Strange Ze reading

[Obbit]

Hi people

I got a strange reading when doing a Ze test which no one else in the company could explain.

When the main earth to the house was disconnected I got a result of 11.08ohms
When I did the test with it reconnected to the house I got a result of 0.25ohms
When I tested the house with the main earth disconnected I got no result

It wasn't poor connection of the probes, or a failing battery in the tester.

When I started electrical testing I tested a few houses with parallel earth paths in place to see what happened, and always there was barely any difference in the results. I'm flummoxed.

The boss says to leave it and just use the 0.25 reading. However, I don't like leaving it if I don't understand the reading, so any opinions would be much appreciated.

 

[trev]

I'd put money on parallel paths as I've seen lots of vastly different readings with bonding connected. What's the earthing system on this mate?

 

[johnboy6083]

it sounds to me like your relying on your main bonding and other paralell paths for your earth path. If you getting 11.08ohm with the main earth disconnected, on a TNS or TNC-S system, then a phonecall to the DNO is in order.

 

[trev]

My second thought is you can't just put the lower figure down willy nilly but it sounds like you know that anyway

 

[Obbit]

Cheers
It's a TNCS
I've never seen that different results when I played around with the tester in the past.
Would you contact the DNO about the Ze or except the Zs as good?

 

[trev]

Measure it with a different tester and main earth disconnected so that you know that it's just the Ze you're measuring, if it's too high for a TN-C-S then get on to the DNO. I wouldn't accept anything that I wasn't certain about mate

 

[Obbit]

Cheers people.
Good to get a bit of reassurance when dis-agreeing with the boss.
I'll get on to the DNO.

 

[plugsandsparks]

weird, can you see the earth cable go into the DNO head from the cu. As its TNCS you would expect a proper reading but that does rely on the earth wire making it back to the head end. Could also be loose / fall out inside the head but seams unlikely. Not right anyway

 

[malcolmsanford]

Hi people

I got a strange reading when doing a Ze test which no one else in the company could explain.

When the main earth to the house was disconnected I got a result of 11.08ohms
When I did the test with it reconnected to the house I got a result of 0.25ohms
When I tested the house with the main earth disconnected I got no result

It wasn't poor connection of the probes, or a failing battery in the tester.

When I started electrical testing I tested a few houses with parallel earth paths in place to see what happened, and always there was barely any difference in the results. I'm flummoxed.

The boss says to leave it and just use the 0.25 reading. However, I don't like leaving it if I don't understand the reading, so any opinions would be much appreciated.

It must be me, because the rest of the lads have had a go at answering, but for me that as totally threw me.

If it is a TNC-S did you test L-N to see what you got? If you did and you got the same results then you need to contact the DNO they have a real problem.

 

[QSGAZ]

parallel paths and the the reason your not getting a reading is its not connected. simples

 

[trev]

When the main earth to the house was disconnected I got a result of 11.08ohms
When I did the test with it reconnected to the house I got a result of 0.25ohms
When I tested the house with the main earth disconnected I got no result

After reading Malolm's post I realise those 3 statements are contradictory 11.8 ohm with main earth disconnected then nothing???

 

[SirKit Breaker]

There are a good few replies. It is as simple as this.

Isolate the consumers side of the Installation at the main switch.
Disconnect the main earthing conductor after the supply head at wherever it is connected to, earth block, CU or whatever.
Do your Ze test, if it is in excess of 0.35 ohm, you have a problem, get the DNO out to it pronto.
If the installation remains energised, then it is imperative on a TNCS system that all the main bonding is in place.
If the main bonding is not in place then you have an even more serious problem, and energising the system could be life threatening.

Cheers..........Howard

 

[ezzzekiel]

I noticed the conflicting statement also, but either way ze is too high ( unless tt) and parallel path through pipe work reducing value