Calculating Ze

[maselec]

While having a pint and a chat with a friend of mine who is also a sparks, I was told that a friend of his (carrying out tenanted, initial verifications for the council) was not meauring Ze but using the calculation:- Zs - (r1+r2)= Ze. I know that this is not allowed and as I have the NICEIC Inspection and Testing book, which backs me up I showed him. However the book does not state why it is not allowed and I just wondered what others view were on this?

 

[Apache]

While having a pint and a chat with a friend of mineI just wondered what others view were on this?

My first thought would be not to go to the pub with you! Couldn't you find some sport/women/weather to talk about?

:Salute

 

[NozSpark]

My first thought would be not to go to the pub with you! Couldn't you find some sport/women/weather to talk about? :Salute

:slap

If you do Ze=Zs-(R1+R2) you'll actually get Zdb, which will include the bonding and as you know Ze is measured will all bonding disconnected.

:C

 

[SPECIAL LOCATION]

While having a pint and a chat with a friend of mine who is also a sparks, I was told that a friend of his (carrying out tenanted, initial verifications for the council) was not meauring Ze but using the calculation:- Zs - (r1+r2)= Ze. I know that this is not allowed and as I have the NICEIC Inspection and Testing book, which backs me up I showed him. However the book does not state why it is not allowed and I just wondered what others view were on this?

To stick a bit more padding around 'Sir Noz's' quite correct answer I will add the following for you to consider......

(i)

Water and Gas pipes etc.. are NOT permitted as a means of earthing...

see regulation 542.2.4 pg 127

(ii)

You have got to have means to measure the resistance of the earthing arrangements....

see regulation 542.4.2 pg 128

So.. it leaves the question....

How are you going to prove that the resistance of the earthing arrangments are adequate for the type of earth method employed on that installtion...

Without disconnecting ALL possible parallel paths...

You could have a situation where a main electrical earth connection has come adrift, but you are getting a reasonable resistance down through the gas and/or water piper via the central heating system etc...

Which obviously don't comply with regs due to 542.2.4

but if you do Ze=Zs-(R1+R2) you could put an apparent compliant Ze value on you cert signing it to say it all is up to BS7671.. which it isn't....

If you can prove there are NO extraneous metal parts on the installation..

e.g. plastic water pipe coming in, No gas No oil etc.. then the calculation Ze=Zs-(R1+R2) would be quite accurate

otherwise you can only measure Ze directly across the supply with the installation disconnected

 

[septiclecky]

:slap If you do Ze=Zs-(R1+R2) you'll actually get Zdb, which will include the bonding and as you know Ze is measured will all bonding disconnected.

:C

I was asked the other day by another electrician where does it say Zdb in the regs, can anyone now the answer without going through the BRB?

 

[maselec]

Thanks for the replies. My general argument was that the measured Zs and R1 R2 could include parellel return paths, though I've never been great at explaining my point of view. I now have some meatier arguments!

 

[maselec]

My first thought would be not to go to the pub with you! Couldn't you find some sport/women/weather to talk about?

I'm married and a Leeds fan, so I've just about given up! Next time I'll remember the weather!

 

[Risteard]

I was asked the other day by another electrician where does it say Zdb in the regs, can anyone now the answer without going through the BRB?

It doesn't. Technically there is no such thing. It is in fact the Zs at the distribution board.

 

[sanny]

We have been told not to do Zs tests anymore unless it can be done with a plug in tester! So we calculate it using Zs=Ze+R1+R2 .i can see the point though it can get quite dangerous up a ladder trying to hold 3 test leads on and press the test button lol

 

[Andy™]

We have been told not to do Zs tests anymore unless it can be done with a plug in tester! So we calculate it using Zs=Ze+R1+R2 .i can see the point though it can get quite dangerous up a ladder trying to hold 3 test leads on and press the test button lol

get a decent tester (like megger) and you dont need to press a test button

 

[ianmacd]

And get some of the kewtech (i think) light fitting adapters and then you only need to find a cpc connection.

 

[Nicky Tesla]

The more usual way would be to measure Zs, measure Ze (Zdb) and then calculate R1R2 much quicker.

What I think you are saying by not measuring Zs, is that it is safer to calculate because the Zs test is a live test.

 

[Nicky Tesla]

Also who would turn off the supply in a commercial 24hr property or hospital for example just to take a Ze reading.

 

[sanny]

That wasnt my point! The EAWR 1989 says

14. No person shall be engaged in any work activity on or so near any live conductor (other than one suitably covered with insulating material so as to prevent danger) that danger may arise unless-

(a) it is unreasonable in all the circumstances for it to be dead; and

(B) it is reasonable in all the circumstances for him to be at work on or near it while it is live; and

© suitable precautions (including where necessary the provision of suitable protective equipment) are taken to prevent injury.

If you have measured Ze and R1+R2 then you have already measured Zs

I have also watched an NICEIC training video which said we shouldnt be doing live Zs tests

 

[Nicky Tesla]

If you measured Ze then you were engaged in a work activity on a live conductor

 

[sanny]

well it would be unreasonable to do a Ze test dead (or impossible) wouldn't it ? unless you knew the conductor size and length back to the sub station or could measure its resistance

All im saying is if you know the the 3 variables which make up Zs why measure it

 

[Andy™]

as far as im concerned, Zs is the single most important test. its the only test that fully verifies a circuit has a working earth.

if you do the Ze & R1R2, what happens if you havent connected the earth properly?

 

[Sidewinder]

Sanny,

Zs will not equal Ze + R1+R2 due to parallel earth paths in the installation.

HSE DO allow and accept that live testing is required as long as it is done in a controlled manner.

I suspect that the H&S personnel who have prevented you undertaking live testing are some of those I come across at IOSH who have no real grasp of electrical engineering.

I take every opportunity to correct them! O)

 

[Nicky Tesla]

(B) it is reasonable in all the circumstances for him to be at work on or near it while it is live;

Is it reasonable to let an electrician do live testing? of course it is.

Is it reasonable to ask an electrician to work on a gas pipe NO but it is reasonable to let a gas engineer to. This reg is basically for anyone who is not competent or confident to be working on live equipment, like a joiner or mechanic but an electrician can be more than capable and safe.

 

[ADS]

We have been told not to do Zs tests anymore unless it can be done with a plug in tester! So we calculate it using Zs=Ze+R1+R2 .i can see the point though it can get quite dangerous up a ladder trying to hold 3 test leads on and press the test button lol

And yet, during the 2391, they have you measure it for every circuit - yet they don't want you to do it?? Makes sense

If you have measured Ze and R1+R2 then you have already measured Zs

No you haven't.

Zs is an AC impedance measurement of the earth fault loop - R1 + R2 is a resistance measurement of two conductors.

Zs measurement also confirms that there actually is an earth path, which calculation doesn't.

What are you going to do on a PIR - start taking the DB apart so that you can measure your R1 + R2.......No, you measure Zs.

I have also watched an NICEIC training video which said we shouldnt be doing live Zs tests

Once AGAIN...... The NICEIC don't make the rules - they just like to think they do.