Manual

54 Gossen Metrawatt GmbH
13 Testing of Breaking Requirements for Overcurrent Protective Devices, Measurement of Loop Impedance
and Determination of Short-Circuit Current (ZL-PE and I
SC
Functions)
Testing of overcurrent protective devices includes visual inspec-
tion and measurement.
Measuring Method
Loop impedance Z
L-PE
is measured and short-circuit current I
SC
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
SC
may not fall below a predetermined
value set forth by IEC 60364, so that reliable breaking of the pro-
tective device (fuse, automatic circuit breaker) is assured.
The measured loop impedance value must therefore be less than
the maximum permissible value.
Tables containing permissible 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 25 from page 100. Maximum device
error in accordance with IEC 61557 / EN 61557 has been taken
into consideration in these tables. See also section 13.2.
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 the limit value for touch voltage is exceeded during this mea-
surement process (> 50 V), safety shutdown occurs for Germany
(65 V applies for Austria – standard: ÖVE/ÖNORM E 8001-1
section 5.3).
The shutdown value can be adjusted within a range of 25 to 65 V
(see section 10.8).
The test instrument calculates short-circuit current I
SC
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, 230
and 400 V systems. This also applies between phases L-L at
500 V. If line voltage does not lie within these nominal ranges, the
instrument calculates short-circuit current I
SC
based upon pre-
vailing 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 con-
ducting 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.
13.1 Measurements with Suppression of RCD Tripping
(PROFITEST MTECH+, PROFITEST MXTRA only)
The test instruments make it possible to measure loop impedance
in TN systems with type A , F and 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 during measurement and does not
affect measurement results.
Note
Loop impedance measurement in accordance with the
procedure for the suppression of RCCB tripping is only
possible 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 N required).
Z
L-PE
Start
t1 t3
Measure
t2
Operation
RCD Disabled!
t
I
F
/mA
Suppression of RCCB tripping for
RCCBs which are sensitive to pulsed
current