UL FAQ

UL issues surge suppressor ratings for TVSS. This includes all equipment installed on the load side of a first AC electrical service and throughout the facility for low voltage AC distribution systems. UL applies a rating to the TVSS based on the suppressed voltage output from a transient surge condition (e.g. what the protected load would see).

Suppression Voltage Ratings are based on the average let through voltage of two 6kV1.2/50m ms, 0.5kA 8/20m ms impulses separated by a Duty Cycle test of ten 6kV 1.2/50m ms, 3.0kA8/20m ms impulses. The average let through voltages of the first and twelfth impulses determine the appropriate UL "Suppression Voltage Rating".

All UL Listed TVSS equipment contains the rating for each applicable protection mode. Manufacturers of TVSS equipment identify UL1449 documentation by way of a nameplate or sticker indicating the specific UL suppression voltage ratings for that particular device. If the UL rating sticker is not affixed to the TVSS, then one may assume that the device is not UL 1449 Listed.

UL Listing documents both product safety and standardized TVSS performance. UL is chartered to determine the safety characteristics of electrical devices. In the case of TVSS, UL is also chartered with evaluating and testing device performance. The testing criteria for these performance levels are based on the ANSI/IEEE C62.41 and C62.45 standards.

TVSS are tested in two ways depending on how the unit will be physically installed. If the device is a self-contained box and will be mounted external to a panelboard, the tests are conducted using six inches of lead length from the enclosure. If the device is integral to a panelboard, it is installed and tested in an appropriate enclosure using a six-inch lead length. Note that integral units are tested to identical standards but receive a UL Registered (UR) designation because they will be installed internal to a UL listed device.

A critical factor concerning TVSS installation is the lead length from the service panel to the device. Every additional foot of lead length adds approximately 170V to the clamping voltage. This is one reason for the industry’s shift toward integrally installed TVSS. Preferred external installation is to place the TVSS as close as possible to the panelboard.