Operation And Features; Sequence Of Operation - Zenith ZTSH Series Operation And Maintenance Manual

Operation and Features

Sequence of Operation

Figure 6 is a typical schematic diagram of a Zenith transfer switch with a Module 9 accessory
package. 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 the Emergency source begins automatically when any
phase of the Normal source falls below the preset 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 the normal 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 the Normal source voltage returns above the pickup set-
ting of the phase relays, the P1 timing cycle is reset to zero by the re-energization of the CR relay.
If the normal source 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. An emergency voltage and frequen-
cy sensing relay VFSM monitors the voltage and frequency of the Emergency source. When
both the voltage and the frequency of the Emergency source reach the preset pickup points,
the VFSM relay initiates the transfer to the Emergency source and timer W begins its timing
cycle. The W timer provides an adjustable transfer delay to the Emergency source as required.
When the W time delay is completed, the Emergency control relay RT energizes and picks
up the power relay, CE1 or CCE. The power relay connects voltage from the Emergency
source to the Emergency switch operating circuit caaausing the switch to transfer its main
contacts to the Emergency source. The switch is mechanically locked in the Emergency
position. The power relay and Emergency 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 the Normal control circuit for retransfer when the Normal source is restored.
The sequence for retransfer to the Normal source begins automatically when the voltage
on all phases of the Normal source reach the preset pickup point and this condition is
detected by the solid state phase relays.
When the Normal source restores, the solid state phase relays B1, B2, B3 pick up and initiate
retransfer of the switch to the Normal source by energizing timer T and beginning its timing
cycle. The T timer provides an adjustable delay to ensure that the Normal source has stabilized
before reconnection to the load. If the Normal source fails before the T time delay is com-
pleted, the phase relays drop out and the T timing cycle is reset to zero.
When the T time delay is completed, the Normal control relay CR energizes and picks up the
power relay, CN1 or CNN. The power relay connects voltage from the normal source to the
Normal switch operating circuit causing the retransfer of the main switch contacts to the
normal source. The switch is mechanically locked in the Normal position. The power relay and
the Normal 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 timefor the engine generator to run without load and cooldown
before shutdown. After the U time delay is completed, the P1 timer is energized and the
engine generator is shutdown.
Zenith Controls, Inc.
ZTSH Operation and Maintenance Manual (43R-1000)
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