Welcome to the ASCO Website

Welcome to our website.

Choose another country or region

UL Standards for Manual Transfer Switches

 
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:

UL 98:Tested at 150% of rated current at 0.75 to 0.8 power factor for 50 cycles, without regard to load type.

UL 1008:(1) Tested with total system load including 100% motor loads. (2) For motor loads, tested at 600% of rated current at 0.4 to 0.5 power factor. (3) 50 cycles for models up to 1600 A, 25 cycles for 1601-2500A, and 3 cycles above 2500.
Temperature

Under each standard, temperature rise tests are conducted at the rated voltage and current. Each specifies temperature limits for various components and materials with limits of 35°C to 125°C. However, UL 98 allows testing of a previously untested sample, while UL 1008 requires a sample that has already undergone overload testing.

Short-Circuit Current

UL 98 does not require short circuit testing on switches rated 10 kiloamps or less that are marked for use with an overcurrent protective device rated less than the current rating of the switch. UL 1008 requires short circuit current testing on all switches. When conducted, the standards require withstand and closing tests under the following short circuit conditions at similar power factors.

UL 98: If the rating of the overcurrent device is greater than the rating of the switch, test shall be conducted at 5,000, 7,500, or 10,000A. Where the available current is greater than 10 kiloamps, the test current shall be greater than 10 kiloamps.

UL 1008:The minimum test rating depends on the switch rating as follows:
- Up to 100A: 5,000A
- 101A to 400A: 10,000A
- Above 400A: 20 times rating but not less than 10,000A
Notably, UL 98 allows use of a new sample for the closing test. UL 1008 requires that the same sample be used for both the withstand and closing tests without the benefit of maintenance.

Endurance

Under UL 98 and UL 1008, endurance testing involves operating the equipment with and without current for prescribed quantities of transfer cycles. Differences in protocol include:

UL 98Making and breaking 100% of rated current with a power factor of 0.75 to 0.8.

UL 1008: Tested for use with total system loads where one-half of the test cycles are at 200% of the rated current and the remainder is at 100%.
UL 1008 typically prescribes greater quantities of cycles with current. For instance, under UL 98, a 400 Amp transfer switch would be subject to 1000 cycles at 100% current. Under UL 1008, the same switch would be subject to 4000 cycles at 100% current, with half of those completed at 200% current. For a 1200 Amp switch, UL 98 prescribes 500 cycles at 100% current; UL 1008 calls for 1500 cycles with current, half of those at 200%.

Application

Manual transfer switches can be used for loads that are classified as optional under the NEC. Nevertheless, optional does not indicate that the associated loads are unimportant. Rather, it indicates that they are not regulated under the safety-related Articles 700 and 701. Depending on the facility and application, optional loads could include equipment that is essential to operational continuity and revenue generation and collection.

In the previously described tests, the UL 1008 testing standards are as rigorous or more demanding on nearly every point. Consequently, UL 1008 manual transfer switching equipment will likely stand up to greater electrical stresses than UL 98 listed units. In many applications, the power availability and reliability need of facility operations make a strong case for providing UL 1008-listed switches. Doing so assures users of obtaining the same robust equipment performance and longevity associated with switches deployed for life-safety and critical loads.

Summary

Manual transfer switches are available with either UL 98 or UL 1008 listings. Comparison of the respective test requirements shows UL 1008 to be more stringent, indicating that units listed to this standard are more likely to provide high reliability and longer service life. Consequently, the use of UL 1008-listed Manual Transfer Switches should be evaluated wherever power availability could impact facility operation and revenue generation.

The information provided herein highlights certain differences between the aforementioned standards for comparative purposes only. For additional detail, always refer to the UL 98 and UL 1008 standards directly.   

References

National Fire Protection Association, NFPA 70 - National Electrical Code, 2020. Quincy, Massachusetts, 2019.
Underwriter Laboratories, UL 98 - Enclosed and Dead-Front Switches, 13th Edition. Northbrook, IL. 2004.
Underwriter Laboratories, UL 1008 - Enclosed and Dead-Front Switches, 8th Edition. Northbrook, IL. 2014.

Get the latest white papers, product news and insights straight to your inbox