Demystifying the UL 1008 testing requirements and ATS ratings

The UL 1008 Standard for Safety – Transfer Switch Equipment encompasses stringent testing for reliable operation under various conditions. This standard helps OEMs and panel builders optimize product safety and performance in critical power applications.

The UL 1008 Standard for Safety-Transfer Switch Equipment outlines the robust testing requirements for validating manufacturer ratings associated with Automatic Transfer Switches (ATS). By selecting ATS that have obtained the UL 1008 rating, OEMs and panel builders can ensure the transfer switch equipment they choose adheres to the necessary quality, safety, and performance criteria required by their application.

In this article, we’ll discuss the UL 1008 testing requirements, particularly the short-circuit testing requirements, and how they influence ATS ratings. Then, we’ll talk briefly about how these ratings can help you choose the right equipment for the job. 

UL 1008 testing requirements

UL 1008 is a well-recognized safety standard for transfer switch equipment in the North American market. This standard aims to ensure the reliability and consistent performance of transfer switches used in electrical power distribution systems. To achieve UL 1008 certification, ATS equipment must pass a series of robust testing requirements to verify manufacturer ratings, including withstand and closing ratings.

  • Normal Operation Test: Verifies the functionality and reliability of the switch under standard or normal operating conditions.
  • Overload Test: Checks the equipment’s ability to carry a specific level of overload current for a particular period without incurring harm.
  • Temperature Test: Verifies/confirms that the temperature of the product is within safe operating conditions as defined by the UL standard when the product is run at 100% of its load capacity.
  • Endurance Test: Tests the longevity and durability of the switch under specified regular operating conditions.
  • Dielectric Voltage-Withstand Test: Subjects the switch to a level of voltage higher than its rating to ensure the switch can withstand such voltage levels without any insulation breakdown.
  • Short-Circuit Withstand Test: Confirms the transfer switch can safely withstand short-circuit fault currents.
  • Short-Circuit Closing Test: Confirms that the transfer switch meets the UL1008 safety requirement after closing the contacts in fault.
  • Short-Time Current Rating Test: A newer, optional test. Assesses the switch’s ability to withstand a longer-duration fault and remain operational afterward. Thus, there is an additional temperature rise test at the conclusion of the Short-Time Current Rating Test.

Shortcircuit ratings explained

The results of these tests, particularly the short-circuit withstand and closing tests, are reflected on the ATS label. These ratings are crucial to selecting the right switch for your application, so let’s dive a bit deeper into the tests and what the results mean.

The two tests are similar, except for the position of the switch. In the short-circuit withstand test, the ATS is closed in the Source 1 position and stays in Source 1 position throughout the duration of the test. In the short-circuit closing test, the ATS contact moves from the Source 2 to Source 1 position (closes in to fault) and stays in Source 1 position throughout the duration of the test.

In both tests, the ATS should be able to transfer electrically and manually to an alternate source after the test. The tested unit should also pass the dielectric test, and there should be no continuity between the normal and the alternate source. Finally, there should be no damage to the alternate side contacts, no breakage of switch/components, and the cabinet door should stay closed during and after the test.

The results of these tests define the ATS short-circuit rating, which is the maximum symmetrical fault current, at a nominal voltage a device or system that can be safely sustained until a fuse or circuit breaker opens and clears the fault. The short-circuit current rating of the transfer switch equipment must be equal to or greater than the available fault current. The chart below shows the typical ATS switch ratings.

Minimum Time
Duration (s)
100A or less 10kA or less 0.008, 0.017, 0.025
401A and greater20x rating, but not less than 10kA0.05
Available Short-circuit rating per UL 1008. Standard reference.

For a more details explanation of ATS labels and what they mean, watch our on-demand webinarImportance of Understanding UL 1008 Transfer Switch Withstand and Closing Ratings (WCR).

Four common ATS certifications and their rating requirements

To round out our discussion, let’s visit four applications in which ATS manufacturers may certify their products.

#1 Protected by an external circuit breaker. This rating is called “Time Based Rating.” The ATS manufacturer certifies their product to a specific short-circuit level for a defined time duration. This rating allow customer to use any manufacturer’s circuit breaker.

This rating was formerly referred to as the “any breaker rating” or “3-cycle rating.” The primary advantage of this rating is that it offers the flexibility to utilize any available breaker in market. The external circuit breaker must be set to clear the fault in a defined time duration as specified by the ATS manufacturer’s certification. For example, if the manufacturer has a 50kA rating for 0.05 seconds, the external circuit breaker must be capable of clearing the 50kA fault within 0.05 seconds.

#2 Protected by a specific manufacturer’s circuit breaker. In this application, the ATS manufacturer has tested and certified their product for use with a specific manufacturer’s circuit breaker, which is mounted outside of the ATS. This usually allows for a higher in short-circuit rating as compared to the time-based ratings in the any-breaker scenario above.

#3 Protected by a fuse. Here, instead of using a circuit breaker, the ATS manufacturer certifies their product for use with an upstream fuse. Fuses have the quickest fault clearing time which allow the ATS manufacturer to achieve higher short-circuit rating compared to all other ratings.

#4 Protected by an external circuit breaker with a short-time rating. This rating has more stringent requirements than any of the other ratings. Short-time rated ATS products are typically applied in complex power systems where there is a benefit from selective coordination of the protection, in order to clear the fault closest to where it originated. The objective is to minimize the impact of the fault within the whole system.

The short-time rating involves the same testing requirements as the time-based rating, but the time duration is longer than 0.05 sec. Typically, the product is tested for either 0.50 sec (30 cycles) or 0.30 sec (18 cycles). In addition, the test unit must pass the UL 1008 Temperature Test after passing the Short-Circuit Withstand and Closing tests.

Short-circuit ratings aren’t the only buying criteria

While it’s vital to choose the right ATS equipment for your application, UL 1008 certification and short-circuit ratings aren’t the only buying criteria to consider. In a previous article, we delved into the benefits of choosing an all-in-one ATS unit for its simplicity and ability to reduce project and maintenance time. In addition, plug and play field installable accessories and the communications capabilities of a modern ATS unit, such as the ABB’s all-in-one TruONE line of ATS switches, can provide a wealth of information about performance and support a predictive maintenance strategy that further lowers the total cost of ownership and overhead costs.

See related blog post 3 New automatic transfer switch technology enables panel builders and OEMs to expand project opportunities.

Babu Chinnasamy

Product Marketing Manager

ABB Electrification Business