Type B Vital Relays - Alstom

Theory of Operation
2.4.4.2.2. Application Considerations
Because the Microchron II timer design is different from that of the DC motor-operated
timer, it always returns to the off position when energy is removed. Thus, it provides full
“set” time from the instant energy is applied. Because of this characteristic, no backcheck
contacts are required.
The Microchron II timer, however, with back-check contacts facilitates its use as a
replacement for the types B2 and KB motor timers without wiring changes. Two backcheck
contacts are in plugboard positions 43-44 and 53-54, as on the Type B2 motoroperated
timer. Two front contacts in plugboard positions 45-46 and 55-56, as on the
Type B2 motor timer, also permit the Microchron II timer to be a replacement for Types
B2 and KB timer relays if the following WARNING is obeyed:
WARNING
THE MICROCHRON II TIMER MAY NOT BE INTERCHANGEABLE WITH
A MOTOR OPERATED TIMER IN EVERY CIRCUIT APPLICATION.
DIFFERENCES IN OPERATION OF THE MOTOR TIMER BACK
CONTACT REQUIRE CONSIDERATION OF THE CHECK CONTACT
OPERATION IN THE SPECIFIC APPLICATION TO ENSURE THAT THE
SAFETY PERFORMANCE OF THE CIRCUIT IS NOT COMPROMISED.
BACK CONTACTS, OR CHECK CONTACTS, ON THE TYPE B2 AND KB
MOTOR TIMERS OPEN AT THE BEGINNING OF THE TIMING CYCLE.
BACK CONTACTS ON THE MICROCHRON II TIMER RELAY OPEN AT
THE END OF THE TIMING CYCLE.
WHILE IT IS DOUBTFUL THAT THERE IS A PROBLEM WITH THE
MAJORITY OF CIRCUITS IN USE, AS AN ADDED SAFETY
PRECAUTION TO PREVENT AN INADVERTENT SUBSTITUTION, THE
MICROCHRON II TIMER RELAY HAS A REGISTRATION PLATE
DIFFERENT FROM ANY OTHER TYPE B2 DC MOTOR TIMER.
P1457, Rev. Nov/07 2–23 Alstom Signaling Inc.