Ground fault detector for gas discharge tubing

Details Ground fault detector for gas discharge tubing Ground fault detector for gas discharge tubing

1999-09-27

2001-07-31

Sherry, Michael J. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Ground fault protection

C361S093700, C361S050000, C361S086000

Reexamination Certificate

active

06268988

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the field of ground fault detectors, and particularly ground fault detectors used in high voltage circuits.
BACKGROUND TO THE INVENTION
Ground fault detectors are used in high voltage circuits such as in ballasts which drive gas discharge tubes such as neon display tubes. A ground fault detector is used to safeguard current carrying circuits, detecting leakage current to ground and shutting off the power supply of the ballast in the event the leakage current exceeds a predetermined value. Excessive leakage current can cause arcing, which can cause fire or can be lethal, and therefore maximum leakage is regulated by standards.
One of the conventional ways of controlling the shut-off of the high voltage is to monitor the alternating current which is returned to ground carried by the center tap of the high voltage secondary winding of a high voltage transformer. In the event the high voltage leads of the secondary winding are conducting to ground, causing current to flow from the center tap to ground, a current transformer reflects this back to a shutdown circuit. Examples of circuits which use this principle are described in U.S. Pat. No. 4,663,571 to Nilssen and U.S. Pat. Nos. 4,613,934 and 5,089,572 to Pacholok.
Another conventional way of controlling shut-down of a high voltage circuit is to detect the inherent phase shift between current and voltage when the high voltage is radiated capacitively to ground. However, the realized circuit requires a phase discriminator circuit and a high parts count, which is costly.
A typical ground fault detector is comprised of a solid state switch which accepts a trigger voltage and conducts to operate a relay, etc. when the trigger voltage is exceeded. The relay operates switch contacts in the power supply, shutting down the power supply. The trigger voltage is derived by detecting the leakage current and converting this current to a voltage which is rectified and is applied to the solid state switch, which will operate if the voltage, and thus the current, is large enough. Ground fault detector circuits of this type are described, for example, in U.S. Pat. No. 4,114,089 to Ahmed, U.S. Pat. No. 3,899,717 to Legatti et al and U.S. Pat. No. 4,138,707 to Gross.
The leakage current detected in the aforenoted structures constitutes radiated or reactive alternating current, similar to current emitted from a radio frequency transmitter. The return energy is purely capacitive to ground. If the energy emitted by both high voltage leads (e.g. the antennae) of the secondary winding of the high voltage transformer of the ballast is not balanced capacitively, a current will flow through the center tap of the secondary winding to ground, creating an A.C. voltage which is detected as leakage current, and causing a false shutdown of the power supply.
It has been determined that hazardous arcing to ground can be detected solely from the D.C. current flowing from a D.C. biased winding to ground, rather than from the A.C. reactive current to ground. The systems described above shut down in the presence of A.C. reactive current, even without the presence of additional resistive current, which causes the false shutdown. The prior art systems are thus not reliable detectors of the hazardous currents.
SUMMARY OF THE INVENTION
The present invention is a ground fault detector circuit which ignores the A.C. leakage current caused by radiation, unbalanced radiation current, etc., and provides a trigger voltage which is caused by true direct current leakage during dielectric breakdown. It generates the trigger voltage by short circuiting A.C. leakage current, and detects only D.C. (resistive path) leakage current, applying a voltage derived from the D.C. leakage current to the trigger input of a solid state switching device such as a programmable shunt regulator.
The present invention thus provides reliable detection and high voltage shut down for D.C. leakage current which would otherwise be hazardous, ignoring A.C. radiated current.
In accordance with an embodiment of the invention, a method of shutting down a power supply which drives a transformer having a center-tapped high voltage secondary winding, comprises short circuiting A.C. leakage current that may flow between the secondary winding and ground, detecting D.C. voltage caused by D.C. leakage current which may be conducted between the D.C. biased secondary winding and ground, applying the D.C. voltage to the control input of a switch, and controlling shut-down of the power supply by means of the switch.
In accordance with another embodiment, a ground fault circuit comprises (a) a power supply, and a transformer having a primary winding driven by the power supply and a high voltage secondary winding, (b) a shutdown control circuit having a controllable switch and a control input coupled to the controllable switch for causing operation of the controllable switch when a trigger voltage applied to the control input is exceeded, the shutdown control circuit being coupled to the power supply for controlling shut-down of the power supply when the switch is in operation, (c) a circuit connected to the high voltage secondary winding for detecting leakage current from the D.C. biased transformer to ground, for short circuiting an A.C. component of the leakage current passing through the detector to ground, and for deriving a D.C. voltage from D.C. leakage current from the transformer to ground, and (d) a circuit for applying the derived D.C. voltage to the control input of the shutdown control circuit, whereby the power supply may be shut down in the presence of leakage current in excess of the trigger voltage which is derived exclusively from D.C. current leakage from the transformer to ground.


REFERENCES:
patent: 4994933 (1991-02-01), Matsuoka

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