zs=ze+r1+r2

[ab1978]

when we measure zs on a circuit and then calculate to back up result using above formula do you get sometimes a different result to your measured zs using earth fault loop meter?
results are within the guides but can be .10ohms - or+ or more out?

 

[sparks1234]

I had a strange thing with zs readings today.
The system was a TT and the ZE reading was 24 ohms, r1/r2 was 0.3 but measured zs was 29 ohms, apparently the RCD can cause this

 

[electricAl]

when we measure zs on a circuit and then calculate to back up result using above formula do you get sometimes a different result to your measured zs using earth fault loop meter?
results are within the guides but can be .10ohms - or+ or more out?

Nothing unusual about this - so long as your measured Zs` are lower that is!
Parallel paths back to earth often reduce a calculated reading to a lower, measured figure. Otherwise, temperature variations can alter your readings either way.
Providing your numbers are within Max Zs allowed then all fine & dandy, so don`t be tempted to adjust back to what Ze + ( R1+R2) works it out to be - `doctored` readings are obvious to a overviewer...
If your measured readings are ok, then they`re ok. Period

 

[NoSparks]

Nobody has mentioned contact resistance yet. Or the difference between a DLock measurement and a straight measurement.

I can't actually remember the last time my figures added up.

 

[electricAl]

Sure, you`ll always get skewed readings on a `No Trip` setting or similiar methods to keep the RCD in. They`ll increase your Zs readings by varying degrees.

As you re-affirmed tho, measured v calculated are often not that close - & the main point is not to try to get them to add up - by all means do the equation to check measured value makes sense, but if it does then have confidence in your test gear & record the result it gave you.

 

[zupos40]

I had a strange thing with zs readings today.
The system was a TT and the ZE reading was 24 ohms, r1/r2 was 0.3 but measured zs was 29 ohms, apparently the RCD can cause this

You also need to be aware that both Zs measured and Ze measured are perfromed using mains Ac voltage at current upto 25A D lock and 15mA notrip, where as the r1+r2 are measured using a dc tester whose output voltage is 4v to 24V at only 200mA as a result trying to add an AC result and a d.c result will result in an error/difference in reading.