TM 10-6640-215-13 1-13. FUNCTIONAL DESCRIPTION OF PETROLEUM LABORATORY SUPPORT SYSTEMS - continued. (3) A3K2, A3K3, A3K4 and A3K4 blower control relays located in explosion proof Distribution Box A3. (4) Two purge intake doors and two purge exhaust doors located on the outside wall of the Petroleum Laboratory. (5) Four purge door limit switches. (6) Two intake dampers and two exhaust dampers located behind the external purge doors. (7) Three ventilation dampers mounted in the ECU plenum located in the mechanical room. (8) Seven purge damper motors. (9) One purge alarm buzzer located at the POWER INPUT PANEL. A purge cycle can be initiated in two ways: by the application of external power after a power shutdown or by an alarm signal from the gas detector system. When external power is first applied at J1, the power required for a purge cycle is supplied  ahead of the Main Circuit Breaker AICBI.  AICBI is held open by power control relay A1KI.  This prevents power from being applied to any equipment or lighting in the Petroleum Laboratory until the purge cycle is completed.  Relay AIKI supplies power to start the 5-minute time delay relay A3K1 and to energize the four blower control relays (A3K2 through A3K5) which provide power to the ECU blowers.  Power is also routed to the purge door damper motors and to the ventilation damper motors so that the purge door dampers are opened and the ventilation dampers are closed.  The ECU blowers move outside air into the ECU ducting creating pressure which forces any gases or contaminated air outside via the purge exhaust doors. After 5 minutes have elapsed, time delay relay A3K1 de-energizes.  This causes power control relay A1KI to de-energize relays A3K2 through A3K5.  Power is removed from the ECU blowers, the dampers are driven to their normal positions and the Main Circuit Breaker may now be closed to supply normal power to the Petroleum Laboratory.  If at the end of the purge cycle the gas detector alarm system is in an alarm condition, the time delay relay is activated causing the cycle to be repeated.  This will continue until the alarm clears.  If when power is applied at J 1 and a purge cycle initiated, and one of the external purge doors is closed, the purge alarm buzzer will be activated by the purge door limit switch and will remain activated until the door is opened. c. Environmental Control.  The four ECUs are identical.  Each has its own controller. They operate on 208V, 60Hz, 3-phase power and are rated at 18,000 BTUs per hour when cooling and 12,000 BTUs per hour when heating.  During the purge cycle, ECU blowers pressurize the Petroleum Laboratory to expel any explosive gases. 1-24