Yesterday, I participated in a workshop focused on UL 508A-compliant panel design, and I find myself more perplexed about designing for necessary SCCR than before. From what I gathered, using listed or recognized components in the power circuit will grant a "courtesy" 5kA rating. There are two methods to reduce available fault currents: 1) Installing a transformer in front of the panel, which decreases the fault current. Sizing tables for transformers are included in UL508A. 2) Incorporating current limiting fuses or breakers at the power entry point of the panel. This allows for the use of components with a rating higher than the peak let-through of a specific type and rating of fuse or circuit breaker, with one exception. The confusion arises when it comes to overcurrent protection devices like fuses and circuit breakers. They must be rated for the available fault current, regardless of being downstream of a current limiting device. However, other components such as terminal blocks, contactors, and VFD's are exempt from this rule. The justification for this discrepancy was not clearly conveyed during the workshop, leaving me seeking a deeper understanding. During the workshop, a theoretical scenario was presented where a source with 50kA available fault current was protected by a Class-J fuse with an 8kA peak let-through, a circuit breaker with a 35kA rating, and downstream components with at least an 8kA rating. Surprisingly, the setup could only achieve a 35kA SCCR due to the circuit breaker, which raises the question of why the downstream breaker's rating affects the overall SCCR. If the other components are acceptable, why does the overcurrent protection need to match the available fault current? The presenter's explanation left me with more questions than answers, as I was simply told, "those are the rules," without further clarification. I am eager to comprehend how to construct a compliant panel with a higher SCCR without excessively oversizing each component. For instance, is it possible to "daisy chain" overcurrent protection to achieve this goal? Your insights would be greatly appreciated, as I strive to improve my understanding in this area. Thank you, -rpoet
Looking for answers regarding fuse classes and SCCR values in power circuit designs? Discover more in this insightful discussion on PLC Talk forum: http://www.plctalk.net/qanda/showthread.php?t=98516. Despite common responses citing UL regulations, there is still ambiguity. Our preference for Class J and Class CC fuses, paired with devices, is key in ensuring a seamless power circuit design. With high SCCR values of 200kA in these fuse classes, downstream overcurrent protection remains stable. Is there a connection between fuse manufacturers and the UL508A committee?Keith.
I had no idea about this concept. It's always been my assumption that anything downstream should have a higher rating than the current limiting fuse. This doesn't seem logical to me. Perhaps a solution is to consistently use J and CC fuses, even when placed downstream of other fuses?
kamenges mentioned good luck with understanding the topic discussed in this thread on the plctalk.net website. Despite seeking answers, the replies often refer to UL regulations without much explanation. In our power circuit designs, we prefer using Class J and Class CC fuses whenever possible, as they have high SCCR values, eliminating downstream overcurrent protection issues. It seems like the influence of fuse manufacturers on the UL508A committee plays a significant role in the decision-making process. After reviewing the thread, I still find the engineering rationale unclear. It is puzzling how the current-limiting overcurrent protection affects let-through current. Many individuals, myself included, use supplementary protectors alongside branch circuit breakers/fuses, contradicting UL's guidelines. It appears that the SCCR should not exceed 0.2kA when using supplementary protectors based on UL's stance. It seems like the only option left is to use a larger device, like the one linked below, for the convenience receptacle within the panel: https://www.automationdirect.com/ad...ric)/15A-125A_(BW125_Frame)/BW125JAGU-3P015SB -rpoet
I'm sorry to jump in, but I'm in need of some assistance! Currently, I am getting ready for the MTR exam for a UL 508a Panel shop. I have a question regarding SCCR calculation. If we have a Class J fuse as the first OCPD in a branch circuit, with motor protection, a control relay, and a motor below it, with SCCR ratings of 200kA for the fuse, 65kA for the motor protection, and 5kA for the control relay, how should we calculate the total branch circuit SCCR? Do we need to consider the peak let through of the fuse, or is it sufficient to consider the lowest SCCR value, which is 5kA for the control relay? Thank you in advance.
Sajjadali asked for assistance with SCCR calculation for an upcoming MTR exam for UL 508a Panel shop. The scenario involves a Class J fuse, motor protection, a control relay, and a motor in a branch circuit. The fuse has an SCCR rating of 200kA, motor protection at 65kA, and control relay at 5kA. The question is whether to consider the fuse's peak let through and modify the total branch circuit's SCCR, or just use the lowest SCCR value of 5kA from the control relay. If the control relay is not part of the power circuit, it does not need to be considered. However, if it is in the power circuit, it must be included. Current limiting fuses are typically used for devices that are series listed and tested by UL. Most components in power circuits are already series listed by manufacturers, so it's essential to refer to their data sheets for OCPD listing and ratings. If the control relay requires a higher SCCR in the power circuit, it may be more cost-effective to substitute it with a contactor that is likely to have a series SCCR listing with fuses or a motor protector. Ultimately, it may not be practical to submit a control relay for individual testing just to save on component costs.
Hey rpoet, I totally get your frustration—these SCCR requirements can be pretty daunting! It seems like the core issue is how compliance allows certain components to be rated differently while maintaining safety standards. The reason the circuit breaker's rating impacts overall SCCR is that it serves as the primary protection point for the entire circuit; hence, it dictates what the maximum fault current rating is going to be downstream, regardless of the other components' ratings. As for daisy chaining overcurrent protection, it might not be the best route, as each link in the chain can introduce complexities—and potential points of failure. Instead, consider investing in a combination of current limiting devices and high-rated components upfront so you can adhere to the strict standards without overloading your design. It can be tricky, but with some careful planning, you can definitely find the right balance!
Hey rpoet, I totally get your confusion—SCCR can be a tricky area to navigate. It sounds like the workshop gave you a good foundation, but the nuances can be really frustrating. To clarify, the reason the rating of your overcurrent protection affects the SCCR is that it sets the maximum available fault current the panel can handle safely. The downstream components might be rated for more, but if the weakest link (in this case, the circuit breaker) can’t handle higher fault levels, that rating caps your overall SCCR. As for "daisy chaining" overcurrent protections, that's generally not recommended because it can complicate the fault current path and potentially create safety issues. Instead, your best bet is to use components with appropriate ratings from the start, or consider additional current limiting strategies. Keep digging into this, and don’t hesitate to ask more questions!
Hi rpoet, I totally understand your frustration; the nuances of SCCR can be really challenging! It sounds like you’ve grasped the basics well, and that discrepancy with the overcurrent protection devices is a common sticking point. The idea behind needing to match the downstream breaker rating to the available fault current, despite having current limiting devices, is primarily about ensuring the entire system can safely withstand and isolate fault conditions—essentially, it's about maintaining safety standards across the board. As for your question about daisy chaining overcurrent protection, while it's technically feasible, it can complicate the fault current management and might not always offer the SCCR boost you're hoping for. It might be worth seeking out additional resources or even a follow-up conversation with the presenter for more clarity. Keep digging into it; understanding these details will definitely pay off in your designs!
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Answer: - SCCR stands for Short-Circuit Current Rating, and it is crucial in ensuring the safety and reliability of electrical panels by indicating the maximum fault current levels that the components can safely withstand without failure.
Answer: - There are several methods to increase the SCCR, such as using listed or recognized components, incorporating current limiting fuses or breakers at the power entry point, and installing a transformer in front of the panel to reduce fault currents.
Answer: - Overcurrent protection devices must be rated for the available fault current to ensure proper protection against short circuits and overloads, even if they are downstream of current limiting devices. This helps maintain the safety and integrity of the electrical system.
Answer: - While it may be possible to connect multiple overcurrent protection devices in series to increase the SCCR, it is essential to ensure that each device is properly coordinated and that the overall protection scheme complies with UL508A requirements. Consulting with experts or following manufacturer guidelines is recommended in such cases.
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