Ze Testing Dilemma

[Nicky Tesla]

When fitting a DB,  ie an install,  measuring the Ze the correct GN3 way is fine,  but measuring Ze during a condition report is not.    During a periodic you are supposed to switch everything off,  then pull out the earthing conductor from the MET and then test, ensuring no pararllel paths.    GN3 also says carry out your tests with minimum dismantleing and dissruption.  Getting a GN3 style Ze on say a health centre or call centre would be very dissruptive.   Imagine announcing you are having to shut the buildings electrics down while you make the test.  Staff and others may have to vacate the premises for H&S reasons or rely on emergency lights while all the computers shut down.

GN3 says that measuring Ze will be easier than using the DNO enquiry values ( if you can get them). I have been in many buildings while contractors have been doing periodic testing and i have never known them to switch off the supply,  but just take the measurement with all the circuits energised and the bonding etc still connected,  as do I in that situation.  Can you just enter 0.35 as standard without measuring on a TNCS and 0.8 on TNS?.  The situation is rediculous.  You basically get forced to cut corners for inconvenience sake  and them someone who doesn't have to do the test themselves will say you have done the test incorrectly.

Not including the above tests I think the whole testing world is a farce.   Very few test sheets i see are ever filled in correctly and the testers dont have a clue about many regs at all.

When doing periodics,  you should be allowed to enter Ze with all conductors connected, and then a Zs.

The other method Zs = Ze + (R1+R2)  relies on Ze.        

 

[NozSpark]

That's the thing you see...

Zenis the external loop impedance, its impossible to do it without disconnecting the parallel earth paths

You could however do a Zdb

 

[steptoe]

yep,

have a read of EAWR

the definition of possibility of death is no consequence to inconvenience,

you have a choice,

either LIM it , or do it properly,

 

[binky]

what's the chance of a fault occurring during the few seconds of taking a reading? and with main bonds still in place there's probably a good earth fault path anyway.

 

[batty]

Does it actually matter it is very unlikely main earth's and main bonds and supplementary bonds will change as long as your disconnection times are in spec just record this and put a note on your certificate that you cannot disconnect earth's due to not being able to disconnect supply.

 

[apprentice87]

Yes, but measure with the bonding in place, and you will not get Ze because of parallel paths. This could be very dangerous..

Say you design bits of an installation with "Ze" taken with bonds in place, say to maybe, an old gas pipe... What is going to happen if next week transco swap the old pipe for a metal one..... For all you know, the DNO earth might be missing completely, but you would not know as you tested with bonding in place and so got a false reading....

Further, no you cannot just assume that the 0.35 and 0.8 figures are correct, these are just for guidance..... I have seen a new install with a Ze of .52 [if not more] on TNCS

Like Steps said, if you cannot do it properly, just put it as a "lim"

john...

 

[SLIPSHOD & SLAPDASH]

In this situation, normally commercial, i record it in the limitations box. Numerous times on NICEIC assessments i have been told to demonstrate a Ze test without any disconnections whilst its compared with my certificate.

 

[apprentice87]

Assessors must be dull!!!! I would refer them to canoes post above!!

 

[Nicky Tesla]

Cheers fellas il try the LIM thing but i dont think the QS will be happy.

 

[SLIPSHOD & SLAPDASH]

Assessors must be dull!!!! I would refer them to canoes post above!!

In my case the assessor knows me from previous visits &  knows that i know the correct procedure. The purpose of the test appears to be to  check my previously recorded values, which on domestic jobs i have found never change much when bonds/earth  are disconnected.

I remember doing the 16th edition upgrade course and the lecturer saying something like  'you domestic guys, worrying about your .2 0r .8, wait till your doing a R1 + R2 on a 500m feed to a factory alteration'

 

[Nicky Tesla]

Never changes on TNCS and rarely but only a tiny tiny amount on TNS

 

[steptoe]

Never changes on TNCS and rarely but only a tiny tiny amount on TNS

what never changes?

Ze ?

:shakehead

 

[apprentice87]

Hi Nicky,

I think you will find it changes a great deal...

john...

 

[kerching]

Surely The Ze values are only accurate at the time that you test it with a suitably calibrated instrument

Example....jointer working on the supply cable from the sub, jumpers the earth ( thus reduces impedance ). Then cuts cable earth. IF his jumper cable is exactly the same as the sheath then there is no difference BUT if his cable has less or more impedance then the value alters.

Xformer tappings get changed

Joints break down

There is a whole host of variables

Merely commenting

 

[Steve3948]

In my case the assessor knows me from previous visits & knows that i know the correct procedure. The purpose of the test appears to be to check my previously recorded values, which on domestic jobs i have found never change much when bonds/earth are disconnected.

I remember doing the 16th edition upgrade course and the lecturer saying something like 'you domestic guys, worrying about your .2 0r .8, wait till your doing a R1 + R2 on a 500m feed to a factory alteration'

Same as.

SSE wont provide an earth if its over 0.35 on TNC-S, further North I am told they will to 0.45.

 

[Apache]

Does anyone else keep reading the title of this thread in a German accent?

 

[Nicky Tesla]

That's due to European harmonisation

 

[Adrian]

Im a bit lost on this post, you talk of not disconnecting the main earth to carry out a Ze because you need to turn off the supply. Don't you turn off the supply to do insulation resistance tests, ring continuity tests ect so whats the problem or do you put lim on everything and just carry out a visual inspection. 

 

[binky]

Never changes on TNCS and rarely but only a tiny tiny amount on TNS

I've had supplies with ******** neutral several times, this test needs to be done along with PSC and PSCC. Dropping out the main earth for 30 seconds to do a test is no bg deal and does not require shutting everything down.

 

[Steve3948]

I've had supplies with ******** neutral several times, this test needs to be done along with PSC and PSCC. Dropping out the main earth for 30 seconds to do a test is no bg deal and does not require shutting everything down.

Hope you have good insurance if your leaving power energised and removing main earth, with building in use  :coat