MIL-HDBK-1027/1B
APPENDIX A (Continued)
EXHAUST FAN SEQUENCE OF OPERATION:
HAND MODE: WHEN THE START PUSHBUTTON, PB1A, IS PRESSED, THE FAN
CONTROL RELAY, CR1, ENERGIZES AND LATCHES IN THE CIRCUIT. AS
THE FAN COMES UP TO SPEED, MINIMUM AIRFLOW DIFFERENTIAL PRESSURE
IS GENERATED, ACTIVATING R1A. THIS CAUSES THE RUN INDICATOR,
L1A, TO LIGHT. PRESSING THE STOP BUTTON, PB1B, UNLATCHES THE
CIRCUIT WHICH DEENERGIZES THE FAN CONTROL RELAY AND RUN
INDICATOR.
AUTO MODE: WHEN THE REMOTE CONTROL RELAY, R1C, IS ENERGIZED,
THE FAN CONTROL RELAY, CR1, IS ALSO ENERGIZED. AS THE FAN COMES
UP TO SPEED, MINIMUM AIRFLOW DIFFERENTIAL PRESSURE IS GENERATED,
ACTIVATING R1A. THIS CAUSES THE RUN INDICATOR, L1A, TO LIGHT.
DEENERGIZING THE DEVICE COMPUTER CONTROL RELAY RELEASES THE FAN
CONTROL RELAY AND TURNS OFF THE RUN INDICATOR.
INTERLOCKS AND ALARMS (HAND AND AUTO MODES): IF NORMAL
DIFFERENTIAL PRESSURE IS SENSED (FAN IS RUNNING), RELAY R1A IS
ENERGIZED. EXCESSIVE DIFFERENTIAL PRESSURE (FAN RUNNING WITH
DAMPER CLOSED) CAUSES R1B TO ENERGIZE. WHEN THE FAN IS
COMMANDED ON AND EITHER INSUFFICIENT OR EXCESSIVE DIFFERENTIAL
PRESSURE IS SENSED FOR 15 SECONDS, RELAY TR1A (TR2A) ENERGIZES
AND LATCHES IN. THIS DEENERGIZES THE FAN CONTROL RELAY, CR1 AND
LIGHTS THE FAN FAILURE INDICATOR, L1B. WHEN THE FAN IS
COMMANDED OFF AND NORMAL DIFFERENTIAL PRESSURE IS SENSED FOR 15
SECONDS, RELAY TR1B ENERGIZES AND LATCHES IN. THIS ALSO CAUSES
INDICATOR L1B TO LIGHT.
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.
________________________________________________________________
Figure A-20B
Exhaust Fan/Crawl Space Power Controls and Interlocks
183
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