Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF TH~ IN~ENTION
Field o~ the Invention:
The invention relates to electrical apparatus and,
more particularly, to a system for disconnecting an electrical
power distribution circuit from a source of electrical power
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durlng e~er~ency cQndltions~
Description of the Prior Art:
Emergency electr~cal po~er interrupting "panic
buttons" are ~idely used in laboratories, shops, factories,
and commercial establishments. Such systems normally con-
sist of large easily seen red pushbutton s~itches located in
readilyaccessible places to provide for rapid interruption
of electrical power should an emergenc~ occur. In prior art
systems, the panic button operates a normally open push-
button switch which is used to energize the trip coil of a
circuit breaker protecting the power distrlbution circuit.
Thus~ separate wiring must be provided between the clrcuit
breaker and each location wherein it is desired to place a
panic button. The cost and complexity of this additional
wlring has often meant that the number of panic buttons
installed has been less than would be desirable. In other
situations, this cost and complexity has completely ruled
out the installation of such an emergency power ~nterruption
system.
In most industrial and commercial installations,
and in many re~idential installations, National Electric
Code requirements call for the inclusion of ground fault
protective capability in addition to overload current pro-
tection. Such ground fault protectlve equipment is normally
designed to deenergize the associated electrical power
~ gr~nd ~u 1~
distribution network upon detection of~current levels much
less than the overload current trip leYels. For example,
many residential ground fault circuit breakers are deslgned
to trlp upon detection of 5 milliamperes of ground fault
current.
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It is desirable to provide an e~ergency power
interruption system which does not re~uire additional wlring
other than the power wiring and which utilizes the sensitive
detection capability of the ground fault current protection
capability already existing in man~ installations.
SUMMARY OF THE INVENTION
In accordance with a preferred embodiment of the
present invention, there is provided an emergency power
interruption system for use on an electrical power line.
The system includes a ground fault circuit breaker adapted
to deenergize the circuit being protected upon detection of
ground fault current above a predetermined level. The
system further comprises means located downstream from the
ground fault circuit breaker for deliberately initiating
ground fault current in the electrical power line which is
above the ground fault tripping current level of the ground
fault circuit breaker.
In a preferred embodiment of the invention, the
electrical power circuit includes a line conductor, a neutral
conductor, and a ground conductor, and a plurality of recep-
tacles adapted to receive cooperating power plugs attached
B to electrical apparatus to be powered from the circuit. The
ground fault current initiating means comprises a plug
~ n~l
adapted for :Insertlon into ~ff~ of the receptacles, and
having a pushbutton switch connected between the pins of the
plug which come in contact with the line conductor and the
ground conductor when the plug is inserted into a receptacle.
By actuating the pushbutton switch, ground fault current is
initiated to caus~i the ground fault circuit interrupter to
trlp and lsolate the circuit from the source of electrical
30~ power.
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B~ D~SCRIPTIO~ OF TH~ D~A~ING
The invention may ~e more readily understood b~
reference to the ~ollowing specification and to the drawin~
which is a diagram, partially schematic and partially pic-
torial, of an electrical power circuit incorporating the
present invention.
DESCRIPTION OF TH~ PREFER~ED EMBODI~ENT
Referring now to the dr~wing, a three-wire electri- -
cal power circuit 12 supplied by a power source (not shown)
upstream to the left comprises a line (or phase) conductor
L, a neutral conductor N, and a ground conductor G. Con~
nected to the circuit 12 are a plurality of three-wire
receptacles 14, only one of ~hich is shown in the drawing.
Cooperating plugs from electrical apparatus are inserted
into the receptacles 14, thereby providing power to the
apparatus. As can be seen, the receptacle 14 includes a
; terminal 22 connected to the line conductor L, a terminal 24
connected to the ground conductor G, and a terminal 26
connected to the neutral conductor N.
A ground fault circuit interrupter 16 is inserted
ln the circuit 12 between the power source and the first of
the receptacles 14. The ground ~ault circuit interrupter 16
B co~ fs /~
includes ~ separable oontaot in series with the line con-
ductor L of the circuit 12. In a manner well known in the
art, current flow from line conductor L to ground is sensed,
and upon occurrence of ground fault current abo~e a predeter-
mined level, the contacts 18 are separated, thereby deenergi-
zing that portion of the circuit 12 downstream from the
circuit breaker 16.
The level of ground fault current which will cause
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the breaker 16 to trip depends upon the particular applica-
tion. For examplet a "people-protecting" ground fault
circuit breaker designed to prevent serious injury to a
person accidentally coming into simultane-ous contact with
the conductor L and ground, is designed to trip at a level
of approximately 5 milliamperes. This is, of course, much
less than the overcurrent trip level of the circuit breaker
16, a function designed merely to protect the conductors
themselves from damage due to overload current levels in the
range of several amperes or higher.
In the present invention, means are provided for
deliberately initiating a flow of current from the line con-
ductor L to ground. When such current flow reaches the
ground fault tripping current level of the circuit breaker
16~ the breaker 16 will trip, thereby deenergizing the
circuit. In a preferred embodiment of the invention a plug
20, externally identical to the pIugs of the power cords of
electrical apparatus being supplied by the circuit 12, is
modified to include a pushbutton switch 34 having contacts
36. The plug 20 a~so includes pins 2~, 30 and 32 cooperating
with corresponding terminals 22, 26, and 24 to make electri-
cal contact with the conductors L, N and G, respectively.
The contacts 36 of the switch 34 are connected in series
with a resistor 3~, and the resulting series combination
connected between the pins 2~ and 32 of the plug 20. As can
be seen, actuation of the switch 34 will thus provide an
electrical connection between the pin 2~ and the pin 32.
When the plug 20 iæ inserted into the receptacle 14, actua-
tion of the ~witch 34 allows current to flow from the con-
ductor L through the terminal 22, the pin 2~, the resistor
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32
B 38, the contacts 36, the Pin ~, and the terminal 24 to the
ground conductor ~. The size of the resistQr 38 is selected
according to the ~oltage upon the circuit 12 to p~ovide a
flow of ground current greater than the ground fault trip-
ping current level of the circuit bre~ker 16. Thus, when-
ever the pushbutton switch 34 is actuated, the circuit
breaker 16 will trip, there~y deenergizing the circuit 12
and isolating that portion of the circuit 12 downstream from
the circuit breaker 16 from contact with the electrical
power source. In a typical application on a 115 VAC circuit,
the circuit breaker 16 can be set to a ground fault tripping
current level of 5 milliamperes, thereby providing protection
against serious in~ury should a human being come in simul-
taneous contact with the line conductor L and ground. By
providing the resistor 38 with a value of 10,000 ohms, a
a~r~ n1N~eJ(1
ground current ofA12 milliamperes will be caused to flow
when the pushbutton 34 is acutated. This level is ~ar in
excess of the level necessary to trip the circuit breaker
16, yet is much below the overcurrent tripping current level
which would cause damage to the conductors of the circuit
12.
By selecting a value for the resistor 38 which
yields a current under normal voltage conditions far in
excess of the ground fault interrupter tripping current
level, sufficient margin is provlded to insure that under
emergency conditions (during which time the line voltage may
drop dramatically) the current through the resistor 38 will
still be sufficient to trip the breaker. The disclosed
invention could, of course, be used on a multiphase circuit
protected by a multiphase ground fault circuit breaker. It
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would then only be necessary to provide a flow o~ current
~rom one Or the phases to ground. r~he Gircuit breaker would
trip regardless of which phase ~as selected.
The resistor 38 i~s~ cf course, not necessary for
the operation of the invention~ Ho~ever, the presence of
the resistor limits the level of ground current which will
flow, thereby allowing the use of inexpensive low current
rating contacts 36. Other impedance means could also be
used in place of the resistor 38 to provide this current-
limiting function.
By installing the ground current initiating meansin the plug 20, a variety of advantages is obtained. The
plug can be placed in any or all of the receptacles 14 upon
the circuit 12. The cost of each plug 20 is much lower than
the cost of providing prior art emergency power interrupting
buttons requiring separate wiring to a circuit breaker trip
coil. In addition, if a temporary remote panic button is re-
quired, it can be provided in a simple, convenient manner
through the use of a standard extension cord plugged into
the receptacle 14 and inserting the plug 20 in the other end
of the extension cord. The plug 20 can then be moved to
wherever emergency power interruption capability is required.
Furthermore, a single plug 20 can be carried by an operator
to any deslred :Location having a receptacle 14.
It can be seen, there~ore, that the present invention
provides an emergency power interruption system which can be
installed at extremely lol~ cost upon existing power circuits
having ground rault circuit protection. Furthermore, the
present inventi.on provides a highly flexi~le and convenient
means Or providing emergency power interruptlon capability
at a wide variety of locations.
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