by calculation

[dingledong]

by calclation does ze +zs equal r2???

 

[spark 68]

Zs = Ze + (R1 + R2)

 

[dingledong]

but its just the r2 that is wanted and i just cant transpose that equation

 

[trev]

Measure r2 mate it's easier

 

[topquark]

but its just the r2 that is wanted and i just cant transpose that equation

You can if you know the relative sizes of your r1 and r2 (assuming that they are the same length overall (which they normally are)).

Think about the relationship between the two ... 2.5mm and 1.5mm for example.

 

[trev]

Why is only the r2 value the one you're after?

 

[dingledong]

i was talking to some one and on a report they said thats what they did for r2

 

[topquark]

i was talking to some one and on a report they said thats what they did for r2

That's really only for design purposes. In the field you would measure it!

 

[telectrix]

well that's arse about face. you measure the resistances and calculate Zs, not the other way round.

 

[trev]

As Topquark said you can work it out from line/cpc sizes but why would you? Just measure it and have done with it

 

[trev]

No matter what the measurement is it's pretty much irrelevant really as the test schedule asks for that or (R1+R2)

 

[Deleted member 26818]

To be honest, this sounds a bit like deducting Ze from Zs and using the result as the R1+R2.
Won't work if there are parallel earth paths connected.

 

[Octopus]

To be honest, this sounds a bit like deducting Ze from Zs and using the result as the R1+R2.
Won't work if there are parallel earth paths connected.

Yes, I completely agree but how many certs have you seen where its obvious that thats how the R1 + R2 was "concluded"?

 

[spark 68]

but its just the r2 that is wanted and i just cant transpose that equation​

R2 = Zs - (Ze - R1)

But as others have said, why would you even want to transpose it? you would still have to measure R1, Zs and Ze, Tel is right it is about face.
Either just measure it (if it is a bonding etc. conductor) or measure R1+R2 and fill in the R1+R2 column for anything else

 

[Deleted member 26818]

I'm assuming that the OP is refering to R2, not really r2.
I've had a few long days recently, so my brain would probably give up the ghost even attempting to work out a method which would give r2 for a RFC by just measuring Ze and Zs.
It's not even really necessary for a PIR.
All that's really needed is to confirm continuit, polarity and that Zs are low enough for the protective device to operate.