What is SCCR and Why is it Needed

From an electrical safety perspective, the past couple of months have been interesting.  Whether from recent posts of properly applying Industrial Machines or Industrial Control Cabinets, the number of electrical safety training programs that I have taught, or the meetings and phone calls, there are questions around short circuit currents, fault-currents, short circuit current ratings (SCCR) and why they are important.

Short circuit current and fault current are often used interchangeably.  However, there are some differences.  A short circuit current is an abnormal condition where two or more conductors of opposite polarity (or ground) are connected together and where the only limiting impedance is that associated with the resistive and inductive parameters associated with the electrical system [1].  Fault current is any abnormal current that flows in an electrical circuit [2].  Fault current can be from an unintended short circuit, transient current, overload current, or other abnormal condition.

Short circuit current rating (SCCR) refers only to equipment or systems and is a parameter associated with the connection to the electrical system.  SCCR is defined as “The prospective symmetrical fault current at a nominal voltage to which an apparatus or system is able to be connected without sustaining damage exceeding defined acceptable criteria” [3]. 

When installing electrical equipment or systems (including machines), the National Electric Code requires that equipment be “selected and coordinated to permit the circuit protective devices used to clear a fault to do so without excessive damage to the electrical equipment of the circuit” [3].  Equipment that is Listed and applied within their Listing requirements are properly applied [3].

Depending on the equipment or machine, SCCRs can be determined through testing or through calculations.  Switchboards, switchgear, panelboards, transfer switch equipment, surge protective devices (SPDs) and the like require testing to obtain their SCCR.  The SCCR for industrial machines and industrial control cabinets can be calculated using Underwriter Laboratories Standard for Safety, Industrial Control Panels, UL 508A; although these calculations are a result of known practices from tested results.

Listing agencies (like UL, ETL, TUV, etc.) that conduct testing to determine the SCCR of equipment are doing so to determine if the equipment is “safe” for installation and operation at the SCCR.  Within product safety standards, Listing agencies have predetermined pass/fail criteria.  While the pass/fail criteria vary slightly standard to standard, common examples include [4]:

• Emission of flame, molten metal, glowing or flaming particles through any openings (preexisting or created as a result of the test) in the product.
• Charring, glowing, or flaming of the supporting surface, tissue paper, or cheesecloth.
• Ignition of the enclosure
• Creation of any openings in the enclosure that result in accessibility of live parts, when evaluated in accordance with the accessibility of live parts
• Loss of structural integrity to a degree that the equipment collapses or experiences such displacement of parts that there is a risk of short-circuiting or grounding of current-carrying parts.
• Opening of an external leads or conductors.

To ensure that the equipment or machine is installed in a manner that reduces electrical hazards, the device must have a short circuit current rating (SCCR) that is equal to or greater than the short circuit current at the point of installation.  Equipment that has a SCCR less than the short circuit current at the point of installation increases the probability of electrical hazards (both Shock and Arcing Hazards), is inconsistent with standards, and can result in injuries to employees or personnel.

Part of engineering equipment or a system includes not only ensuring that it functions properly, but also ensuring that it is installed in a manner that does not create hazardous conditions.  Knowing what the short circuit current at the point of installation and the SCCR of the equipment is one step in determining if the equipment is properly applied.

 

1. Wikipedia.  Short Circuit Current.  Retrieved 2013 March 23.  Available [on-line] at http://en.wikipedia.org/wiki/Short_circuit
2. Wikipedia.  Fault Current.  Retrieved 2013 March 23.  Available [on-line] at http://en.wikipedia.org/wiki/Fault_(power_engineering)
3. National Fire Protection Association (NFPA), National Electric Code, NFPA 70-2011, Quincy, MA USA
4. Underwriters Laboratories (UL), Standard for Safety, Surge Protective Devices, UL 1449 3^rd edition, Northbrook, IL USA