Elmer was an excellent forklift operator. He had been driving accident-free for five years.
That is, until today.
Product delivery was slow, and the work-in-progress was piling up, causing material to be stored in less-than-ideal locations. As Elmer moved material near the production line, he accidentally snagged the electrical conduit leading to a 10 hp 480 VAC induction motor. In a single moment, the forklift pulled the conduit from the running motor’s junction box. This caused a short circuit across all three phases.
A fireball erupted from the motor’s control panel as illustrated by Figure 1. This wasn’t a component failure. It was a design error that neglected to address Short Circuit Current Rating (SCCR).
Note: This document uses the word circuit breaker in a generic sense. For increased technical clarity for SCCR calculations, we must consider clarifiers such as “current limiting” or “instantaneous trip.” Recommend you consult the UL 508A standard or explore the space with the UL inspector.
Figure 1: Simulated control panel explosion generated by OpenAI ChatGPT 4o dated 22 Apr 2025.
How are industrial control panels designed?
Company technicians Alice and Bob had designed the control panel ten years ago. In truth, it was their first design. Today, both admit that they had no idea what they were doing. They simply copied other panels in the facility without understanding the underlying Short Circuit Current Ratings (SCCR) safety concepts.
Like thousands of other technicians and aspiring control panel designers, they needed to start somewhere. In fairness, they had done a good job as their control panel had operated flawlessly for a decade. Yet, they had inadvertently embedded a dormant weakness deep within the system. The catastrophic fault revealed itself to Elmer when he inadvertently performed the SCCR test with a misplaced forklift tine.
What is Short Circuit Current Rating (SCCR)?
The Short Circuit Current Rating (SCCR) has two general definitions:
-
Component level: SCCR is the maximum fault current a single device may withstand. This includes all components in the power path such as fuses, circuit breakers, terminal blocks, motor starters, overload relays, wire, transformers, receptacles, and other current-carrying component.
-
Control panel level: SCCR is the maximum fault current for a control panel. It is determined by the weakest link (component) of all components contained in the panel.
As an example, consider the Siemens 3RT20262BB40 contactor – a pair of which is shown in Figure 2 configured as a three-phase reversing motor controller. The datasheet reveals that the contactor can drive a 480 VAC 15 hp motor. The 21 A contacts can handle the motor’s approximate 14 A full load current. With respect to our opening narrative, Alice and Bob had properly selected the contactor.
However, the contactor must be properly shielded with a circuit breaker or fuse. By itself, the contactor’s ability to withstand Elmer’s three-phase fault-inducing forklift is limited. Without proper upstream protection, it cannot withstand the full rated current of the factory’s electrical distribution.
A properly designed control panel will not explode!
Figure 2: Image of a pair of Siemens 3RT20262BB40 contactors configured as a reversing motor controller.
Tech Tip: When we think about SCCR, we should think in terms of the weakest link. We recognize that energy flows from the utility’s transformer, to the factory’s distribution center, to the control panel, through all control panel components, and finally to the individual load(s). We also recognize that things can and do go wrong.
If the 480 VAC feeder can provide a 100 kA fault current, the control panel must be designed to protect itself from the same 100 kA fault. This is where Alice and Bob went wrong. They had unknowingly designed a 20 kA panel connected to a 100 kA source. The panel innards were the weakest link.
Boom!
How do we increase the SCCR of a control panel?
Conceptually, this is an easy task as we install protection in the form of transformers, circuit breakers and fuses. In practice it can be difficult to meet the needs.
While we design a control panel for a given SCCR at a given voltage, the SCCR of individual components is voltage dependent. Stated another way, SCCR is always defined for a given voltage. This should not be surprising as higher voltages tend to create longer and stronger arcs which are increasingly difficult to extinguish. Recall that this inability to extinguish the arc leads to excess heat generation which destroyed the panel as designed by Alice and Bob.
As an example, suppose we placed the matching Siemens S0 frame 3RV20214BA20 circuit breaker upstream of the contactors. This circuit breaker is a good match for our 480 VAC 10 hp motor. However, we need to be very careful with the SCCR. The datasheet reveals that the system provides protection of 100 kA in a 240 system but only 5 kA in a 500 V system. That is far too low for our desired 100 kA panel. It appears that Alice and Bob have backed themselves into a corner as 100 kA fuses with a let-through current less than 5 kA are hard to find.
An alternative is to jump to the next larger size for the matching circuit breaker and contactor preceded by type RK5 time delay fuses. The Siemens 3RV20314SA10 Motor Starter Protector (MSP) and a 3RT20351KB40 contactor have a rating up to 12 kA on a 500 V system. According to the conservative UL 508A methodology, a 30 A RK5 time delay fuse such as the Littlefuse FLSR030.T provides a peak let-through current of 11 kA which is just enough to squeak by with a panel rating of 100 kA. Recall that the MSP is rated for 12 kA, making the fuse the weakest link.
I’ll leave it to you to decide if both the MSP and RK5 fuse are required. Also, be sure to select the appropriate thermal or solid-state overload relay and a power monitor if desired.
Tech Tip: As an alternative, it may be desirable to completely redesign the panel to operate on a lower voltage. As a rule, component selection increases with a lower voltage such as when 480 is changed to a 208 three-phase system.
Figure 3: Image of the Siemens 3RV20314SA10 MSP and a 3RT20351KB40 contactor.
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.
Parting thoughts
This narrative provides a glimpse of the complexity associated with industrial control SCCR. Elmer’s forklift incident demonstrates the need for protection. Meanwhile, Alice and Bob demonstrate the iterative nature of panel design. Their initial decisions were wrong and were refined through several design iterations.
Do you agree with their final analysis for a 100 kA SCCR given a 480 VAC 10 hp motor? Please leave your comments and suggestions in the space below.
Best wishes,
APDahlen
P.S. Out of curiosity, how many of you share the passion, initiative, and mistakes embodied by Alica and Bob?
Related information
Please follow these links to related and useful information:
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.