Choosing the Correct Transfer Switch

Abstract

Standby generation systems in low- and medium-voltage applications connect to the utility power system in a number of different ways. Typically, an automatic transfer switch is a part of most power system connections. Engineers can choose three- or four-pole transfer switches for the intertie point. The application's sensitivity to a power interruption during switching helps determine transfer-switch selection. The transfer switch can be contactor or circuit breaker based, or it can be a bypass design. At medium voltage, special requirements must be considered. Ultimately, the correct arrangement must be chosen depending upon grounding locations and neutral bonding. Incorrect power system bonding and grounding can cause unwanted transients or hazardous multiple grounds (improper ground-fault protection) depending upon the transfer-switch selection. Improper design can create unintentional current paths and defeat ground-fault protection. A transfer switch must be chosen for robust operation and maintenance at the beginning of the design process.