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Issue 1
Technical Description
The surveillance tone is fed (via the power amplifier) to the loudspeaker line.
At the end of each line (or at the end of each spur on that line) a BEL1IP detects this signal and
super imposes a DC current with reference to ground.
Where there are no spurs on the line the single BEL1IP is set to produce 1mA. When there are
spurs, the BEL1IP units are set to produce a total current of 1mA. Eg If there are two spurs each
unit is set for 0.5mA. This current is monitored by the Fault Detection module.
The constant current source is designed so that if one side of the line becomes disconnected no
current flows.
A voltage doubling circuit is incorporated which improves the overall system sensitivity, and
care must be taken as high voltages may exist.
The recommended surveillance tone level is 10 Volts but detection is not affected when higher
speech or music signals are present. The DC line current produced by the BEL1 is extracted by the
centre tap of the amplifier's output transformer, which is fed to the input of the fault detector. This
input is terminated with a 6.8K resistor and assuming a line current of 1mA produces 6.8 Volts
across it. This DC signal is buffered and fed into a window detector via a sample and hold gate.
The lower voltage threshold is 5 Volts and the upper is 10 Volts and providing the input signal is
within this range the fault detector will indicate a normal condition. The input circuit also includes an
AC detector providing a fault condition should the loudspeaker line be unbalanced due to a fault
condition.
Switch Settings
The switches on the unit are set depending on the number of spurs and hence BEL1IPs on the line,
as shown in the following table.
SW1
1 2 3 4
1
on *
on *
on *
on *
2 off on off on
3 on off on off
No. of spurs
on line
4 off off off off
* Default as supplied from the factory is for a single spur and a single BEL1IP per line.
