Application
Altitude
Power Break® II circuit breakers are designed for operation
at altitudes from sea level to 6000
ft .
( 1800 meters). Reduced
air densities at altitudes above 6000 ft. affects the ability of
the circuit breaker to both transfer heat and interrupt short
circuits.
Determine the altitude of the breaker's instal lation, and se
lect the altitude rating factor "D" from the following table.
Table 45.1
Factor "D" Altitude
Altitude (Feet)
Altitude (Meters)
Factor D
From 100 To 6,000
From 30 To 1.800
1.00
From 6,001 To 10,000
From 1,801 To 3,000
1.04
Above 10,001
Above 3,001
1.08
load Class Rating Factor
The type of load and its duty cycle must be considered in the
application of Power Break II circuit breakers. Loads, such
as capacitors and electromagnets, require a substantial and
continuous derating factor if the breaker is normally used to
switch the load. With loads such as resistance welders, the
breaker's continuous current rating must be no less than
125% of the welder's 100% duty-cycle rating.
In general, insulated case circuit breakers a re intended for
the protection of insulated cable. Where a circuit breaker is
intended to protect load equipment, prudent engineering
practices call for obta ining factory review and concurrence
with the selection of a specific protective device. Load class
selection factors "E" for typical application are listed in the
fol lowing table.
Table 45.2
Factor "E"
-
Load Class or Type
Load Type
Switching Electromagnets
Single Motor Branch Circuit Protec
tion ( Normal Duty) CD
Switching Capacitors
Single Motor Branch Circuit Protec
tion (Heavy Duty)
G)
All
other load Types (Normal Duty)
Factor E
1.50
1.50
1.50
175
1.0
(j)
Use this factor to either plugging duty or starting more than 25 times per hour.
where the rms current cannot be easily calculated.
Safety Factor
A safety factor is used to provide a design margin between
the rating of a circuit breaker and the derived operating
current using all of the selection factors described in the
equation on page 42. A safety factor of 10% is often used to
prevent nuisance tripping.
Factor "F" - Safety Factor
A safety factor of 10% is equivalent to a Factor F of 1.10.
Interrupting Ratings
I n addition to current rating considerations, Power Break I I
circuit breakers must b e able t o automatically trip, o r open,
the protected circuits under overload conditions. The breaker
must have sufficient interrupting capacity to interrupt the
maximum short-circuit that can flow under worst-case
conditions.
The fol lowing paragraphs describe the interrupting rating of
Power Break II circuit breakers.
Basis of I nterrupting Ratings
Short-circuit current
i nterrupting ratings depend on know
ing the magnitude of short-circuit current that may flow
through the circuit breaker. Devices rated in accordance with
Standard UL 489 list their interrupting rating in terms of "rms
symmetrical amperes."
Power factor or X/R ratio.
Interrupting ratings are based on
a specific ratio of resistance - to reactance, or power factor
in the faulted circuit. Practical ac circuits contai n some reac
tance, so there is some displacement between current and
voltage wave-forms. Since a short circuit can occur at a ny
point of the voltage wave, the actual trace of short-circuit
current may d isplay considerable initial displacement from
the zero current axis.
The magnitude of the momentary peak current to be inter
rupted is a function of the maximum peak current displace
ment from the zero current axis. That displacement is a
function of the X/R (ratio of reactance to resistance) or power
factor of the faulted circuit. The higher the X/R rating, the
lower the power factor and the greater the magnitude of
peak current displacement.
Zero axis
Symmetric a! ac wave
Symmetrical ac waveform
Envelopes of peaks
of sine wave are
symmetrical about
zero ax1s
45
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