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TM 55-1905-220-14-9
(c) Circuit diagram.
1 From a study of the wiring diagram (Foldout 1), it will be seen that when the circuit is in normal
operating condition, the supervisory relay is energized and pulled in.  The supervisory
resistor associated with the contact maker is in the circuit, and while the alarm relay is also in
the circuit, it is not energized sufficiently to close its armature. Short-circuiting of the resistor
by closing the contact maker will increase the current in the circuit. This increase in current
flowing is enough to increase the magnetic energy of the alarm relay to a point where it will
operate its armature, thereby causing the alarm bell to ring and an indicator to be set to
indicate the particular circuit in alarm condition. Since the normal operating current is just
sufficient to hold the supervisory relay armature operated, an open circuit in the wiring
causes the supervisory relay to drop out, which in turn will operate a buzzer and cause a
yellow target to show, indicating the particular circuit in trouble.
2 The resistor is the supervisor of the circuit. Because of the presence of this resistor it is
possible to maintain the supervisory relay energized and the alarm relay to all intents not
energized. (The alarm relay is, of course, partially energized but not sufficiently to operate.)
By this arrangement, it is possible to make the system a closed circuit system that will give a
positive warning in case a contact maker closes, and will also supervise itself and give an
equally positive warning if for any reason a break or a failure should occur.
The current flowing under normal operating conditions, that is with the supervisory relay
operated, the supervisory resistor in the circuit, and the alarm target relay open, is called the
supervisory current, and in this system is approximately .012 amperes.
b. Operation of an Alarm System.
(1) Placing of System in Service.
(a) Assuming that an alarm system is properly installed and connected, all that is necessary to
place the system in service is: first, make sure that all switches on the alarm switchboard are in the normal
position; and second, close the alarm supply switch located on the interior communication switchboard. Closing
this supply switch will energize the entire alarm system.


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