8 TGH1402en/11.2020
MF loop for monitoring the PE conductor
Frequency selective current monitor SRCM420
The device monitors the generator current circuit for undercurrent. The measuring current trans-
former W20 or W35 supplies the required measuring current Imfor SRCM420. Its input filter, which
is set to 1 kHz, registers the transformer signal with a signal-to-noise ratio of approx. 40dB in rela-
tion to the fundamental component of 50 Hz. When the value drops below the preselected res-
ponse value, the device outputs an alarm. This causes the alarm LEDs AL1 and AL2 to light up and
the changeover contacts K1 and K2 switch over. When K2 switches over, the contactor (BS) swit-
ches off.
2.2.3 Switching off the loop monitor or loader
Pressing the OFF button of the BG12-1 interrupts the MF conductor loop. This lowers the genera-
tor current IGen to below the response value I2. The SRCM420 detects the undercurrent and out-
puts the associated alarm. In other words, the LEDs AL1 and AL2 light up and the alarm relays K1
and K2 switch over. Among the changeover contacts, which operate in N/C MF loop for monito-
ring the PE conductor operation, the contacts 11-12 and 21-22 are connected. When K2 switches
over, this causes the contactor (BS) to shut down.
2.2.4 Switching on the loop monitor or loader
Pressing the ON button of the BG12-1 closes the MF conductor loop. This increases the generator
current IGen to 50…60 mA. The SRCM420 detects a current outside the alarm range and terminates
the alarm output. In other words, the LEDs AL1 and AL2 go out and the alarm relays K1 and K2
switch over. Among the changeover contacts which operate in N/C operation, the contacts 11-14
and 21-24 are now closed. When K2 switches over, this causes the contactor (BS) to switch on.
2.2.5 Loop monitor detects longitudinal discontinuity
If a longitudinal discontinuity occurs inside the MF loop and drops the generator current below
the response value I2, the SRCM420 outputs an alarm. This shuts down the contactor (BS).
2.2.6 Meaning of the displayed current values
Generator current IGen when the operator unit is switched on
Standard: 50…60 mA
When the operator unit BG12-1 is switched on, the size of the generator current IGen is determined
by the system leakage capacitance Ceand the ohmic resistance of the entire MF loop. The resul-
ting impedance Zdetermines the size of the generator current IGen. This current must be measu-
red after installing the MF loop monitor and is used as a reference value for assessing the system.
Generator current IGen when the operator unit is switched off
Standard: 10…30 mA
When the operator unit BG12-1 is switched off, the size of the generator current IGen is mainly de-
termined by the system leakage capacitances Ce. This current must be measured after installing
the MF loop monitor and is used as a reference value for assessing the system.
