Instructions

GMC-I Messtechnik GmbH 27
8 Testing of Breaking Requirements
Overcurrent Protective Devices,
Measurement of Loop Impedance and
Determination of Short-Circuit Current
(functions Z
L-PE
and I
K
)
Testing of overcurrent protective devices includes visual inspec-
tion and measurement. Use the PROFITEST MASTER or SECULIFE IP
to perform measurements.
Measuring Method
Loop impedance Z
L-PE
is measured and short-circuit current I
K
is
ascertained in order to determine if the breaking requirements for
protective devices have been fulfilled.
Loop impedance is the resistance within the current loop (utility
station – phase conductor – protective conductor) when a short-
circuit to an exposed conductive part occurs (conductive connec-
tion between phase conductor and protective conductor). Short-
circuit current magnitude is determined by the loop impedance
value. Short-circuit current I
K
may not fall below a predetermined
value set forth by DIN VDE 0100, so that reliable breaking of the
protective device (fuse, automatic circuit breaker) is assured.
Thus the measured loop impedance value must be less than the
maximum allowable value.
Tables containing allowable display values for loop impedance
and minimum short-circuit current display values for ampere rat-
ings for various fuses and circuit breakers can be found in the
help texts and in section 21 beginning of page 92. Maximum
device error in accordance with VDE 0413 has been taken into
consideration in these tables. See also section 8.2.
In order to measure loop impedance Z
L-PE
, the instrument uses a
test current of 3.7 to 7 A (60 to 550 V) depending on line voltage
and line frequency. At 16 Hz, the test has a duration of no more
than 1200 ms.
If dangerous contact voltage occurs during measurement
(> 50 V), safety shut-down occurs.
The test instrument calculates short-circuit current I
K
based on
measured loop impedance
Z
L-PE
and line voltage. Short-circuit
current calculation is made with reference to nominal line voltage
for line voltages which lie within the nominal ranges for 120 V,
230 V and 400 V systems. If line voltage does not lie within these
nominal ranges, the instrument calculates short-circuit current I
K
based on prevailing line voltage and measured loop impedance
Z
L-PE
.
Select Measuring Function
Connection:
Schuko / 3-Pole Adapter
Connection:
2-Pole Adapter
Note
Loop impedance should be measured for each electrical
circuit at the farthest point, in order to ascertain maxi-
mum loop impedance for the system.
Note
Observe national regulations, e.g. the necessity of conduct-
ing measurements without regard for RCCBs in Austria.
3-Phase Connections
Measurement of loop impedance to earth must be performed at
all three phase conductors (L1, L2, and L3) for the testing of over-
current protective devices at three phase outlets.
8.1 Measurements with Suppression of RCD Tripping
Measuring Method with Suppression of RCD Tripping
The test instruments PROFITEST MTECH+, PROFITEST MXTRA and
SECULIFE IP make it possible to measure loop impedance in TN
systems with type A, F
and type AC RCCBs (10, 30, 100,
300, 500 mA nominal residual current).
The test instrument
generates a direct
current to this end,
which saturates the
RCCB’s magnetic
circuit.
The test instrument
then superimposes
a measuring cur-
rent which only
demonstrates half-
waves of like polar-
ity. The RCCB is no
longer capable of
detecting this mea-
suring current, and
is consequently not tripped during measurement.
A four conductor measuring cable is used between the instru-
ment and the test plug. Cable and measuring adapter resistance
is automatically compensated for during measurement and does
not effect measurement results.
Note
A loop impedance measurement by using the method of
suppression of RCD tripping can only be performed with
type A and F RCDs.
Note
Bias Magnetization
Only AC measurements can be performed with the 2-
pole adapter. Suppression of RCD tripping by means of
bias magnetization with direct current is only possible via
a country-specific plug insert, e.g. SCHUKO, or the 3-
pole adapter (neutral conductor necessary).
Z
L-PE
Start
t1 t3
Measurement
t2
Operation
RCD Disabled!
t
I
F
/mA
Suppression of RCCB tripping for RCCBs
which are sensitive to pulsating current