5.3. ERMU SR 12-11-2007 - Laserfiche WebLink

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Open Transition (Break-Before-Make) Transfer Switch -With this transfer switch, the load to be supplied from the Customer Generator is fast disconnected from the Distribution System and then connected to the Customer Generator. This transfer can be relatively quick, but voltage and frequency excursions are to be expected during transfer. Computer equipment and other sensitive equipment may shut down and need to be reset. The transfer switch typically consists of a standard UL approved transfer switch with mechanical interlocks between the two source contactors that drop the Distribution System before the Customer Generator is connected to supply the Customer load. To qualify as an open transition switch with limited protective requirements, mechanical interlocks are required between the two source contacts to ensure that one of the contacts is always open and the Customer Generator is never operated in parallel with the Distribution System. If the mechanical interlock is not present, the protection requirements are as if the switch is a closed transition switch. As a practical point of application, this type of transfer switch is typically used for loads less than SOOkW due to possible voltage flicker problems created on the Distribution System when the Customer load is removed from or returned to the Distribution System. This level may be larger or smaller than the SOOkW level depending on the Distribution System stiffness. Quick Open Transition (Break-Before-Make) Transfer Switch -The load to be supplied from the Customer Generator is first disconnected from the Distribution System and then connected to the Customer Generator, similar to the open transition. However, this transition is typically much faster (under 500 milliseconds) than the conventional open transition transfer operation. Voltage and frequency excursions will still occur, but some computer equipment and other sensitive equipment will typically not be affected with a properly designed system. The transfer switch consists of a standard UL approved transfer switch with mechanical interlocks between the two source contacts that drop the Distribution System source before the Customer Generator is connected to supply the Customer load. Mechanical interlocks aze required between the two source contacts to ensure that one of the contacts is always open. If the mechanical interlock is not present, the protection requirements are as if the switch is a closed transition switch. As a practical point of application, this type of transfer switch is typically used for loads less than SOOkW due to possible voltage flicker problems created on the Distribution System when the Customer load is removed from or returned to the Distribution System. This level may be larger or smaller than the SOOkW level, depending of the stiffness of the Distribution System. A synchronization check relay is required for quick open transition transfer Switches.