This document addresses Frequently Asked Questions (FAQ) about peak let-through current as related to fuses. It is biased toward industrial control and automation systems with additional emphasis on the UL 508A standard.
What is peak let-through current?
Peak let-through current is defined as the maximum current passed through a fuse or circuit breaker in the process of interrupting a fault. Figure 1 presents a representative example using the Littlefuse FLSR030.T class RK5 cartridge fuse.
Tech Tip: This narrative addresses safety standards associated with high-powered electronic systems. While the material has been prepared with care, it may contain unintentional errors or misinterpretation of the standards. Refer to DigiKey’s Terms of Service for official guidance. We appreciate any feedback you can provide to improve the narrative and factual content.
Figure 1: The representative Littlefuse RK5 fuse has a 11 kA peak let-through current according to UL 508A SB4.4.
Tech Tip: UL 508A umbrella standard table SB4.2 provides conservative peak let-through rating for fuse classes CC, CF, G, J, CF, L, RK1, RK5, and T. The table data may differ from the manufacturer’s data. The manufacturer’s stated test results are often better as shown in Figure 1, where Littlefuse rates the fuse as 4.3 kA yet UL lists the fuse at 11 kA. Panel shop Manufacturer Technical Representatives (MTR) should generally follow the conservative UL standards.
Why peak let-through current matters
Peak let through is a critical consideration when designing an industrial control panel as it relates to the Short Circuit Current Rating. Without delving too deeply into the specifics, fuses and circuit breakers provide upstream protection for components in the control panel. For example, a motor starter such as the Schneider unit shown in Figure 2 has a limited SCCR. Without upstream protection, it could be damaged or even destroyed in a fault condition. Properly installed fuses or circuit breakers would protect the unit.
For maximum protection, the peak let-through current of the upstream interrupter must be less than the withstand rating of downstream components. In our example, the fuse must protect the contactor and not the other way ‘round. This fact cannot be overstated for high-energy power circuits. This keeps the high energy melting and arcing contained with the fuse or circuit breaker body. Contrast this with an unprotected motor starter which could explode. Also note that there is a time component to the peak current. The upstream protector must contain the molten materials and heat-generating plasma for the duration of the fault. This is good reason to err on the side of caution using conservative values.
Figure 2: Image of a Schneider reversing motor starter.
Tech Tip: The peak let through current rating is essential to the continued safety and integrity of a control panel. For continued protection, replace fuses with the correct type as indicated by the marking in a UL 508A certified panel. As an example, consider a 30 A type T fuse with a peak let-through of 7.5 kA, the panel integrity could be compromised if an 11 kA RK5 fuse were installed. Technically, the improper fuse has violated the panels’ SCCR and could result in an explosion under extreme fault conditions as described in this engineering brief.
What happens when a high current passes through a circuit breaker?
The circuit breaker and fuse are closely related. However, they are often handled differently from a code compliance and overall panel SCCR. Like the fuse, circuit breakers are rated in terms of SCCR and voltage. For example, a UL 489 listed circuit breaker may be rated at 7.5 kA at 400 volts.
Clarification of UL circuit breaker listing
We must recognize the distinction between traditional and current-limiting circuit breakers.
Unless specifically marked as “current-limiting,” we should not expect a fuse-like peak let-through current. Unless specifically marked, the SB4.3.2 modifications do not apply to circuit breakers.
For the sake of completeness, we will continue our exploration with the Phoenix Contact circuit breaker shown in Figure 3. Once again, we recognize that this is not a current-limited interrupter. Consequently, it will have a different time-overload characteristic than the fuse as shown in Figure 1.
Tech Tip: Most miniature circuit breakers including the one shown in Figure 3 are not rated for current limiting operation in a branch circuit. Be sure to consult the breaker’s datasheet, UL Product IQ, and explore potential solutions with your panel shop’s UL inspector.
Figure 3: Teardown of a Phoenix Contact circuit breaker with marking to identify critical components.
Tech Tip: From a UL perspective the device shown in Figure 3 is classified as a UL 1077 supplementary protector. It should not be used for branch circuit protection. It is a common UL 508A error to confuse a UL 1077 protector with a UL 489 listed circuit breaker. On the surface, they appear similar, yet confusion could lead to loss of certification or result in costly rework.
Operation of a representative circuit breaker
Most circuit breakers are spring-loaded so that the contact moves as quickly as possible. The Phoenix Contact circuit breaker shown in Figure 3 provides a representative example showing the operation of the breaker. The spring itself is hidden underneath the upper right mechanism (yellow dot).
This circuit breaker has both electromagnetic and thermal trip mechanisms. The red dot solenoid’s plunger will release the mechanical spring-loaded trip mechanism. The green dot heater is wound on a bimetallic strip that will bend also releasing the trip mechanism. We can think of the solenoid as a fast reflexive mechanism while the thermal element is slow. In a high current fault condition, the circuit breaker is tripped by the solenoid before the heater’s natural time constant has time to respond.
Electrical arc complicates the ability to quickly extinguish an arc
Fuses melt and then arc as they open a circuit fault. This is especially true for inductive loads such as motors which will tend to feed the arc, making a bad situation worse. Circuit breakers are designed to quickly move (snap) a contact from a closed to an open position.
The speed of circuit opening followed by arc extinguishing determines the time-current response characteristic.
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Fuse: The fuse link must first melt; the arc must then be extinguished.
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Circuit breaker: The spring-loaded mechanism must be tripped; the arc must then be extinguished.
For our purposes, we recognize that the fault opening mechanism is unique to each device. It is also complicated by the fuse class such as the distinction between a type T or a time delay such as an RK5. The circuit breaker design can also have a profound influence on the time-current characteristics. Finally, we recognize that the arc is increasingly difficult to extinguish as the voltage increases. It is for these reasons that the UL 508 A standard exists with carefully crafted sections for each type of fault current-interrupter.
As we conclude, note large arc chute installed in the circuit breaker (red lightning bolt). This arc chute is responsible for capturing the arc. It then breaks it into smaller pieces distributed across the arc chute. This tends to cool the arc as the energy is dissipated into the heavy metal plates. The size and weight of the arc chute reflects the amount of energy dissipated in a fault condition.
Parting thoughts
I’ll admit the more I explore the UL 508A standard, the more my eyes are opened to the wonderful complexity of control panel design. I salute the Manufacturer Technical Representatives (MTRs) and army of UL inspectors for tirelessly working through the technical requirements to ensure safe control panel construction and installation.
Best wishes,
APDahlen
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About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.