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Transfer Switch Basics

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Power Distribution Equipment

Transfer Switch Basics

During actual or threatened power failure, transfer switches are engaged to transfer critical electrical load circuits from the normal source of power to the auxiliary (emergency) power source. When normal power is restored, the transfer switches either automatically route their load circuits to the normal supply or must be transferred manually. Voltage
and frequency-sensing relays are provided to monitor each phase of the normal supply. The relays initiate load transfer when there is a change in voltage or frequency in any phase outside of predetermined limits. Additionally, the relays initiate retransfer of the load to the normal source as soon as voltage is restored in all the phases beyond the predetermined pick-up value of the relay. A transfer switch obtains its operating current from the source to which the electric/current load is being transferred.

Types of transfer switches:

There are two types of transfer switches: electrically operated or manually operated. Electrically operated transfer switches also come with an optional bypass function.

References: “Joint Departments of the Army and the Navy, Operation Maintenance and Repair of Auxiliary Generators, 26 August 1996”

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