4.1. ERMUSR 09-10-2019 - Laserfiche WebLink

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Mechanical interlocks are 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 500kW 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 500kW level, depending of the stiffness of the Distribution System. A synchronization check relay is required for quick open transition transfer Switches. Closed Transition (Make-Before-Break) Transfer Switch—The Customer Generator is synchronized with the Distribution System prior to the transfer. The transfer switch then parallels with the Distribution System for a short time (100 milliseconds or less) and then the Customer load is disconnected from the Distribution System. This transfer is less disruptive than the Quick Open Transition because it allows the Customer Generator a brief time to pick up the Customer load before the support of the Distribution System is lost. With this type of transfer,the Distribution System or the Customer Generator is always supplying the Customer load. As a practical point of application, this type of transfer switch is typically used for loads less than 500kW due to possible voltage flicker problems created on the Distribution System when the Customer load is removed from or returned to the Distribution System. Depending on the Distribution System stiffness, this level may be larger or smaller than the 500kW level. The closed transition switch must include a separate parallel time limit relay which is not part of the Customer Generator control PLC and trips the Customer Generator from the Distribution System for a failure of the transfer switch and/or transfer switch controls. Soft Loading Transfer Switch (1)With Limited Parallel Operation,the Customer Generator is paralleled with the Distribution System for a limited amount of time (generally less than 1-2 minutes)to gradually transfer the Customer load from the Distribution System to the Customer Generator. This minimizes the voltage and frequency problems by softly loading and unloading the Customer Generator. Protective devices are required to permit safe and proper operation of the Distribution System while interconnected with a Customer Generator. In general, an increased degree of protection is required for a larger Customer Generator due to the greater magnitude of short circuit currents and the potential impact to Distribution System stability. Large installations may require more sensitive and faster protection to minimize damage and ensure safety. Customer shall provide protective devices and systems to detect the 5 135