Iso-Tech IMT-3300 Manuel D'instructions page 8

The ISO-TECH IMT-3300 draws a small current from the supply
and measures the difference between the unloaded and loaded
supply voltages. The loop resistance is then calculated and
displayed.
For a TT system, the earth fault loop impedance is the sum of
the following impedances:
Impedance of the power transformer secondary winding.
Impedance of the phase conductor resistance from the power
transformer to the location of the fault.
The impedance of the protective conductor from the fault
location to the earth system.
Resistance of the local earth system (R).
Resistance of the power transformer earth system (Ro).
Figure 4 below shows the fault loop impedance path for a TT
system as a dotted line.
For a TN systems the earth fault loop impedance is the sum of
the following impedances:
1. Impedance of the power transformer secondary winding.
2. Impedance of the phase conductor from the power
transformer to the location of the fault.
3. Impedance of the protective conductor from the fault
location to the power transformer.
Figure 5 below shows the fault loop impedance path for a TN
system as a dotted line.
In accordance with the international standard IEC 60364 for
a TT system the following condition shall be fulfilled for each
circuit.
RA must be ≤ 50/Ia where;
RA is the sum of the resistances of the local earth system R
and the protective conductor connecting it to the exposed
conductor part. 50V is the maximum voltage limit (it may be
25V in certain circumstances).
Ia is the value of current that causes automatic disconnection
of the protective device within 5 seconds.
When the protective device is a residual device (RCD), Ia is
the rated residual operating current IΔn. For example in a TT
system protected by an RCD the maximum RA values are as
follows:
Fig 4
Rated residual
operating current
IΔn mA
Ra (at 50V) Ω
Ra (at 25V) Ω
— 8 —
10 30 100 300
5000 1667 500 167
2500 833 250 83
Fig 5
500 1000
100 50
50 25