Transfer Switches
A transfer switch connects the load to either the utility supply or the generating set, and prevents both being connected at once. Automatic types switch on loss of mains; manual and changeover types are operated by hand. Rating is by current, poles and utilisation category.
Transfer switches are specified by rated current, number of poles (three or four), and whether neutral is switched. Four-pole switching is required where the neutral must be broken for earthing arrangements to remain correct on each source.
Mechanical interlocking is the safety-critical feature: it must be impossible to close both sources simultaneously, whether the switch is operated automatically or manually.
Common questions
Do I need a three-pole or four-pole transfer switch?
Four-pole is required where the installation earthing arrangement means the neutral must be switched with the phases — common where the generating set has its own neutral earth. Three-pole is used where a common neutral is retained. The site earthing arrangement decides it, not the set.
What is a changeover switch?
A changeover switch is a manually operated transfer device with mechanical interlocking. It performs the same isolation function as an ATS but requires an operator, so it is used where automatic transfer is not needed or not wanted.
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