Application
V. I ndividual Pole Short-Circuit Test Sequence (lsu)
1.
Individual Pole Short-Circuit Brea king Capacity:
Power Break® I I Circuit breakers are tested to complete 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 amp Power Break I I circuit breakers are tested
using two phases of a three-phase 25,000 amp 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 minute.
3.
Verification of Overload Release:
Following the dielectric verification, the circuit breaker
overload protection is verified by applying 2 50% of rated
current through each pole individually. The breaker must
trip within the trip time curve.
Table 42.1
Test Tolerance: Unless Otherwise Specified
All Tests
No Load, Normal
Short Circuit
Load or Over Load
Current
+5%-0%
-
3500
Voltage
+5%-0%
-
2500
Frequency
-
±5%(47.5-52 5 Hz)
±5%(475-52 5 Hz)
Power Foetor
-
±0.05
+0 -0.05
Power Break®
I I -
100 °/o
Rated Circuit Breakers
Power Breaker I I circuit breakers are U L Listed to carry 100%
of rating-plug nameplate current, continuously®, when
installed in accordance with guideline printed on the breaker,
and as detailed in the following Current Rating Factors por
tion of this section.
Standard Rated Devices
A "Standard rated" device is rated to carry 100% of its name
plate current for short periods of time (up to 3 hours maxi
mum) when enclosed in equipment , such as a switchboard.
The enclosed device has a continuous rating of 80% of rating
according to current versions of the N ational Electric Code
(Article 2 20-10).
100% Rated Devices
100% rated devices are designed to carry 100% of their
nameplate current rating when enclosed and tested in distri
bution equipment.
(})
4000
A stationary
80 %
42
Breakers with 100% rating a re so marked on the front of
the devices. This marking includes minimum enclosure size,
minimum ventilation (if a ny) , minimum cable sizes and tem
perature ratings to be used. Current versions of the National
Electric Code permit such use i n the exceptions to Articles
2 10-22(c) and 2 20-10 (b).
Factors Affecting the Current Ratings
of Installed Devices
There are six application factors to consider in selecting cur
rent rating on installed Power Break
I I
insulated case circuit
breakers. These are: 1) the size of the cable or bus used in
the line and load connections, 2) the actual i nstalled ambient
temperature, 3) the system operating frequency, 4) the a lti
tude of the installation, 5) the type of loading of the protected
circuit, and 6) the design safety factor.
The following simple relationship combines these six a pplica
tion factors into a single equation:
ICB
=
l
A
X A X B X
c
X D X E X F
lcs
= I nstalled circuit breaker current rating, amps
lA
= Actual load current, amps
A = Cable of bus sizing factor
B = Ambient temperature rating factor
C = Frequency rating factor
D = Altitude rating factor
E = Load class rating factor
F = Safety factor
Cable or Bus Size
The thermal design of a circuit breaker takes into account
the ability of the line and load cables o r buses to act as heat
sinks. Standard UL 489 has assigned specific cable or bus
sizes for each current rating. Generally, these assignments
are coordinated with specific conductor temperature ratings.
I ncreasing a conductor's temperature rating decreases both
the cross-sectional area and its ability to conduct heat from
the circuit breaker. The chart below illustrates the effect of
changing cable areas on the current-ca rrying ability of the
circuit breaker.
Current rating
% of change
+50%
% Change in current carrying ability
+25
�
·25%
-50%
Rated
200% of rated
conductor
conductor size
Effects of changing load and line conductor sizes
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