FIND IT – Look for... the
Weakest Link
The SCCR marking for the assembly will be based
upon the lowest short-circuit current or withstand rating
of any power circuit component contained within the
assembly — or the “
weakest link
”. Many components
used in industrial control panels, industrial machinery
and HVAC equipment may be the weakest link,
including:
• motor controllers
• definite purpose contactors
• supplemental protectors
• circuit breakers (with low interrupting ratings)
• power distribution blocks.
FIX IT — How To Get A Higher SCCR So
Equipment Isn’t Limited
By fixing these
weakest links
, higher SCCRs can be
achieved. In general, there are three options:
1.
Use overcurrent protective devices with
higher interrupting ratings
— Overcurrent
protective devices with low interrupting ratings often
become the weakest link and limit the SCCR of the
assembly. The interrupting ratings of the overcurrent
protective devices used directly affects the assembly
rating. The only fix is to use devices with higher
interrupting ratings.
2.
Use current limitation to fix low rated
components
— It only takes one component with a
low SCCR to limit the entire assembly. Many of the
components used in industrial control applications have
low short-circuit current ratings. Fix these low rated
components and the assembly rating by using
current limitation. Since current-limiting fuses reduce high
level short-circuit currents to a minimal level, they allow
the use of these lower rated devices while
providing a higher SCCR.
3.
Buy more expensive components with
higher ratings
— A 65kA circuit breaker will more
than double your cost compared to a 14kA circuit
breaker. The fusible solution provides an interrupting
rating as high as 200kA, at a comparable cost to your
existing design, making it the most economical solution.
FORGET IT — Use Current-Limiting Fuses
Not only can modern current-limiting fusible solutions
fix undesired weak links and obtain a high assembly
short-circuit current rating, they can also provide
enhancements to your design in the areas of flexibility
of installation, increased workplace safety, and reliable
protection through the life of your equipment. These
enhancements take care of the hassle associated with
equipment with limited ratings so you can
FORGET IT
!
Benefits of Using Cooper Bussmann Current-
Limiting Fuses
Straight Voltage Ratings:
• Straight voltage ratings allow for more installations where
equipment can be installed. Straight voltage rated
equipment can be installed in any type of electrical system.
With a straight voltage rating you can ship it anywhere and
not have to worry.
High Assembly SCCRs:
• High SCCRs allow for more applications where equipment
can be installed. With a high SCCR you can ship it
anywhere and not have to worry.
• A wider range of fault current levels provides assurance that
the equipment will be installed correctly and safely.
• Meets the UL508A and NEC
®
requirements for SCCR
marking.
• Marketing advantage over competitive equipment with a
lower rating.
Increased Flexibility In Panel Use and Installation:
• Valuable time that was spent gathering information for
proper application is drastically reduced with fuses because
modern current-limiting fuses can provide high assembly
SCCRs and straight voltage ratings since they have:
- High Interrupting Ratings of 200,000A or more
- Excellent current limitation to protect low rated
components at high short-circuit current levels
- Straight voltage ratings so they can be installed in any
type of application independent of the grounding scheme
used, unlike devices with a slash voltage rating which are
limited to installation in ONLY a solidly grounded wye
type system
Find It, Fix It, Forget It
Fuse & Motor Starter
Combination
• Fusible disconnect with
Cooper Bussmann LP-CC
fuses marked interrupting
rating 200kA
• Motor starter marked
50kA
when protected by fuses or
circuit breakers
• The SCCR is
50kA
Circuit Breaker &
Motor Starter
Combination
• Three-pole circuit
breaker marked
interrupting rating
14kA
• Motor starter marked
50kA when protected by
fuses or circuit breakers
• The SCCR is
14kA
