Shorting switch and system to eliminate arcing faults in low voltage power distribution equipment

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to commonly assigned, concurrently filed:

U.S. patent application Ser. No. 10/172,208, filed Jun. 14, 2002, entitled “Shorting Switch And System To Eliminate Arcing Faults In Power Distribution Equipment”;

U.S. patent application Ser. No. 10/172,651, filed Jun. 14, 2002, entitled “Shorting Switch And System To Eliminate Arcing Faults In Power Distribution Equipment”;

U.S. patent application Ser. No. 10/172,238, filed Jun. 14, 2002, entitled “Shorting Switch And System To Eliminate Arcing Faults In Power Distribution Equipment”;

U.S. patent application Ser. No. 10/172,622, filed Jun. 14, 2002, entitled “Bullet Assembly For A Vacuum Arc Interrupter”;

U.S. patent application Ser. No. 10/172,080, filed Jun. 14, 2002, entitled “Vacuum Arc Interrupter Having A Tapered Conducting Bullet Assembly”;

U.S. patent application Ser. No. 10/172,209, filed Jun. 14, 2002, entitled “Vacuum Arc Interrupter Actuated By A Gas Generated Driving Force”;

U.S. patent application Ser. No. 10/172,628, filed Jun. 14, 2002, entitled “Blade Tip For Puncturing Cupro-Nickel Seal Cup”; and

U.S. patent application Ser. No. 10/172,281, filed Jun. 14, 2002, entitled “Vacuum Arc Eliminator Having A Bullet Assembly Actuated By A Gas Generating Device”.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is directed to shorting switches and, in particular, to shorting switches for eliminating arcing faults in low voltage power distribution equipment. The invention is also directed to shorting systems for eliminating arcing faults in low voltage power distribution equipment.

2. Background Information

There is the potential for an arcing fault to occur across the power bus of a motor control center (MCC), another low voltage (LV) enclosure (e.g., an LV circuit breaker panel) and other industrial enclosures containing LV power distribution components. This is especially true when maintenance is performed on or about live power circuits. Frequently, a worker inadvertently shorts out the power bus, thereby creating an arcing fault inside the enclosure. The resulting arc blast creates an extreme hazard and could cause injury or even death. This problem is exacerbated by the fact that the enclosure doors are typically open for maintenance.

It is known to employ a high-speed shorting switch, placed between the power bus and ground, or from phase-to-phase, in order to limit or prevent equipment damage and personnel injury due to arc blasts. It is also known to employ various types of crowbar switches for this purpose. The switches short the line voltage on the power bus, eliminating the arc and preventing damage. The resulting short on the power bus causes an upstream circuit breaker to clear the fault.

Examples of medium voltage devices include a stored energy mechanism with vacuum interrupter contacts, and a mechanism to crush a conductor magnetically.

An example of a low voltage device is a stored energy air bag actuator, which drives a conductive member having a pin and a flange, in order to short two contacts. The first contact is in the form of a receptor for capturing the pin of the driven conductive member. The second contact has an opening, which allows the pin to pass therethrough, but which captures the flange of the driven member.

There is room for improvement in shorting switches and systems that respond to arcing faults and switch fast enough in order to protect workers and equipment from arc blasts associated with LV power distribution equipment.

SUMMARY OF THE INVENTION

These needs and others are met by the present invention, which provides a shorting switch for low voltage applications, which switches fast enough to protect personnel and equipment, while maintaining suitable electrical contact in order to eliminate subsequent arcing. The shorting switch employs a simple and low cost spring energy storage and fractured release member mechanism. The fractured release member actuates a spring-loaded contact, which then electrically engages two or more contacts. The spring has a suitably high force in order to maintain suitable contact force to prevent contact blow-apart and subsequent arcing.

As one aspect of the invention, a shorting switch for eliminating arcing faults in low voltage power distribution equipment comprises: a base supporting a first side and an opposite second side; a spring having a first end engaging the first side of the base, the spring also having a second end; a release member having an opening therein, the release member having a first end engaging the first side of the base, the release member also having a second end; a charge disposed in the opening of the release member, the charge for fracturing the release member; a bridging contact biased by the second end of the spring toward the second side of the base, the second end of the release member normally maintaining the spring in a compressed state; two contacts supported by the second side of the base for electrical engagement by the bridging contact after fracture of the release member; and two terminals respectively electrically connected to the two contacts.

As another aspect of the invention, a shorting switch for eliminating arcing faults in low voltage power distribution equipment comprises: a base supporting a first side and an opposite second side; a spring having a first end engaging the first side of the base, the spring also having a second end; a release member having an opening therein, the release member having a first end engaging the first side of the base, the release member also having a second end; a charge disposed in the opening of the release member, the charge for fracturing the release member; a bridging contact biased by the second end of the spring toward the second side of the base, the second end of the release member engaging the bridging contact to normally maintain the spring in a compressed state; at least three contacts supported by the second side of the base for electrical engagement by the bridging contact after fracture of the release member; and at least three terminals respectively electrically connected to the at least three contacts.

As another aspect of the invention, a shorting system for eliminating an arcing fault in low voltage power distribution equipment comprises: a base supporting a first side and an opposite second side; a spring having a first end engaging the first side of the base, the spring also having a second end; a release member having an opening therein, the release member having a first end engaging the first side of the base, the release member also having a second end; a charge disposed in the opening of the release member, the charge for fracturing the release member; a bridging contact biased by the second end of the spring toward the second side of the base, the second end of the release member normally maintaining the spring in a compressed state; two contacts supported by the second side of the base for electrical engagement by the bridging contact after fracture of the release member; two terminals respectively electrically connected to the two contacts; and means for detecting an arcing fault and responsively activating the charge disposed in the opening of the release member, in order that the activated charge fractures the release member, which releases the spring, which drives the bridging contact to short across the two contacts to eliminate the arcing fault.

BRIEF DESCRIPTION OF THE DRAWINGS

A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:

FIG. 1

is a plan view of a high-speed low voltage (LV) shorting switch including a spring mechanism, bridging contact, release bolt, contacts and terminals in accordance with the present invention.

FIG. 2

is a side view of the high-speed LV shorting switch of FIG.

1

.

FIG. 3

is a cross-sectional view along lines III—III of FIG.

2

.

FIG. 4

is a plan view of the release bolt of

FIG. 3

, as fractured after the charge is activated.

FIG. 5

is a plot of breaking torque versus breakline depth for the release bolt of FIG.

3

.

FIG. 6

is a plan view of a bridging contact engaging four contacts of a three-phase plus ground LV shorting switch in accordance with another embodiment of the invention.

FIG. 7

is a block diagram of a shorting system including the shorting switch of FIG.

1

.

FIG. 8

is a schematic diagram of a sensor suitable for use with the shorting switch of FIG.

1

.

FIG. 9A

is a schematic diagram of another sensor suitable for use with the shorting switch of FIG.

1

.

FIG. 9B

is a schematic diagram of a modified form of the sensor of FIG.

9

A.

FIG. 10

is a schematic diagram of sensor electronics suitable for use with the shorting switch of FIG.

1

.

FIG. 11

is a diagram illustrating application of the invention to arc protection in switchgear.

FIG. 12

is a plot of voltage and current waveforms of a test at 500 V/38 kA showing a switching time of about 972 &mgr;s.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to

FIGS. 1 and 2

, a high-speed low voltage (LV) shorting switch

2

is illustrated. The exemplary shorting switch

2

advantageously eliminates arcing faults in LV power distribution equipment (not shown). The shorting switch

2

includes a base

4

, which supports a first side

6

and an opposite second side

8

. A spring mechanism, such as compression spring

10

, has a first end

12

engaging the first side

6

of the base

4

, and a second end

14

.

As shown in

FIG. 3

, a release member, such as release bolt

16

, has a first end

18

engaging the first side

6

of the base

4

and a second end

20

. A charge

22

is disposed in an opening

24

of the release bolt

16

. As discussed below in connection with

FIGS. 3 and 4

, activation of the charge

22

results in the fracture of the release bolt

16

. A bridging contact

26

is biased by the second end

14

of the spring

10

toward the second side

8

of the base

4

. Prior to activation of the charge

22

, the second end

20

of the release bolt

16

holds the bridging contact

26

and normally maintains the spring

10

in a compressed state.

As best shown in

FIGS. 1 and 3

, two contacts

28

,

30

are supported by the second side

8

of the base

4

for electrical engagement (as shown in

FIG. 1

) by the bridging contact

26

after fracture of the release bolt

16

. Otherwise, prior to fracture of the release bolt

16

, a suitable gap

31

is disposed between the bridging contact

26

and contacts

28

,

30

when the spring

10

is compressed. Two terminals

32

,

34

(e.g., without limitation, power bus connections) are respectively electrically connected to the two contacts

28

,

30

. As shown in

FIG. 3

, the two terminals

32

,

34

are adapted for electrical connection (e.g., by suitable electrical terminations

35

) to two power lines

36

,

38

, respectively. For example, the power lines

36

and

38

may respectively include a LV power line and a ground or neutral. After the charge

22

is activated and the release bolt

16

is fractured, the spring-loaded bridging contact

26

, which was maintained by the release bolt

16

, is released at a suitable high-speed, in order to short across the two contacts

28

,

30

. The spring

10

forces the bridging contact

26

to electrically engage the contacts

28

,

30

, thereby shorting the power lines

36

and

38

, in order to effectively quench an arc. In turn, the short circuit is suitably cleared by an upstream circuit breaker (not shown) in the power circuit.

As best shown in

FIG. 3

, the exemplary base

4

is a steel block, although any suitable base structure (e.g., without limitation, an insulated box or tube (not shown), which encloses a spring mechanism, a bridging contact, a release bolt, and three corresponding terminals, each of which is attached to a corresponding bus in a three-phase system) and/or any suitable base material (e.g., conductive; non-conductive; magnetic; non-magnetic) may be employed. In the exemplary embodiment, a suitable insulator

40

is suitably secured by fasteners, such as screws

42

,

44

, through respective openings

46

,

48

in one end

50

of the base

4

to form the second side

8

thereof.

Also, as shown in

FIG. 3

, the contact

28

and the terminal

32

are formed by a single conductive member

52

, which is suitably secured by fasteners, such as screws

54

,

56

, through respective recessed openings

58

,

60

in the insulator

40

. Similarly, the contact

30

and the terminal

34

are formed by a single conductive member

62

, which is suitably secured by fasteners, such as screws

64

,

66

, through respective recessed openings

68

,

70

in the insulator

40

. It will be appreciated that the insulator

40

is not required, and that an insulating base structure may be employed. As another alternative, only one of the conductive members

52

,

62

may be insulated from the base

4

.

In the exemplary embodiment, a suitable member

72

is suitably secured by fasteners, such as screws

74

,

76

, through respective openings

78

,

80

in the other end

82

of the base

4

to form the first side

6

thereof. Although a steel member

72

is shown, any suitable material (e.g., conductive; non-conductive; magnetic; non-magnetic) may be employed. The body

86

of the release bolt

16

passes through an opening

88

in the member

72

. The head

18

of the release bolt

16

engages the member

72

and is received within an opening

90

of the end

82

of the base

4

.

As best shown in

FIG. 3

, the bridging contact

26

has a threaded opening

92

which engagingly receives the threads

94

on the end

20

of the release bolt

16

, thereby holding the bridging contact

26

at that end

20

. In turn, the bridging contact

26

captures the spring

10

between the surface

96

of the member

72

at the spring end

12

, and the receptive portion

98

of the bridging contact

26

at the opposite spring end

14

.

The opening

24

of the release bolt

16

is a longitudinal cavity along the longitudinal axis thereof. The exemplary charge

22

is a small electrically activated, chemical charge, such as model number RP-501 made by Reynolds Industries Systems, Inc. (RISI). The RP-501 is a standard, end lighting, exploding bridge wire (EBW) detonator for use in general purpose applications (e.g., it is capable of detonating compressed TNT and COMP C-4). Although an exemplary detonator charge is employed, any suitable charge may be employed to fracture any suitable release member.

The charge

22

includes an electrical input, such as a pair of conductors

100

, which pass through the opening

24

of the release bolt

16

and through an opening

102

of the base end

82

. The charge

22

is suitably activated by an electrical signal on the conductors

100

to provide a shock wave to fracture the release bolt

16

. Preferably, the release bolt body

86

has a breakline

104

disposed thereon with a predetermined depth in such body

86

.

In the exemplary embodiment, the bolt body

86

has a 0.5-inch diameter and the bolt cavity

24

has a 0.295-inch diameter. The exemplary bolt

16

is 4.5 inches in length, with the cavity

24

being 2.0 inches deep from the bolt head

18

, and the breakline

104

being 1.9 inches deep from the bolt head

18

. The exemplary breakline

104

is employed to locate and control the fracture zone when the shock wave, created from the charge

22

, fractures the metal release bolt

16

.

The release bolt

16

is normally employed to compress the spring

10

. After activation of the charge

22

inside the release bolt

16

, the bolt fractures at or about the breakline

104

(as shown in FIG.

4

), thereby releasing the bridging contact

26

and, thus, the spring

10

. The spring

10

has a predetermined compression force. The release bolt

16

is structured to maintain the compressed state of the spring

10

until after the charge

22

is activated. In turn, the bridging contact

26

, on the spring end

14

, is moved thereby to close and hold closed the contacts

28

,

30

. In the exemplary embodiment, the bridging contact

26

is held closed with a force of about 512 pounds across the contacts

28

,

30

. This holding force prevents the contacts

28

,

30

from re-opening and vaporizing, while maintaining a suitably low contact resistance.

FIG. 5

is a plot of breaking torque versus breakline depth for the release bolt

16

of FIG.

3

. The vertical line

106

represents the minimum torque on the release bolt

16

suitable to fully compress the spring

10

. The plot shows the maximum depth of the breakline

104

while still maintaining a spring force of 1200 pounds plus a suitable safety factor. The exemplary release bolt

16

is “grade

5

” and can safely withstand a tensile stress of about 120,000 PSI without fracturing. An optimum breakline depth of about 0.025 inch or 0.03 inch at position

108

is preferably employed to reliably fracture the exemplary bolt

16

with the exemplary charge

22

and still allow the spring

10

to be compressed solid and held with a suitable safety margin.

Although the exemplary shorting switch

2

electrically connects two terminals

36

,

38

by the bridging contact

26

, the invention is applicable to other applications, such as three, four or more terminals being electrically connected by a bridging contact. Such applications may include, for example, shorting switches for three phases to ground, three phases to neutral, three phases shorted together, two power legs to ground, and two power legs to neutral. For example,

FIG. 6

shows a bridging contact

110

(shown in phantom line drawing) engaging four contacts

112

,

114

,

116

,

118

of a shorting switch

120

. The shorting switch

120

is similar to the shorting switch

2

of

FIG. 1

, except that it has four contacts and four corresponding terminals

122

,

124

,

126

,

128

for three phases (A,B,C) and ground (G). Preferably, the contacts

112

,

114

,

116

,

118

have a gap therebetween and a solid insulator

130

is disposed within the gap. In a similar fashion, a suitable solid insulator (e.g., thermal set polyester; a thermal plastic, such as Delrin or Nylon) (not shown) may be disposed within the rectangular gap

132

between the contacts of

28

,

30

of

FIGS. 1 and 3

. Such insulation increases the over air creepage distance and, thus, permits the bus contacts

28

,

30

to be positioned closer to each other.

FIG. 7

shows a shorting system

140

including one or more shorting switches

2

(only one switch (SW)

2

is shown in

FIG. 7

) of FIG.

1

. The shorting system

140

eliminates an arcing fault

142

in low voltage power distribution equipment

144

. The shorting system

140

also includes a detection and activation circuit

146

for detecting the arcing fault

142

and responsively activating the shorting switch charge (C)

22

, in order that the activated charge

22

results in the elimination of the arcing fault as discussed above in connection with

FIGS. 1-3

. The circuit

146

includes a detection (OD) circuit

148

for detecting the arcing fault

142

and responsively outputting one or more trigger signals

150

, and an activation circuit (ACT)

152

for detecting the one or more trigger signals

150

and responsively outputting the activation signal

154

to the electrical inputs

155

of the charges

22

.

The detection circuit

148

utilizes photovoltaic cells in a sensor unit. One form of the sensor unit

201

is illustrated in FIG.

8

. The sensor unit

201

includes the first photovoltaic device

203

including at least one, or a plurality of series connected photovoltaic cells

205

, and a first filter

207

which filters light incident upon the photovoltaic cells

205

. This first filter

207

has a passband centered on the characteristic wavelength, e.g., 521.820 nm, of the arcing material.

The sensor

201

includes a second photovoltaic device

209

, which also includes one or more series connected photovoltaic cells

211

, and a second filter

213

which filters light incident upon the photovoltaic cells

211

and has a passband that does not include the characteristic wavelength of the arcing material, e.g., centered on about 600 nm in the exemplary system.

The first photovoltaic device

203

generates a sensed light electrical signal in response to the filtered incident light, and similarly, the second photovoltaic device

209

generates a background light electrical signal with an amplitude dependent upon the irradiance of light in the passband of the second filter

213

. An electric circuit

215

, having a first branch

215

1

connecting the first photovoltaic cells

203

in series and a second branch

215

2

similarly connecting the second photovoltaic cells

211

in series, connects these two electrical signals in opposition to a light-emitting device such as a light-emitting diode (LED)

217

. When arcing is present, the sensed light electrical signal generated by the first photovoltaic device

203

exceeds the background light electrical signal generated by the second photovoltaic device

209

by a threshold amount sufficient to turn on the LED

217

. While in the absence of arcing, the first photovoltaic device

203

will generate a sensed light electrical signal due to some irradiance in the passband of the first filter

207

, it will be insufficient to overcome the reverse bias effect of the background light signal generated by the second photovoltaic device

209

on the LED

217

. In fact, where the background light is fluorescent, from an incandescent bulb or a flashlight all of which have lower irradiance in the passband of the first filter

207

, but higher irradiance in the passband of the second filter

213

, the background light electrical signal will significantly exceed the sensed light electrical signal and strongly reverse bias the LED

217

. The filters

207

and

213

can be interference filters, although lower cost bandpass filters could also be utilized.

An alternate embodiment of the sensor unit

201

′ shown in

FIG. 9A

adds a bias generator

219

in the form of one or more additional photovoltaic cells

221

connected in series with the first photovoltaic device

203

in the first branch

215

1

of the electrical circuit

215

. This puts a forward bias on the LED

217

so that fewer or smaller filtered photovoltaic cells

205

and

211

can be used. This also reduces the size and therefore the cost of the filters

207

and

213

. As the additional photovoltaic cells

221

are not provided with filters, the total cost of the sensor is reduced. The embodiment of

FIG. 9A

can be modified as shown in

FIG. 9B

to place the bias generating cells

221

of the sensor

201

″ in series with both filtered photovoltaic cells

205

and

211

, but still provide the same effect of forward biasing the LED

217

.

Through their utilization of photovoltaic.cells

205

,

211

and

221

, the sensors

201

and

201

′ of FIGS.

8

and

9

A-

9

B are self-energized.

FIG. 10

illustrates an example of an arcing fault detector

222

. The sensor unit

201

(or

201

′) is connected to a response device

223

, which includes a photoelectric circuit

225

. This photoelectric circuit includes a photo diode

227

, which is activated by the light signal generated by the sensor

201

. The light signal is transmitted from the sensor

201

to the photo detector

227

by an optic fiber

229

. This permits the photoelectric circuit

225

to be remotely located from the component being monitored where the arcing fault detector is used, for instance, in switchgear. This removes the photoelectric circuit

225

from the vicinity of voltages that could otherwise produce electromagnetic interference in the electronics. Thus, the optic fiber

229

provides electrical isolation for the photoelectric circuit

225

. As the light signal generated by the sensor

201

is essentially a digital signal, that is it is on when an arcing fault is detected and off in the absence of arcing, a low-cost optic fiber is suitable for performing the dual functions of transmitting this digital optical signal and providing electrical isolation for the photo-electric circuit

225

.

The photodetector

227

is energized by a suitable DC supply voltage such as +V

cc

. The light signal generated by the LED

217

in the presence of arcing turns on the photo detector

227

, which causes current to flow through the resistor

231

. The voltage across this resistor

231

generated by the current is amplified by the op amp

233

sufficiently to turn on a transistor

235

. The transistor

235

provides the trigger signal to a one-shot multi-vibrator

237

. Normally, the transistor

235

is off so that a pull-up resistor

239

applies +V

s

to the trigger input of the one-shot multi-vibrator

237

. When the sensor provides a light signal through the optic fiber

229

to turn on the photodetector

227

, the transistor

235

is turned on pulling the trigger input of the one-shot multi-vibrator

237

essentially down to ground. This causes the output Q of the multi-vibrator V

out

to go high. An RC circuit

241

formed by the capacitor

243

and resistor

245

resets the one-shot multi-vibrator

237

to go low again so that V

out

is a pulse signal. The arcing fault signal represented by V

out

can be used to set an alarm, and/or trip a circuit breaker, or otherwise trigger the charge

22

of the shorting switch

2

or initiate a notification action. The time constant of the RC circuit

241

is selected to produce a pulse of sufficient duration to actuate the desired output device.

The output Q of the multi-vibrator

237

provides a trigger pulse V

out

of suitable amplitude (e.g., about 9 V) and duration (e.g., about 1 to 10 &mgr;s; about 5 &mgr;s) and is electrically connected to a pulse amplifier

246

. The output of the pulse amplifier

246

, which provides a suitable amplitude (e.g., about 180 V), is electrically connected by a suitable coaxial cable (e.g., RG-58)

247

to a high power pulser

248

. The exemplary pulser

248

is a Model 619 made by Cordin Company of Salt Lake City, Utah. The output of the pulser

248

, which provides a suitable amplitude (e.g., about 4000 V), is electrically connected by a suitable coaxial cable (e.g., RG-8)

249

to the charge

22

of the shorting switch

2

of FIG.

1

.

FIG. 11

illustrates schematically an application of the optical arcing fault detector

222

to distribution systems switchgear. The switchgear

250

includes a metal switchgear cabinet

251

. Typically, the cabinet

251

is divided into a forward-compartment

252

, a middle compartment

253

, and a rear compartment

255

. The forward compartment

252

is divided vertically into cells

257

in which are housed electrical switching apparatus such as circuit breakers (CBs)

259

. The middle compartment

253

houses rigid buses including a horizontal three-phase bus

261

which is connected to a set of vertical buses (only one visible)

263

. The vertical buses are connected to the circuit breakers

259

through upper quick disconnects

265

. Lower quick disconnects

267

connect the circuit breakers through runbacks

269

to cables

271

extending from the rear compartment

255

.

The optical arcing fault detector

222

can be used to protect the switchgear

250

from arcing faults, which can occur between any of the conductors

261

-

271

or between such conductors and the metal cabinet

251

. Thus, sensors

201

can be inserted into the cells

257

, the middle compartment

253

and the rear compartment

255

where they can monitor for arcing faults. Each of the sensors

201

is connected by an optic fiber

229

to the photoelectric circuit

225

that can be contained in the top-most cell

257

of the forward compartment

252

or any other convenient location. Upon detection of an arcing fault, the arc signal generated by the photoelectric circuit

225

can be applied as a trigger signal through a trip lead

273

to each of the high-speed shorting switches

2

.

FIG. 12

shows a plot of voltage and current waveforms at 500 V/38 ka

RMS

for the shorting system

140

of

FIG. 7

showing a mechanical switching time (MST) of about 972 &mgr;s after the charge

22

is activated. The shorting system

140

can hold the bridging contact

26

and the contacts

28

,

30

closed at the rated current and voltage. The spring-loaded switch

2

successfully switches in 972 &mgr;s on a 500V/38 kA

RMS

circuit. This switch

2

and the terminals

32

,

34

are adapted for electrical connection to a low voltage (e.g., 690 VAC) power system wherein the threat of an arcing fault might occur. The contacts

28

,

30

and the bridging contact

26

are adapted for operation with a 500 VAC/38 kA

RMS

power source for the power distribution equipment. The actual closing times depend on the amount of mass being moved and on the spring force being applied. For example, a relatively large moving bridge contact

26

may be employed with a 0.5-inch gap

132

being maintained between the contacts

28

,

30

.

Table 1, below, shows a summary of timing results for the exemplary spring-loaded low voltage shorting switch

2

and shorting system

140

of FIG.

7

. The arcing fault detector

222

of

FIG. 10

has a detection and activation time (DAT) of about 550 &mgr;s, and the bridging contact

26

has a switching time of about 972 &mgr;s, although as discussed above, the DAT may vary from about 300 &mgr;s to about 2 &mgr;s.

TABLE 1

Detection and Activation

Mechanical

Total System

Time

Switch Time

Operating Time

(&mgr;s)

(&mgr;s)

(ms)

550

972

˜1.52

Various enhancements can be made, for example, to increase the fault current rating. For example, a spring with a relatively larger spring force may be employed in order to suitably provide a better shorting contact. To reduce switching time, corresponding weight reductions of the moving mass (e.g., mass of the bridging contact

26

; spring

10

; diameter of the release bolt

16

; length of the bolt left in the bridging contact

26

) may be employed.

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 invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.