Combination transfer and bypass isolation switch utilizing drawout protective devices and key interlocks

An apparatus selectively connects a normal power line and an emergency power line to a load, and includes two interlocked switching mechanisms, each having individual interlocks, for selectively connecting the power lines to the load, and an interlock mechanism for interlocking the switching mechanisms in order that only one of the power lines at a time is connected to the load. The switching mechanisms may each have two drawout cassettes including associated switching devices for switching the power lines to the load and, also, including an isolated position for isolation of the associated switching device from at least one of the switched power lines and the load. Each switching device may be a circuit breaker having a trip unit for sensing current flowing from a power line to the load and tripping the circuit breaker. The interlock mechanisms may include keylock mechanisms which selectively disable closing of the switching devices. Each keylock mechanism may have individual locks on associated cassettes and a single key for operating only one of the locks. The individual interlocks of the switching mechanisms may cooperate with the keylock mechanisms to selectively disable connection of a power line to the load. Each key may be retained by an unlocked associated lock and may be removed from a locked associated lock.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is directed to a combination transfer and bypass isolation 
switch using switching devices, and more particularly to such a switch 
using two interlocked drawout circuit interrupters for transfer switch 
operation, two interlocked drawout circuit interrupters for bypass 
operation, and key interlocks for each circuit interrupter. 
2. Background of Information 
Transfer switches are well known in the art. Transfer switches operate, for 
example, to transfer a power consuming load from a circuit with a normal 
power supply to a circuit with an emergency power supply. Applications for 
transfer switches include stand-by applications, among others, in which 
the emergency power supply stands-by if the normal power supply should 
fail. Generally, a transfer switch selects either a normal primary power 
line or an emergency backup power line for connection with the load. The 
transfer switch typically comprises a pair of circuit interrupters 
combined with a drive mechanism, a linkage mechanism and a selection 
mechanism. 
The drive and linkage mechanisms may either be mechanical or electrical. In 
mechanical systems, usually motors are preferred, but at other times, 
there is a clear preference for manually-operated drive mechanisms. In 
either type of mechanical system, the linkage mechanism extends from the 
drive mechanism to handles of the circuit interrupters. The linkage 
mechanism couples the drive mechanism with the handles in order that drive 
force is translated into position changes between progressive positions of 
the handles. These forces open one circuit interrupter and close the other 
circuit interrupter. In electrical systems, a first electrical signal is 
provided to open one circuit interrupter and a second electrical signal is 
provided to close the other circuit interrupter. 
The selection mechanism may either be automatic or manual. In automatic 
systems, the transfer switch senses the voltages of the normal power line 
and the emergency power line. Whenever the normal voltage drops below a 
first predetermined value, and the emergency voltage is above a second 
predetermined value, the normal power line is disconnected from the load 
and the emergency power line is connected to the load. Otherwise, whenever 
the normal voltage is above the second predetermined value, the normal 
power supply is reconnected (or remains connected) to the load independent 
of the state of the emergency voltage. Thus, there is a preference for the 
normal power line. Connection and disconnection are accomplished by the 
drive and linkage mechanisms. In manual systems, for example, an operator 
opens the normal circuit interrupter and closes the emergency circuit 
interrupter. 
Bypass switches are also well known in the art. Bypass switches operate, 
for example, to transfer a power consuming load from a first circuit 
interrupter to a parallel-connected second circuit interrupter. Bypass 
switches are used with either a normal power supply or an emergency power 
supply. Applications for bypass switches include stand-by applications, 
among others, in which the bypass circuit interrupter stands-by if the 
normal circuit interrupter requires maintenance or is otherwise 
unavailable for operation. In such applications, interlocks between bypass 
and normal circuit interrupters are not required because both circuit 
interrupters connect the identical power line to the same load. 
When bypass switches are used with transfer switches, additional isolation 
and interlocking mechanisms are required to prevent the simultaneous 
connection of a normal power line and an emergency power line. These power 
lines have similar, but different, voltages, frequencies and phases, and 
thus, a connected power line must be disconnected before the other power 
line is connected. An isolation mechanism selectively disconnects or 
isolates transfer switching devices from the power lines and the load. The 
isolation mechanism also enables selection of an appropriate bypass 
switching device. The combined system of transfer switches, bypass 
switches and the isolation mechanism typically comprises seven circuit 
interrupters and associated interlocking mechanisms. In such combined 
system, two circuit interrupters are used for transfer switching, two 
circuit interrupters are used for bypass switching, and three circuit 
interrupters are used for isolation. 
The interlocking mechanisms cooperate with the seven circuit interrupters 
to ensure that the normal and emergency power lines are not 
interconnected. The interlocking mechanisms, for example, enable closing 
of a bypass circuit interrupter whenever the isolation circuit 
interrupters are open. On the other hand, the isolation circuit 
interrupters must be closed in order to connect a selected power line to 
the load using the transfer circuit interrupters. Such interlocking 
mechanisms typically comprise three locks having a single key. Each lock 
has a locked position to disable closing of a circuit interrupter and an 
unlocked position to enable closing of the circuit interrupter. Typically, 
a first lock is used on the bypass normal circuit interrupter, a second 
lock is used on the bypass emergency circuit interrupter and a third lock 
is used for the three isolation circuit interrupters. The three isolation 
circuit interrupters also have a common linkage, enabled and disabled by 
the third lock, for simultaneously opening or closing the three isolation 
circuit interrupters. 
There remains a need, therefore, for an improved combination transfer and 
bypass isolation switch for normal and emergency power lines that 
minimizes the required number of circuit interrupters. 
SUMMARY OF THE INVENTION 
This and other needs are satisfied by the invention which is directed to a 
combination transfer and bypass isolation switch using two interlocked 
circuit interrupters for transfer switch operation, two interlocked 
circuit interrupters for bypass switch operation, and key interlocks on 
each circuit interrupter. In accordance with the invention, for use with a 
normal power line and an emergency power line, a transfer switch having 
two interlocked circuit interrupters selectively connects a load to only 
one of the power lines. A normal circuit interrupter is closed to connect 
the normal power line to the load. Similarly, an emergency circuit 
interrupter is closed to connect the emergency power line to the load. The 
transfer switch also has mechanical interlocks between the normal and 
emergency circuit interrupters. The interlocks prevent the emergency 
circuit interrupter from closing when the normal circuit interrupter is 
closed and prevent the normal circuit interrupter from closing when the 
emergency circuit interrupter is closed. 
A bypass switch having two interlocked circuit interrupters is connected in 
parallel with the transfer switch and selectively connects the load to 
only one of the power lines. A bypass normal circuit interrupter is closed 
to connect the normal power line to the load. Similarly, a bypass 
emergency circuit interrupter is closed to connect the emergency power 
line to the load. The bypass switch also has mechanical interlocks between 
the bypass normal and bypass emergency circuit interrupters. These 
interlocks of the bypass switch operate in a comparable manner as the 
mechanical interlocks of the transfer switch described above. 
A first key interlock mechanism interlocks operation of the normal 
(transfer) and bypass emergency circuit interrupters. A second key 
interlock mechanism interlocks operation of the emergency (transfer) and 
bypass normal circuit interrupters. The key interlock mechanisms provide 
an effective interlock between transfer switch and bypass switch 
functions. Each key interlock mechanism includes a lock on each circuit 
interrupter and a single key. Each lock has a locked and an unlocked 
position. Only when a lock is locked, which prevents an associated circuit 
interrupter from closing, may the key be removed. On the other hand, 
whenever a lock is unlocked, which permits an associated circuit 
interrupter to be closed, the key is retained by the lock. Each lock also 
cooperates with the associated mechanical interlock. For example, whenever 
the emergency circuit interrupter is open and the normal circuit 
interrupter is unlocked, the normal circuit interrupter may be closed. 
Otherwise, when the emergency circuit interrupter is closed or when the 
normal circuit interrupter is locked, the normal circuit interrupter 
cannot be closed. 
In one embodiment of the invention, the transfer and bypass switches, for 
maintenance reasons, have drawout circuit interrupters which may be 
isolated from the power lines and the load of the associated transfer or 
bypass switch. 
It is an object of the invention to provide a transfer and bypass isolation 
switch using a minimum number of four interlocked switching devices, in 
which two switching devices are used for transfer operation, two switching 
devices are used for bypass operation, and in which interlocks prevent the 
interconnection of the normal and emergency power lines. 
It is also an object of the invention to provide a transfer and bypass 
isolation switch using drawout circuit interrupters for isolation from an 
associated power line and load by removal or drawout of a bypassed circuit 
interrupter.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates a combination transfer and bypass isolation switch 10 
for connecting only one of a normal power line 1 and an emergency power 
line 2 to a load 3. In the exemplary embodiment, transfer switch 20 
includes interlocked circuit breakers N,E and bypass switch 30 includes 
interlocked circuit breakers BN,BE. The transfer switch 20 selectively 
connects the load 3 to only one of the power lines 1,2. The power lines 
1,2 are three phase AC power lines having a fourth neutral line (see FIG. 
3) and the circuit breakers N,E,BN,BE are four-pole devices for switching 
the three phase AC power and neutral lines. It being understood that the 
invention is applicable to any type of power line, or any type of 
switching device, having any number of phases, or poles, respectively. 
A typical example of a circuit breaker may be found in U.S. Pat. No. 
4,240,053 issued Dec. 16, 1980 to Hugh A. Nelson et at. entitled "Circuit 
Breaker Utilizing Improved Current Carrying Conductor System" and assigned 
to the assignee of the present invention, which is herein incorporated by 
reference. 
In the exemplary embodiment of FIG. 1, each of the circuit breakers 
N,E,BN,BE has a trip unit 110 (see FIG. 4) which senses current flowing 
from the associated power line to the load 3, generates a time-current 
parameter, and trips the circuit breaker open in the event various 
predetermined time-current trip characteristics are exceeded. A typical 
example of a trip unit for a circuit breaker may be found in U.S. Pat. No. 
4,351,013 issued Sep. 21,1982 to Joseph J. Matsko et al. entitled "Circuit 
Interrupter with Multiple Display and Parameter Entry Means" and assigned 
to the assignee of the present invention, which is herein incorporated by 
reference. It being understood that the invention is applicable to other 
types of circuit interrupters not having a trip unit (e.g., a molded case 
switch). 
The normal circuit breaker N of FIG. 1 is closed to connect normal power 
line 1 to load 3. Similarly, the emergency circuit breaker E is closed to 
connect emergency power line 2 to load 3. A mechanical interlock 13 of 
transfer switch 20, having normal interlock 13N and emergency interlock 
13E, interlocks circuit breakers N and E, respectively. Emergency 
interlock 13E prevents emergency circuit breaker E from closing when 
normal circuit breaker N is closed. Similarly, normal interlock 13N 
prevents normal circuit breaker N from closing when emergency circuit 
breaker E is closed. 
The bypass switch 30, which is connected in parallel with transfer switch 
20, also selectively connects the load 3 to only one of the power lines 
1,2. A bypass normal circuit breaker BN is closed to connect normal power 
line 1 to load 3. Similarly, bypass emergency circuit breaker BE is closed 
to connect emergency power line 2 to load 3. A mechanical interlock 
14,14N,14E of bypass switch 30 interlocks circuit breakers BN,BE and 
operates in a like manner as mechanical interlock 13,13N,13E of transfer 
switch 20 described above. 
The transfer and bypass switches 20,30 also have key interlock mechanisms 
K1 and K2 for interlocking operation of circuit breakers N,BE and E,BN, 
respectively. The key interlock mechanisms K1,K2 include locks 
K1A,K1B,K2A,K2B on circuit breakers N,BE,E,BN, respectively. The key 
interlock mechanisms K1 and K2 each include a single key 251 and 261 (see 
FIG. 2), respectively. Each lock has a locked position, which prevents the 
associated circuit breaker from closing, and an unlocked position. Each 
lock permits an associated key to be removed in the locked position. On 
the other hand, each lock retains the associated key in the unlocked 
position. Each lock also cooperates with an associated mechanical 
interlock. For example, whenever emergency circuit breaker E is open and 
lock K1A of normal circuit breaker N is unlocked, the normal circuit 
breaker may be closed. Otherwise, when emergency circuit breaker E is 
closed or when lock K1A is locked, normal circuit breaker N cannot be 
closed. The remaining locks K1B,K2A,K2B operate in a manner similar to the 
operation of lock K1A. 
In the exemplary embodiment of FIG. 1, each circuit breaker has terminals 
24,25 that are part of a drawout mechanism or cassette 22 (see FIG. 2) for 
isolating the circuit breaker from the power lines and load of the 
associated transfer or bypass switch. When terminals 24 are separated from 
terminals 25, circuit breaker BN, for example, is isolated from normal 
power line 1 and load 3. A typical example of a drawout cassette circuit 
interrupter may be found in U.S. Pat. No. 4,565,908 issued Jan. 21, 1986 
to Fred Bould entitled "Drawout Switchgear Apparatus with Retractable 
Shutter Mechanism for Terminal Stabs" and assigned to the assignee of the 
present invention, which is herein incorporated by reference. It being 
understood that the invention is applicable to other types of circuit 
interrupters, switches or breakers not having drawout or isolation 
mechanisms. 
Referring now to FIG. 1a, a schematic diagram of a prior art transfer and 
bypass mechanism is illustrated. A transfer switch 20A, having normal 
(transfer) circuit breaker N and emergency (transfer) circuit breaker E, 
cooperates with three isolation circuit breakers IN,IE,IL in order to 
selectively connect only one of normal power line 1 and emergency power 
line 2 to load 3. Whenever normal circuit breaker N and isolation circuit 
breakers IN and IL are closed, then normal power line 1 is connected to 
load 3. Similarly, when emergency circuit breaker E is closed and 
isolation circuit breakers IE and IL are closed, then emergency power line 
2 is connected to load 3. The normal circuit breaker N and emergency 
circuit breaker E are interconnected by a common linkage, in a break 
before make configuration that is well known in the art, in order that 
only one of the circuit breakers is closed at a time. A bypass normal 
circuit breaker BN and a bypass emergency circuit breaker BE selectively 
connect only one of normal power line 1 and emergency power line 2, 
respectively, to load 3. 
The bypass normal circuit breaker BN, the bypass emergency circuit breaker 
BE and the interlock circuit breakers IN,IE,IL each have respective key 
interlock mechanisms KL1,KL2,KL3 for locking operation of these five 
circuit breakers. The key interlock mechanisms KL1,KL2,KL3 generally have 
a single key (not shown) for operating the mechanisms. In particular, the 
isolation circuit breakers IN,IE,IL must first be jointly opened by the 
common linkage and, then, must be jointly locked by locking mechanism KL3 
before either of the bypass circuit breakers BN,BE may be closed. Thus, 
for example, after locking mechanism KL3 is locked, the single key may be 
removed and used to unlock locking mechanism KL1, and bypass circuit 
breaker BN may be closed to connect load 3 to normal power line 1. No key 
interlock mechanism is provided for normal circuit breaker N and emergency 
circuit breaker E. These circuit breakers are isolated from the power 
lines 1,2 and the load 3 by isolation circuit breakers IN,IE,IL which 
remain locked in an open position by locking mechanism KL3. 
Referring now to FIG. 2, transfer switch 20 is mounted in steel enclosure 
21, bypass switch 30 is mounted in steel enclosure 31 and automatic 
transfer circuitry (ATC) 40 is mounted in steel enclosure 41. In the 
exemplary embodiment, ATC 40 controls opening and closing of normal 
circuit interrupter N and emergency circuit interrupter E over control 
lines 131-134 (see FIG. 6). An example of automatic transfer in an 
uninterruptable power supply may be found in U.S. Pat. No. 5,081,367 
issued Jan. 14, 1992 to George A. Smith et al. entitled "Electric Power 
System with Maintenance Bypass for Uninterruptable Power Supply Using 
Closed Transition Operation" and assigned to the assignee of the present 
invention, which is herein incorporated by reference. 
Each of the drawout cassettes 22,23,32,33 of FIG. 2 includes a circuit 
interrupter N,E,BN,BE, respectively, which may be isolated from its 
associated power lines and load. The circuit interrupters include key 
interlock mechanisms K1A,K2A, K2B,K1B, respectively. 
Referring now to FIG. 3, a schematic diagram illustrates power 
interconnections and power terminations of an embodiment of the invention 
including emergency and normal power lines which each have three phases 
and a neutral. In particular, four terminals NN,N1,N2,N3 are provided for 
the neutral and three phases of normal power line 1. In a like manner, 
four terminals EN,E1,E2,E3 are provided for the neutral and three phases 
of emergency power line 2, and four terminals LN,L1,L2,L3 are provided for 
the neutral and three phases of load 3. Four jumpers JT interconnect load 
side terminals N3LD,N2LD,N1LD,NNLD of normal circuit breaker N with load 
side terminals E3LD,E2LD,E1LD,ENLD of emergency circuit breaker E. 
Similarly, four jumpers JB interconnect load side terminals 
BN3LD,BN2LD,BN1LD,BNNLD of bypass normal circuit breaker BN with load side 
terminals BE3LD,BE2LD,BE1LD,BENLD of bypass emergency circuit breaker E. 
Load power busses 3LD, 2LD, 1LD, NLD interconnect the load terminals of 
transfer circuit breakers N,E with the respective load terminals of bypass 
circuit breakers BN,BE. In a comparable manner, normal line power busses 
3LNN,2LNN, 1LNN,NLNN interconnect the line terminals of normal circuit 
breaker N with the respective line terminals of bypass normal circuit 
breaker BN. Emergency line power busses 3LNE,2LNE, 1LNE,NLNE interconnect 
the line terminals of emergency circuit breaker E with the respective line 
terminals of bypass emergency circuit breaker BE. Those skilled in the art 
will appreciate that the terminals, jumpers and power busses are selected 
in accordance with the rated voltage and current carrying capacity of the 
circuit breakers. 
Referring now to FIG. 4, a front view of drawout cassette 22 is 
illustrated. A switching device 100 of drawout cassette 22 includes a trip 
unit 110, an operator interface 140 having an open pushbutton 146 for 
opening the device, a dose pushbutton 144 for closing the device, a 
charging handle 129 for manually charging a switching mechanism (not 
shown), and a mechanical L-shaped interlock 101 for disabling closing of 
the device. A vertical rod 254 engages the L-shaped interlock 101 near 
bend 102. Whenever rod 254 (see FIG. 5b) forces interlock 101 in upward 
direction U, a trip bar or lever (not shown) of switching device 100 
rotates and disables latching of the switching device in a closed 
position. The interlock 101 is mechanically biased to assume a lower 
position 103 (shown in shadow) whenever rod 254 or roller 179 (see FIG. 
4c) do not force the interlock in direction U. Thus, when rod 254 and 
roller 179 are not in place, interlock 101 assumes lower position 103, and 
switching device 100 may be closed. 
Referring to FIG. 4a, a rear view of switching device 100 is illustrated. 
Rear surface 28 of the exemplary switching device 100 includes mechanical 
interlock pin 150 and eight sets of terminals 24. It being understood that 
each set of terminals 24 may include plural individual terminations. As 
understood by those skilled in the art, such plural terminations are 
numbered in accordance with the rated current carrying capacity of 
switching device 100. The terminals 24 include four sets of line terminals 
and four sets of load terminals for three power line phases and a neutral 
line. Whenever switching device 100 is closed, pin 150 protrudes through 
rear surface 28 to engage interlocking mechanism 160 (see FIG. 4b). The 
pin 150 is pushed and forced to protrude through the surface by a tapered 
end of a pusher rod (not shown) surrounded by an opening spring (not 
shown) within switching device 100. On the other hand, when switching 
device 100 is open, pin 150 is generally flush with rear surface 28 and 
interlocking mechanism 160 is not engaged. 
FIG. 4b illustrates drawout cassette 22 with normal circuit interrupter N 
(see FIG. 2) removed. In the exemplary embodiment, rear surface 29 of the 
cassette includes eight terminals 25 and interlocking mechanism 160. The 
eight terminals 25 are interconnected with the corresponding eight sets of 
terminals 24 (see FIG. 4) when normal circuit interrupter N is installed 
(see FIG. 1). The interlocking mechanism 160 includes a mounting bracket 
174 attached to the rear surface 29 of cassette 22. A generally L-shaped 
operating mechanism 162 is pivotally connected to mounting bracket 174 at 
pivot point 164. An engaging arm 161 and an operating arm 163 extend from 
pivot point 164. Engaging arm 161 has an engaging surface 165 for engaging 
pin 150 of normal circuit interrupter N (see FIG. 4a). Mounting bracket 
174 also includes a mounting tab 167 having a hole 168 for mounting an 
interlocking cable 170. The cable has an outer sheath 171 and an inner 
operating wire 172. Operating wire 172 is connected to a connecting 
surface 169 of operating arm 163 by connection 166. Thus, for example, 
when normal circuit interrupter N (see FIG. 2) is closed, pin 150 (see 
FIG. 4a) protrudes and engages surface 165 which rotates engaging arm 161 
and operating arm 163 about pivot point 164. In turn, operating arm 163 
moves operating wire 172 left in direction L of FIG. 4b. 
FIG. 4c illustrates drawout cassette 23 with emergency circuit interrupter 
E (see FIG. 2) removed. An inside surface 27 of the cassette includes an 
interlocking mechanism 180 which cooperates with the interlocking 
mechanism 160 of FIG. 4b. The interlocking mechanism 180 is mounted to a 
plate 181 which is attached to inside surface 27 and includes interlocking 
cable 170, mounting tab 182, spring 176 and an L-shaped operating 
mechanism 177. In the exemplary embodiment described in FIGS. 4b-4c, 
interlocking cable 170 extends from normal circuit interrupter N to 
emergency circuit interrupter E. Mounting tab 182 has a hole 175 for 
mounting the interlocking cable 170. Operating mechanism 177 has an 
operating arm 183 and a roller 179 rotatably mounted to operating 
mechanism 177 at rotation point 184. The operating mechanism 177 is 
pivotally mounted to mounting plate 181 at pivot point 178. Operating wire 
172 of cable 170 passes through spring 176 and is connected to operating 
arm 183 by connection 173. The spring 176 generally resists movement of 
operating arm 183 and operating wire 172, in rear direction R of FIG. 4c, 
except under influence of pin 150 (see FIG. 4a). 
Thus, for example, when normal circuit interrupter N (see FIG. 2) is closed 
and one end of operating wire 172 moves in direction L of FIG. 4b, another 
end of operating wire 172 and operating arm 183 move in direction R of 
FIG. 4c. In turn, operating mechanism 177 rotates about pivot point 178 
and roller 179 moves in a general upward direction U. Whenever roller 179 
forces interlock 101 (see FIG. 4), shown in shadow in FIG. 4c, in upward 
direction U, latching of the circuit interrupter in a closed position is 
disabled. As discussed above, interlock 101 is mechanically biased to 
assume a lower position 103 (see FIG. 4) when either rod 254 (see FIG. 4) 
or roller 179 do not force the interlock in direction U. 
In a like manner, interlocking mechanisms 160, 180 in circuit interrupters 
N,E operate to prevent closing circuit interrupter N whenever circuit 
interrupter E is closed. Similarly, the interlocking mechanisms in circuit 
interrupters BN,BE of bypass switch 30 prevent closing circuit interrupter 
BN whenever circuit interrupter BE is closed and, also, prevent closing 
circuit interrupter BE whenever circuit interrupter BN is closed. 
Referring now to FIGS. 5-5a, keylock 250 is shown in an unlocked position 
in FIG. 5. The keylock 250 includes a key 251, a keyhole 255 for the key 
and a lock bolt 252. Whenever locking surface 253 is lifted above lock 
bolt 252 and key 251 is rotated in clockwise direction CW, then keylock 
250 is locked and the lock bolt protrudes from the keylock to support the 
locking surface (see FIG. 5a). In this case, key 251 may be removed from 
the keylock. On the other hand, when key 251 is rotated in 
counter-clockwise direction CCW, keylock 250 is unlocked, lock bolt 252 
retracts into the keylock, locking surface 253 falls below the lock bolt, 
and the key is retained and cannot be removed from the keylock. 
The keylock 250 in FIG. 5a is shown with key 251 in the locked position. 
The lock bolt 252 protrudes from keylock 250 and supports locking surface 
253 of vertical rod 254. The rod 254 engages interlock 101 near bend 102. 
In this position, as discussed above, rod 254 forces interlock 101 in 
upward direction U and switching device 100 cannot be closed. Whenever key 
251 is turned in counter-clockwise direction CCW (see FIG. 5), lock bolt 
252 retracts into keylock 250, no support is provided for locking surface 
253 of vertical rod 254, and rod 254 disengages interlock 101. In this 
position (see FIG. 5), as discussed above, switching device 100 may be 
closed. 
Referring now to FIG. 6, a system including normal power line 1, emergency 
power line 2, normal circuit breaker N, emergency circuit breaker E and 
ATC 40 is illustrated. The ATC 40 includes a voltage sensing circuit 42 
for detecting an undervoltage condition of the power lines 1,2, a normal 
power logic circuit 44 for normal circuit breaker N and an emergency power 
logic circuit 46 for emergency circuit breaker E. The ATC 40 may also 
include ancillary power circuitry (not shown) for powering indicators (not 
shown) of the circuit breakers of transfer switch 20 and bypass switch 30 
whenever the circuit breakers are isolated for test purposes. The voltage 
sensing circuit 42 provides six signals, representative of an undervoltage 
condition of the three phases of each of power lines 1,2 to the logic 
circuits 44,46. Leads 131-134 connect electrical signals from ATC 40 to 
circuit breakers N,E to open and close each circuit breaker. Logic circuit 
44 provides open 135 and close 136 signals on leads 131 and 132, 
respectively, to normal circuit breaker N. Similarly, logic circuit 46 
provides open 137 and close 138 signals on leads 133 and 134, 
respectively, to emergency circuit breaker E. 
Each of the close signals 136 and 138 are received by an electrical 
solenoid device having a closing coil (not shown) in each of the circuit 
breakers N and E, respectively. The closing coil rotates a latch (not 
shown) which releases energy stored in the switching mechanism, which is 
charged by charging handle 129 (see FIG. 4), in order to close the circuit 
breaker. Each of the open signals 135 and 137 are received by an 
electromechanical shunt trip or opening coil (not shown) in each of the 
circuit breakers N and E, respectively. The opening coil rotates the trip 
bar or lever which releases a toggle lever (not shown) in order to open 
the circuit breaker. 
Whenever any of the three voltages of normal power line 1 are determined by 
voltage sensing circuit 42 to have a voltage less than a first 
predetermined value, and the three voltages of emergency power line 2 are 
determined by voltage sensing circuit 42 to have a voltage greater than a 
second predetermined value, then, after a first predetermined delay, logic 
circuit 44 outputs open signal 135 and logic circuit 46, after a break 
before make delay, outputs close signal 138. In the exemplary embodiment, 
the predetermined values are generally set at 70% and 90% of rated line 
voltage, respectively, using potentiometers (not shown) in voltage sensing 
circuit 42. 
On the other hand, when the three voltages of normal power line 1 are 
determined by voltage sensing circuit 42 to have a voltage greater than 
the second predetermined value, then independent of the voltages of 
emergency power line 2 and after a second predetermined delay, logic 
circuit 46 outputs open signal 137 and logic circuit 44, after a break 
before make delay, outputs close signal 136. In the exemplary embodiment, 
the predetermined delays range from 1-60 seconds and 0.2-30 minutes, 
respectively. 
Referring again to FIGS. 1, 2 and 5b, in order to bypass normal power line 
1 to load 3, prevent connection of emergency power line 2 to load 3, and 
isolate normal circuit interrupter N from power line 1, an operator of 
system 10 would: (1) lift locking surface 253 and turn key 261 to lock 
locking mechanism K2A; (2) remove key 261 from lock K2A; (3) insert key 
261 in lock K2B; (4) turn key 261 to unlock locking mechanism K2B; (5) 
close bypass normal circuit interrupter BN; and (6) open and drawout 
normal circuit interrupter N. 
Subsequently, in order to selectively disconnect normal power line 1 from 
load 3, and to connect emergency power line 2 to load 3: the operator 
would: (7) open bypass normal circuit interrupter BN; (8) lift locking 
surface 253 and turn key 261 to lock locking mechanism K2B; (9) lift 
locking surface 253 and turn key 251 to lock locking mechanism K1A; (10) 
remove key 251 from lock K1A; (11) insert key 251 in lock K1B; (12) turn 
key 251 to unlock locking mechanism K1B; and (13) close bypass emergency 
circuit interrupter BE. 
Alternatively, in order to selectively disconnect normal power line 1 from 
load 3, and to connect emergency power line 2 to load 3: the operator 
would: (7) open bypass normal circuit interrupter BN; (8) lift locking 
surface 253 and turn key 261 to lock locking mechanism K2B; (9) lift 
locking surface 253 and turn key 251 to lock locking mechanism K1A; (10) 
remove key 261 from lock K2B; (11) insert key 261 in lock K2A; (12) turn 
key 261 to unlock locking mechanism K2A; and (13) close emergency circuit 
interrupter E. 
In order to selectively connect, independent of ATC 40, emergency power 
line 2 to load 3 and to prevent reconnection of normal power line 1 to 
load 3, an operator of system 10, which has locks K1A and K2A unlocked, 
would: (1) open normal circuit interrupter N; (2) lift locking surface 253 
and turn key 251 to lock locking mechanism K1A; (3) remove key 251 from 
lock K1A; (4) insert key 251 in lock K1B; (5) turn key 251 to unlock 
locking mechanism K1B; and (6) close bypass emergency circuit interrupter 
BE. Alternatively, the operator would: repeat (1) through (5); and (6) 
close emergency circuit interrupter E. 
Subsequently, in order to selectively disconnect emergency power line 2 
from load 3, and to reconnect normal power line 1 to load 3, the operator 
would: (7) open bypass emergency circuit interrupter BE; (8) lift locking 
surface 253 and turn key 251 to lock locking mechanism K1B; (9) lift 
locking surface 253 and turn key 261 to lock locking mechanism K2A; (10) 
remove key 261 from lock K2A; (11) insert key 261 in lock K2B; (12) turn 
key 261 to unlock locking mechanism K2B; and (13) close bypass normal 
circuit interrupter BN. Those skilled in the art will recognize that other 
methods of selectively connecting and disconnecting the circuit 
interrupters are possible. 
While specific embodiments of the invention have been described in detail, 
it will be appreciated by those skilled in the art that various 
modifications and alternatives to those details could be developed in 
light of the overall teachings of the disclosure. Accordingly, the 
particular arrangements disclosed are meant to be illustrative only and 
not limiting as to the scope of the invention which is to be given the 
full breadth of the appended claims and any and all equivalents thereof.