Assembly SCCR Made Simple

Equipment SCCR compliance is critical for the safety of equipment and those who work around it, even if many struggle to maintain it. Dan Neeser, Senior Field Application Engineer at Eaton, makes it simple, explaining what assembly SCCR is, how it’s calculated, and the processes and product solutions that can help you quickly, easily, and economically improve equipment SCCR. 

Dan Neeser, Senior Field Application Engineer, Eaton

Short-circuit current ratings (SCCRs) are the prospective symmetrical fault current that a device or equipment can be connected to at a nominal voltage without sustaining damage exceeding defined acceptable criteria. The overcurrent protective device’s IR, which is marked on the overcurrent protective device along with the corresponding voltage rating, identifies the maximum fault current that can be safely interrupted. The equipment SCCR considers the overcurrent protective device’s IR if incorporated into the equipment, but it also considers the SCCR of other power circuit components in the equipment, based on the overcurrent protective device protecting the component, to achieve an overall assembly SCCR.

To determine assembly SCCR, you can use the Eaton Bussmann Series “Two Sweep” Method, which is based on the Supplement SB in UL 508A, the Standard for Industrial Control Panels.

Sweep 1: Verify Component SCCRs

1. Determine the component SCCR for each branch circuit.
2. Determine the component SCCR for each feeder circuit, including supply, feeders, and sub-feeders.
3. If you’re using a power transformer in a feeder circuit, modify the transformer secondary branch circuit overcurrent protective device IR and the component SCCRs if possible.
4. If you’re using a current-limiting overcurrent protective device in the feeder circuit, only modify the branch circuit component SCCRs if possible.
5. Determine the assembly SCCR for Sweep 1.

Sweep 2: Verify Assembly SCCRs

1. Determine the IR of all the overcurrent protective devices used in the feeder and branch circuits, the SCCR of combination motor controllers in the branch circuits, and the IR of overcurrent protective devices protecting the control circuits.
2. Identify the lowest overcurrent protective device IR or SCCR.
3. Compare the lowest overcurrent protective device IR or SCCR with the component SCCRs from step five of Sweep 1. The lowest rating encountered is the assembly SCCR.

One of the biggest challenges to effectively addressing proper equipment SCCRs in industrial electrical installations is that the equipment SCCR is often overlooked or misunderstood. This can pose serious risks to personnel and equipment, including electric shocks, burns, and equipment and facility damage. So, it’s crucial to be aware of code requirements and properly apply them.

Additionally, since equipment SCCRs apply to new and existing electrical installations, as well as to equipment that’s moved to a new location, you must consider the electrical system’s available fault current and equipment SCCR every time you install equipment and revise production lines. Verifying adequate equipment SCCR is not something that you can do once and never think about again.

SCCR Requirements

Key SCCR code requirements to keep in mind include:

• NEC 110.9. Overcurrent protective devices, such as fuses and circuit breakers, must have a suitable IR for the available fault current where the equipment is installed.
• NEC 110.10. Overcurrent protective device IRs and the component SCCRs in equipment must be selected and coordinated to be suitable for the available fault current where the equipment is installed.
• NEC 409.110(4), 430.8, 440.4(B), 620.16(A) and 670.3(A)(4). Industrial control panels, motor controllers, HVAC equipment, elevator control panels, and industrial machinery must be clearly marked with its SCCR.
• NEC Article 409.22(A), 430.83(F), 440.10(A), 620.16(B) and 670.5(A). Industrial control panels, motor controllers, HVAC equipment, elevator control panels, and industrial machinery cannot be installed anywhere that the available fault current exceeds its marked SCCR.
• OSHA 1910.303(b)(4). Contains similar language to NEC 110.9 for both new and existing installations.
• OSHA 1910.303(b)(5). Contains similar language to NEC 110.10 for both new and existing installations.

SCCR Challenges

Although there’s an abundance of detailed resources designed to support SCCR compliance, it’s not a straightforward task. So, it’s not uncommon for confusion, misunderstandings, or unintentional ignorance of the proper application of equipment SCCR to result in noncompliance. However, noncompliance can result in significant fines and reputation damage and — even more importantly — can compromise both personnel and equipment safety. And per OSHA 1910.303(b)(5), equipment installed before equipment SCCR requirements were implemented must still comply, so there’s simply no way of getting around it.

In cases where equipment predates equipment SCCR regulations and there’s no manufacturer guidance, your options are to acquire sufficient knowledge of the terminology and assessment methods required to determine the equipment SCCR or hire a consultant to determine the equipment SCCR. Option one can result in inaccurate assessments, especially if you’re unclear about the UL 508A Supplement SB SCCR analysis method. So, it’s often best to hire a consultant when you’re trying to bring equipment with an unknown assembly SCCR into compliance, since you’ll still be tasked with making any necessary changes to the overall assembly and conducting a field evaluation of the equipment. The field evaluation must be performed by a recognized, qualified electrical testing laboratory, which will inspect and mark the equipment SCCR for the specific installation.

Unfortunately, new equipment is not without equipment SCCR compliance risks either. For example, equipment manufacturers often standardize their products on a minimum SCCR of 5kA or 10kA, even though the majority of estimated available fault currents at the utility transformer secondaries supplying industrial installations are typically between 35kA and 100kA. Although it’s likely to be insufficient for standards compliance in many electrical installations, it’s a popular approach since available fault currents may be unknown and the equipment manufacturers are not required to assure the equipment SCCR is adequate for the specific electrical installation. To ensure compliance, the installer and user must perform fault current calculations to determine if the equipment SCCR in question is appropriate for the available fault current where the equipment is installed. Luckily, since these calculations can be extremely complex, several device and equipment suppliers offer resources including compliance applications and fault current calculators.

Additionally, available fault current may not remain consistent over time. Any changes with the utility transformer or the electrical distribution system can raise or lower available fault current levels, which could result in inadequate equipment SCCRs. So, anytime changes to the electrical distribution system are made, equipment SCCR should be verified for adequacy.

Developing an Effective Equipment SCCR Plan

Considering the potential risks and numerous challenges at hand, industrial manufacturers should require a comprehensive equipment SCCR compliance plan for equipment installation and assembly upgrades. This can be expensive and time-consuming without some planning, but there are a variety of product solutions available to help you develop a seamless, easy-to-follow roadmap for equipment SCCR compliance. Together, these solutions will minimize both short- and long-term potential safety risks.

Fuses

The first major challenge you’re likely to face when designing equipment with a desired SCCR is resolving any weak SCCR links, such as components with lower SCCRs and overcurrent protective devices with lower IRs than the desired equipment SCCR. When one link falls below the desired SCCR, then the SCCR of the entire assembly will reflect the rating of that weak link.

The easiest way to avoid this issue is to do your due diligence at the outset of the equipment design by choosing components with tested high-fault ratings (SCCR) and overcurrent protective devices with high IR. This often requires the use of current-limiting fuses to achieve both high component SCCR and high IR. Documentation of the component SCCR is based on the component labeling or installation instructions. Manufacturer documentation, such as catalogs, datasheets, and website information, can be used as a resource but is not considered acceptable for documenting the component SCCR.

Some common fuse classes and their applications include:

• Class CC Fuses. Designed to withstand the sustained starting currents of small motors and transformers, Class CC fuses provide short-circuit protection for motor branch circuits and pair well with low HP IEC- and NEMA-rated motor controllers, contactors, and variable speed drives.

Eaton Bussmann Series Class CC fuses set the standard in Class CC fuse performance in the industry. These branch-circuit-rated, 600V, 200,000A IR fuses offer exceptional protection in the smallest package of any UL class of fuses and are available in low-peak (motors), time-delay (transformers), and fast-acting variations for broad application suitability.

• Class R Fuses. Used in general applications and motor circuits, Class R fuses provide excellent time-delay performance and good current-limiting performance. Typical applications include electrical distribution equipment, enclosed combination motor controllers, and motor control centers.
• Class T Fuses. These fuses are ideally suited for applications that require compact size and excellent current-limiting performance, such as protecting variable speed drives, rectifiers, and power conversion equipment.
• Class J Fuses. These fuses are ideally suited for applications that require compact size and excellent current-limiting performance, such as protecting branch and feeder circuits in HVAC equipment, industrial control panels, elevator control panels, and industrial machinery.
• Class CF Fuses or CUBEfuses. Exclusively available from Eaton Bussmann Series, these Class CF fuses, or CUBEfuses, offer the protection of Class J fuses and also feature a finger-safe construction and a compact size.

Eaton Bussmann Series CUBEFuse is a revolutionary finger-safe, Class J performance fuse solution. Low-Peak CUBEFuses (left) have the smallest footprint of any Class CC, J, or R fuse and require up to 70% less space when combined with our unique TCFH fuse holders (right) or UL98 listed compact circuit protector (CCP) fused disconnect switches. CUBEFuses also offer up to a 300kA interrupting rating that enables higher equipment SCCR and are available from 1–400A.

Eaton is one of the industry’s most trusted fuse suppliers and offers an extensive portfolio of Class CC, R, T, J, and CF fuses to help you satisfy desired equipment SCCR requirements.

Fuse Blocks and Fuse Holders

Proper fuse selection is key, but it’s not the only solution you need to focus on. Every component contributes to the SCCR, including fuse holders and fuse blocks. To pick the right solution for your application, review the product documentation and consider:

• The SCCR of the fuse block or fuse holder.
• The fuse class it will hold and the IR for the fuse.
• The amperage rating, including maximum and minimum amperage ratings, that can be installed.
• The size of the solution in relation to the available space in the assembly.
• Installation requirements (e.g., DIN-rail or panel mount).

Eaton Bussmann Series fusible power distribution blocks combine a fuse block and power distribution block, which reduces panel space requirements by up to 57% and installation time and labor by an average 33% compared to two individual solutions. They’re available in one-, two-, and three-pole configurations and have a 200 kA withstand rating that helps achieve higher assembly SCCRs.

Fusible Disconnect Switches

Eaton Bussmann Series compact circuit protector (CCP) fused branch circuit disconnect switches require up to 69% less space, offer ratings up to 400A, and have a high 200kA SCCR that can help improve assembly SCCR. The CCP has an ampere rating rejection feature that prevents over-fusing and allows lower-ampere-rated fuses to be used without the need of fuse reducers. For instance, a 30A or less CUBEFuse can be applied in a 60A or 100A CCP or a 60A or 100A TCFH fuse holder. These CCPs are also available with through-the-door operating mechanisms and other accessories, such as multi-wire lugs.

Circuit Breakers

Circuit breakers provide assemblies with resettable overcurrent protection and may not need to be replaced after operating in an overcurrent condition. For standard equipment SCCR of 5kA or 10kA, circuit breakers are a cost-effective solution. For higher equipment SCCR, circuit breakers are not as cost-effective as current-limiting fuses and may not have been tested with components for the desired equipment SCCR.

Trusted solutions include Series C, Series G, and the new Power Defense molded case circuit breakers from Eaton. Series C molded case circuit breakers are available with thermal magnetic, electronic, and magnetic-only trip units and ideal for panelboard, switchboards, motor control centers, and busway applications rated for up to 600VAC or 250VAC and 800A. They are also widely installed throughout the U.S. and offer high domestic familiarity. Series G molded case circuit breakers are globally rated and have a compact, field-configurable form factor suitable for a wide range of regional and industry requirements. They also offer high interruption ratings, a selection of thermal magnetic and electronic trip units, and high SCCR listings for Eaton motor control components and are rated for up to 690VAC and 15–2,500A. Power Defense molded case circuit breakers are globally rated and equipped with PXR trip units that offer features such as breaker health, multiple load alarms, ground fault alarms, intuitive user interface, embedded communications and metering, and field installable components and trip units.

Eaton’s Series C molded case circuit breakers (left), Series G molded case circuit breakers (middle), and Eaton Power Defense circuit breakers (right) are all available at RS.

Trust Eaton and RS for your SCCR Solutions

A true commitment to equipment SCCR compliance means paying attention to overcurrent protective devices and components in equipment — new ones, old ones, new locations, and certainly not just the components addressed here. If it affects the equipment SCCR, it needs to be a part of your equipment SCCR plan, and if anything in the equipment changes, you’ll have to revisit and potentially adjust that plan.

Eaton has more than 100 years of experience in electrical power management and the expertise to see beyond today, which allows us to offer end-to-end solutions that satisfy the various demands of industrial infrastructure ranging from legacy equipment to cutting-edge. Our proven technologies improve safety, increase productivity, ensure compliance, and help reduce energy consumption in support of creating a more sustainable world.

So, if you’re thinking about improving your assembly SCCR, you can trust us to provide the solutions you need. For example, our team recently helped a Wisconsin-based chiller equipment manufacturer improve the SCCR of their standard design from 5,000A to 100,000A to meet a customer requirement — and with minimal component changes comparable to original material costs. Success stories like these are common at Eaton, and we’re proud to partner with RS to help even more customers overcome challenges.

RS offers everything you need for quick, easy, and economical SCCR compliance so you can protect what matters most. Eaton SCCR solutions available at RS include fuses, circuit breakers, contactors, power distribution blocks, and terminal blocks.

For assistance identifying and deploying Eaton products that can help you improve your assembly SCCR, please contact your local RS representative at 1.866.433.5722 or reach out to the RS technical product support team.