TM 3-4230-209-20&P
1-20
(11) In the holding relay circuit a fail-safe circuit forms an important
(10) If ignition has occurred, as soon as a fire is going the temperature
function. Shown here is the fail-safe circuit. It starts basically at relay K2-A1
in the exhaust flue will rise sharply. When the exhaust flue reaches 300F,
and stops at S4. An over temp condition at S1 or a low pressure at S3 will
the flame switch (S2) contacts will open. This will remove the circuit voltage
cause an immediate shutdown of the heater. If the exhaust temperature is less
through S4 contacts 2 and 3, the heaters of both TD-1 and TD-2 to ground.
than 300 F, S2 will close. This would allow voltage to be applied to TD1 and
When TD-1 and TD-2 cools down, TD-1 will open its bimetallic contacts. With
TD2 heaters. If this were to occur in approximately 120 seconds, the TD2
TD-1 contacts now open, they no longer form a bypass circuit from K2-A2 over
bimetallic contacts would open up and the circuit between pins 5 and 7 would
to A1 and the whole circuit acts as a holding relay. The heater should heat the
open. The fault would have-to be corrected before the circuit would hold
water.
again.