Automatic transfer switch are used to automatically change over from the main electrical supply to stand-by generators or generate on failure of the main supply. Some transfer switches are manual, in that an operator effects the transfer by throwing a switch, while others are automatic and trigger when they sense one of the sources has lost or gained power.
When the main supply is restored, the system automatically changes back and stops the generator. The transfer switch isolates the backup generator from the electric utility when the generator is on and providing temporary power. The control capability of a transfer switch may be manual only, or a combination of automatic and manual.
The stand-by generator will eventually be shut down after a short cooling down period. Some transfer switches allow for load shedding or prioritization of optional circuits, such as heating and cooling equipment. More complex emergency switchgear used in large backup generator installations permits soft loading, allowing load to be smoothly transferred from the utility to the synchronized generators, and back; such installations are useful for reducing peak load demand from a utility.
Automatic transfer switch are provided with mechanically or electrically interlocked pole contactors or MCCB.
- Operated both automatically and manually.
- Employ a Breaker closing time delay instilled in order to avoid current and voltage surges.
- Employ automated sensing for:
- Under voltage
- Phase failure
- Phase sequence
- Instilled with Generator start/stop commands
- And Indicate:
- Incoming voltage
- Engine running/faulty conditions
- MCCB On/Trip conditions
- Engine tests run position for routine test of engines.
- Status signals for remote indications.