Ze - zs = r1 + r2?

[conorchapple]

Hi I'm new here, I'm a second year apprentice.

When we do domestic testing such as a eicr my boss says that he can 'work out' R1 + R2 by taking the ZE from the ZS however i don't think this is correct as everyone would just do it?? I just wanted to confirm that this isn't correct as i do need to learn the correct way to test new installations for the future and for my college exams.

THANKS

 

[Darkwood]

Welcome to the forum

 

[Sintra]

Unfortunately your boss is wrong. You should not calculate the R1 + R2 from the Zs although it is deemed acceptable to calculate Zs from a measured R1 + R2.

 

[NDG Elecs]

Zs usually has parallel paths to earth, often through gas and water pipework, which would lower the Zs and reduce the calculated R1R2. So it would be incorrect to calculate R1R2 in that instance. I was taught that R1R2 should always be measured.

 

[TJ Anderson]

It is definately not the correct way due to parallel paths as already said. In fact, the cpc could be completely open circuit and totally relying on parallel path for continuity. Your boss is not alone though, there are many that calc like this though as it gives them a figure for the box and makes out they carried out the dead test. He would be better to just LIM the R1 & R2 box and just record Zs on a EICR. That is perfectly acceptable if agreed with client and very common on commercial EICR's. He should really be doing it all on a domestic though due to easy isolation and small amount of circuits tbh.

 

[telectrix]

agreewith above posts. R1+R2 should be measured before energising circuit/s. then Zs measured when circuit/s is/are live.

 

[Martin Kerr]

agreewith above posts. R1+R2 should be measured before energising circuit/s. then Zs measured when circuit/s is/are live.

You are on a roll this morning Tel , did she throw you out of bed?

 

[conorchapple]

Thanks everyone!!! Thought there was more to it that what we do

 

[telectrix]

no, i fell out. someone'sgotto stir and let the dogs out for a pee.

https://www.youtube.com/watch?v=Qkuu0Lwb5EM

 

[Darkwood]

You are on a roll this morning Tel , did she throw you out of bed?

Tel wears the pants in his house.... and the skirts, knickers, high he--- Oopps! I think I said too much :biggrin5:

 

[telectrix]

Tel wears the pants in his house.... and the skirts, knickers, high he--- Oopps! I think I said too much :biggrin5:

shhh. she thinks we had burglars in her knicker drawer.

 

[HandySparks]

Hi I'm new here, I'm a second year apprentice.

When we do domestic testing such as a eicr my boss says that he can 'work out' R1 + R2 by taking the ZE from the ZS however i don't think this is correct as everyone would just do it?? I just wanted to confirm that this isn't correct as i do need to learn the correct way to test new installations for the future and for my college exams.

THANKS

As a second year apprentice (or anyone working in this industry), you need to get your notation correct. In this case we know what you meant, but an examiner may not be so lenient.

You've used Ze, ZE, ZS, zs, r1, r2, R1, R2, some of which have meanings other than the ones you needed.

 

[rich.250]

Zdb + (R1+R2) will give you your actual Zs.
That's the way it should be calculated.
In my opinion low current loop testers are completely inaccurate so I would prefer to see it done this way.
Definitely never the other way around!

 

[Toneyz]

Agree with the above Ze/Zdb + (r1+r2) you cannot go wrong with measuring a cable resistance with its connections.You can do a loop test get a aceptable result however with other paths fails to show the cpc that has come out /loose of a terminal.I is the electrical installation that should be tested.A paralell path could go tomorrow with the removal of a copper water pipe.
Also agree with the comment on loop meters we sent 4 off t be calabrated/checked within spec got them back plugged them into a known socket and got a range of readings but all within tolarance.

 

[boberty]

One thing I also find is that you can get higher earth loop readings when doing a non trip test due to the R.C.D. So doing the dead test gives you a better idea of what your actual resistance is.

 

[telectrix]

you can bypass the RCD to do a high current loop test. richard burns posted an excellent diagram of how to do it a couple of weeks ago.

 

[Pete999]

Hi I'm new here, I'm a second year apprentice.

When we do domestic testing such as a eicr my boss says that he can 'work out' R1 + R2 by taking the ZE from the ZS however i don't think this is correct as everyone would just do it?? I just wanted to confirm that this isn't correct as i do need to learn the correct way to test new installations for the future and for my college exams.

THANKS

No mate your boss is talking out of his back passage, unfortunately some unscrupulous practitioners of our beloved trade, think they are called cowboys, do this, sad really

 

[dcwuk]

Sorry to drag this up again and I always measure R1+R2 but why do some testers like the MFT1730 have the facility to measure R1+R2 using Ze as a reference. Or have I had a bit of a mental block?

 

[TJ Anderson]

Sorry to drag this up again and I always measure R1+R2 but why do some testers like the MFT1730 have the facility to measure R1+R2 using Ze as a reference. Or have I had a bit of a mental block?

I have a 1720 myself. I know there are sparks who do calc Zs (Zs-Ze=R1&R2), but it can potentially be very inaccurate when you take into account parallel earth paths. The megger is basically doing this for you and it encourages the bad practice.

 

[dcwuk]

I have a 1720 myself. I know there are sparks who do calc Zs (Zs-Ze=R1&R2), but it can potentially be very inaccurate when you take into account parallel earth paths. The megger is basically doing this for you and it encourages the bad practice.

Indeed so why do the manufactures include this facility ?
Surely there is some consultation between the manufacturer and the relevant bodies at the design stage.

 

[TJ Anderson]

They do it cause some people want it and it is an extra feature that other testers don't have.

 

[DuaneMHunt1976]

Zs

Ze + R1 + R2

dont know why they bother with breckets () dosnt do any thing for it
not keen on using = symbol as it is not pure

 

[dcwuk]

They do it cause some people want it and it is an extra feature that other testers don't have.

So at the end of the day even though its bad practice its still acceptable to the powers that be?

 

[TJ Anderson]

So at the end of the day even though its bad practice its still acceptable to the powers that be?

No not really, if you had a gun you dont have to shoot someone with it. At the end of the day, as a qualified electrician, you can put whatever you deem fit in the R1 + R2 box, it is only a model form taken from non statutory regulation.

 

[dcwuk]

Thanks for your replies, I'm in a protracted discussion about this subject and the people involved are using the fact that the meter
has the facility makes it acceptable practice. I disagree for all the reasons given in this thread.

 

[HappyHippyDad]

Thanks for your replies, I'm in a protracted discussion about this subject and the people involved are using the fact that the meter
has the facility makes it acceptable practice. I disagree for all the reasons given in this thread.

I measured a Ze on a TT the other day and it was 197Ω, Zs on the ring was 73Ω so by using Ze - Zs = R1 + R2 I get an R1 + R2 of -124Ω that's a damn fine conductor!

I can't see how it could ever be acceptable to calculate R1+R2 this way.

 

[hightower]

I measured a Ze on a TT the other day and it was 197Ω, Zs on the ring was 73Ω so by using Ze - Zs = R1 + R2 I get an R1 + R2 of -124Ω that's a damn fine conductor!

I can't see how it could ever be acceptable to calculate R1+R2 this way.

Is the title in this thread wrong? Shouldn't this (unacceptable) practice be Zs - Ze = R1 + R2

 

[telectrix]

Is the title in this thread wrong? Shouldn't this (unacceptable) practice be Zs - Ze = R1 + R2

well spotted, hawkeye.

 

[hightower]

well spotted, hawkeye.

YES! I FEEL LIKE A GOD! Pfft, all you puny humans replying to an equation that doesn't even make sense. I pity you all.

(DISCLAIMER: For those that don't know me too well, yes, this is obviously said in massive jest. I am not obnoxious enough to class myself as a God. If you forced my hand, I would say demi-God at best.)

 

[LeeH]

I have a 1720 myself. I know there are sparks who do calc Zs (Zs-Ze=R1&R2), but it can potentially be very inaccurate when you take into account parallel earth paths. The megger is basically doing this for you and it encourages the bad practice.

Do you disconnect all the parallel paths when measuring then??

Bad practice my arse.

 

[Simonslimline]

Do you disconnect all the parallel paths when measuring then??

Bad practice my arse.

How do you measure R1+R2 then?

 

[telectrix]

Do you disconnect all the parallel paths when measuring then??

Bad practice my arse.

of course it's bad practice. R1+R2 should be measured before the circuit is energised. otherwise, how do you ensure that it's safe to power up?

 

[YoungBart]

Just to stick my two penneth worth in... In my opinion it is bad practice to calculate R1+R2. This is a basic dead test that proves the installation is safe and correct.
However, I think It is perfectly acceptable (and maybe even safer) to calculate Zs (Zs= R1+R2 + Ze) as You will always get the worse case scenario and It saves removing any live accessory covers etc.
So IMO if you can be sure no one has 'tampered' with any of the install between Dead and live tests then I cant see a reason why You cant calculate Zs.

 

[LeeH]

of course it's bad practice. R1+R2 should be measured before the circuit is energised. otherwise, how do you ensure that it's safe to power up?

My bad, I quickly read this and thought it was a debate on calculating vs measuring Zs, not reverse engineering Zs by subtraction.

Apologise.

 

[jetandy]

Hello
I may be simple missing something here but if you put a link in at the board to measure your R1R2 and test at the last point. Surely you could still get a parallel path measurement. As the earth is connected to the met at the board. Or is that simple incorrect. As a z's is testing the earth loop and not just a low ohs resistance test?

 

[Risteard]

Hello
I may be simple missing something here but if you put a link in at the board to measure your R1R2 and test at the last point. Surely you could still get a parallel path measurement. As the earth is connected to the met at the board. Or is that simple incorrect. As a z's is testing the earth loop and not just a low ohs resistance test?

Indeed you could. We can both pretend that this is a very current thread.

 

[jetandy]

Indeed you could. We can both pretend that this is a very current thread.

Well it was just the most relevant one. So what if you can explain is the difference then between measuring the parallel path with a Z's as opposed to the R1+R2?

 

[jetandy]

Well it was just the most relevant one. So what if you can explain is the difference then between measuring the parallel path with a Z's as opposed to the R1+R2?

Are we suggesting that one should really remove the cpc and test the circuit individually. Otherwise I am still in the dark as to why the parallel path causes an issue for one test but not the other.