Emerson 7000 Open-Transition 2-position Automatic Transfer Switches, Load Retransfer To Normal

4--2 Description of Operation

Open–Transition (2–position) Automatic Transfer Switches continued

Load Retransfer To Normal

NORMAL OK

Load on Emerg

NORMAL OK

TD Emerg>Normal

__min __s

NORMAL OK

TD Engine Cooldown

__min __s

NORMAL OK

Load on 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 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. If Feature 27 inphase monitor control is enabled, the controller will inhibit transfer until the sources are in phase.

Load Retransfer. To retransfer the load to the normal source the controller de–energizes ER relay and energizes SE relay. The transfer switch TS coil energizes, and all transfer switch contacts (mains, controls, auxiliaries) reverse position. 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.