GE Industrial Solutions ZBTSD Series - Manual (Page 33)

71R -4000B 31 ■ ZBTS / ZBTSD Operation and Maintenance Manual
Sequence of Operation
The information package supplied with the transfer switch
includes a schematic diagram and description of operation
of all accessories provided.
The sequence for load transfer to Source 2 begins
automatically when any phase of Source 1 falls below
the preset “fail” or dropout point and this undervoltage
failure condition is detected by the solid state phase
relays B1, B2, B3.
The phase relays drop out, de-energizing Source 1 control
relay CR, and engine start timer P1 begins its timing cycle.
The P1 time delay is provided to override momentary
outages and to prevent nuisance starting of the engine
generator. If Source 1 voltage returns above the “restore”
or pickup setting of the phase relays, the P1 timing cycle is
reset to zero by the re-energization of the CR relay.
If Source 1 voltage does not return before the P1 time delay
is completed, the P1 timer drops out and sends a starting
signal to the engine generator. Source 2 voltage and fre-
quency sensing relay VFSM monitors the voltage and
frequency of Source 2. When both the voltage and the
frequency of Source 2 reach the preset “restore” or pickup
points, the VFSM relay initiates the transfer to Source 2
and timer W begins its timing cycle. The W timer provides
an adjustable transfer delay to Source 2 as required.
When the W time delay is completed, Source 2 control
relay RT energizes and picks up the power relay, CE1 or
CCE. The power relay connects voltage from Source 2 to
the Source 2 switch operating circuit causing the switch
to transfer its main contacts to Source 2. The switch is
mechanically locked in Source 2 position. The power
relay and Source 2 operator are de-energized when the
SE limit switch is activated by the operator. The SN limit
switch is deactivated at the same time, readying Source 1
control circuit for retransfer when Source 1 is restored.
The sequence for retransfer to Source 1 begins automat-
ically when the voltage on all phases of Source 1 reach
the preset “restore” or pickup point and this condition
is detected by the solid state phase relays.
When Source 1 restores, the solid state phase relays B1,
B2, B3 pick up and initiate retransfer of the switch to
Source 1 by energizing timer T and beginning its timing
cycle. The T timer provides an adjustable delay to ensure
that Source 1 has stabilized before reconnection to the
load. If Source 1 fails before the T time delay is completed,
the phase relays drop out and the T timing cycle is reset
to zero.
When the T time delay is completed, Source 1 control
relay CR energizes and picks up the power relay, CN1 or
CNN. The power relay connects voltage from Source 1
to Source 1 switch operating circuit causing the retransfer
of the main switch contacts to Source 1. The switch is
mechanically locked in Source 1. The power relay and
Source 1 operator are de-energized when the SN limit
switch is activated by the operator. The SN limit switch also
energizes the engine over-run timer U and begins its timing
cycle. The timer provides a period of time for the engine
generator to run without load and cool down before
shutdown. After the U time delay is completed, the P1
timer is energized and the engine generator is shutdown.