MIL-HDBK-1027/1B
APPENDIX A (Continued)
CRAWL SPACE POWER SEQUENCE OF OPERATION:
HAND MODE: WHEN EITHER FAN IS ON FOR 10 MINUTES WITHOUT A
DETECTED FAILURE, THE PURGE COMPLETE RELAY, TR3A, IS ENERGIZED.
WHEN THE ON PUSHBUTTON, PB3B, IS PRESSED AND TR3A IS ENERGIZED,
THE CRAWL SPACE POWER RELAY, CR3, ENERGIZES AND LATCHES IN THE
CIRCUIT. THIS PROVIDES POWER TO THE CRAWL SPACE, ENERGIZING
RELAY R3A AND LIGHTING THE POWER INDICATOR, L3A. PRESSING THE
OFF PUSHBUTTON, PB3B, UNLATCHES THE CIRCUIT WHICH DEENERGIZES
THE POWER CONTROL RELAY AND POWER INDICATOR.
AUTO MODE: WHEN THE REMOTE CONTROL RELAY, R3B, IS ENERGIZED,
THE FAN CONTROL RELAY, CR3, IS ALSO ENERGIZED. THIS PROVIDES
POWER TO THE CRAWL SPACE, ENERGIZING RELAY R3A AND LIGHTING THE
POWER INDICATOR, L3A. WHEN THE DEVICE COMPUTER CONTROL RELAY
RELEASES, THE CRAWL SPACE POWER RELAY AND INDICATOR DEENERGIZE.
ALARMS (HAND AND AUTO): IF RELAY R3A DOES NOT ENERGIZE WITHIN 2
SECONDS AFTER POWER IS COMMANDED ON, POWER FAILURE RELAY TR3B
ENERGIZES AND LATCHES ON. IF RELAY R3A DOES NOT DEENERGIZE
WITHING 2 SECONDS AFTER POWER IS COMMANDED OFF, POWER FAILURE
RELAY TR3C ENERGIZES AND LATCHES ON. WHENEVER EITHER POWER
FAILURE RELAY IS COMMANDED THE POWER CONTROL FAILURE INDICATOR,
L3B, LIGHTS.
NOTES: 1.
CONTROL LOGIC MAY BE IMPLEMENTED IN DISCRETE HARDWARE
OR IN A SMALL PROGRAMMABLE LOGIC CONTROLLER.
2. TIMER RELAYS PROVIDE A DELAY AS NOTED AFTER POWER IS
APPLIED. WHEN POWER IS REMOVED THE CONTACTS RESET
IMMEDIATELY.
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Figure A-20D
Exhaust Fan/Crawl Space Power Controls and Interlocks
185
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