TGH1402en/11.2020 7
MF loop monitor
2
MF loop for monitoring the PE conductor
2.1
Intended use
• The MF loop monitor is used to monitor mobile electrical equipment and the associated
supply cables in non-gassy mines underground and in opencast mines. The loop moni-
tor is suitable for IT systems running on 3-phase AC 0…1000 V and 50…60 Hz. The pro-
tective earth conductor is monitored.
• The IT systems must not contain operating equipment which generates interference fre-
quencies in the 1 kHz range, e.g. frequency converters.
• The cable capacitance
C
e
of the cable being monitored must not be greater than 0.6 μF
per phase.
• The MF loop monitor is not intended to and must not replace any safety measures!
2.2
Function
2.2.1 Principle
In order to monitor the trailing cable PE conductor, a 1 kHz current is fed into the conductor loop
consisting of the PE conductor and the virtual HL conductor. HL is the parallel circuit consisting of
L1, L2 and L3 relating to the generator current
I
Gen
. Details are shown on the principle circuit dia-
gram on page 9. Changes in the loop current caused by interruptions or increased resistance
per unit length are recorded by means of a measuring current transformer and the frequency se-
lective SRCM420 residual current monitor. As soon as the current drops below the response value,
an alarm is output and the contactor (BS) shuts down.
2.2.2 Components of the MF loop monitor
Signal generator AF460
The equipment has two equivalent signal outputs to supply two MF loops. The generator supplies
a sinwave output current
I
Gen
of 80 mA maximum at a frequency of 1kHz at both outputs.
I
Gen
is fed
into the loops PE, L1, L2 and L3. The coupling device AG410 is connected upstream to supply the
current to the outer conductor.
Coupling device AG410
Two coupling devices connect the virtual conductor HL, consisting of L1, L2 and L3, to the rest of
the measurement frequency loop. They connect the current circuit of the generator to 3 current
circuits of the IT system.
Operator unit BG12-1
The contactor (BS) is switched on or off indirectly by means of the operator unit in the loader. The
generator current circuit (MF loop) is connected ohmically by pressing the ON button and
I
Gen
in-
creases to approx. ca. 50…60 mA. This current causes the contactor (BS) to switch on via
SRCM420.
The OFF button must be pressed to switch off the contactor (BS). The generator current circuit is
interrupted and the current drops to 28 mA, for example. The size of the current is determined by
the local system leakage capacitance
C
e
. SRCM420 registers this “undercurrent” and switches off
the contactor (BS).
Summary of Contents for AF460
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