MXPA06004178A - Single-sensor microcontroller-based approach for ground fault circuit interrupters - Google Patents

Abstract

A microcontroller-based temperature compensated circuit for ground-fault circuit interrupter to meet the requirements of UL 943 using a single sensor to detect both ground-fault and grounded-neutral fault conditions in both full-wave and half-wave AC power supplies as part of a ground-fault circuit breaker or a receptacle device.

Description

PROCEDURE BASED ON SINGLE SENSOR MICROCONTROLLER FOR FAULT CIRCUIT CIRCUIT BREAKERS FOR EARTH CONNECTION BACKGROUND OF THE INVENTION Existing designs for ground fault protection devices, such as circuit breakers and outlets, typically use an analog circuit and two current sensors that meet the requirements of UL 943. A first sensor it is necessary for the detection of the current imbalance characteristic of a ground fault, and a second sensor is used as part of a latent oscillating circuit that detects the neutral condition with ground connection that can degrade the capacity of the failure detection by ground connection. It is required that these sensors are of high precision with respect to a wide range of temperatures and that they have a low part-to-part variation because the analog circuit offers few compensation or calibration capabilities. Also, the analog procedure could not work well if the supply was discontinuous because a non-volatile memory function is not available.

SUMMARY OF THE INVENTION In short, the present invention uses the combination of a single low cost current sensor and a small low cost microcontroller, which are designed for use as part of a ground fault circuit interrupter or Outlet device that meet all the requirements of UL 943 while addressing the problems of existing designs. According to another embodiment of the invention, the cost is reduced when compared to the two sensor method if the functions of ground fault detection and neutral detection are combined with grounding in a sensor. In accordance with yet another embodiment of the invention, a simple measurement and temperature compensation scheme, which corrects non-linearities of the sensor with respect to temperature, allows the sensor to be designed using low-cost materials and a simple manufacturing process. . Another embodiment of the present invention utilizes a programmable device that provides software-based calibration during the electronic assembly process that overcomes the part-to-part variation in the detection circuitry. This allows a wider range of acceptable tolerance for the components of the detection circuit and decreases the amount of rejected component material.

In accordance with another embodiment of the present invention, an analog memory function is provided that resumes a trip condition or circuit disconnection based on a detected fault if the power was temporarily lost before having time to activate the trip circuit. This feature allows the circuit of the present invention to operate from a half-wave rectified energy source or other source of discontinuous energy.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 is a schematic diagram of a ground fault circuit interrupter including the invention, Figure 2 is a timing diagram illustrating the use of the memory capacitor in the Figure 1, Figure 3 is a series of waveforms illustrating the detection of ground fault with a half wave power supply, Figure 4 is a series of waveforms illustrating the detection of failure by grounding with a full-wave power supply, Figures 5a and 5b illustrate the detection of a condition where there is no ground-neutral condition, and Figures 6a and 6b illustrate the detection of a ground condition. neutral with ground connection.

DETAILED DESCRIPTION OF THE ILLUSTRATED MODALITY Next, with reference to the drawings, and initially to Figure 1, a digital circuit to interrupt the ground fault circuit (GFCI) 10 based on an icrocontroller is located in order to detect the conditions of failure by grounding and neutral with grounding in the linear and neutral conductors 30 and 32, respectively, using a single current transformer TI as a sensor. The Ul digital microcontroller is a device such as the PIC12CE673 microcontroller, or a digital signal processor or an ASIC device with features such as: a card RAM, a non-volatile memory, an internal time regulator or synchronizer, an internal converter from analog-to-digital (A / D) and analog and digital ports. The direct current (DC) energy for the GFCI circuit 10 is supplied from the power supply circuit 20, which extracts the energy from the linear and neutral conductors 30 and 32, and a reference circuit 22 that produces the required levels of regulated DC voltage. The complete supply of electrical energy consists of a disconnect solenoid Ll, a MOVÍ varistor, a rectifier CR1, a capacitor Cl, a voltage reduction resistor Rl, a chain of diodes CR2-CR5, a reference diode CR6 and a capacitor of exit C3. The disconnect solenoid Ll, the capacitor Cl and the varistor MOVÍ perform the input filtering and protection against overvoltages. The disconnect solenoid Ll serves multiple functions by providing input filtering, i.e., a series of pedals for overvoltage and noise suppression, as well as a means that opens the main contacts (not shown) in a failure of short circuit in the power supply or for the intended function of disconnection in the event of a detected condition of ground fault or neutral fault with ground connection. The rectifier CR1 transforms the alternating current of the AC input, and the capacitor Cl provides additional energy storage and the suppression of the high frequency overvoltage currents. The voltage reduction resistor R1 is suitably sized to maintain a sufficient forward negative bias current in the diode chain CR2-CR6 in the voltage reference circuit 22, in addition to providing the required operating current for the circuit in the minimum input voltage. The DC direct current voltage level required for the operation of the microcontroller and another set of circuits is regulated by the diode chain CR2-CR6. The reference voltage necessary for the stable operation of the detection circuit 24 is provided by CR6 and C3. Capacitor C3 provides a small amount of energy storage in accordance with overvoltage current conditions. The regulated Vref output is available according to an input range of -66 to -132 Volts of Alternating Current (VAC). The output voltage Vref and the input interval can be adjusted by changing the component values, as is well understood by those skilled in the art. A C2 capacitor and a Controlled Rectifier Silicon (SCR) Ql perform a disconnect function. When a fault is detected by the microcontroller Ul, the digital output "disconnection" plug of the microcontroller Ul is set, which turns on the SCR Ql and creates a current circuit through the solenoid Ll, the rectifier CR1 and the SCR Ql . The current that originates is at a sufficient level to activate the disconnect solenoid Ll and to open the main contacts (not shown). The capacitor C2 provides the noise suppression for the gate of SCR Q1 and stores the voltage during the disconnection operation to maintain the "on" state of Q1 for a longer period of time. A manual test circuit 18 consists of a manual push-to-test PTT switch and a pair of resistors Rll and R12. When the PTT switch is pressed, sufficient current flow occurs causing the GFCI circuit 10 to detect a fault and use the disconnect function to open the main contacts (not shown). The current sensing circuit 24 consists of a current transformer TI connected with a linear conductor 31 and a neutral conductor 32 and an amplifier circuit composed of an operational amplifier U2 and a pair of resistors R7 and R8. A bias voltage resistive divider circuit is formed by a pair of resistors R3 and R4, which establishes a circuit voltage that is 1/2 Vref. This ensures that the "Zero" level of the output of the detection circuit 24 is placed halfway between the bars of the A / D input of the microcontroller Ul to facilitate detection of the envelope. The permeability of the CT current transformer is affected by the changes in ambient temperature that are preferred to be compensated for both ground fault and ground fault threshold levels.

An optional temperature sensing circuit 26 utilizes the base-to-emitter voltage of the small signal bipolar connection transistor Q3 to provide a reading of the ambient temperature conditions adjacent to the current transformer TI. The bias connection current of transistor Q3 is adjusted by a resistor R13 connected to the reference supply voltage Vref. The reference voltage Vref and the voltage at the base of the transistor Q3 are sampled by the microcontroller Ul, and the sampled value is used to adjust the fault threshold value by grounding and the reference value of neutral detection with connection to ground to compensate for changes in the operation of the current transformer TI with respect to temperature. During the manufacturing process, the microcontroller could be programmed to calculate the ground fault and neutral fault values with a ground connection at a given temperature and can also store the threshold values in a non-volatile memory. Another method of temperature compensation is discussed below with reference to Figures 5 and 6. An analog short-term memory circuit 28 consists of a capacitor C6, a load resistor R9 and a voltage regulating resistor RIO. The microcontroller Ul uses a bidirectional terminal Mem cap, as an analog input for reading the voltage of the memory circuit 28 and as a digital output for charging the capacitor C6 of the memory circuit 28. If a fault was detected, the software that running on the Ul microcontroller would cause a load to be placed on capacitor C6. If the power was lost before the trip solenoid valve was able to open the contacts, the trip memory (ie, the voltage at capacitor C6) will remain for a short time and will cause the trip function to be reactivated (eg the Ul) microcontroller based on the resumption of supply voltage. The memory circuit 28 allows the GFCI circuit 10 to operate from a half-wave rectified power source or from another discontinuous power source. Next, with reference to Figure 2, the timing or regulation diagram shows the use of an analog memory circuit 28 during normal operation (no fault is detected) for synchronization purposes to determine when to perform the neutral checks with grounding and grounding fault. The memory circuit 28 allows the synchronization of the neutral checks with grounding so that they remain consistent even if a rectified half-wave power supply (discontinuous) was used. When the voltage of the memory circuit reaches the almost unloaded state, the microcontroller Ul loads the capacitor C6 to a voltage level less than the amount required to indicate a pending disconnection, as discussed previously and executes a continuous fault detection mode by continuous ground connection for a period of time until the voltage capacitor C6 once again reaches the nearly discharged state. When the voltage of the memory circuit 28, sampled by the microcontroller Ul, reaches the almost discharged state, a neutral check with ground connection is executed during the intermediate time or the space interval. This cycle occurs a few times per second as illustrated and can be adjusted by varying the values of the C6 memory capacitor and the RIO voltage regulation resistor. Next, with reference to Figure 3, the operation of the ground fault detection is illustrated by starting a circuit disconnection based on a rectified half-wave power supply. In 100a the power supply is started, and in 102a the microcontroller Ul is initialized and the memory capacitor C6 is read to determine if there is an unfulfilled condition of disconnection from a previous cycle as discussed previously. At 104, the ground fault detection function turns on switch Q2, then places the overloaded low impedance resistor R6 in the circuit through the secondary winding of the CT. The operational amplifier U2 amplifies the voltage through the resistor R6 to a level that allows 5 mA of fault current per ground connection to be read by the A / D converter in microcontroller card Ul. The results are compared in the software with a ground fault failure threshold value to determine if the trip or disconnection threshold has been exceeded, indicating a failure. If there was no fault, then at 106, the memory capacitor C6 would be charged to indicate a pending disconnection condition, and at 108a the disconnection function would be activated in an attempt to cause a circuit disconnection in the remaining time. However, at 110 the half-wave power supply is disconnected. At 100b the power supply starts once again, and the microprocessor Ul is initialized once again at 102b, although the load on the memory capacitor C6 indicates a pending disconnection condition, so that the disconnection function is activated at 108b for cause an immediate disconnection of the circuit. When power is supplied continuously with a full-wave power supply, as in Figure 4, the circuit disconnection could happen more quickly because the power is available to activate the disconnection function during the negative cycle. half line. Using a full-wave power supply, start cycles 100 and 102 of Figure 3 are only performed once based on ignition / reconnection, and are not shown in Figure 4. During the detection of the fault by connection to On the ground, the microcontroller Ul turns on the switch Q2, placing the overloaded impedance resistor of low impedance R6 in the circuit through the secondary IT winding. The operational amplifier U2 amplifies this signal to a level that allows 5 mA of the fault current per ground connection to be read by the A / D converter on the microcontroller card Ul. The results are compared in a software with a ground fault fault reference value to determine if the disconnection threshold has been exceeded. If there was no fault, then, at 106 the memory capacitor would be charged to indicate a pending disconnection condition, and at 108 the disconnection function would be activated to cause an immediate disconnection of the circuit. In case the circuit of the main line is interrupted before the circuit has been disconnected, the memory function for a short time can help to disconnect immediately based on the restoration of energy.

Next, with reference to Figures 5-6, the waveforms of the output of the current detection circuit 24 are illustrated for the operation of the neutral detection function with grounding when there is no neutral condition with connection to earth and when a neutral with a 1-Ohm earth connection is present, respectively. A neutral detection mode with ground connection is entered when the voltage in the C6 memory capacitor reaches the nearly discharged state. That happens when the circuit is first energized and every few hundred milliseconds after that, as determined by the memory circuit 28 for both full-wave and half-wave power supplies. In a neutral detection mode with ground connection, the switch Q2 is turned off by the auto-start output of the microcontroller Ul, which changes the gate voltage of the switch Q2 from high to low and generates a disturbance in the secondary winding of the transformer of IT current through capacitor C5. With the R6 disconnected from the circuit, the secondary winding of the transformer TI and the capacitor C4 are allowed to resonate with a small amount of damping provided by the overloaded high impedance resistor R5, as shown in Figure 5b. The neutral condition with ground connection changes the impedance of the secondary winding of the CT transformer and dampens the oscillations with precision, as shown in Figure 6b. The peak-to-peak envelope or amplitude of the damped oscillatory waveform as it changes over time is amplified by the operational amplifier U2 and is measured by the A / D input of the microcontroller Ul after a previously adjusted delay. The peak-to-peak amplitude of the waveform, or envelope, measured by the microcontroller Ul is compared to a stored threshold of the neutral condition with ground connection. If the peak-to-peak amplitude were larger than the threshold, then the primary impedance would be above the neutral threshold level with ground connection, for example, it would be > 2.5 Ohms In this case, the C6 memory capacitor is charged for the next synchronization interval, the low impedance overloaded resistor R6 is switched back to the circuit by the Q2 switch, and the software program initiates the verification of a fault condition by ground connection. If the measured peak-to-peak amplitude were less than the neutral threshold value with ground connection, then there would be a neutral condition with ground connection, the C6 memory capacitor would be charged to indicate a pending condition of disconnection and the disconnection function would be activated. Figure 6b illustrates the damping effect of the neutral condition with ground connection 34 caused by a neutral with a 1-Ohm ground connection, which causes the envelope of the oscillatory waveform to decay rapidly, if compared with the enclosure shown in Figure 5b, where there is no neutral condition with ground connection. The damped oscillations mentioned above can be expressed in the form of an exponential equation multiplied by a sinusoid as follows: A sin (ÜT) X e_a? 'represents the initial amplitude of the sinusoid,? represents the frequency of oscillation, T represents time and a represents the decay factor. This a is the combination of the elements that cause the oscillation to decay. The neutral-to-ground resistance is directly related to this parameter a. As the neutral-to-ground resistance decreases, the parameter a increases causing the decay to be faster. In order to determine the presence of a predetermined neutral-to-ground resistance value, this parameter a can be calculated or estimated by means of a number of methods. Each method offers benefits and commitments in terms of processing requirements and noise susceptibility. Once estimated, the estimate could be compared with a set point for the detection of a neutral fault with ground connection. Each of the following methods can be implemented only with the positive, negative or both values or the absolute value of the oscillation cycles. These methods are described below: Method 1: Peak Envelope - Noting that the form of the expression that describes the decay of the oscillation contains a sinusoid and an exponential function, this method seeks to find the exponential envelope function. The peaks of the oscillation are located by sampling the signal at a high speed. This peak-to-peak amplitude can be measured to determine the envelope of the waveform. The envelope measured at a specific time from the start of the oscillatory waveform can then be used to measure the rate of decay of the exponential function. Method 2: Peak Polynomial Envelope - This method is like Method 1, although it uses an estimate of the second order of the function in the form of y = Ax2 + Bx + C. A is used to estimate the parameter a. A multi-order polynomial function could also be used. Method 3: Estimated Linear Envelope - This method is also like Method 1, except that a linear adjustment of the peak values is found. The envelope that originates from the best adjustment line is used to estimate parameter a. Method 4: Cycle Area - This method is like Method 1 although it uses an estimate of the area below the signal waveform instead of the peak values. The resulting points are adjusted in a model. A parameter of this model is used to estimate the parameter. This method could use an exponential, linear or polynomial function model like the previous methods 1, 2 or 3. Method 5: Inclination of Half Cycle - This method estimates the inclination of the leading or trailing edge of a half cycle by measuring two or more points . Parameter-to-base decisions could be the mid-cycle tilt N, where N is 1, 2, 3, 4 ... Method 6: Tilt Half Cycles Function -This method requires tilt calculation M half cycles and subsequently, the use of a parameter such as the inclination of the M inclinations resulting from half a cycle. Method 7: Threshold on the Inclination of the Half Cycles - This method requires the calculation of the inclination of the M half cycles and subsequently, the use of a threshold to count the number of half cycles above a predetermined threshold. The number of half cycles with an inclination above a threshold is used as the decision parameter.

Method 8: Peak Counting Above a Threshold - A fixed number of half cycles or a fixed period of time is monitored. During this time, the number of half cycles crossing over a preselected threshold is counted. A decision parameter based on the number of peaks above the threshold is used. According to another embodiment of the present invention, the temperature effect on the operation of the current transformer TI can be determined, during the detection of neutral fault with ground connection, by measuring the frequency of the damped oscillatory waveform of the transformer of current TI. By measuring the resonant frequency with a known value of capacitance, the frequency changes can be directly related to the changes in the inductance of the current transformer TI. The change in inductance is a direct indication of a change in permeability in the core material of the transformer and also refers to the output characteristics of the current transformer TI. According to one embodiment of the present invention, the microcontroller is programmed during the manufacturing process at a baseline temperature to initiate the production of a damped oscillatory waveform in order to produce a reference frequency value, and to store the reference frequency value in a non-volatile memory. The reference frequency value obtained is directly related to the inductance of the current transformer TI at a baseline temperature. During the normal operation of the present invention, the reference frequency value is compared to an optionally measured resonant frequency, for the purpose of calculating the modified values of ground fault and neutral grounding fault levels for use in the process of failure detection. In this way, changes in the operation or performance of the current transformer TI, with respect to a temperature range, can be effected by observing the resonant frequency instead of the optional temperature detection circuit 26. While the modes and particular applications of the present invention have been illustrated and described, it being understood that the invention is not limited to the interpretation and precise compositions described herein and that various modifications, changes and variations could be apparent from the foregoing description without departing from of the spirit and scope of the invention as defined in the appended claims.

Claims (8)

1. CLAIMS 1. A system based on microcontroller for the detection of ground fault and neutral conditions with ground connection in a power distribution system that has linear and neutral conductors, characterized in that it comprises: a detection circuit that contains a single current transformer that produces an output signal in response to the current flow in both the linear and neutral conductors of the electric power distribution system, a microcontroller that receives the output signal from the sensor and initiates the generation of a disconnection signal based on the detection of the ground fault or neutral condition with grounding in the power distribution system, the microcontroller is programmed to use the sensor output signal in order to detect the conditions of failure by grounding during spaced intervals of time, and to use the signal of detector output in order to perceive the neutral condition with ground connection during intermediate intervals of time between the spaced intervals of time, a circuit breaker that interrupts the current flow in the power distribution system in response to the signal disconnection, and an analog memory circuit that can be operated with both full-wave and half-wave power supplies to provide a synchronization function in order to regulate the spaced intervals of time and intermediate intervals of time, and an adjustment of memory function in response to detection of ground fault or neutral grounding conditions to resume circuit disconnection if power was temporarily lost before activating the circuit breaker.
2. 2. A system based on microcontroller for the detection of ground fault and neutral conditions with ground connection in a power distribution system having the linear and neutral conductors, characterized in that it comprises: a detection circuit which contains a single current transformer that varies non-linearly with temperature, producing an output signal in response to the current flow in both the linear and neutral conductors of the electric power distribution system, a microcontroller that receives the output signal of the sensor and that initiates the generation of a disconnect signal based on the detection of the ground fault or neutral condition with ground connection in the power distribution system, and a non-volatile memory associated with the microcontroller , where the microcontroller is programmed during manufacturing to receive the sensor output signal at a given temperature, and calculating a predetermined value of ground fault failure based on the sensor output and for storing the predetermined value of ground fault failure in the non-volatile memory, and calculating a predetermined value Neutral threshold with ground connection based on the sensor output and store the default neutral threshold value with ground connection in the non-volatile memory.
3. 3. A microcontroller based system for the detection of ground fault and neutral conditions with ground connection in a power distribution system having linear and neutral conductors, characterized in that it comprises: a detection circuit that it contains a single current transformer that produces an output signal in response to a current flow in both the linear and neutral conductors of the electric power distribution system, the current transformer has an inductance that varies with temperature, a detection circuit of ambient temperature placed next to the current transformer, which produces a voltage that varies linearly with the ambient temperature conditions, a programmable microcontroller having a predetermined value of failure threshold per ground connection and a predetermined value of neutral threshold with ground connection stored in a non-volatile memory, the microcontroller is programmed to calculate a modified value of failure threshold per ground connection based on the default value of ground fault fault and the output of the ambient temperature detection circuit, calculate a modified value of neutral threshold with ground connection based on the default value of neutral threshold with connection to ground and the output of the ambient temperature detection circuit, use the modified value of the ground fault fault that detects the ground fault condition, use the modified value of the neutral threshold with ground connection that detects the neutral condition with ground connection, and start the generation of a disconnection signal in based on the detection of the ground fault or neutral condition with ground connection in the power distribution system and a circuit breaker that interrupts the current flow in the power distribution system in response to the signal of disconnection.
4. 4. A system based on microcontroller for the detection of ground fault and neutral conditions with ground connection in a power distribution system having the linear and neutral conductors, characterized in that it comprises: a detection circuit which provides an output signal, the detection circuit contains a current transformer having an inductance that varies with temperature and a resonant circuit, a programmable microcontroller containing a predetermined value of ground fault failure and a predetermined value of neutral threshold with ground connection stored in a non-volatile memory, the microcontroller is programmed to initiate a pulse signal that produces a resonant oscillation in the detection resonant circuit during a neutral test with ground connection, measure the frequency of the resonant oscillation to determine the change in the inductance of the current transformer, calculate a modified value of fault threshold per ground connection based on the default value of ground fault failure and the change in inductance of the current transformer, calculate a modified value of failure threshold per connection to ground based on the neutral threshold value with ground connection and the change in. inductance of the current transformer, use the modified value of the ground fault fault that detects the ground fault condition, use the modified value of the neutral threshold with ground connection that detects the neutral condition with connection to the ground ground, and initiate the generation of a disconnect signal based on the detection of the ground fault condition or the neutral condition with grounding in the power distribution system, and a circuit breaker that interrupts the current flow in the power distribution system in response to the disconnect signal.
5. 5. A method for detecting the ground fault and neutral conditions with ground connection in a power distribution system having the linear and neutral conductors, characterized in that it comprises: producing a signal with a single transformer of current, in response to the current flow in both the linear and neutral conductors of the electric power distribution system, supply the signal to a microcontroller that is programmed to use the signal in order to detect the fault conditions by grounding or neutral with ground connection in the power distribution system and initiate the generation of a disconnection signal based on the detection of the ground fault or neutral condition with ground connection, interrupt the current flow in the power distribution system in response to the disconnect signal, and use an analog memory to provide a synchronization function to control the intervals for testing the ground fault conditions or neutral with ground connection and memory function adjustment in response to the detection of ground fault condition or neutral with ground connection to resume the disconnect condition if power was temporarily lost before the current flow in the power distribution system is interrupted.
6. 6. A method for detecting ground fault and neutral conditions with grounding in a power distribution system having the linear and neutral conductors, characterized in that it comprises: producing a signal with a single transformer of current that varies non-linearly with respect to temperature, which is in response to the current flow in both the linear and neutral conductors of the electric power distribution system, and supply the signal to a microcontroller that is programmed during manufacturing to receiving the signal at a reference temperature and calculating a predetermined value of the ground fault fault based on the reference temperature, and storing the predetermined value of the ground fault failure threshold in a non-volatile memory associated with the microcontroller and receive the signal at a reference temperature and calculate the predetermined threshold value of n eutro with ground connection based on the temperature reference, and store the default value of neutral threshold with ground connection in a non-volatile memory associated with the microcontroller.
7. 7. A method for detecting ground and neutral fault conditions with grounding in a power distribution system having linear and neutral conductors, characterized in that it comprises: producing a signal with a sensor, which it varies non-linearly with temperature, in response to the current flow in both the linear and neutral conductors of the electric power distribution system, produce a reading of the environmental temperature of the sensor, and supply the signal to a microcontroller that has a predetermined value of ground fault and a predetermined neutral threshold value with ground connection, the microcontroller is programmed to use the ambient temperature reading to calculate the modified value of ground fault value based on the value Default fault threshold by ground connection, use the ambient temperature reading to calculate the Modified value of neutral threshold with ground connection based on the default value of neutral threshold with ground connection, use the signal that detects the ground fault conditions based on the modified value of ground fault failure threshold , use the signal that detects the neutral conditions with ground connection based on the modified value of neutral threshold with ground connection, start the generation of a disconnection signal based on the detection of a ground fault condition or neutral with ground connection, and interrupting the flow of current in the power distribution system in response to the disconnect signal.
8. 8. A method for detecting ground fault and neutral conditions with grounding in an electrical power distribution system having the linear and neutral conductors, characterized in that it comprises: producing a signal in response to the flow of current in both the linear and neutral conductors of the electric power distribution system with a sensor that contains a resonant circuit and a current transformer that has an inductance that varies with temperature, supplying the signal to a microcontroller having a predetermined value of failure threshold per ground connection and a predetermined value of neutral threshold with ground connection, and the microcontroller is programmed to initiate a pulse signal that produces a damped oscillation in an output signal from the sensor during the neutral test with ground connection, measure the frequency of the damped oscillation to determine the change in the inductance of the current transformer, calculate the modified value of the ground fault failure threshold based on the Default value of ground fault failure threshold and change in inductance of the current transformer, calculate a modified value of ground fault failure threshold based on the neutral threshold value with ground connection and change in the inductance of the current transformer, use the modified value of the ground fault fault that detects the ground fault condition, use the modified value of the neutral threshold with ground connection that detects the neutral condition with ground connection, and start the generation of a disconnection signal based on the detection of the condition of failure due to earth or neutral connection with ground connection in the power distribution system.