Electricity: electrical systems and devices – Safety and protection of systems and devices – Ground fault protection
Patent
1986-10-03
1988-01-26
Leung, Philip H.
Electricity: electrical systems and devices
Safety and protection of systems and devices
Ground fault protection
361 42, 361100, 361180, 361181, 318345D, H02H 317
Patent
active
047220210
ABSTRACT:
To detect if a tool bit, such as a drill, a saw blade, or the like, upon being worked into a wall, meets a metallic obstruction, such as a conduit, reinforcement rod, hydraulic pipe or the like, an alternating voltage, preferably between 2 and 20 kHz, is coupled through a coupling capacitor (9) to the tool bit, and current flow to the tool bit is detected by a current measurement stage (10; 31, 32) to disconnect energy to the drive motor (5), for example by disabling firing of a thyristor (15), or closing a valve of a compressed-air tool. The coupling capacitor should have a capacity small enough to provide high impedance to network power frequency, and to insure a substantial change in current being supplied to the tool bit holder, typically a chuck, if the tool bit meets a metallic object in the wall. The a-c supply circuit can be formed as part of a trigger circuit for the thyristor, by pulse-energizing a trigger diode (28) upon repetitive charge of a capacitor (26) through a charging network (22, 23, 24, 25).
REFERENCES:
patent: 3171062 (1965-02-01), Rowe, Jr.
patent: 3786328 (1974-01-01), Bos
patent: 4040117 (1977-08-01), Houser
patent: 4089031 (1978-05-01), Stevens
patent: 4224652 (1980-09-01), Fiorentzis
patent: 4528557 (1985-07-01), Braun
Gerschner Martin
Gunther Klaus
Hornung Friedrich
Schadlich Fritz
Leung Philip H.
Robert & Bosch GmbH
Williams Howard L.
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