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Short-circuit current rating (SCCR) is the amount of available fault current that an electrical component or equipment can safely withstand, when properly applied. Adequate SCCR is imperative to support a safer workplace and protect equipment. Consequently, OSHA and NFPA 70: National Electrical Code (NEC) have specific SCCR requirements for equipment, such as industrial control panels, industrial machinery, general equipment, and HVAC equipment. Failure to provide equipment with adequate SCCR can have serious consequences including exposure to arc blast, flying debris, electric shock, burns, and others. A properly applied equipment SCCR is an important element in the safety of personnel and surrounding equipment.

Realizing properly applied equipment SCCR begins with a solid equipment SCCR strategy. There are several challenges that should be considered to achieve code compliance:

Develop an effective SCCR strategy

An effective SCCR strategy strives to achieve the necessary equipment SCCR with minimal costs, resources, and effort. Unfortunately, an approach that works for one industry or application may not work for others. Installation sites typically vary in terms of fault current depending on the location of the device in the facility’s electrical distribution system.

That said, there are a three basic concepts for developing an effective SCCR strategy:

The first step is understanding the available fault current level—the amount of current that would be present in the event of a short-circuit event. Determining the amount of available fault current depends on a variety of variables, including available fault current from the utility (typically, an unknown and subject to change), size of the upstream transformers, fault generation from motors, circuit impedances, voltage drop, short-circuit power factor, and more. The complexity and interaction of these variables can make an exact calculation difficult. That said, a conservative approach can be used to quickly and cost-effectively estimate fault current levels and the amount of SCCR protection required for a specific location.

A "worst-case" available fault current calculation assumes a worst-case condition and is intended to minimize the risk that the calculated available fault current level is too low. This equation assumes unlimited current from the utility. It is also commonly accepted to estimate the motor contribution by multiplying the total motor full load amps (FLA) by four.

Ifc = Isc trans + Isc motors

Ifc = "Worst-case" fault current

Isc trans = Fault current contribution from transformer

Isc motors = Fault current contribution from motors

UL 1561-listed transformers that are 25 kVA and larger have a ±10% impedance tolerance, which can affect fault current levels. Thus, a 0.9 factor is applied to the transformer’s impedance percentage (%). The fault current contribution of a 3-phase transformer (ISC trans) is calculated using the following equation:

Isc motors = Total motor FLA x 4

An alternative to calculating the available fault current is to use a Web-based application to calculate available fault current. Look for an application that considers transformer and motor contribution as well as conductor/bus impedance when determining a fault current level.

The second step is to determine the minimum acceptable equipment SCCR. There are several factors to consider regarding equipment SCCR:

Finally, it is critical that all equipment suppliers provide an analysis of the control panel’s equipment SCCR determination. The surveys mentioned earlier also revealed that half of design engineers struggle with interpreting or applying the UL 508A standard regarding SCCR. It is not uncommon for current-limiting devices to be misapplied in the panel’s equipment SCCR determination. One mistake can mean the difference between achieving the required SCCR and only achieving the minimum 5 kA. An analysis detailing how the component SCCRs and interrupting ratings are applied in conjunction with current-limiting devices helps support and validate the panel’s claimed SCCR.

The approach and implementation costs of an effective SCCR strategy can vary; however, it is critical to meet evolving equipment-protection requirements. Complying with the latest OSHA and NEC requirements can help enhance personnel safety and system reliability while protecting equipment investments.

Additional information regarding SCCR ratings and requirements can be found on via NFPA, NFPA 70E, and Electrical Code Coalition websites.

Erik Barnes is OEM marketing manager at Eaton.

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