How to specify an automatic transfer switch

What the ATS actually does

A transfer switch monitors the normal source, signals the generator to start when acceptable power is lost, transfers the connected load after the alternate source stabilizes, and returns the load when normal power is restored. Its controller logic, sensing, time delays, and mechanical switching device must operate as one coordinated system.

UL 1008 covers transfer switch equipment used for emergency and other power systems. The listing, application, and short-circuit rating must match the installation.

Choose the transition method

Open transition breaks from one source before making the other and creates a brief interruption. Delayed transition adds an intentional neutral period that can help residual motor voltage decay. Closed transition momentarily parallels acceptable sources to reduce interruption, but it requires utility coordination and additional control and protection considerations.

Bypass isolation allows maintenance or replacement of the automatic switching mechanism while an alternate path continues to carry the load. It is often selected where taking the load down for ATS maintenance is unacceptable.

Coordinate poles, neutral, and service entrance

Three pole versus four pole selection depends on whether the neutral must be switched and how the generator grounding arrangement is designed. Service entrance rated equipment adds the disconnecting, overcurrent, and grounding provisions required when the ATS is used at the service.

These decisions should be made by the project engineer from the grounding and protection design, not guessed from the load amperage.

Verify short-circuit performance

Provide the available fault current at the ATS and the upstream protective device. The withstand and closing rating depends on the tested combination and may vary with fuse or circuit breaker type. A generic amp rating does not establish suitability for the available fault duty.

Write the sequence of operation

Define source-failure thresholds, engine start delay, transfer delay, return delay, cooldown, exercise schedule, load shed priorities, generator inhibit, manual controls, alarms, and communications. A short written sequence gives the supplier a far better target than a catalog description.