Last month we discussed the operation and ratings of the automatic transfer switch (ATS). In this column we’ll review three significant specification options, and continue our discussion next month with application of the ATS to motor loads.

Three-Pole vs. Four-Pole vs. Overlapping What?

When applying an ATS on a solidly grounded (208Y/120V or 480Y/277V) system, it is important that you do not create a second neutral-to-ground current path for either source. Such a dual path can result in electromagnetic interference from currents flowing in the ground, and can interfere with the proper operation of ground fault protection systems. ATSs are available in three contact configurations, with selection dependent upon whether the load requires a neutral conductor.

The three-pole transfer switch, having provisions to switch the three-phase conductors but not the neutral, is always suitable for use with three-wire loads that do not require a neutral conductor. It may also be used with four-wire loads if it includes a neutral bus with a solid connection of both source and load neutrals. However, this requires that the neutral of the standby source not be directly bonded to ground, which can create safety concerns and should only be specified after careful consideration by the designer. The preferred application for four-wire loads is use of either a four-pole switch or a three-pole switch with overlapping neutral contacts.

The four-pole ATS has a neutral switching pole identical to the phase poles, with the same electrical ratings. This arrangement has the advantage of providing the same short circuit withstand and load switching capability in all parts of the switch but sometimes raises concerns about overvoltages during the short period of time when the load neutral may not be connected to either source neutral. This concern may be alleviated by adjusting the mechanism to make the neutral pole first to close and last to open.

An overlapping neutral is a separate set of contacts designed to momentarily connect both source neutrals together to the load neutral during the transfer to ensure that there is no opportunity for overvoltages due to an open neutral. Because it never fully breaks rated current, this contact does not require the same heavy-duty construction as a fourth pole and is consequently smaller and less expensive. Both the four-pole and overlapping neutral configurations are commonly applied successfully for four-wire loads.

Open- vs. Closed-Transition

When the normal source fails, there is obviously an interruption of power to the load until the alternate source is available and the ATS transfers to the emergency position. Traditionally, transferring back to the restored normal source also resulted in a second interruption as switching was done on a break-before-make or open-transition basis. The closed-transition ATS, a relatively recent innovation, eliminates the need for any interruption when switching between two energized sources.

A closed-transition ATS transfers between energized sources that are synchronized to each other on a make-before-break basis, with the sources connected for a period of less than a quarter second. This permits transfer back to a restored normal source without a second interruption of the load. The ATS incorporates a phase monitor circuit to verify that both sources are in phase before permitting a closed-transition operation. If the standby source is a generator, a delay is experienced until the generator drifts into an acceptable phase angle with respect to the normal source.

This feature is also attractive when the load will not tolerate the brief interruptions associated with periodic testing of a standby engine-generator. For mechanical reliability, engine-generators should be tested under enough load to bring them to normal operating temperature and the closed-transition ATS can make it easier to implement this mode of testing. It should be noted however, that this provides only a test of the generator; it should not substitute for periodically testing the complete system by interrupting the normal source.

Bypass/Isolation Capability

If reliability of service is a concern, specification of bypass/isolation capability may be considered. This provides a second, manually operated switching mechanism arranged to permit direct connection of the load to either source. After the bypass switch has been activated, the ATS assembly and its controls can be isolated and disconnected for service without interrupting the load. The bypass switch can also be used to manually transfer the load in an emergency if the ATS fails to do so. Because two complete switching mechanisms and isolation provisions are required, this feature approximately doubles the cost and size of an ATS. It is therefore typically specified only for hospitals, data centers, and other critical loads.