22 GMC-I Messtechnik GmbH
7.4 Testing Residual Current Circuit Breakers in TN-S Systems
Connection
RCCBs can only be used in TN-S systems. An RCCB would not
work in a TN-C system because PE is directly connected to the
neutral conductor in the outlet (it does not bypass the RCCB).
This means that residual current would be returned via the RCCB
and would not generate any differential current, which is required
in order to trip the RCCB.
As a rule, the display for contact voltage is also 0.0 V, because
the nominal residual current of 30 mA together with minimal loop
resistance result in a very small voltage value:
Measuring resolution is 0.1 V, and the display value is thus
rounded down to 0.0 V.
7.5 Testing Residual Current Circuit Breakers (RCDs) in IT
Systems with High Wiring Capacitance (e.g. in Norway)
The system type (TN/TT or IT) can be adjusted for U
IN
(I
N
, t
a
)
RCD tests and for earth resistance measurements (R
E
).
For measurements in IT systems, a probe is indispensable as the
resulting contact voltage U
IN
cannot be measured without a
probe.
If „2-pole“ has been adjusted for connection
type instead of „2-pole with probe“ and if sys-
tem type IT is then selected, the following error
message pops up:
Set Parameters – Select System Type
Start Measurement
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