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duty imposed. 
Large air circuit breakers may 
be used either in selective tripping systems or in cascade systems. Selec- 
tive tripping systems, Fig. 3.5, are those in which the circuit breakers are 
set to trip selectively so that the one nearest the fault operates first so 
that only the faulty portion of the circuit is deenergized. In this case all 
circuit breakers should have adequate interrupting ratings, that is, their 
rating should be equal to or greater than the short-circuit duty a t the 
Selective Tripping vs. Cascading. 
156 A-C SHORT-CIRCUIT PROTECTIVE DEVICES AND ClRCUlT EQUIPMENT 
point of application. There are additional problems of selecting the 
time-current settings which are discussed more fully in Chap. 9. 
In cascaded operation, Figs. 3.6 and 3.7, circuit breakers may he used 
under certain circumstances beyond their interrupting rating. This 
applies where the main circuit breaker (commonly referred to in applica- 
tion tables as the A' circuit breaker) has adequate interrupting rating, 
that is, its rating is equal to or greater than the short-circuit duty imposed 
a t the point of application. The feeder circuit breakers (commonly 
referred to in application tables as the B circuit breaker) in this case, 
Fig. 3.6, may be used to twice their interrupting rating provided that the 
following conditions are met. The total kva of connected synchronous 
motors should not exceed 25 per cent of the supply transformer or 
I 
A / A LOAD CENTER 
nln UNIT SUBSTATION 
GENERATOR 
- A 
Y 
MAIN SECONDARY 3 BREAKER 
SU0 - DISTRI0UTlON 
CENTER 
FURNACE LOAD MOTOR WELDER 
200 HP 
FIG. 3.4 One-line diogrom showing typical applications of large air circuit breakers. 
ELECTRICALLY 
OPERATED 
I 5 7 A-C SHORT-CIRCUIT PROTECnVE DEVICES AND CIRCUIT EQUIPMENT 
generator rating. In addition to the main circuit breakers having ade- 
quate interrupting ratings, their instantaneous tripping attachment must 
be set t o operate when the current through the backed-up or B circuit 
breakers is not more than 80 per cent of the interrupting rating of the 
backed-up or B circuit breakers. This ensures that the main circuit 
breakers will operate whenever the short-circuit duty exceeds the inter- 
rupting rating of the B circuit breakers. 
& 1500 KVA LOAD CENTER - UNIT SUESTDTION 
A 
MDIN CIRCUIT BREAKER 
RATE0 DT LEAST 50.000 
DMP INTERRUPTING 
ERANCH FEEDER 
CIRCUIT BREDK- 
/ 
SHORT CIRCUIT DUTY 
DT THIS POINT 32.000 ~ U E - B U ~ 
DMP RMS DSYMMETRICDL 7 
FEEDER CIRCUIT EREDKERS 
RDTED 50,000AMP INTERRUPTING 
SHORT CIRCUIT DUTY DT THIS POINT 
50.000 DMP RMS ASYMMETRICDL 
WOOD DMP FROM THE TRANSFORMER 
DND 9000 DMP FROM THE MOTORS 
+FEEDER CABLE 
A /I' ERDNCH FEEDER CIRCUIT 
DMP INTERRUPTING 
V 
)BREDKERS RATED 25000 
SHORT CIRCUIT DUTY 
DT THIS POINT 22000 
DMP RMS DSYMMETRICDL 
* 
NOTE: SHORT CIRCUIT LEVELS 
DT SUB E u s s E s n am 
REDUCED DUE FEEDER 
CAELE IMPEDDNCE 
FIG. 3.5 
tripping system. 
tain selectivity. 
One-line diagram showing large air circuit breakers applied in selective 
Time settings of overcurrent trip elements must be properly set to ob- 
158 A-C SHORT-CIRCUIT PROTECTIVE DEVICES AND CIRCUIT EQUIPMENT 
I 
i 
& I 5 0 0 KVA LOAD CENTER - UNIT SUBSTATION 
MAGNETIC TYPE 
SHORT CIRCUIT DUTY 
AT THIS POINT 50.000AMP 
RMS ASYMMETRICAL 
I ' I 
MOTOR 
CONTRIBUTION 
9000AMP 
I 
NOTE: INSTANTANEOUS TRIP ELEMENT ON MPIN 
BREAKER A MUST BE SET TO TRIP AT 
16400 AMP THIS IS 0.8 OF 2 0 5 0 0 AMP 
20500 AMP IS THE CURRENT FLOWING 
FROM THE MAIN TRANSFORMER THRU 
BREAKER A WHEN CURRENT FLOWING 
THRU FEEDER BREAKER 0 IS 25.000 AMP 
THE RAyiNGOF BREAKEdB 
FIG. 3.6 One-line diagram showing large oir circuit breakers applied in cascade with 
only one source of low-voltage power. 
Motor contribution must be considered. The duty including motor 
contribution should not exceed twice the interrupting rating of the 
backed-up circuit breaker. However, the motor contribution may not 
come through the main circuit breaker. Therefore, the main A' circuit- 
breaker instantaneous trip setting may be less than 80 per cent of the 
backed-up circuit-breaker interrupting rating because the main A' circuit 
breaker must trip instantaneously when the total rms asymmetrical short- 
circuit current through the backed-up circuit breaker is 80 per cent or 
more of the interrupting rating of the backed-up B circuit breaker. For 
example, in Fig. 3.6 if the backed-up or B circuit breakers are rated 
25,000 amp interrupting rating, the short-circuit duty a t the point of 
application of the B circuit breaker should not exceed 50,000 amp rms 
asymmetrical. This may he made up of 41,000 amp from the main 
source and 9000 amp from the motors. The main-source circuit breaker 
must trip instantaneously a t 0.8 X 20,500 or 16,400 amp rms asym- 
metrical. It makes no difference whether the circuit breaker is applied 
a t the bus or a t some point remote from the bus. When the backed-up 
circuit breakers are applied a t points remote from the bus, such as circuit 
breakers B' in Fig. 3.7, the interrupting duty a t the circuit breaker ahead, 
A-C SHORT-CIRCUIT PROTECTIVE DEVICES AND CIRCUIT EQUIPMENT 159 
B in Fig. 3.7, may be in excess of twice the interrupting rating of the 
backed-up B' circuit breaker, but because of cable impedance the short- 
circuit current a t the point of application of the backed-up circuit breaker 
B' must be limited to twice its interrupting rating. Circuit breakers 
operating a t beyond their interrupting rating in cascade mustbe inspected 
after each operation and may require more than normal maintenance 
after interrupting currents beyond their rating even though the main 
circuit breaker does open. 
Another qnalification is that the circuit breakers must be of the same 
manufacture and of similar characteristics. Feeder circuit breakersshould 
be electrically operated because the forces incident to closing against 
short circuits in excess of the circuit-breaker rating may preclude success- 
ful manual closing. Circuit breakers of two widely different interrupting 
I 
f 
&I000 KVA LObD CENTER 
-UNIT SUBSTATION 
I I I -knunar 
I 
HERE 26000 AMP RMS 
NOTE! INSTbNTANEWS snom CIRCUIT DUTY 
TRIP ELEMENT ON 
FEEDER BREIKER B ISYMUETRICbL 
MUST BE SET bT 
12000 bMP(OQ X I50001 
FIG. 3.7 
remote from the main source of power. 
One-line diagram showing large air circuit breakers in cascade applied 
160 A-C SHORT-CIRCUIT PROTEtTlVE DEVICES AND CIRCUIT EQUIPMENT 
ratings cannot be cascaded. As a guide to this, refer to Table 3.2 which 
shows the maximum interrupting rating circuit breaker which can he 
used to back up any given interrupting rating feeder circuit breaker. 
The ratio of the columns may be more than 2 : 1. The higher interrupt- 
ing duty in the main circuit breaker often comes about because of having 
to select it for continuous current-carrying rating rather than inter- 
rupting rating. Regardless of the interrupting rating of the main cir- 
cuit breaker, the duty cannot exceed twice the interrupting rating of the 
cascaded B feeder circuit breaker. 
TABLE 3.2 Range of Large Air Circuit Breakers Which Can Be Cascaded with 
Main Circuit Breaker A' 
Interrupting Rating, 
Each Other 
Minimum Interrupting Rating of 
Coscaded Feeder Circuit Breaker B, 
Amp Rmr Amp Rmr 
25,000 15.000 
30,000 15,000 
50.000 15.000 
60,000 25,000 
75,000 25,000 
100,000 50.000 
120,000 75,000 
150.000 100.000 
Where there are two or more sources of current to a bus with cascaded 
feeder rircuit breakers, the following rule applies, Fig. 3.8. All main A' 
circuit breakers (i.e., A : , A: , A : ) must be tripped instantaneously when 
the total short-circuit current through the hacked-up