HT instruments SIRIUS87 - Manual (Page 82)

SIRIUS 87
EN - 80
11.8. EARTH RESISTANCE MEASUREMENT IN TT SYSTEMS
Purpose of the test is to check 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.
INSTALLATION PARTS TO BE CHECKED
The earth installation under working conditions. The check is to be effected without
disconnecting the earth plates.
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 obtained by the measurement of contact voltage U
t
(see (**)).
- Earth resistance obtained by the 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 the case of
RCD S 2 I n ).
50= Safety limit voltage (reduced down to 25V in special rooms).
(*) 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 provide values resulted to be indicative of the earth resistance .
EXAMPLE FOR 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 and multiply the result by 0.03A (30
mA). If the result is lower than 50V (or 25V for special rooms) the installation can be
considered as coordinated as it respects the above-said relation.
When we face 30 mA RCDs (the most of civil installations) the maximum earth resistance
allowed is 50/0.03=1666
 permitting to use even simplified methods which even though
they do not provide extremely precise values give a value approximate enough for the
calculation of the co-ordination.