Amprobe GP-2 Geo-Test - Manual (Page 35)

GP-2 GeoTest
EN - 35
11. PRACTICAL REPORTS FOR ELECTRICAL TESTS
11.1. EARTH RESISTANCE MEASUREMENT
PURPOSE OF THE TEST
This test is carried out to make sure that the protective device is coordinated with the earth
resistance value. You cannot take off hand a limit earth resistance value as a reference for
the test result: occasionally you must make sure that the prescribed coordination is
respected.
EQUIPMENT PARTS TO BE TESTED
The earth equipment and its working conditions. The test must be carried out without
disconnecting the earth references.
ADMISSIBLE VALUES
The earth resistance value, however measured, must meet the following requirement:
R
E
< 50 / I
a
where: R
E
= measured resistance of the earth equipment. The value can be
determined with the following tests:
- earth resistance with 3-wire voltmetric method
- loop impedance (see (*))
- 2-wire earth resistance (see (**))
- 2-wire earth resistance in the socket (see (**))
- earth resistance given by the contact voltage U
t
(see (**)).
- earth resistance given by tripping time of RCDs (A, AC), RCD S (A,
AC) (see (**)).
I
a
= tripping time in 5s increments of the automatic switch; nominal tripping
current of the RCD (in case of RCD S 2 I
∆ n
).
50= Limit safety voltage (reduced to 25V for special places).
(*) In presence of an RCD the test must be carried out on the RCD
upstream or downstream, bypassing it to avoid its intervention.
(**) These methods, even if not yet prescribed for by the CEI 64.8 standard,
give values which many 3-wire comparison tests proved to be revealing
for earth resistance.