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The Transfer Switch Ratings AHJs Want to See

UL 1008 – Standard for Safety – Transfer Switch Equipment sets forth safety and performance requirements for transfer switches. A transfer switch can only be listed to the UL 1008 standard if it passes a specified series of tests and marked with the resulting electrical ratings. These are the ratings that an Authority Having Jurisdiction (AHJ) will want to see when inspecting a newly installed transfer switch.

Background Information

When fault currents flow through a transfer switch, it will be subjected to stresses that could compromise its function. If the currents are excessive, these effects include electromagnetic forces that could cause the closed power source contacts of the switching mechanism to open or prevent their closure. If either of these scenarios occurs, the switch could be damaged, and the open contacts would result in a power outage to downstream loads. In addition, excessive heat rise could cause the materials inside the equipment to deform or deteriorate, resulting in a potential fire hazard.

To someone new to transfer switching, the opening of a circuit may seem like a desired outcome when overcurrents or faults occur. However, it is important to consider the function of a transfer switch, which is to connect an electrical load to an active power source under all foreseeable conditions, including the maximum fault current at a switch location. Safely clearing faults by opening a circuit is specifically the function of overcurrent protection devices such as circuit breakers or fuses. To promote the highest levels of reliability, transfer switches must be able to withstand and close on the available fault currents at the location where they are installed. For additional information, see the ASCO Technical Brief entitled Basic Automatic Transfer Switch Functions.

Types of Ratings and Their Markings

To evaluate transfer switch safety under fault conditions, UL 1008 specifies mandatory tests that manufacturers perform to obtain a UL 1008 Listing. These establish the switch’s Withstand and Closing Ratings. UL 1008 also specifies optional tests that describe extended fault management capabilities. Each of these values must be marked on a transfer switch by its manufacturer in the manner specified by the UL standard. A further description of UL 1008 testing requirements is found in the ASCO White Paper entitled UL1008 Transfer Switch Withstand and Closing Ratings.

Withstand and Closing Ratings

ASCO documents that describe UL 1008 tests for verifying both withstand and closing ratings are listed at the end of this document. The ratings verified by a UL 1008 testing program must be marked on the switch. The first of these will be identified as the ratings for Short-Circuit Current.

Using Breaker Generally

Most commonly, circuit breakers are used upstream of a transfer switch. A Short-Circuit Current Rating is marked for these devices. Because this rating applies to breakers of any manufacture or model, they are commonly referred to as an “Any Breaker Rating”. Figure 1 shows markings for a transfer switch rated for up to 100,000 Amps at up to 600 Volts for up to 0.050 seconds (equivalent to 3 cycles in a 60 Hertz application). This rating means that the transfer switch is suitable for use if the upstream breaker will open at a lower short-circuit current and/or sooner than the maximum duration marked on this transfer switch.
Using Specific Breakers

Specific circuit breaker models may offer especially quick trip times, which enable a transfer switch to withstand and close on greater faults than might be achieved using a different breaker of the same ampacity. UL 1008 allows manufacturers to list these higher values if the switch passes actual testing when protected by a specific breaker. For 480 Volt applications, Figure 2 shows that this switch can handle up to 125,000 Amps when protected by a 3000-Amp Square D NW-L Breaker and up to 200,000 Amps when protected by 2000-Amp Square D MTZ breaker. Using one of these breakers could avoid having to specify a larger and more costly transfer switch.
Using Fuses

Fuses often open faster than breakers. Because the amount of current that can be conducted through a fuse is inverse to the time the current must be held, using a fuse instead of a breaker can maximize the amount of current that a transfer switch can safely handle. UL 1008 enables manufacturers to test and mark transfer switches for use with a fuse of a specific class and ampacity. The markings in Figure 3 show this switch can carry up to 200,000 Amps at up to 600 Volts when protected by a Class L fuse rated for 3000 Amps.
Short-Time Ratings

Many power distribution systems limit the amount of equipment depowered by overcurrent protection devices by applying a selective coordination strategy. This approach places the breakers or fuses with the quickest opening times closest to loads to reduce the amount of downstream equipment affected by the opening of a protective device. This reduces disruption to a facility and its operations when a fault is cleared. To enable selective coordination, a transfer switch must carry a greater fault current for a longer duration than its nearest upstream protective device. A more detailed description of coordination strategies is found in the ASCO White Paper entitled Selective Coordination Basics.

To provide a range of selective coordination options, UL 1008 allows manufacturers to test switches under fault for longer durations than are used to establish the short-circuit current ratings. When manufacturers elect to complete this testing, they can publish Short-Time Ratings to describe a switch’s performance under extended fault conditions. These ratings will reflect the inverse relationship between the amount of current and the duration for which a switch can carry it. Figure 4 shows the amounts of current and corresponding voltages as well as the durations for which they can be held. Any upstream fuse or breaker must be rated below these values to ensure it will clear a fault before the indicated maximum duration marked on the transfer switch.
Qualitative Markings

To ensure that AHJs and users can understand the context for the aforementioned UL 1008 ratings, the standard specifies that certain qualitative text be included with the ratings. These inform the reader of important information describing the intended use of the switch and its ratings. Regulated entities should expect AHJs to verify that a transfer switch has been installed in compliance with the marked notices. Consequently, specifiers should become familiar with the meaning of these notices before selecting transfer switch equipment. For reference, Figure 5 provides the entire UL 1008 label that was used to develop Figures 1 through 4 above.