UL 497D OUTLINE

Description / Abstract: This Outline covers secondary protector components for communications circuits that are intended to be used in installations or circuits with specified voltage suppression. These components are intended to be used in circuits that are protected by a voltage suppression device that has a suppression voltage, including the high end of any specified tolerance, less than the peak value of the maximum applied rms fault voltage for the component. This includes circuits with integral voltage suppression components and circuits protected by a known, appropriate primary protector where the installation of such a primary protector is controlled.

These requirements cover components intended for use in circuits electrically connected to signaling and communications circuits located on the equipment side (protected side) of a primary protector.

These components are investigated using a reduced fault voltage for the Overvoltage Test as recorded in the test report and identified in the individual certification information. The maximum suppression-voltage rating, including the high end of any specified tolerance, of the required voltage-suppression device is also included. (Common terms for suppression voltage ratings for various technologies include but are not limited to "Rated Impulse Voltage", "Maximum Clamping Voltage", "Clamping Voltage (Vc), "Let-Through Voltage", "Maximum Continuous Operating Voltage", "Switching Voltage (Vs)" and "Breakdown Voltage".)

These components are intended to suppress abnormal voltage and/or current conditions that bypass the preceding voltage suppression device. Current limiting components limit currents to less than the current-carrying capacity of communication wire employed in the communication loop of the protected premise. Voltage limiting components may be subjected to excessive currents during their normal operation and usually require additional overcurrent protection in the end product application.

These components do not normally provide protection for the voltage suppression device needed in the circuit to limit the voltage to less than the component ratings. They also do not provide protection against excessive currents that may flow during the operation of the preceding voltage suppression device. Additional overcurrent protection prior to the voltage suppression device may be required. Any such overcurrent protection shall not disable the voltage suppression without also disconnecting the components from the signaling or communications circuit.