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 TM 55-1905-220-14-9
4-48. ALARM SWITCHBOARD - MAINTENANCE INSTRUCITONS (Continued).
(c) With everything normal, the supervisory current will maintain all of the circuits of the alarm
system in the condition just described. Closure of the contact maker will cause the system to operate as an
alarm. A failure of the supervisory current in any circuit will cause the associated supervisory relay to release
and notify the attending personnel by both the audible and a visual indication that the circuit is out of order.
See paragraph b(3).
(3)
Alarm Circuits.
(a) Operation of the system as an alarm is as follows: If any contact maker closes, thereby
shunting the supervisory resistance out of the circuit, the current flowing in the line circuit will rise to a value
(.043 amperes) sufficient to cause the complete energization of the alarm target relay.
(b) The armature of the alarm relay mechanically operates a rotating target which exposes a red
signal on the front of one of the two line alarm units. This red signal designates by means of the nameplate
above it, the equipment in which contact maker has closed.
(c) The armature of the alarm target relay closes a circuit from one side of the supply (terminal
"L1") through the coil of the extension signal relay, K, at the top of the panel, through the normally closed
contact of the silent alarm test switch, S, at the top of the panel to terminal "A". The circuit continues to the
closed contact of the alarm relay, terminal "2F" of the two line unit, "2F" of the alarm switchboard, and thence
to "L2" terminal, which is the other side of the system supply. This will cause the extension signal relay to
operate.
(d) The operation of the extension signal relay, K, closes the circuit from one side of the A.C.
supply to Terminal "L1" through the alarm bell through the extension signal relay closed contact to terminals
"2F" and "L2" on the other side of the supply, causing the alarm bell to operate.
(e) Terminals and wiring are provided for extension signals if required. There are four such sets
of terminals marked: "EX1 EX2"; "EX3 EX4"; "EX5 EX6" and "EX7 EX8". Each leg of each extension signal
circuit is fused with a 3 ampere fuse mounted in the fuse indicator on the top panel.
(4) Trouble Circuits.
(a) Operation as a trouble alarm is as follows: Failure of the supervisory current in any circuit will
de-energize the supervisory relay on that circuit, allowing the relay armature to drop out, showing a yellow
target. This closes the circuit to the common trouble buzzer.
4-1230
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