Specifications

Engineering Standard –NSW
Signalling SMS 02
Cerberus Level Crossing Monitor Equipment
March 2005 Version 1.2 © Australian Rail Track Corporation Page 33 of 118
This document is uncontrolled when printed.
3.3.2 Function
The current sensor uses a Hall Effect device to monitor the current
in the wire that is threaded through its aperture. It does not affect
the circuit in which the current is being measured. The Hall Effect
device measures the magnetic field created by current flowing in
the wire. Tong and clamp meters also use Hall Effect devices for
measuring DC currents.
3.3.3 Operation
Power is connected to the current on terminals 1 and 2. Terminal 1
is 0 volts and terminal 2 is +12 VDC.
The output is a voltage between terminals 1 and 4. Terminal 4
being positive in respect to terminal 1. The output voltage is linear in
respect to the current in the circuit being measured.
Output Voltage = Current (A) / 4.
Level Crossing lamps draw approximately 2.5 amps each (10V 25
watt globes). Therefore the output voltage of the current sensor for
a 2 lamp a side level crossing (crossing without gates) while lamps
are flashing is about 5 amps / 4 which is 1.25 volts.
The current sensor can measure currents in the range of 0.28 amps
to 20 amps. Note that because of characteristics of the Hall Effect
device the current sensor typically has an output voltage of about
70 millivolts when their is no current following in the circuit being
measured.
3.3.4 Configuration and Set-up
Normally the link on the current sensor PCB is in the DC position
(LK1 on some units). It may be necessary to move the link to the
AC position (LK2 on some units) if there are 12 Volt DC Signal
lamps fed from the location. When the Signal changes aspect there
may be considerable induction into other circuits which the current
sensor is sensitive enough to pick up. The monitor will then alarm
because it detected lamp current when the crossing lamps should
have no current. The current sensors are susceptible to electrical
noise including radio frequency interference which causes the
sensors to indicate a false higher current reading. If interference
does occur than it will be required to move the link to the AC
position. The sensor wiring may have to be moved and it may be
necessary to move the whole sensor to another position to reduce
the interference.
The current sensors are calibrated by the manufacturer but
occasionally there is one that has been set wrong or missed. This will
mean that field checks and possible re-calibration may be required.