Ground fault interrupter

Details Ground fault interrupter Ground fault interrupter

2001-03-15

2004-04-13

Leja, Ronald W. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Ground fault protection

Reexamination Certificate

active

06721151

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority to Japanese Patent Application No. 2000-072992, filed Mar. 15, 2000, entitled “Ground Fault Interrupter”. The contents of that application are incorporated herein by reference in their entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a ground fault interrupter.
2. Discussion of the Background
Due to the recent increase in use of inverter devices and development of high capacity inverter devices, the current leak caused by high frequency electric leak has increased. A conventional ground fault interrupter (B) includes a zero-phase current transformer
5
, a resistor
6
for detection and an electric leak detection circuit
8
as shown in FIG.
4
. The zero-phase current transformer
5
detects an unbalanced current flowing in electric lines
2
,
2
between a grounded commercial electric source and a load. The resistor
6
is connected to the output side of the zero-phase current transformer
5
and converts a current flowing in the zero-phase current transformer
5
to a voltage signal. The electric leak detection circuit
8
, functioning as an electric leak control unit, opens and closes switch
4
in the electric lines
2
,
2
based on the voltage signal from the resistor
6
. The electric leak detection circuit
8
is comprised of an integrated circuit and includes an amplification unit (
8
a
) and an electric leak determination unit (
8
b
). The amplification unit (
8
a
) amplifies the signal from the resistor
6
and the electric leak determination unit (
8
b
) opens the switch
4
in the electric lines
2
,
2
when grounding is detected based on the signal from the amplification unit (
8
a
).
The conventional ground fault interrupter (B) shown in
FIG. 4
further includes a clipping circuit and a resistor (R
1
) for limiting an input current to the electric leak detection circuit
8
. The resistor (R
1
) is positioned between the zero-phase current transformer
5
and the electric leak detection circuit
8
, and prevents the electric leak detection circuit
8
from being damaged by a large output generated by the zero-phase current transformer
5
, for example, due to electrical surge. The clipping circuit is comprised of a pair of diodes (D
1
), (D
2
) and connected in parallel to the resistor
6
.
In such a conventional ground fault interrupter, the electric leak determination unit (
8
b
) determines that electric leak occurs when the electric leak detection circuit
8
receives a high frequency signal even though the ground fault does not actually occur. Consequently, when the load is an inverter device, the conventional ground fault interrupter (B) unnecessarily opens the switch
4
.
Furthermore, to prevent such a malfunction, another conventional ground fault interrupter (C) shown in
FIG. 5
includes a filter (
8
c
) provided in an electric leak detection circuit
8
′. The filter (
8
c
) cuts high frequency elements in a signal outputted from a zero-phase current transformer
5
. As in the previous conventional ground fault interrupter (B), this electric leak detection circuit
8
′ is comprised of an integrated circuit.
Although the filter (
8
c
) of the ground fault interrupter (C) sufficiently cuts high frequencies, the ground fault interrupter (C) still malfunctions because of a large amount of high frequency electric leak current, thus unnecessarily opening the switch
4
.
When high frequency electric leak occurs, a voltage across a resistor
6
connected to the zero-phase current transformer
5
gets clipped by a clipping circuit. Specifically, because a voltage drop of approximately 0.7 V occurs across diodes (D
11
), (D
12
), the voltage across the resistor
6
becomes a constant at 0.7 V for a duration when the output of the zero-phase current transformer
5
is large. The voltage signal during this clipping period thus has a low frequency. As a result, a current which is almost DC passes though the filter (
8
c
) and causes the malfunction described above.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, a ground fault interrupter includes a switch, a zero-phase current transformer, a detection resistor, a controller and at least one filter. The switch is provided in electric lines connecting an electric power source and an electric load. The zero-phase current transformer is configured to detect unbalanced current flowing in the electric lines. The detection resistor is connected in parallel to the zero-phase current transformer and configured to convert current outputted from the zero-phase current transformer to voltage. The controller is configured to determine based on the voltage of the detection resistor whether an electric leak occurs and to open the switch when the controller determines that an electric leak occurs. The at least one filter is provided between the detection resistor and the controller and configured to remove high frequency elements in the voltage of the resistor. The at least one filter includes an input side resistor which is connected in series to the controller and which is configured to limit current input to the controller; and a capacitor which is connected in parallel to the controller.


REFERENCES:
patent: 3962606 (1976-06-01), Burns et al.
patent: 6025980 (2000-02-01), Morron et al.

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