Circuit Breakers; Standard Rated Devices; 100% Rated Devices; Factors Affecting The Current Ratings - GE Power Break II Manual

Application
V. I ndividual Pole Short-Circuit Test Seq uence (lsu)
1 .
I n d ividual Pole Short-Circuit Brea king Capacity:
Power Break® I I Circuit breakers a re tested to com plete a
series of short-circuit interruptions with a 3-minute rest
period between interruptions. The breakers must com­
plete an "open" operation followed by a "close-open"
operation on each pole individually. The circuit breakers
a re tested with an open circuit voltage sufficient to en­
able a recovery voltage of the rated voltage plus 5%. The
800-2000 a m p Power Break I I circuit breakers a re tested
using two phases of a three-phase 2 5,000 a m p rms
symmetrical, 50 Hz, 25% power factor short circuit.
2. Verification of Dielectric Withstand:
Following the short-circuit interruptions, the breakers
main circuit is verified to withstand a voltage equal to
twice its rated insulation voltage; however, not less than
1000 V for 1 min ute.
3.
Verification of Overload Release:
Following the dielectric verification, the circuit breaker
overload p rotection is verified by a pplying 2 50% of rated
current through each pole individually. The breaker must
trip withi n the trip time cu rve.
Table 42.1
Test Tolerance: Unless Otherwise Specified
All Tests
Load or Over Load
Cu rrent +5%-0%
+5%-0%
Voltage
Frequency -
Power Foetor
-
Power B reak®
I I -
100 ° / o Rated Circuit Breakers
Power Breaker I I circuit breakers a re U L Listed to ca rry 100%
of rating-plug nameplate current , continuously ® , when
insta lled in accordance with guideline pri nted on the brea ker,
and as detailed in the following Cu rrent Rating Factors por­
tion of this section.

Standard Rated Devices

A "Sta ndard rated" device is rated to carry 100% of its name­
plate cu rrent for short periods of time (up to 3 hours maxi­
mum) when enclosed in equipment , such a s a switchboard.
The enclosed device has a conti nuous rating of 80% of rating
according to current versions of the N atio nal Electric Code
(Article 2 20- 10).

100% Rated Devices

100% rated devices a re designed to ca rry 100% of their
nam eplate cu rrent rating when enclosed and tested in distri­
b ution equipment.
(})
4000 80 %
A stationary
42
No Load, Normal Short Circuit
3500
-
2500
-
±5%(47.5-52 5 Hz) ±5%(475-52 5 Hz)
±0.05 +0 -0.05
Breakers with 100% rating a re so m arked on the front of
the devices. This marking includes minimum enclosure size,
minimum venti lation (if a ny) , minimum cable sizes and tem­
perature rati ngs to be used. Current versions of the National
Electric Code permit such use i n the exceptions to Articles
2 10-2 2(c) and 2 20-10 (b).

Factors Affecting the Current Ratings

of I nstalled Devices
There a re six application factors to consider in selecting cur­
rent rati ng on instal l ed Power Break
b reakers. These a re: 1) the size of the cable or bus used in
the line and load connections, 2) the actua l i nstalled am bient
temperature, 3) the system operating frequency, 4) the a lti­
tude of the insta llation, 5) the type of loading of the protected
circuit, and 6) the design safety factor.
The fol lowing simple relationship combines these six a ppl ica­
tion factors into a single equation:
I CB c
= X A X B X X D X E X F
l
A
= I nsta lled circuit brea ker cu rrent rati ng, a m ps
l
cs
= Actual load current , amps
l A
A = Cable of bus sizing factor
B = Am bient temperature rating factor
C = Frequency rating factor
D = Altitude rating factor
E = Load class rating factor
F = Safety factor
Ca ble or Bus Size
The thermal design of a circuit breaker takes i nto account
the abil ity of the line and load cables o r buses to act as heat
si nks. Sta ndard U L 489 has assigned specific cable or bus
sizes for each curre nt rating. General ly, these assig nments
a re coordinated with specific conductor temperature ratings.
I ncreasing a conductor's temperature rating decreases both
the cross-sectional a rea and its ability to cond uct heat from
the circuit b reaker. The chart below ill ustrates the effect of
changing cable a reas on the cu rrent-ca rrying ability of the
circuit breaker.
% Change in current ca rrying ability
+50%
Current rating
+ 25
% of change
�
·25%
-50%
Rated
conductor
Effects of changing load a n d l i n e conductor sizes
I I
insu lated case circuit
200% of rated
conductor size