Manual transfer switches are sometimes used in backup power systems that serve loads classified as optional by the National Electrical Code® (NEC®), and both UL 98 and UL 1008-listed devices are used in these systems. The following article compares important aspects of the respective testing standards.
Background Information
Articles 700.5 and 701.5 of the 2020 National Electrical Code, which respectively address life-safety and legally-required loads, require that transfer switches operate automatically, be listed by a rating agency, and be marked for emergency use. Because UL 1008 - Standard for Safety - Transfer Switch Equipment addresses automatic transfer switches, UL 1008 transfer switches are used to comply with NEC requirements for Article 700 and 701 applications. However, for optional loads, NEC Article 702.4(B)(1) states the following:
Manual Transfer Equipment.
Where manual transfer equipment is used, an optional standby system shall have adequate capacity and rating for the supply of all equipment intended to be operated at one time. The user of the optional standby system shall be permitted to select the load connected to the system.
UL 98-listed double-throw switching devices are sometimes considered for transferring loads between power sources serving optional loads. However, UL 98 - Standard for Safety - Enclosed and Dead-Front Switches is a standard for general purpose switches, and not primarily intended for transferring load between power sources. Conversely, a primary function of a UL 1008 transfer switch is to maintain connection between a power source and a load, even under short circuit conditions. As such, UL 1008 test requirements are more robust than those of UL 98. Some of the primary differences are enumerated below.
Test Requirements Compared
UL 98 and UL 1008 each specify tests for (1) Overload, (2) Temperature (3) Short-Circuit Current, and (4) Endurance. The following sections compare the respective criteria for each test.
Overload
Under UL 98, general purpose switches are tested for overload at 90 to 100% of their rated voltage. UL 1008 requires overload testing at a minimum of 110% of rated voltage. The test parameters compare as follows:
Background Information
Articles 700.5 and 701.5 of the 2020 National Electrical Code, which respectively address life-safety and legally-required loads, require that transfer switches operate automatically, be listed by a rating agency, and be marked for emergency use. Because UL 1008 - Standard for Safety - Transfer Switch Equipment addresses automatic transfer switches, UL 1008 transfer switches are used to comply with NEC requirements for Article 700 and 701 applications. However, for optional loads, NEC Article 702.4(B)(1) states the following:
UL 98-listed double-throw switching devices are sometimes considered for transferring loads between power sources serving optional loads. However, UL 98 - Standard for Safety - Enclosed and Dead-Front Switches is a standard for general purpose switches, and not primarily intended for transferring load between power sources. Conversely, a primary function of a UL 1008 transfer switch is to maintain connection between a power source and a load, even under short circuit conditions. As such, UL 1008 test requirements are more robust than those of UL 98. Some of the primary differences are enumerated below.
Test Requirements Compared
UL 98 and UL 1008 each specify tests for (1) Overload, (2) Temperature (3) Short-Circuit Current, and (4) Endurance. The following sections compare the respective criteria for each test.
Overload
Under UL 98, general purpose switches are tested for overload at 90 to 100% of their rated voltage. UL 1008 requires overload testing at a minimum of 110% of rated voltage. The test parameters compare as follows: