HT instruments SIRIUS89N - Manual (Page 116)

SIRIUS89N
EN - 114
16.4. Earth Resistance Measurement in TT Systems
PURPOSE OF THE TEST
Check that the RCD is co-ordinated with the earth resistance value. It is not possible to
assume an earth resistance value as reference limit (for example 20
Ω according to the
art. 326 of DPR 547/55) 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, even though not provided by the standards CEI 64.8,
provide values resulted to be indicative of the earth resistance .