HT instruments GEO416 - Manual (Page 25)

GEO416 - GEO416GS
EN - 24
11. PRACTICAL REPORTS FOR ELECTRICAL TESTS
11.1. EARTH RESISTANCE IN TT SYSTEMS
The test is aimed at checking that the RCD is coordinated with the earth resistance value.
It is not possible to assume an earth resistance value as reference limit when controlling
the test result, while it is necessary to check every time that the co-ordination complies
with the requirements of the Standards.
The parts to be checked are represented by the whole earth installation under working
conditions. The check is to be effected without disconnecting the earth rods.
The earth resistance value measured shall meet the following relation R
A
< 50 / I
a
where:
R
A
= resistance of the earth installation whose value can be set with the following
measurements:
 Three-wire earth resistance with volt ampere method
 Two-wire earth resistance with volt ampere method
 Phase to earth fault loop impedance (*)
 Two-wire earth resistance in the socket with volt ampere method (**)
 Earth resistance obtained by the measurement of contact voltage U
t
(**)
 Earth resistance obtained by the tripping time test of the RCDs (A, AC),RCD S
(A, AC) (**)
I
a
= tripping current in 5s of the automatic RCD; rated tripping current of the RCD (in
case of RCD S 2 I
n
) in ampere
50 = safety limit voltage (reduced down to 25V in special environments)
(*) If the installation is protected by an RCD the measurement shall be effected
upstream or downstream the RCD short-circuiting it to avoid its tripping
(**) This method, even though not presently provided for by standards, provide values,
which compared with numberless reference 3-wire tests resulted to be reliable for
earth resistance
Example
Let's assume an installation protected by an RCD I
a
= 30 mA. The earth resistance is
measured using one of the methods quoted above. To evaluate whether the installation
resistance is complying with the standards in force multiply the result by 0.03A (30 mA). If
the result is lower than 50V (or 25V for special environments) the installation can be
considered as coordinated as it meets the above said relation.
In case of 30 mA RCDs (most civil installations) the maximum earth resistance allowed is
50 V/ 0.03=1666Ω permitting to use even simplified methods which though do not provide
extremely accurate values, give values approximate enough to calculate the coordination.