r1+r2 vs Zs

[securespark]

r1r2 + Ze = Zs.


Does that automatically make Zs a bigger number than the r1r2 resistance reading, or do the multiple earth paths make Zs a smaller figure?

Or can Zs sometimes be bigger and sometimes smaller?

Discuss.

 

[GaryMo]

I've seen it both ways.

I've measured R1+R2 then measured Zs and found Zs to be lower than the actual R1+R2 value which is almost certainly due to parallel earth paths.

I generally measure R1+R2 and add that to Ze for a calculated Zs figure.

 

[Spark123]

Tis Ze + (R1+R2) = Zs
Afaik this equation usually errs on the side of caution.

 

[securespark]

Yeah, forgive me 123....I always get my r's and R's mixed up...

 

[electronicsuk]

If there are only parallel paths via the DB through main bonding at the like, and not anywhere on the circuit itself, then it would be impossible for Zs to be smaller than R1+R2. It could easily be smaller rather than equal to R1+R2+Ze, though, after taking such paths into account. It would be fair to expect Zs = R1 + R2 + Zdb.

Of course, if there are parallel paths on the circuit you're testing at anywhere other than the DB, you could end up with a Zs that's lower than R1+R2.

 

[securespark]

Was having a conversation with someone who was trying to figure it out in his head.

His way of looking at it is that the R1+R2 must be smaller because it does not include the Ze.

My experience is generally that Zs is lower due to PEP's.

 

[Spark123]

Yeah, forgive me 123....I always get my r's and R's mixed up...

Not so much the Rs, the way you wrote it in the OP means multiply r1 and r2 together

 

[Lectrician]

It is fairly irrelevent though....

When doing an EIC you do the R1+R2 and can calculate the Zs (no point doing both). You actually HAVE to do the R1+R2 because you HAVE to prove you have an earth BEFORE doing any live tests.

When doing a PIR you can measure the Zs and calculate the R1+R2.

If the circuits are ring finals, you should always do the r1+r2, and use that to calculate the R1+R2, and then use this for the Zs.

The NIC have made it clear they prefer to see R1+R2 measured for most circuits as they would like to minimise live working.

 

[bongos]

OOI, when measuring the Ze (to confirm an earth path), am I correct in assuming you would still calculate Zs using the figure set by the DNO for the type of earthing installation?

IE - You measure a Ze of 0.16Ω on a TN-C-S system. But when completing Zs = Ze + (R1 + R2) you still use the DNO figure of 0.35Ω for Ze and not your measured figure of 0.16Ω?

 

[Spark123]

When doing a PIR you can measure the Zs and calculate the R1+R2.

I didn't think you were allowed to do it this way around, i.e. R1+R2 = Zs - Ze

 

[Spark123]

OOI, when measuring the Ze (to confirm an earth path), am I correct in assuming you would still calculate Zs using the figure set by the DNO for the type of earthing installation?

IE - You measure a Ze of 0.16Ω on a TN-C-S system. But when completing Zs = Ze + (R1 + R2) you still use the DNO figure of 0.35Ω for Ze and not your measured figure of 0.16Ω?

Nah, use the real figure as you'll find you'll never be able to use some of the breakers, for example a C63 when the supply is a TN-S

 

[Lectrician]

You can do it that way around for a PIR as the site is already live. However, as stated, the NIC prefer to see R1+R2 measured in preference.

No point using the DNO max figures or 0.35 or 0.8 if you have a measured value.

 

[Adam_151]

When doing a PIR you can measure the Zs and calculate the R1+R2.

Not sure I follow the logic of calculating an R1+R2 (which may or may not be some distance off the real figure) for a PIR, doesn't seem a lot of point!, I generally just mark it as lim

 

[Spark123]

Is it not pointless calculating or recording R1+R2 from a measured Zs, as PEBs are present when measuring Zs and not when measuring Ze there is a margin for error?

 

[JohnD]

Yeah, forgive me 123....I always get my r's and elbow mixed up...

(joke)