Active ground fault disconnect

Details Active ground fault disconnect Active ground fault disconnect

1999-06-17

2001-08-21

Sherry, Michael J. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Ground fault protection

C361S058000, C361S093700, C361S101000, C361S106000

Reexamination Certificate

active

06278596

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to circuitry for providing protection against ground fault currents in electronic instruments that might damage the circuit-under-test or be harmful to personnel, and more particularly such circuitry providing multiple levels of protection.
BACKGROUND OF THE INVENTION
Existing designs for operator and circuit-under-test protection in the presence of potentially dangerous or damaging ground fault currents has been accomplished using positive temperature coefficient (PTC) circuit elements and mechanical relays and switches. While such protection can prevent fire and explosions by limiting extreme currents, they may not respond sufficiently to marginal or transient amounts of energy to fully protect the operator's safety and the circuit-under-test under all circumstances.
Referring to
FIG. 1A
, while “floating” (ungrounded) operation is relatively simple and inexpensive, under fault conditions a very high current flow can arise from a relatively small voltage, because of the extremely low impedance from one probe common (ground) to another. Once the instrument is “floating”, the error of inadvertently connecting one of the commons to a high voltage can make all other exposed commons unexpectedly “hot”, potentially dangerous to either personnel or circuitry-under-test.
Even if the user does not receive a shock by coming into contact with the unexpectedly “hot” lead or exposed floating regions of the instrument, connecting a second common to earth ground can cause an explosive and terrifying flash and arcing. The user may be burned or incur secondary injuries as a reaction to being startled by the sudden and explosive flow of large currents where they were not expected.
Introducing an unexpectedly high voltage into parts of the user's circuit which are normally near zero can also have damaging effects on the circuit under test. Even if such contact is inadvertent, the effects can be catastrophic to unprotected elements of the circuit under test. Even dropping or dragging such a floating “hot” common line so that it comes into contact with other circuit elements, can have unexpected consequences that cause permanent damage to the user's circuit. Non-floating, earth grounded operation, can still lead to unfortunate surprises, affecting both the operator's safety and the integrity of the elements of the circuit under test.
Referring next to
FIG. 1B
, the common leads are protected against anything more than transient and limited voltages by the incorporation of a PTC resistor in series with the ground path. Now unexpected current surges raise the resistance of the PTC resistor, and dramatically limit the maximum current that can flow, at least for any significant length of time. This circuit is still rather simple, yet it prevents the occurrence of the arcs and explosions of the previously discussed prior art circuit. Providing this circuitry with a second level of protection requires that other steps be taken to limit the exposure of all instrument commons to sudden and unexpected currents. Probes must be specially insulated. To achieve true double isolation, all of the other input/output functions and exposed conductors must also be protected against unexpectedly high voltages.
U.S. Pat. No. 4,583,146 to Howell for “Fault Current Interrupter”, hereby incorporated by reference, discloses a fault current interrupter achieved by the parallel combination of a PTC resistor and a voltage dependent resistor connected across a pair of mechanical contacts to permit the interruption of a flowing current without arcing during the separation of the contacts. The PTC resistor is selected to have a relatively low resistance at room temperature and a substantially higher resistance at higher temperatures. This allows the current to transfer away from the contacts through the PTC resistor until the voltage across the voltage dependent resistor causes the voltage dependent resistor to become conductive and thereby transfer the current away from the PTC resistor.
U.S. Pat. No. 4,967,176 to Horsma et al. for “Assemblies of PTC Circuit Protection Devices”, hereby incorporated by reference, discloses a device assembly in which a plurality of PTC circuit protection devices are connected in series. Assemblies of this type are useful in providing protection where a lone device would not suffice. In a preferred system, the device assembly is connected in series with a circuit breaker. U.S. Pat. No. 5,644,461 to Miller et al. for “High Voltage D-C Current Limiter”, is hereby incorporated by reference. This patent describes a current limiter for protecting a circuit at a predetermined driving voltage in excess of 50 volts. A plurality of PTC resistors are connected in series, with a zener diode connected in parallel with each of the PTC resistors. All of the PTC resistors, taken together, and all of the zener diodes, taken together, have a voltage range larger than the predetermined driving voltage.
U.S. Pat. No. 5,864,458 to Duffy et al. for “Overcurrent Protection Circuits Comprising Combinations of PTC Devices and Switches”, hereby incorporated by reference, discloses electrical circuit protection arrangements with PTC devices to switch voltages and currents in normal circuit operations, wherein the voltage and/or current ratings of the mechanical switches and PTC devices are much less than the normal operating voltages and currents of the circuits. This feature permits the use of smaller and less expensive mechanical switches and PTC device than would otherwise be required in such circuits. The arrangements of switches and PTC devices also permit the PTC devices to limit the magnitude of the fault current passed to the circuit.
U.S. Pat. No. 4,068,281 to Harnden for “Thermally Responsive Metal Oxide Varister Transient Suppression Circuit”, hereby incorporated by reference, describes a temperature responsive resistor that is thermally connected to a body of metal oxide varistor material in transient suppression and other applications. In the event of excess energy dissipation in the varistor, the temperature of the thermally responsive resistor increases to provide a trigger signal to a triac or other similar electronic switch which may be utilized to provide an alarm, provide increased cooling of the varistor body, to divert or limit current flow from the varistors. Current flow through the temperature responsive resistor may be provided from a third terminal on the metal oxide varistor in which case the circuit is responsive to both total energy dissipation and peak energy dissipation levels in the varistor.
U.S. Pat. No. 5,379,176 to Bacon et al. for “Protective Input Circuit for an Instrument”, is hereby incorporated by reference. This patent describes a protective circuit for the input of a multimeter is provided with series connected thermistors partially shunted by a varistor whereby overload voltage is distributed between the thermistors enabling protection at higher voltage levels. Shunt connected varistors protect primarily against transient overloads and are thermally coupled to the aforementioned thermistors to bring the thermistors more rapidly to their high resistance condition such that damage to the varistors is avoided and continued protection is provided.
U.S. Pat. No. 5,250,893 to Gambill et al. for “Digital Voltmeter”, hereby incorporated by reference, describes a voltage measuring instrument which generally includes signal conditioning circuitry coupled to input probes for protection against overvoltages and processing of input voltage signals. A VAC/VDC control circuit determines whether AC or DC voltages are present, and it will positively indicate when no voltage is present in a test circuit. The metering circuit of the invention will thereby automatically determine the proper mode of operation for the type of voltage present in a test circuit so as to eliminate user selection of operating parameters and possible errors. Further, the metering circuit is adapted to generate a proper reference voltage for the measure

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