Cooper Bussmann Electronics Short-Circuit Current Rating - Manual (Page 2)

What Is A Short-Circuit Current Rating
(SCCR)?
SCCRs on components and equipment represent the
maximum level of short-circuit current that the
component or equipment can withstand and is used for
determining compliance with NEC ® 110.10. This rating
can be marked on individual components or
assemblies. The new SCCRs represent the maximum
amount of fault current that the assembly can withstand
under fault conditions. Assembly ratings take into
account all components contained within the equipment
rather than just the main overcurrent protective device.
A common mistake is to assume that the interrupting
rating of the overcurrent protective device protecting
the circuit represents the SCCR for the entire circuit.
Interrupting ratings, used for compliance with NEC ®
110.9, apply solely to the overcurrent protective device.
It is the characteristics of the overcurrent protective
device (e.g. opening time, let-through energy) that
need to be used in determining compliance with NEC ®
110.10, not the interrupting rating.
Who Is Affected By The SCCR Markings
The 2005 NEC ® has requirements for anyone building
equipment listed to the 2005 NEC ® or to UL508A and
requires the following to be marked with an SCCR:
• Industrial Control Panels [409.110]
• Industrial Machinery Electrical Panels [670.3(A)]
• Certain HVAC Equipment [440.4(B)]
• Meter Disconnect Switches [230.82(3)]
• Certain Motor Controllers [430.8]
Effective April, 2006, all equipment listed to UL508A will
be required to be marked with an assembly SCCR.
Inspectors and installers need this information in order
to ensure compliance with NEC ® 110.10. Equipment
installed where fault current levels exceed their short-
circuit current limit can be hazardous to persons and
property. SCCRs marked on components and
equipment make it easier to verify proper protection for
components and equipment for specific applications —
whether it be the initial installation or relocation of
equipment.
How Is SCCR Determined?
For meter disconnect switches and motor controllers,
this withstand level, or SCCR, is often determined by
product testing. For assemblies, the marking can be
determined through product listing or by an approved
method. With the release of the UL508A Industrial
Control Panel standard, an industry method is now
available. Any method used, whether UL508A or
another approved method, should be based on the
“weakest link” approach. In other words, the assembly
should be limited to installation where fault levels do
not exceed the withstand rating of devices with the
lowest SCCR. The marking determined should
represent the limits of the assembly for a safe
installation. Current-limiting overcurrent protective
devices can be used in the feeder or branch circuits to
increase the assembly SCCR where lower rated
components are used. When current-limiting devices
(that limit fault current levels to within the lower rated
components' ratings) are used, they provide protection
for the assembly — provided the current-limiting
Overcurrent Protective Device (OCPD) is used within
its ratings.
Are You Ready For The New SCCR Marking Requirements?
CAUTION: Short-circuit current ratings (SCCRs) are
different than interrupting ratings marked on
overcurrent protective devices.
Who Is Affected How Are They Affected
Facility/Process Engineers/ Need to specify end use equipment with ratings adequate for available
Consulting Engineers short-circuit current. Need adequate ratings so equipment has flexibility to be
moved around.
Control Panel Builders/ Need to determine and mark the assembly short-circuit current rating on the
Machine Builders/ equipment they build. Need to market equipment with ratings as least as high
HVAC Manufacturers as their competition.
Electrical Contractors Need assurance that the equipment they install is adequate for the available
short-circuit current at the point of installation, to avoid red tags, and lost time
and labor.
Electrical Inspectors Need to assure that the available short-circuit current where equipment is
being installed does not exceed the rating marked on the equipment.