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