JPH06300806A - Device for monitoring ground fault of electric installation - Google Patents

Abstract

PURPOSE:To rapidly carry out precautions of a ground fault, by detecting an omen of the ground by an omen detecting circuit to generate an alarm, and simultaneously by storing and outputting the zero phase voltage and zero phase current in a fixed period before and after the omen detection necessary to investigate the cause thereof by a waveform storage circuit. CONSTITUTION:The zero phase voltage V0 and zero phase current I0 of customer's power receiving facilities, etc., are monitored by an omen detecting circuit A, and the omen of the ground is detected. In the magnitudes and phases of the voltage V0 and current I0 obtained by Fourier transforms 6, 7 through A/D converters 4, 5, when three conditions that both the voltage V0 and current I0 are over a set value, the phase difference of the voltage V0 and current I0 is in a fixed angle range, and the magnitude of the high frequency component of the voltage V0 is over a set value are simultaneously brought into existence and continued during a fixed time, the omen is detected by a judging circuit 8, and the alarm is given by an alarm output 9. Next, in a waveform storage circuit B, the waveforms of the voltage V0 and current I0 during a fixed period before and after the omen detection are stored in a memory 11 and read through an output part 12 in cause analyzing time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention detects a warning sign of a ground fault accident by a zero-phase voltage V ₀ and a zero-phase current I ₀ of an electric equipment and gives an alarm, and a zero-phase voltage V ₀ and zero before and after the warning. The present invention relates to a ground fault monitoring device for electric equipment, which stores a phase current I ₀ , prevents a ground fault from occurring, and enables quick maintenance work by facilitating search for a fault point.

[0002]

2. Description of the Related Art A ground fault may occur in a power distribution system or the like due to insulation deterioration or the like. As a countermeasure against this, conventionally, a ground fault protection relay has been installed to disconnect the accident equipment from the system when a ground fault occurs.

This ground fault protection relay detects the zero-phase voltage V ₀ and the zero-phase current I ₀ of the equipment to be protected, both magnitudes of which exceed the reference level, and the zero-phase voltage V ₀ and the zero-phase current. I ₀
When the phase difference of is within a predetermined range and a ground fault is occurring on the self-equipment side continues for a certain period of time, it is determined to be a ground fault accident and a trip operation is performed.

[0004]

The above-mentioned ground fault protection relay is an after-action device that trips accident equipment after a ground fault occurs, and does not make advance forecasts. Moreover, since it does not record the zero-phase current at the time of a ground fault as a record, it is not useful for searching the cause and accident point.

From the standpoint of security management of electrical equipment, advance measures are required. In other words, it is necessary to find a place where a ground fault accident is likely to occur in advance and repair the portion to prevent the ground fault accident.

By the way, a ground fault accident generally occurs as a result of progress of insulation deterioration. From this point of view, the present inventor has found that, in a place where a ground fault is likely to occur, the zero-phase voltage V ₀ and the zero-phase current I ₀ change as a sign of the change, which is characterized by the type of ground fault (in the ground fault protection relay, I thought that a small level that could not be detected would appear.

[0007] Therefore, in equipment for security management,
An attempt was made to install a measuring instrument for zero-phase voltage V ₀ and zero-phase current I ₀ and observe the waveforms. According to this method, it is possible to obtain a preliminary forecast of a ground fault and narrow down the expected location of the fault based on the type of ground fault identified by the waveform, thereby achieving the purpose of preventing a ground fault. I understood.

However, the above sign occurs only in a short period immediately before the occurrence of the ground fault, and it is practically impossible to constantly monitor it for security management.

In order to utilize this sign as a preventive measure for a ground fault, it is necessary to detect this sign accurately and issue an alarm, and record the zero-phase voltage V ₀ and the zero-phase current I ₀ during this period. is there.

It is also conceivable that the sign detection is performed by using the detection circuit of the ground fault protection relay to improve its sensitivity. However, this is unsuitable in operation principle.

That is, the detection circuit of the ground fault protection relay is composed of an analog circuit, and a filter is adopted to extract the commercial frequency components of the zero-phase voltage V ₀ and the zero-phase current I ₀ to be detected. Therefore, in general, phase delay and waveform distortion occur in the filter. Therefore, if it is attempted to detect a sign that is a change that is considerably smaller than the change that occurs in the case of a ground fault, it is difficult to distinguish it from noise caused by guidance, etc., and an accurate warning cannot be issued.

Even if the sign can be detected, if the recording is started after the sign is detected, it is not sufficient to elucidate the cause of the defect and specify the defective portion. This is because the change that characterizes the cause of the ground fault most often appears in the waveform itself at the start of the change that is determined to be a precursor.

[0013] Therefore, the present invention detects high accuracy to distinguish it likely changes cause ground fault and noise, and at the same time alarm output, necessary and sufficient zero-phase voltage V ₀ and zero-phase current I of the period cause investigation By proposing a device that stores ₀ before and after detection, it is an object to promptly take preventive measures against a ground fault in protection management of electric equipment.

[0014]

The basic configuration (corresponding to claim 1) of the ground fault monitoring apparatus 1 provided by the present invention is a sign detection circuit A, which detects a sign of a ground fault of an electric equipment and issues an alarm. At the same time, the waveform memory circuit B stores the zero-phase voltage V ₀ and the zero-phase current I ₀ for a predetermined period before and after the sign detection in order to examine the cause, and when called by the administrator, this The stored contents are output.

The sign detection circuit A and the waveform storage circuit B are
It consists of the following elements.

That is, the sign detection circuit A includes a zero-phase voltage sensor 2 and a zero-phase current sensor 3 installed in an electric equipment to be monitored, and an A / D converter 4 for digitizing outputs of both sensors 2 and 3. , 5 and the digitally converted zero-phase voltage V ₀ and zero-phase current I ₀ are Fourier-transformed to calculate the magnitude and phase of the commercial frequency component.
And the magnitudes of the commercial phase zero-phase voltage V ₀ and zero-phase current I ₀ output from the Fourier transform circuits 6 and 7 both exceed the values set to detect the sign of the ground fault, and When the phase difference between the zero-phase voltage V ₀ and the zero-phase current I _{0 at} the commercial frequency is within the set range, and the state indicating that the ground fault cause is on the monitored equipment side continues for a predetermined time, it is determined as a ground fault. And a warning output section 9 for outputting the trigger signal as an alarm signal to the outside.

Further, the waveform storage circuit B includes a buffer circuit 10 for constantly holding the zero-phase voltage V ₀ and the zero-phase current I ₀ output from the A / D converters 4 and 5 for a certain period of time in the past.
When the trigger signal is generated, the buffer circuit 10
By taking out the held data of, the waveform storage memory 11 for storing the zero-phase voltage V ₀ and the zero-phase current I ₀ for a predetermined period before and after the generation of the trigger signal, which is necessary for examining the cause of the defect, and the waveform storage memory 11 are recorded. Zero phase voltage V ₀ and zero phase current I ₀
And a stored data output unit 12 for outputting

Further, the following circuit (corresponding to claims 2, 3 and 4 respectively) is provided as a circuit added to the above basic configuration to improve detection accuracy and reliability.

PLL circuit 13 ... Generates a power supply synchronization signal, determines the sampling timing of the A / D converters 4 and 5 by a clock signal obtained by multiplying the power supply synchronization signal, and uses the power supply synchronization signal to determine the Fourier transform circuits 6 and 7. Determine the calculation cycle.

Pulse count circuit 14 ... Monitors the zero-phase current I ₀ output by the zero-phase current sensor 3, and when a predetermined number of excessive pulses exceeding a certain level occur within a predetermined period, it is determined that a ground fault has occurred. The trigger signal is compulsorily generated as an alarm signal and a storage start signal.

Lightning noise removal circuit 15: causes the Fourier transform circuits 6 and 7 to calculate a harmonic of a predetermined order that does not occur due to the arrival of lightning noise, and when this does not exceed a certain level, the trigger signal of the sign detection circuit A Is prohibited. This circuit 15 is incorporated in the determination circuit 8.

[0022]

In the basic configuration described above, when the determination circuit 8 detects a phenomenon that is a precursor of a ground fault, the zero-phase voltage V ₀ and the zero-phase current I _{0 for} a predetermined period before and after detection, which are necessary for examining the cause, are determined. The waveform is digitized and stored.

In the sign detection in the judgment circuit 8, the magnitudes of the commercial frequency components of the zero-phase voltage V ₀ and the zero-phase current I ₀ obtained by Fourier transform both exceed a predetermined value indicating the sign of the ground fault. The condition that the phase difference between the current and the voltage is within the angle range indicating that it is on the equipment side to be monitored is performed under the condition that the predetermined time has continued.

Since this judgment is performed on the data obtained by the Fourier transform, the accuracy is high, and even if the margin (margin) of the judgment level (size, phase, duration) is reduced, the influence of induced noise It is hard to receive. Therefore, erroneous detection can be reduced, and a sign can be detected with high sensitivity and high speed.

In the waveform storage before and after the sign detection, the buffer circuit 10 always temporarily stores the immediately preceding data, and when a trigger signal is generated by the sign detection, the data held in the buffer circuit 10 for a certain period before. Then, the data for a predetermined period following this is recorded in the waveform storage memory 11. As a result, only the data necessary for pursuing the cause of the defect are recorded, the structure of the memory circuit can be simplified, and at the same time, the waveform analysis becomes easy.

With the addition of the PLL circuit 13 described above, the sampling timing of the A / D converters 4 and 5 and the operation cycle of the Fourier transform circuits 6 and 7 are synchronized with the system voltage. This improves the accuracy of the sign detection and the accuracy of the stored zero-phase voltage V ₀ and zero-phase current I ₀ .

In the case where the above pulse count circuit 14 is added, when a ground fault accident occurs due to a sudden cause, it is detected at high speed (for example, one cycle of commercial frequency), the cause is investigated and the accident point is searched. It enables the storage of the waveforms before and after the ground fault that is most necessary for.

With the lightning noise removing circuit 15 added, even if the predetermined conditions of magnitude and phase for the zero-phase voltage V ₀ and the zero-phase current I ₀ at the commercial frequency are satisfied due to the occurrence of lightning noise, As a result, the trigger signal is not generated in the sign detection circuit A, and only the necessary alarm and stored data are reported to the administrator.

[0029]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration shown in FIG. 1 is the basic configuration described above.
It incorporates all the additional circuits.

A sign detection circuit A detects a sign of a ground fault by monitoring the zero-phase voltage V ₀ and the zero-phase current I ₀ of the customer power receiving equipment or the like. The components of this circuit are described next.

Reference numeral 2 is a zero-phase voltage sensor, and reference numeral 3 is a zero-phase current sensor, which are respectively installed in the power receiving system to be monitored as shown in FIG. In FIG. 2, 16 is a power distribution cable, 17 is a power receiving transformer, 18 is power receiving equipment side wiring (inside wiring, etc.), 19 is a customer's ground fault protection relay, and 20 is load equipment.

Reference numerals 4 and 5 are A / D converters, which digitize the outputs of the sensors 2 and 3 at a predetermined sampling timing. The A / D conversion scale is made to correspond to the amplitudes of the zero-phase voltage V ₀ and the zero-phase current I ₀ used as targets of the sign detection, in order to effectively use the memory capacity of the waveform storage circuit B. . That is, it overflows with respect to an excessive pulse due to lightning noise or the like and outputs a constant maximum value that can be stored.

Fourier transform circuits 6 and 7 Fourier transform the digitized zero-phase voltage V ₀ and zero-phase current I ₀ to calculate the effective value (magnitude) and phase of the required frequency. In this Fourier transform, for example, one cycle period of the commercial frequency is set as one calculation cycle, and the DFT calculation is performed on the commercial frequency and the harmonic component of the specific order.

Reference numeral 8 is a judging circuit, which detects a sign of a ground fault.
This sign detection has the following conditions regarding the magnitude and phase of the zero-phase voltage V ₀ and the zero-phase current I ₀ obtained by the Fourier transform.
(A) (b) (c) are established at the same time, and if this state continues for a predetermined time, it is judged as a sign of a ground fault.

(A) The magnitudes (for example, effective values) of the zero-phase voltage V ₀ and the zero-phase current I _{0 at} the commercial frequency both exceed the set values. This set value is set to a value considerably lower than the operation level of the ground fault protection relay for the purpose of sign detection.

(B) The phase difference between the zero-phase voltage V ₀ and the zero-phase current I _{0 at} the commercial frequency is within a predetermined angle range showing that there is a ground fault on the side of the own equipment. This range is the same as the range determined by the ground fault protection relay, but it can be set to a more strict value because it is determined by highly accurate data obtained by Fourier transform.

(C) The magnitude of the harmonic component of a predetermined order (for example, the 32nd order or the 64th order) of the zero-phase voltage exceeds a set value (provided as a function of the lightning noise removing circuit 15). ). This condition is provided to prevent the detection operation against lightning noise, and the set value is a value that meets this purpose. This utilizes the fact that the above harmonic components are not included because lightning noise occurs regardless of the system power supply.

The above continuation time is set in order to increase the reliability of the sign detection.

That is, the determination operation of the determination circuit 8 is performed the same number of times because the Fourier transform is performed for each cycle of the commercial frequency, and the duration is the number of times the above condition continues in this determination. That is the condition setting. If the number of times is too small, the probability of false alarm by detecting noise that is not a sign of ground fault increases, and if the number of times is increased, the detection speed slows down, and the beginning of the abnormality that is most necessary for clarifying the cause of the ground fault occurs. Make it hard to record. Therefore, this condition also corresponds to the standard of the ground fault protection relay that it should not operate in an abnormal state of 3 cycles, for example, 2
It is set to 3 cycles.

FIG. 3 is a circuit diagram showing the above judgment conditions.
become that way. That is, (a) commercial-frequency zero-phase voltage V
A level determination circuit 21, which detects that both the magnitude of ₀ and the zero-phase current I ₀ (for example, the effective value) exceed a set value,
(B) A phase difference determination circuit 22 that detects that the phase difference between the zero-phase voltage V ₀ and the zero-phase current I ₀ is within a predetermined angle range, and (c) the harmonic level of the specific order does not exceed the set value. A lightning noise removing circuit 15 for outputting when
Obtained at D-gate 23. Then, the output of the AND gate 23 is input to the duration determination circuit 24, and a trigger signal is generated when the determination condition continues for 2 to 3 cycles.

Reference numeral 9 is an alarm output section which ORs this trigger signal.
The signal is received through the gate 25 and output as an alarm signal to the outside. Specifically, an alarm is given by a display lamp and a sounder installed near the monitoring device 1, and at the same time, a security management center connected by a telephone line or the like is notified.

Next, the waveform storage circuit B will be described. This circuit B stores the waveforms of the zero-phase voltage V ₀ and the zero-phase current I ₀ for a predetermined period before and after the sign detection in digital amounts and outputs them when analyzing the cause of abnormality. The components of this circuit B are as follows.

Reference numeral 10 denotes a buffer circuit, which always holds the zero-phase voltage V ₀ and the zero-phase current I ₀ output from the A / D converters 4 and 5 by going back a certain period in the past. This storage method takes in the current data while discarding the oldest data,
The storage capacity should be at least large enough to hold data for the duration of the determination circuit 8 (2 to 3 cycles). With this capacity, it is possible to store data after the beginning of the abnormality.

Reference numeral 11 denotes a waveform storage memory, which is a zero-phase voltage V for a predetermined period before and after the trigger signal is generated, which is necessary for examining the cause of the defect.
The waveforms of ₀ and zero-phase current I ₀ are stored in digital quantities. The memory 11 takes in the data held in the buffer circuit 10 when the trigger signal is generated, and further stores the outputs of the A / D converters 4 and 5 for a predetermined period thereafter. To give an example of this storage period, for example, it is represented by a commercial frequency, and is set to 2 to 3 cycles before the sign detection and 32 cycles (0.5 seconds) after the sign detection. The front side period is to secure the data immediately after the start of the abnormality as described above, and the rear side period corresponds to the ground fault protection relay of the substation whose operating time is set to the longest. This is because meaningful data cannot be obtained after this. The storage capacity of the memory 11 is such that one block of storage before and after the sign detection can be stored a plurality of times.

Reference numeral 12 is a storage data output unit, which stores the above waveforms.
Zero-phase voltage V recorded in the memory 11 ₀And zero-phase current I₀Wave of
Shape data to RS-232C according to the request from the outside.
It outputs through the interface circuit.

Reference numeral 13 is a PLL circuit for producing a power supply synchronizing signal, which is connected to the power supply circuit 26. This PLL circuit 1
3 generates a clock signal obtained by multiplying this power supply synchronizing signal (for example, dividing one cycle period into 256), and A / D
It outputs to converters 4 and 5 as a sampling signal.
As a result, the waveforms of the zero-phase voltage V ₀ and the zero-phase current I ₀ can be digitized with high accuracy. The power supply synchronizing signal is also given to the Fourier transform circuits 6 and 7 as a signal for determining one operation cycle. This allows the Fourier transform to be accurately performed for each cycle period of the commercial frequency,
Secure the calculation accuracy.

Reference numeral 14 is a pulse counting circuit for detecting a ground fault accident at high speed. Even if a sudden ground fault accident occurs,
In order to examine the cause, zero-phase voltage V ₀ and zero-phase current I ₀
Is provided to store the. The pulse counting circuit 14 monitors the zero-phase current output from the zero-phase voltage sensor 3 and when a predetermined number of excessive pulses exceeding a certain level occur within a predetermined period (for example, one cycle period of commercial frequency),
A trigger signal is generated when it is determined to be a ground fault. This trigger signal is input to the OR gate 25, becomes an alarm signal, and causes the waveform storage memory 11 to start the storage operation.

The trigger signal is also generated during a power failure and when the ground fault protection relay 9 of the own equipment operates. This intends to effectively utilize the storage function of the monitoring device 1. For this purpose, a power failure detection circuit 27 connected to the power supply circuit 27 and a GR operation signal detector 28 connected to the ground fault protection relay 19 of the equipment are provided, and each output is input to the OR gate 25. There is.

The ground fault monitoring device 1 is installed in the electrical equipment to be monitored and used as follows.

As described above, when the ground fault is detected and the trigger signal is generated, the alarm output unit 9 issues an alarm. The security personnel who rushed in response to the alarm read the waveforms of the zero-phase voltage V ₀ and the zero-phase current I ₀ before and after detection of the sign, which were stored in the waveform storage memory 11 of the waveform storage circuit B, through the storage data output unit 12. , The cause of the ground fault and the defective part are estimated from the waveform analysis.

The reading and the waveform analysis are carried out by connecting a laptop personal computer 29 as shown in FIG. 4 carried by the security personnel. A program for graphically displaying the read digital data of the zero-phase voltage V ₀ and the zero-phase current I ₀ as shown in FIGS. 6, 7, and 8 is installed in the personal computer 29. Has been done.

The waveform analysis is performed by comparing with a model waveform for each ground fault form prepared in advance by an experiment. There are various types of ground faults, such as a complete ground fault, a resistance ground fault, an arc ground fault, a surface leak, a gap discharge, and an intermittent ground fault of a cable, each of which exhibits a unique waveform. The data read out and displayed graphically is the waveform of the entire period required for the analysis from the start of the abnormality to the steady state. Therefore,
The overall waveform (FIG. 6) is displayed while changing the scale from the waveform of the characteristic portion (FIGS. 7 and 8) to determine the type of ground fault.

For example, the data shown in FIG. 6 is recorded data when a utility pole falls down to cause a ground fault, and the front side of the portion surrounded by a square frame in FIG. 6 is enlarged and displayed. It can be seen from the waveform of FIG. 8 that an arc ground fault occurs at the time point a, and the rear side portion is enlarged and displayed next.

The personal computer 29 has a function of calculating and displaying the effective value and the phase difference between the zero-phase voltage V ₀ and the zero-phase current I ₀ from the read digital data (see FIGS. 6, 7 and 8). It also has a display on the right side).

By knowing the type of ground fault, it is possible to estimate in which equipment to be monitored the defect has occurred. This makes it possible to quickly repair a defective part by checking the corresponding part.

The waveform analysis by this reading can also be carried out by a computer of a security management center connected through a telephone line with an automatic remote notification device 30 as shown in FIG.

[0057]

According to the present invention, the predetermined frequency components of the zero-phase voltage V ₀ and the zero-phase current I ₀ are extracted with high accuracy by Fourier transform, and the sign of the ground fault is detected by this.
The determination level can be set precisely, and high-speed detection is possible with few false detections.

A / D of zero-phase voltage V ₀ and zero-phase current I ₀
Since the conversion is performed by using the clock signal synchronized with the power supply created by the PLL circuit as the sampling timing, the accuracy of the data stored in the waveform storage circuit can be increased and the reliability of the waveform analysis can be improved.

The waveform storage circuit causes the buffer circuit to always hold the data of a certain period before, so that when the sign of the ground fault is detected and the trigger signal is generated, the zero-phase voltage V _{0 in} the predetermined period before and after the detection is generated. The digitized data of the zero-phase current I ₀ is stored. With this storage method, it is possible to store immediately after the occurrence of the abnormality. Further, in this storage method, only the data necessary for the waveform analysis is left, so that it is possible to reduce the cost by reducing the required storage capacity and facilitate the waveform analysis work. This is clear if one imagines recording all the data during the monitoring period.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a ground fault monitoring device of the present invention.

FIG. 2 is a diagram showing a normal monitoring state in which the ground fault monitoring device of the present invention is attached to electric equipment.

FIG. 3 is a circuit diagram showing the function of the determination circuit of FIG.

FIG. 4 is a diagram showing a state where waveform analysis is performed by a personal computer after detection of a ground fault sign.

FIG. 5 is a diagram showing an embodiment having an automatic remote notification function to a security management center.

FIG. 6 is a zero-phase voltage V ₀ stored at the time of detecting a sign of ground fault.
Waveform diagram of the PC and zero-phase current I ₀

FIG. 7 is a waveform diagram in which the front portion of the square frame in FIG. 6 is enlarged and displayed.

FIG. 8 is a waveform diagram in which the rear portion of the rectangular frame in FIG. 6 is enlarged and displayed.

[Explanation of symbols]

 A Sign detection circuit B Waveform memory circuit 1 Ground fault monitoring device 2 Zero-phase voltage sensor 3 Zero-phase current sensor 4,5 A / D converter 6,7 Fourier transform circuit 8 Judgment circuit 9 Alarm output unit 10 Buffer circuit 11 Waveform memory memory 12 memory data output unit 13 PLL circuit 14 pulse count circuit 15 lightning noise removal circuit 29 personal computer

Claims (4)

[Claims] 1. A sign detection circuit for detecting a sign of a ground fault of an electric equipment, and a zero-phase voltage V ₀ and a zero-phase current I ₀ for a predetermined period before and after sign detection necessary for examining the cause of the sign detection. A ground fault monitoring device having a waveform storage circuit for storing, wherein the sign detection circuit is a zero-phase voltage sensor and a zero-phase current sensor installed in an electric facility to be monitored, and outputs of both sensors are digitized. A / D converter, a Fourier transform circuit for Fourier transforming the digitally converted zero-phase voltage V ₀ and zero-phase current I ₀ to calculate the magnitude and phase of the commercial frequency component, and a commercial output of the Fourier transform circuit Frequency zero-phase voltage V ₀
When the magnitudes of the zero-phase current I ₀ and the zero-phase current I ₀ both exceed values set to detect the sign of the ground fault, the phase difference between the zero-phase voltage V ₀ and the zero-phase current I _{0 at} the commercial frequency is set. Within range,
When a state indicating that the ground fault is on the monitored equipment side continues for a predetermined time at the same time, it is determined that a ground fault has occurred, and a determination circuit that outputs a warning signal and a trigger signal that is a signal to start storage is configured. Then, the waveform storage circuit, a buffer circuit for always holding the zero-phase voltage V ₀ and the zero-phase current I ₀ output from the A / D converter by going back a certain time in the past, and when the trigger signal is generated, By taking out the data held in the buffer circuit, a waveform storage memory for storing the zero-phase voltage V ₀ and the zero-phase current I ₀ in a predetermined period before and after the trigger signal is generated, which is necessary for examining the cause of the defect, and the waveform storage memory are recorded. And a stored data output section for outputting the zero-phase voltage V ₀ and the zero-phase current I ₀ . 2. A clock signal obtained by multiplying a power supply synchronizing signal extracted by a PLL circuit determines a sampling timing of an A / D converter, and the power supply synchronizing signal determines a calculation cycle of a Fourier transform circuit. Item 1. A ground fault monitoring device for electric equipment according to Item 1. 3. A trigger that monitors a zero-phase current and, when a predetermined number of excessive pulses exceeding a certain level occur within a predetermined period, determines a ground fault and forcibly outputs an alarm signal and a signal to start storage. The ground fault monitoring device for electric equipment according to claim 1, further comprising a pulse counting circuit for generating a signal. 4. A lightning noise removal circuit for causing a Fourier transform circuit to calculate a harmonic of a predetermined order that does not occur due to the arrival of lightning noise, and prohibiting the generation of a trigger signal of the sign detection circuit when this does not exceed a certain level. The ground fault monitoring device for electrical equipment according to claim 1, further comprising: