GB1343819A - Fire alarm monitor - Google Patents

Abstract

1343819 Fire alarms WALTER KIDDE CO Ltd 15 Feb 1972 [25 May 1971] 17066/71 Heading G4N In an alarm system having a plurality of units 3, 4 each having a normally open contact AA, BB, connected across a line 1 and closed in response to an unwanted condition and a normally closed contact A, B, connected in series with line 1 and opened in response to a fault in the units 3, 4, at least one of the normally closed contacts B is bridged by a circuit including a fault indicator 11 and having an impedance such that operation of normally opened contacts, such as AA, will still be detected despite opening of contact B. As described, the standing current in line 1, as determined by resistor 2, is sufficient to maintain transistor 7 conductive and fault relay RL1 energized. In the event of a fire, one of contacts AA, BB, closes so as to energize alarm relay RL2, which is held in by contact 5 and operates bells or a lamp. A fault in one of alarm units 4 causes opening of the normally closed contact B which would prevent detection of the closure of subsequent normally open contacts such as AA. A circuit comprising thyristor 8 and resistor 9 is therefore included which operates an indicator lamp 11 when contact B opens. Resistor 9 has such a value that the opening of contacts B will be detected by the de-energization of transistor 7 and fault relay RL1, but the alarm relay RL2 will still be operated in the event of closure of contact AA. Diode 10 protects the gate circuit of thyristor 8. Thyristor 8 may be replaced by a reed relay or Darlington pair.