Altitude; Load Class Rating Factor; I Nterrupting Ratings; Basis Of I Nterrupting Ratings - GE Power Break II Manual

Application

Altitude

Power Break® II circuit breakers a re designed for operation
at altitudes from sea l evel to 6000
air densities at a ltitudes a bove 6000 ft . affects the ability of
the circuit breaker to both transfer heat and interrupt short
circuits.
Determine the a ltitude of the breaker's instal lation, and se­
lect the a ltitude rating factor "D" from the following table.
Table 45.1
Factor "D" Altitude
Altitude (Feet) Altitude (Meters)
From 100 To 6,000 From 30 To 1.800
From 6,001 To 10,000 From 1,801 To 3,000
Above 10,001 Above 3,001

load Class Rating Factor

The type of load and its duty cycle must be considered in the
application of Power Break II ci rcuit brea kers. 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 cu rrent rating must be n o 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 b reaker is
intended to protect load equipment, prudent engineering
p ractices call for obta ining factory review and concurrence
with the selection of a specific p rotective device. Load class
selection factors "E" for typical a p plication a re listed i n the
fol lowi ng table.
Ta ble 45.2
Factor "E" Load Class or Type
-
Load Type
Switching Electromagnets
Single Motor Branch Circuit Protec­
tion ( Normal Duty) CD
Switc h i ng Capacitors
G)
Single Motor Branch Circuit Protec­
tion (Heavy Duty)
(j)
All other load Types (Normal Duty)
Use this factor to either plugging duty or starting more than 25 times per hour.
where the rms current cannot be easily calculated.
ft .
( 1800 meters). Reduced
Factor D
1.00
1.04
1.08
Factor E
1.50
1.50
1 . 50
175
1.0
Safety Factor
A safety factor is used to p rovide a design margin between
the rating of a circuit breaker and the derived operating
current using all of the selection factors described in the
eq uation on page 42. A safety factor of 10% is often used to
p revent nu isance tripping.
Factor "F" - Safety Factor
A safety factor of 10% is eq uivalent to a Factor F of 1.10.

I nterrupting 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 circu its under overload conditions. The b reaker
must have sufficient interrupting capacity to i nterrupt 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 nterru pting Rati ngs
i nterrupting ratings depend o n know­
Short-circuit current
ing the magnitude of short-circuit cu rrent that may flow
through the circuit breaker. Devices rated in accordance with
Standard UL 489 list their interrupting rating in terms of "rms
sym metrical a m peres."
Interrupting ratings a re based on
Power factor or X/R ratio.
a specific ratio of resistance - to reactance, or power factor
in the faulted circuit. Practical ac circu its contai n some reac­
tance, so there is some displacement between cu rrent 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 momenta ry 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
fu nction of the X/R (ratio of reactance to resistance) or power
factor of the faulted circuit. The higher the X/R rati ng, the
l ower the power factor and the g reater the mag nitude of
peak current d isplacement.
Zero axis
Symmetric a! ac wave
Symmetrical ac waveform
Envelopes of peaks
of sine wave are
symmetrical about
zero ax1s
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