you reconnected the earth properly after Ze?
It sounds like a loose live tail at isolator, meter or cut-out, or loose main earth conductor.

Hi, Thanks for your thoughts. 2 wire test at main isolator 0.07 I think. 3 wire 33.3. I think it must be a bad connection just wondering what & why such a big disparity.

 

[timhoward]

Hi, Thanks for your thoughts. 2 wire test at main isolator 0.07 I think. 3 wire 33.3. I think it must be a bad connection just wondering what & why such a big disparity.

Oh sorry, didn't twig you were measuring 2 and 3 wire at same point. As you say they are a mile apart and that is indeed interesting.
As the 2 wire test uses Live and Earth, and the 3 wire obviously adds Neutral, I'm just wondering what effect a loose N conductor would have on the test result....
I wouldn't be happy until I knew why either.

 

[telectrix]

Most likely is uplift caused by RCD or MCB. try a 3 wire no trip on the outgoing of the RCD.

 

[timhoward]

Most likely is uplift caused by RCD or MCB. try a 3 wire no trip on the outgoing of the RCD.

We are reading it differently - think he’s saying at isolator at intake he’s getting wildly different reading for 2 wire and 3 wire tests.
Upstream of CU by sounds of it.

 

[Strima]

Have you done a two lead test between L&N?

 

[Bruno Branson]

Most likely is uplift caused by RCD or MCB. try a 3 wire no trip on the outgoing of the RCD.

That was my initial thought, but such a high figure. Then found I was getting pretty much the same reading everywhere. Seems to be related to the income as all else isolated off.

 

[Bruno Branson]

Have you done a two lead test between L&N?

No, brain wasn't in gear & thought of that after I left.