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Common Types of Transfer Switches
Transfer switches can be electromechanical or solid state or “static” transfer switches. Here we are focused on electromechanical units.
Transfer switches come in multiple types and configurations. The simplest type is a safety switch, often referred to as a double-throw (DT) safety switch. It is a three-position switch with a center OFF position and two ON positions, each side receiving power from a different supply source. This is a manually opened and manually switched device. There is no separate control scheme. Usually there is no protection involved, although a DT switch can be fusible. This DT switch is one of those we refer to as a manual transfer switch or MTS. Other types of MTS include arrangements with two manually operated circuit breakers or non-automatic switches operated by two concurrent breaker operators and interlocked with a rocking beam interlock. This same arrangement can be electrically operated so that opening and closing can be performed with pushbuttons.
Automatic transfer switches, or ATS, must open and close electrically. They most often have integrated transfer programmable logic systems built into their freestanding cabinet. They can also be integrated into switchgear lineups. These can be assembled around circuit breakers or (usually proprietary types of) contactors, or in the case of medium voltage, they may be electrically operated load break switches. They can be single or three phase and can be designed for use at any voltage. At ROMAC we build and work with transfer switches through 15kV.
Some key configurations are open transition, break before make, closed transition, make before break; in 3 phase applications) 3 pole-3 wire delta, 3 pole-4 wire wye and 4-pole units with full capacity neutral. There are industry-standard protective suites built into the programmers that allow detailed customizing of power system conditions that will trigger a transfer, start up an engine generator set or return to main power source. This involves careful monitoring of system voltage, current, frequency, phase alignment, phase failure and ground conditions.
Wherever power interruption is unacceptable, even for minutes, you will find transfer switches in service. Some of the most common applications are hospitals, data centers, telephone switching centers, internet server centers and cellular server and relay stations.