HT instruments M71 - Manual (Page 24)

M71
EN – 23
8. PRACTICAL REPORTS FOR ELECTRICAL TESTS
8.1. EARTH RESISTANCE MEASUREMENT IN TT SYSTEMS
PURPOSE OF THE TEST
Make sure that the RCD is coordinated with the earth resistance value. It is not possible to
take an earth resistance value as reference limit when controlling the measurement result,
while it is always necessary to check that the coordination complies with the standards'
requirements.
INSTALLATIOIN PARTS TO BE CHECKED
The earth installation under working conditions. The check is to be effected without
disconnecting the earth rods.
ALLOWABLE VALUES
The earth resistance value measured shall meet the following relation:
R
A
< 50 / I
a
where:
R
A
= Resistance of the earth installation, the value can be set with the following
measurements:
- Earth resistance with three-wire volt-ampere method
- Fault loop impedance (see (*))
- Two-wire earth resistance (see (**))
- Two-wire earth resistance in the socket (see (**))
- Earth resistance achieved by measurement of contact voltage U
t
(see (**)).
- Earth resistance achieved by measurement of tripping time test of the RCDs (A,
AC),RCDs S (A, AC) (see (**)).
I
a
= Tripping current in 5s of the RCD, rated tripping current of the RCD (in case of RCD S
2 I
∆n
).
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.
(**) These methods, even though not presently provided for by standards, provide values,
which compared with numberless reference tests resulted to be reliable for earth
resistance.
EXAMPLE OF EARTH RESISTANCE TEST
Let's assume an installation protected by a 30 mA RCD. Let's measure the earth
resistance 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
50/0.03=1666Ω permitting to use even simplified methods which though do not provide
extremely accurate values, give values approximate enough to calculate the coordination.