Emerson 4000, 7000 Delayed-Transition Automatic Transfer Switches, Load Retransfer To Normal

Description of Operation 4--7

Delayed–Transition Automatic Transfer Switches continued

NORMAL OK

Load on Emerg

NORMAL OK

TD Emerg>Normal

__min __s

TEST MODE

TEST CIRCUIT 5

TD Load Disconnect

__min __s

NORMAL OK

TD Engine Cooldown

__min __s

NORMAL OK

Load on Normal

Load Retransfer To Normal

The sequence for load retransfer to the normal source begins automatically when the controller detects a restored normal source or a cancelled transfer test signal.

Normal Source Restoration. The Normal source is considered acceptable again when all six voltage, frequency, or phase rotation conditions occur (see page 3–1).

Cancel Transfer Test. Removal of the test transfer signal can be by the Transfer Control switch (Feature 5), engine–generator exerciser clock (Feature 11C), or via serial port (Feature 72A). When using the Transfer Control switch, it must be released from the Transfer Test position.

The controller begins the load retransfer sequence by starting the Feature 3A time delay. Feature 3A time delay on retransfer to normal allows the normal source to stabilize. If the normal source fails while the Feature 3A time delay is running, the controller waits for the normal source again to become acceptable and restarts the Feature 3A time delay. If the emergency source fails during while Feature 3A is running, the controller bypasses the time delay for immediately load retransfer. To bypass Feature 3A time delay, turn the Transfer Control switch to the Retransfer Delay Bypass position

At the conclusion of the Feature 3A time delay, the controller is ready to transfer the load to normal.

Load Retransfer. To retransfer the load to the normal source in a delayed–transition mode the controller de–energizes the ER and ER2 relays and energizes the SE2 relay. The transfer switch CE coil energizes and opens the CE transfer switch main contacts. The load is disconnected from both sources. The load disconnect time delay starts. When this time delay ends the controller energizes the ER relay. The transfer switch CN coil energizes and closes the CN transfer switch main contacts. The transfer switch is now supplying the load from the normal source again

Engine Cooldown & Stop. After load retransfer to the normal source, the controller starts Feature 2E time delay. Feature 2E time delay provides an unloaded cooldown running period for the engine–generator. At the end of the time delay, the controller energizes the NR relay and the engine–generator is signalled to shutdown.