Using control-panel components designed to perform in the worst-case scenario will help keep people and equipment safe should an incident occur.
By Dave Scheuerman, technical training manager, Littelfuse Inc., Industrial Business Unit
When operating in its current-limiting range, a current-limiting fuse is designed to clear a short circuit current in less than one half cycle. Current-limiting fuses operating in their current-limiting range will limit the maximum instantaneous peak (Ipeak) current to a value substantially less than the peak current that could flow if the fuses weren’t in the circuit.
To increase the short-circuit current rating (SCCR) and reliability of industrial control panels (ICPs) while simultaneously increasing the safety of electrical workers, it’s recommended that you do the following:
• Use components or approved combinations with higher SCCRs.
• Use current-limiting fuses or other current-limiting components in the feeder circuit (refer to UL® 508A Supplement SB for more information).
• Use overcurrent protective devices with higher interrupting ratings in branch and control circuits.
• Submit the panel to UL or another nationally recognized testing laboratory (NRTL) for approval testing using the testing method.
Every component within an ICP must be designed to perform in the worst-case scenario, which helps verify people and equipment remain safe should an incident occur.
Currently, UL 508A Supplement SB does not use the apparent root mean square (RMS) let-through voltage of current-limiting overcurrent protective devices to determine the SCCR of industrial control panels. Instead, the supplement compares the UL maximum allowable Ipeak let-through current during the first half cycle of current-limiting fuses with the RMS SCCR of the various components used within the panel.
Current-limiting fuses can optimize a panel as long as the SCCR is considered in its design. Most molded case circuit breakers can’t provide adequate SCCRs. Traditionally, the best way to increase the SCCR of a panel with a low rating has been to switch from a molded case circuit breaker to current-limiting fuses. But after switching to current-limiting fuses, how do you improve the panel’s SCCR even further?
How Does Current Limitation Increase SCCRs?
According to NEC® Article 240.2, a current-limiting overcurrent protective device interrupts currents in its current-limiting range, thus reducing the current flowing in the faulted circuit to a magnitude substantially less than that obtainable in the same circuit if the device were replaced with a solid conductor having comparable impedance.
Depending on the power factor and when the fault occurs, the maximum possible Ipeak without current limitation can be as high as 2.3 times the available RMS fault current when the fault occurs. However, when current-limiting fuses are used, the maximum Ipeak is only a small fraction of what the potential flow would otherwise be. The area under the curve is known as the I²t let-through energy of the fuse. The lower the I²t, the lower the potentially destructive thermal energy will be that passes through the circuit when it’s interrupted.
UL 508A Supplement SB assigns a maximum SCCR of only 200 amperes to unmarked supplementary protectors. UL 508A Article SB3.2 prohibits supplementary protectors in primary control circuits. If any supplementary protectors are used in branch circuits, replacing the supplementary protector with Littelfuse UL Class CC fuses in touch-safe DIN rail-mounted fuse holders can dramatically increase the SCCR of an ICP and, in turn, increase safety.
Determining the SCCR of an ICP
NEC Article 409.110 states that the SCCR of an industrial control panel is based either on the SCCR of a listed and labeled assembly or by using an approved method.
UL 508A Supplement SB is an example of such an approved method. Other approved methods are left to the discretion of the local electrical inspector and may involve a stamped engineered analysis by a licensed professional engineer or the local authority having jurisdiction (AHJ).
Some original-equipment manufacturers prefer to submit a panel to UL or to another NRTL to be tested and listed if the panel is used for a specific purpose and continuously manufactured, or if the panel is part of a larger piece of listed equipment.
Another reason to submit a panel for approval testing may be to obtain higher SCCRs than other approved methods can provide.
Determine SCCR with UL 508A Supplement SB
Since 2006, custom control panels and components containing a UL-listed certification mark must follow the construction standards and the marking requirements of UL 508A Supplement SB. Under this method of assigning the SCCR to an ICP, the panel is not required to be listed.
Although UL 508A Supplement SB only refers to listed ICPs, the standard’s requirements can be applied to any control panel powering any type of utilization equipment. Manufacturers that build a few panels at a time or produce panels with several optional configurations often prefer to use the UL 508A Supplement SB method to establish the panel’s SCCR.
UL 508A Supplement SB provides a method of assigning the SCCR to an ICP based on the types of overcurrent protective devices and the ratings of components used within the panel. Supplement SB says that the SCCR of an ICP is equal to the SCCR of the:
• lowest rated component.
• lowest rated UL approved series rated combination.
• SCCR of an approved series combination according to the standard.
However, the UL 508A Supplement SB method might not provide an adequate SCCR for the intended application. In these situations, the panel must be re-engineered or listed and approved by a different method.
When panels with fuses and circuit breakers are listed by an NRTL with a high fault SCCR, the equipment instructions and labels must indicate that overcurrent protective devices must be replaced only by a specified fuse class and rating, or by a specific make, model and rating of a circuit breaker.
Panels not listed may be scrutinized by the local electrical inspector or AHJ for the required NEC Article 409 SCCR markings. If requested by the local AHJ, manufacturers must keep detailed records of their bill of materials, SCCRs of all components, and the method that was used to determine the overall SCCR of the panel.
Five Key Points
Remember these five key elements that can help support equipment and worker safety:
• All industrial control panels must be marked with an SCCR.
• Panels can be submitted and tested by an NRTL to establish the SCCR.
• UL 508A Supplement SB may be used to establish the SCCR.
• Using current-limiting fuses is one of the easiest ways to help increase the SCCR of a panel.
• Current-limiting fuses help increase safety and reliability.
If a panel is UL listed and labeled, or if UL 508A Supplement SB is used to establish the high-fault SCCR of the panel, then all components and ratings must be identified and documented.
To maintain a marked SCCR, warning labels and instructions must clearly indicate that overcurrent protective devices may only be replaced with the specific components originally designed for use in this application.
Littelfuse Inc., based in Chicago, is a participating Encompass™ Product Partner in the Rockwell Automation PartnerNetwork™ program. The company offers current-limiting fuses and accessories to increase the SCCR of industrial control panels and reduce arc flash and other electrical hazards for electrical workers.
The Journal From Rockwell Automation and Our PartnerNetwork™ is published by Putman Media, Inc.