Measuring earth electrode resistance

[Norv]

To measure the earth electrode resistance is it a case of doing a Ze test as normal ? Is the limit 21 ohms according to NIC ?

I dont understand the calc as per bottom p78 OSG "For TT systems, the value of the earth electrode resistance Ra in ohms multiplied by the opertaing current in amps of the protective device 'I (delta) n' shall not exceed 50V e.g. if Ra = 200ohm, then the max RCD operating current should not exceed 250mA"

Could someone please explain.

 

[securespark]

21 Ohms is in the OSG as a guide to what the Impedance is on the DNO's side of things. OSG says Max 200 Ohms Zs, NIC 100 Ohms.

I would do a EFL test on it to get a reading, yes.

 

[Spark123]

Where you are installing an electrode for a system which has a return path to a distributors electrode then a simple Ze test can be done.
If you were to be doing this to provide an earth for a generator etc, you need to use an earth electrode resistance tester.
The 21 ohms you refer sounds like the figure quoted for the distributors side of things in the OSG.
A system where I/\n x Ra =200ohms allows an RCD up to 250mA.(Half this for a special locn where touch voltage should not exceed 25v) You would however take into account things such as supplementary protection against direct contact (use 30mA) and available RCD sizes (do they make a 250mA?). IIRC the NICEIC also require Ra to not exceed 100 ohms.

 

[Adam_151]

As for the calculation, multiply the impedance of the electode by the operating current of the RCD (in amps) and you'll get the maximum touch voltage you can have before the RCD trips, in most situations, this should not exceed 50v. (its ohms law if you look closely enough )

If you had an electrode of 100ohm and an rcd of 300ma, the touch volatage would be 100 x 0.3 = 30v

If you have multiple RCDs and they arn't downstream of each other (eg. two boards off the same supply, both with 30ma devices), then logic would have you add the operating currents say you had an electrode of 180ohm, it'd be 180 x (0.03 + 0.03) = 10.8v