Direct measurement of Zs

Question from Exam Success in Inspection and Testing 2391-10-the IEE Citry and Guilds-

Describe briefly how to carry out,by direct measurement an earth loop impedance test on the workshop lighting circuit.

Answer-

Secure safe area ariund last luminaire on the circuit

use earth loop impedance tester

leads to gs38

furthest point in circuit test between switched line and cpc

with circuit energised test and record result

reinstate the luminaire

This is surely against EWR 1989 working on live circuit ?

This is surely against EWR 1989 working on live circuit ?

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It must be unreasonable in all the circumstances to work on it dead, and it must also be reasonable in all the circumstances to work on it live, and appropriate PPE / risk assessment etc taken.

I don't see the problem if you're using GS38 leads - I am not convinced that live testing is illegal.

Our tutor expected us to conduct Zs tests on all circuits for the 2391 practical exam.

Hi Harvey

No the regulations only stipulates "Unless unreasonable to do so" or something similar.

There is no other way to carry out a Zs test on a lighting circuit.

Thats why the test leads should comply with GS38, no need if it was not "Live".

Many of us now obtain Zs another way which is acceptable and safer, and recommended by the NICEIC.

Zs = Ze + (R1+R2) so we do a Ze at the origin and add it to our R1 + R2s that we have had to obtain when DEAD testing to prove it is safe to energise the circuit.

That way we only have to do ONE Live test to obtain all the Zs values required.

Many on a Periodic would do Zs direct measurement, but ALWAYS VERY CAREFULLY.

Ok?

TBH with RCD's on every circuit (well almost) I always do the calculated method.. I find that the "soft" loop tests are not that accurate.

TBH with RCD's on every circuit (well almost) I always do the calculated method.. I find that the "soft" loop tests are not that accurate.

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at least with Zs it will show if you have no reconnected earth properly

Thanks to you all for the quick response

TBH with RCD's on every circuit (well almost) I always do the calculated method.. I find that the "soft" loop tests are not that accurate.

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Probably more accurate than the calculation - R1 + R2 is the resistance of the two conductors added together, end of.

Zs is impedance of the fault loop, including parallel paths.

The calculation should only really be used as a guide to what the approx. Zs measurement should be.

Understanding needs to be gained of the difference between 'working live' - e.g. connecting up a light switch without isolating first, and 'live testing', which is a necessity.

As stated in an earlier post - the 2391 exam has you Zs testing every circuit.......why?.......because they want to be sure you know how to do it.

Anyone can calculate it

Harvy,

i have nhe nic testing dvd, they say that testing Zs on light circuits is against EWR as you are working with live conductors, but how do you test Ze then? if it was me i would say dont test a live circuit, though in reality i would test it, i would hope that i am competent enough to test a light fitting, though i am goint to invest in some lightmates

My 2391 instructor and NICEIC assesor both have no problem with calculated Zs..

Next from the 'elf ' n' Safety brigade , " All live tests will be carried out with the power off"

why not also ban Zs from a socket, since its 'live working'?

and whilst your at it, ban use of any electrical equipment used at work, because 'its live'

you have to draw the line somewhere. in most cases, it is reasonable to carry out Zs live. obviously, if your perched ontop of a ladder, with an old test kit with insulation falling off leads etc, then its best to calculate...

You've seen my test kit then ? :Blushing

Calculated and measured Zs readings will always be different.

On a new install I would say that it is reasonable to calculate the Zs, though in reality I would always measure as its so easy at the install stage.

On any periodic I would always live test.

Just my way of doing things I know.

Not this old chestnut again.

To all those that don't agree with calculating..... It is accepted, so accept it. If you want to do it live then do so, but don't keep trying to make others feel like they're breaking the law or something. Either way is fine.

Hi Harvey

Please have a gander at:

http://www.talk.electricianforum.co.uk/topic/21961-live-or-dead-testing-a-question-for-members/page-7#entry304321

I wish I had found your post before asking the same question which lead to over 100 response posts, but we went off subject to other interesting differences of pre-conceived ideas.

Stay safe and work dead if reasonable/possible.

Cheers

Les

Green-Hornet

I think its all about the Electricity at Work Regs., and nothing to do with accuracy of the Zs. No one will thank you for working live measuring Zs for a more accurate value, but the world will cry at your funeral. Do you wear a seat belt?

Cheers

Les

says-les said:

 Do you wear a seat belt?

Cheers

Les

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that would all depend on whether the law requires me to or not,

I am exempt, [by law], from wearing one in certain countries.

I use a plug in style adapter

Plug in push test leads in relevant holes, clip cpc clip on, do test, done

http://www.google.co.uk/search?hl=en&safe=off&tbo=d&biw=768&bih=928&site=imghp&tbm=isch&sa=1&q=socket+and+see+lamp+tester&oq=socket+and+see+lamp+tester&gs_l=img.3...261643.266044.0.266770.15.15.0.0.0.0.218.1425.6j7j1.14.0...0.0...1ac.1.F-BS_FV7vh0

There is no requirement I have found in law that precludes live TESTING, only live WORKING, there is a BIG difference, as long as the tester is competent.

Well said Sidey, it amazes me the different ways in which words are interpreted.

Generally live testing doesn't not require the cables to be disconnected (work done to allow test) from their terminals as the probes are merely touched on terminals as appropriate, hence why probes have to be suitable shielded.

To work on something involves the use of tools hence why certain equipment has to be placed within an enclosure that requires tools to access/work on it.