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Current Limitation
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Current Limitation – NEC 240.2
• NEC 240.2 offers the following definition of a
current-limiting overcurrent protective device:
• “A device that, when interrupting currents in its currentlimiting range, reduces the current flowing in the faulted
circuit to a magnitude substantially less than that
obtainable in the same circuit if the device were
replaced with a solid conductor having comparable
impedance.”
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Current Limitation
• A current-limiting protective device
clears a fault current in less than
one-half cycle.
• Thermal energy is proportionate to the
square of “RMS” current
• Mechanical stresses are proportionate
to the square of “peak” current.
• The degree of current-limitation
depends upon the type of currentlimiting overcurrent protective
device selected (circuit breaker or
fuse).
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Current Limiting Devices
• Circuit Breaker
• Can be labeled current limiting only if current limiting
requirements of UL 489 are met
• Requirement states – the circuit breaker must limit
the asymmetrical I2t to a value below the equivalent
symmetrical I2t and clear within ½ cycle.
• Fuse
• Current-limiting requirements are based upon the
fuse type (Class) and corresponding I2t and Ipeak
let-through requirements at various fault levels.
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Current-Limiting Circuit Breakers
• If the overcurrent
protective device is
a current-limiting
circuit breaker
• Per UL 489, the
RMS asymmetrical
current is required
to be reduced to the
equivalent RMS
symmetrical current
or less.
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Current-Limiting
Fuses
• If the overcurrent protective
device is a current-limiting
fuse
• Per UL/CSA/ANCE 248,
the fuse is required to limit
the I2t let-through to a
value equal to or less than
the value indicated based
upon the fuse type (Class),
ampere, voltage rating and
fault current level.
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Current Limiting Overcurrent Devices (CB
vs. Fuses)
• C.L.C.B.
• 50,000A Fault (15% Power Factor), 100A C.L.C.B. must clear the
fault in ½ cycle and let-through 50,000A RMS symmetrical or less
• I2t of CL CB per above = 50,0002 * 0.00833 = 20,750,000 A2
seconds or less
• Class RK5 Fuse
• From previous chart, at 50,000A fault, I2t let-through must equal
500,000 A2 seconds or less
• Maximum permitted by standards requirements, based on
I2t, the RK5 fuse is required to let through 41.5 times less
“energy” than a current-limiting circuit breaker.
• Class RK1, J, CC, & T Fuses
• Have much less let through limits than RK5 Fuses
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Current Limiting Overcurrent Devices
• The degree of current-limitation can be determined
from the standards or from let-through charts.
• Let-through charts are
• Plotted from actual test data.
• Based upon fault current level.
• Available from circuit breaker or fuse manufacturers.
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Let-Through Charts
• Example – C.L. Fuse Let-Through Chart
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Applying Let-Through Charts
• KRP-C800SP LOW-PEAK® current-limiting timedelay fuse
• Determine peak and equivalent RMS let-through
current
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Applying Let-Through Charts
• Determine the peak let-through
current.
• Start at the prospective short-circuit
current, 86,000 amperes (point A)
and proceed up to the 800 ampere
fuse curve.
• Follow left to the instantaneous peak
let-through current (point D)
• Read the peak let-through current 49,000 amperes.
• If a fuse had not been used, the peak
current would have been 198,000
amperes (point C).
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Applying Let-Through Charts
•
Determine the equivalent RMS letthrough current.
• Start at the prospective short-circuit
current, 86,000 amperes (point A) and
proceed up to the 800 ampere fuse curve.
• Follow left to the A-B Line.
• Proceed down to equivalent RMS letthrough current scale (point B).
• Read the RMS let-through current as
21,000 amperes.
• If a fuse had not been used, the RMS letthrough current would be 86,000 amperes.
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© 2015 Eaton. All Rights Reserved..
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