Develop an effective industrial equipment SCCR strategy

Technology Update: Short-Circuit Current Rating (SCCR) compliance requires attention to multiple considerations. See the SCCR regulations. Online extra: Below, learn five steps to define equipment SCCR.

By Erik Barnes October 7, 2015

To meet equipment short-circuit current rating (SCCR) needs, there are many considerations. In addition to regulatory and code requirements, protecting against short-circuit events is critical to enhancing workplace safety and protecting equipment. Original equipment manufacturers (OEMs) can play a crucial role in helping customers understand and plan for appropriate equipment SCCR levels.

This effort can yield powerful short- and long-term business advantages by enhancing safety, minimizing risk, and reducing costs. General discussion that raises awareness of equipment SCCR requirements can help establish the protection levels needed to support a safe working environment. A solid equipment SCCR plan can minimize risks and save customers the time and costs in the long term, even as utility upgrades, maintenance, equipment relocation, and other factors impact available fault current levels. Effective SCCR plans also help avoid oversized components and increased material costs. 

Meeting code

SCCR requirements apply to control panels used to operate machinery and equipment. Although protection from short-circuit events in electrical switchgear and distribution equipment is often properly specified and applied, these same requirements are often misunderstood or misapplied in machinery and equipment. The Occupational Safety and Health Administration (OSHA) and the National Electrical Code (NEC) require sufficient equipment short-circuit current ratings of control panels at the point of installation to protect equipment and personnel from certain risks in the event of a short circuit.

Summary code, regulations

In summary:

  • The NEC requires industrial control panels be marked with
the equipment SCCR (Sections 409.110, 670.3(A), 440.4(B)). 

  • NEC 110.10 requires that electrical equipment is sufficient for the available fault current.
  • OSHA regulation (Section 1910.303(b)(5)) requires all electrical equipment, including equipment that is already installed and new equipment being installed, meet this requirement. OSHA does not provide for any exemptions.
  • The NEC (Section 409.22) prohibits installing industrial control panels in locations where available fault current exceeds the equipment’s short-circuit current rating.

5 kA equipment design limits

Manufacturers of industrial control panels and machinery face a difficult task when it comes to standardizing an equipment SCCR design; one that works for one industry or application may not work for others. Installation sites can vary widely with differing equipment SCCR needs. The required equipment SCCR can even vary within the same facility, depending on its location in the electrical distribution system.

With so much variation, how can a machinery OEM arrive at an acceptable equipment SCCR strategy? If a control panel manufacturer chooses
to standardize on 5 kA equipment SCCR for all designs, and only designs a higher equipment SCCR when required, significant design changes may be necessary to meet
a new equipment SCCR need. Often this results in reengineering efforts and increased material costs. Another risk of standardizing a 5 kA equipment SCCR is revealed if it is found to be inadequate after it is installed; a scenario that may not be easily corrected.

An effective strategy strives to achieve the necessary equipment SCCR for the majority of installations with minimal costs, resources, and effort. No one plan works for every industry or application. Each manufacturer must carefully consider the various factors when developing its equipment SCCR strategy. To develop an effective strategy:

  • Understand available fault current and the range typically encountered in an industry
  • Define an equipment SCCR level that best meets the majority of these installations
  • Develop and compare designs at this equipment SCCR level and the worst case scenario (highest fault current) to determine the difference in cost, installation, and features
  • Standardize an equipment SCCR design that best meets the largest range of application requirements
  • Validate and document the industrial control panel’s SCCR.

With the right tools and resources, defining a competitive solution that meets equipment SCCR is attainable. Consider component SCCR selection tools or equipment SCCR calculators when developing equipment SCCR solutions.

Developing an equipment SCCR plan can yield significant benefits for the OEM. A carefully considered equipment SCCR strategy not only avoids excessive reactive costs (engineering time, costly material changes, project delays), but also helps differentiate from competition, as well as enhances personnel safety.

Learn more about five steps to define equipment SCCR.

Five steps to define equipment SCCR

The understanding of available fault current and how equipment SCCR is specified vary among different industries. This variation by industry can make it difficult to establish a strategy for equipment SCCR, but there are significant benefits that can be realized from a solid equipment SCCR strategy, for personnel safety and for cost avoidance and efficiency. 

1. Establish equipment SCCR to meet the vast majority of fault current levels found in various applications

Equipment manufacturers need to be competitive in terms of both cost
and features. It may be feasible to design an industrial control panel to meet the highest expected fault current level and still be competitive. However, this may not be possible in some cases. Many OEMs will need to establish
an equipment SCCR design that meets 80% to 90% of the available fault current levels likely to be encountered and develop an additional design to meet the maximum expected available fault current level.

A distribution chart is a useful analytical tool to understand the range of available fault currents at the substation transformer’s secondary. The example shows a bell curve that displays the range of transformer fault current levels that may be found in an industry. A cumulative percentage is applied, and from this it is determined (in this example) that equipment SCCR designs standardized at 65 kA will cover 83% of the applications. If possible, a backup design is desirable to accommodate installations where the available fault current is 200 kA and covers virtually all the market. If a 200 kA design is deemed prohibitive, then a 100 kA design could be implemented to cover 97% of the market. Cost may not necessarily impact this aspect of the design; some panel designs can reach SCCRs of 200 kA at the same cost as 5 kA or 10 kA designs. 

2. Compare designs at targeted equipment SCCR levels

Define and prioritize equipment design attributes, such as material costs, enclosure size, installation, and minimized downtime. Compare these attributes when designing to various targeted equipment SCCR levels. 

3. Evaluate late-point definition design options for high available fault current applications

An ideal solution that achieves a high equipment SCCR involves minor alteration to a standardized design, resulting in maximum efficiency with minimal effort. These solutions can be realized by one of three means: 

  • Modifying a standard design to increase the equipment’s SCCR 

  • Adding external assemblies/components to raise equipment SCCR 

  • Adding external assemblies/components that lower the available fault current level. 

Modifying the standard design is typically achieved by part substitution or the addition of components within the panel that result in a higher equipment SCCR. The objective is to identify and resolve "weak links" that keep equipment SCCR low with minimal impact on the standard design. This can be accomplished by substituting weak links with higher SCCR components or introducing a current-limiting solution
as permitted by Underwriters Laboratories (UL) 508A. An example of applying a current-limiting solution would be to replace a standard feeder circuit breaker with a current-limiting fuse or circuit breaker that raises the SCCR of a branch circuit component in the panel. UL 508A must be followed when implementing current-limiting solutions.

It may be possible to standardize a control panel design so that a higher equipment SCCR can be applied if it is fed by a current-limiting fuse or circuit breaker. This type of solution greatly improves manufacturing efficiency and increases total flexibility in the field. This type of solution is also achieved by applying the current-limiting rules per UL 508A. The components inside the standardized panel design must be coordinated in conjunction with an external current-limiting overcurrent protective device. The control panel’s SCCR marking must also be updated to reflect this field addition. More planning may be involved during the panel design, but the major benefit is that no changes inside the panel will be required in order to achieve the higher equipment SCCR and code compliance.

The third option is to install equipment or components ahead
of the control panel that effectively lower the available fault current. One example is a transformer, which can only deliver a limited amount of fault current on the secondary based on its size and impedance. Thus, a properly sized transformer for the application could partially resolve or completely resolve inadequate equipment SCCRs by limiting the available fault current to the equipment.

One, some, or all of these solutions may be beneficial in preparing for a high available fault current situation. Proper design planning is a significant factor in delivering a solution that meets equipment SCCR requirements while retaining a competitive edge. 

4. Choose the option that best meets the objectives

Having analyzed all equipment SCCR designs, determine the one that delivers the required protection while maintaining competitiveness. No one solution will work best for every scenario, as needs and the types of equipment SCCR solutions vary based on applications and industries served. A solid equipment SCCR plan enables regulatory and code compliance, maintains competitiveness, and differentiates designs. 

5. Validate and document equipment SCCR analysis

Once equipment SCCR designs have been determined, it is important to verify and document their analysis. Surveys of OEMs show that four out of 10 struggle to some degree when interpreting and/or applying the UL 508A standard for equipment SCCRs. Verification can be accomplished by third-party applications.

It is also beneficial to document the equipment SCCR analysis. Inspectors
or clients may request to see an explanation of how the equipment SCCR was determined.

Exceed expectations

Regulations and good business require that working environments are safe for employees. Protecting against short-circuit events is one aspect of these safety requirements. Helping customers to understand and plan for short-circuit events and high-fault current levels can exceed their expectations by enhancing safety, protecting equipment investments, simplifying maintenance and reducing costs-in both the short and long term.

– Erik Barnes is OEM marketing manager, Eaton. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering,

ONLINE extra

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