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.
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.
