Testing Breaking Conditions For Overcurrent Protective Devices; Function) - Gossen MetraWatt ProFiTEST C-GB int. Operating Instructions Manual

7 Testing Breaking Conditions for
Overcurrent Protective Devices,
Measuring Loop Impedance and
Calculating Short-Circuit Current (Z
Testing overcurrent protective devices includes visual inspection and
measurement. Use the PROFiTEST
Measuring Method
Loop impedance Z is measured and short circuit current I
Loop
calculated in order to determine whether or not breaking conditions are
fulfilled by the overcurrent protective device.
Loop impedance is the resistance of the current loop (power distribution
station – phase conductor – protective conductor) when a short-circuit to
frame occurs (conductive connection between phase conductor and pro-
tective conductor). The loop resistance value determines the magnitude of
short-circuit current. Short-circuit current I
value set forth in DIN VDE 0100, in order to assure that the system's pro-
tective device (fuse, circuit breaker) breaks in a reliable fashion.
For this reason, measured loop impedance must be less than the
maximum allowable value.
Tables including allowable display values for loop impedance, as well as
minimum display values for short-circuit current for various fuse and
breaker current ratings can be found in chapter 11 starting on page 34.
Maximum instrument error is taken into consideration in these tables (see
also chapter 7.3).
In order to measure loop impedance Z Loop , the instrument uses a test
current of 740 mA and a test duration of approximately 400 ms regardless
of line voltage and line frequency.
If dangerous contact voltage occurs during this measurement (> 50 V),
safety shutdown ensues. The measuring and test instrument calculates
short-circuit current I based upon measured loop impedance Z Loop
sc
and line voltage. Short-circuit current is based upon nominal voltage
230 V (170 V  265 V) or on nominal voltage 120 V (80  170 V).
Loop impedance can be measured either with the positive or the negative
®
half-wave with the PROFiTEST
By using this measuring method in combination with the

PROFiTEST DC-II measuring adapter, loop impedance can be measured
in systems equipped with RCCBs without causing them to trip.
26

function)

Loop
®
C for measurement.
sc
may not drop to below the
sc
C.
Connection
is
R
F
Measurement of loop impedance must be performed from all three
phases (L1, L2 and L3) to the protective conductor PE for 3-phase
systems in order to assure flawless functioning of the overcurrent
protective device.
➭
Select loop impedance measurement with the Z
wave should be displayed as the active waveshape. Refer to the next
chapter regarding measurements at RCCBs.
➭
Start the measurement with the START key.
L1
L2
L3
N
PE
START
key. A solid sine
Loop
GMC-I Messtechnik GmbH