Ze - zs = r1 + r2?

[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.