JUPITER
IT - 39
9.2. OVERALL EARTH RESISTANCE 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 measurement results. 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 plates.
Allowable values
The value of overall earth resistance, however measured, must satisfy the following
relationship:
R
A
< 50 / Ia
where:
Ia = tripping current of the automatic RCD or rated tripping current of RCD In
expressed in A
50 = Safety limit contact voltage (reduced down to 25V in special environments)
EXAMPLE OF EARTH RESISTANCE CHECK
System protected by a 30mA differential switch.
Measurement of overall earth resistance
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 to calculate coordination.
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