Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
An automatic transfer switch (ATS) is designed to provide a continuous source of power for critical loads by automatically transferring from a normal power source to an emergency power source when one or more predetermined events occur (e.g., the normal power source falls below a preset limit). ATSs are in widespread use in, for example, airports, subways, schools, hospitals, military installations, industrial sites, and commercial buildings equipped with secondary power sources and where even brief power interruptions can be costly (or perhaps even life threatening). Transfer switches operate, for example, to transfer a power consuming load from a circuit with a normal power supply to a circuit with an auxiliary power supply. For instance, a transfer switch can control electrical connection of utility power lines and a diesel generator to facility load buses. In certain installations, the transfer switch automatically starts a standby generator and connects the standby generator to the load bus upon loss of utility power. In addition, the transfer switch can automatically reconnect the utility power to the load bus if utility power is reestablished.
ATSs are typically of two types: (i) an ATS comprised of a single fixed switching apparatus; and (ii) a removable main transfer switch interconnected with a redundant fixed bypass manual transfer switch. The second type is typically referred to as a “bypass-isolation transfer switch,” and in this second type, the transfer switch includes a main transfer switch and a bypass feature. The bypass feature typically includes a secondary electro-mechanical switching device (bypass switch) that can route power to the load in a fashion which circumvents the main transfer switch. This bypass feature allows, for example, (i) switch redundancy if a problem arises with the main transfer switch, (ii) exercising the main transfer switch without a load connection, and (iii) isolation for maintenance of the main transfer switch while ensuring the continuity of power to the load or loads.