HT instruments MACROTEST G3 - Manual (Page 97)

MACROTESTG3 - COMBIG3
EN - 96
12.7. VERIFY OF PROTECTION AGAINST INDIRECT CONTACTS IN TT SYSTEMS
Purpose of the test
Checking that the protection device is coordinated with the value of earth resistance. We
cannot assume a priori a reference limit value for earth resistance as a reference when
checking the measurements result. It is necessary to check each time that the coordination
prescribed by the standard is met.
Parts of the system to be checked
Earth installation in operating conditions. The test must be performed without
disconnecting the earth rods.
Allowable values
The value of earth resistance, however measured, must satisfy the following relation:
R
A
< 50 / I
a
where: R
A
= resistance measured of earth installation whose value can be determined
with the following measurements:
 Three-wire earth resistance with voltammetric method
 Impedance of the fault ring (*)
 Earth resistance with two wires (**)
 Earth resistance with two wires in socket (**)
 Earth resistance obtained by the measurement of contact voltage U
t
(**)
 Earth resistance obtained by the tripping time test of the RCDs (A, AC, B),
RCD S (A, AC) (**)
I
a
= tripping current of the automatic RCD or rated tripping current of the RCD (in
case of RCD S 2 IdN) in ampere
50 = safety limit voltage (reduced down to 25V in special environments)
(*) If the system protection is obtained through a differential switch, the measurement
must be performed upstream of this switch or downstream of it by short-circuiting the
switch in order to prevent it from tripping.
(**) These methods, although not currently foreseen by guidelines provide values that
have been proven indicative of the earth resistance by numerous comparisons with the
three-wire method.
EXAMPLE OF EARTH RESISTANCE CHECK
System protected by a 30mA differential switch.

Let us measure the earth resistance by using one of the above-mentioned methods.
 In order to understand if the system resistance is to be considered as compliant with
the standards, we need to multiply the value found by 0.03A (30mA).
 If the result is lower than 50V (or 25V for special environments), the system can be
considered as coordinated, as it satisfies the relationship indicated above.
 When dealing with 30mA differential switches (as in almost all civil systems), the
maximum allowable earth resistance is 50/0.03=1666 . This enables using also the
indicated simplified methods which, although they do not provide an extremely precise
value, provide a sufficiently approximated value for coordination calculation.