Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection
Reexamination Certificate
1999-06-17
2002-08-20
Leja, Ronald W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Ground fault protection
Reexamination Certificate
active
06437951
ABSTRACT:
TECHNICAL FIELD
The present invention relates to electrical equipment that in use is subject to fault conditions that can cause harm to users of the equipment. More particularly, it relates to the provision of an electrical ground fault protection circuit that monitors the electrical equipment and its installation and in response to the detection of a ground fault will disconnect the equipment from its power supply.
BACKGROUND OF THE INVENTION
There are electrical ground fault protection circuits available that detect and provide protection against some electrical fault conditions, such as leakage of current to ground. These circuits are termed ground fault interrupters (GFIs). These protective circuits detect leakage of current to ground by comparing the input current to the output current. This comparison, however, fails to detect all harmful conditions that may occur. For example, if a primary leg of the power source is shorted across the primary ground, standard GFIs will not detect this condition. This is because the input and the output current could remain the same. In addition, known GFI's do not detect an open ground or an elevated voltage on the primary ground or equipment housing.
What is needed is an electrical ground fault protection system that continuously tests for numerous conditions to determine whether one or more conditions exist that could cause harm to a user. The continual testing for potentially harmful conditions would provide desirable safeguards to the user. In addition, the system should alert a user to some harmful condition or conditions before operation is commenced. This would provide an additional safeguard to the user. Herein, the term “user” refers to and includes any and all persons in the vicinity of the equipment and/or potential ground fault condition.
The present invention is directed to the provision of an electrical fault protection system that tests for several conditions to determine whether any individual condition or simultaneous conditions exist that would provide a harmful condition or conditions to the user of the equipment.
An object of the present invention is to detect harmful conditions including 1) current leakage of one of the primary legs; 2) current through the primary ground; 3) voltage leak from a primary leg to primary ground or case ground; 4) open primary ground; 6) lack of ground to work area continuity; and 7) elevated voltage on the work area. By continually testing for these potentially harmful conditions, this invention provides desirable safeguards to the user. In addition, this system alerts a user to some harmful conditions before operation is commenced. This provides an additional safeguard to the user.
BRIEF SUMMARY OF THE INVENTION
The electrical ground fault protection circuit of the present invention is basically characterized by power and ground LINE connections that are connected to power and ground lines of an electrical distribution system and power and ground LOAD connections that are connectable to a load. Power and ground paths extend from the power and ground LINE connections to the power and ground LOAD connections and include an interrupter having a connect position in which it allows current flow from the LINE connections to the LOAD connections and a disconnect position in which it interrupts such current flow.
According to an aspect of the invention, a ground line monitor is provided for detecting the presence or absence of a fault condition in the ground line. In response to the presence of a such a condition, the circuit switches the interrupter from its connect position to its disconnect position. Also, the circuit includes a power path monitor for detecting a fault condition in the power path. In response to the presence of such a fault condition, the circuit switches the interrupter from its connect to its disconnect position.
According to a further aspect of the invention, the circuit includes a plural power paths and a ground path extending from the power and ground LINE connections to the power and ground LOAD connections. A voltage sensor is interconnected between each power path and the ground path. Each voltage sensor detects a voltage drop in the power path. In response to the presence of a voltage drop of a predetermined amount, the circuit switches the interrupter from its connect position to its disconnect position.
In some embodiments, the circuit is connectable to an electrical distribution system that includes three primary legs and a primary ground. The circuit includes three power paths, one for each primary leg, each connected to a separate one of the primary legs, and a ground path connected to the primary ground.
In a preferred embodiment, the circuit includes a transformer connected to receive power from the power paths and to supply power to the voltage sensors.
According to another aspect of the invention, the circuit includes a ground continuity monitor for detecting the ground continuity of the circuit. The circuit may also include an elevated voltage monitor for detecting an elevated voltage at the load that is above a predetermined voltage. In response to the presence of such an elevated voltage, the circuit will switch the interrupter from its connect position to its disconnect position.
A further object of the invention is to provide an electric welding installation that includes an electrical fault protection circuit of the type described.
Other objects, advantages and features of the invention will become apparent from the description of the best mode set forth below, from the drawings, from the claims and from the principles that are embodied in the specific structures that are illustrated and described.
REFERENCES:
patent: 3886409 (1975-05-01), Scarpino
patent: 4011483 (1977-03-01), Meadows
patent: 4441135 (1984-04-01), Drain
patent: 4507699 (1985-03-01), Rohm
patent: 4835651 (1989-05-01), Li et al.
Ahlstrom Michael R.
Robbins Dennis R.
Barnard Delbert J.
Leja Ronald W.
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