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3.6. LOOP IMPEDANCE MEASUREMENT (Z
S
)
In a TN or TT type installation, the loop impedance measurement is used to calculate the short-circuit current and to size the
protections of the installation (fuses or RCDs), especially their breaking capacity.
In a TT type installation, the loop impedance measurement makes it easy to determine the earth resistance without planting any
rods and without cutting off power to the installation. The result obtained, Z
S
, is the loop impedance of the installation between
the L and PE conductors. It is barely greater than the earth resistance.
From this value and the conventional touch voltage limit (U
L
), it is then possible to choose the rated differential operating current
of the RCD: I
D
N
< U
L
/ Z
S
.
This measurement cannot be made in an IT type installation because of the high earthing impedance of the supply transformer,
which may even be completely isolated from earth.
3.6.1. DESCRIPTION OF THE MEASUREMENT PRINCIPLE
The device starts by generating pulses having a duration of 300 µs and an amplitude of at most 3.5 A between the L and N ter-
minals. This first measurement is used to determine Z
L
.
It then applies a low current, 6, 9 or 12 mA at the user’s discretion, between the L and PE terminals. This low current serves to
avoid tripping residual current devices of which the nominal current is greater than or equal to 30 mA. This second measurement
is used to determine Z
PE
.
The device then calculates loop resistance Z
S
= Z
L-PE
= Z
L
+Z
PE
, and short-circuit current Ik = U
LPE
/Z
S
.
The value of Ik serves to check the proper sizing of the protections of the installation (fuses or RCDs).
For greater accuracy, it is possible to measure Z
S
with a high current (TRIP mode), but this measurement may trip the RCD of
the installation.
3.6.2. MAKING A MEASUREMENT
Connect the measuring cable to the device, then to the socket outlet of the instal-
lation to be tested.
L
PE
N
Rb
R
E
R
N
R
L
L
PE
N
Rb
R
N
R
L
Ra
Case of a TT installation
Case of a TN installation
Set the switch to Z
S
(R
A
/S
EL.
).
If possible, first disconnect all loads from the network on which you make the loop
measurement.
It is possible to eliminate this step if you use a measurement current of 6 mA, which
allows a leakage current of up to 9 mA for an installation protected by a 30 mA
residual current device.
SET UP
OFF
in trip mode, it is not necessary to connect the N terminal.
At the time of connection, the device first checks that the voltages present
on its terminals are correct, then determines the position of the phase (L) and
of the neutral (N) with respect to the protective conductor (PE) and displays
it. If necessary, it then automatically switches the L and N terminals so that
the loop measurement can be made without modifying the connections of
the device.
