Title of Invention	ANALOG CIRCUITRY TO MONITOR EARTH LEAKAGE CURRENT
Abstract	An electronic current detection circuit is disclosed that comprises a multiphase power supply and a logic circuitry having a user settable circuit electrically connected to said multiphase power supply, an ac/dc generator electrically connected to said user settable circuit wherein under fault condition said ac/dc generator generates an ac signal upon sensing any leakage current value greater than a threshold value and said ac signal is used in a trip system, to isolate any device from the mains or/and is used in remote indications.

Title of Invention

ANALOG CIRCUITRY TO MONITOR EARTH LEAKAGE CURRENT

Abstract

An electronic current detection circuit is disclosed that comprises a multiphase power supply and a logic circuitry having a user settable circuit electrically connected to said multiphase power supply, an ac/dc generator electrically connected to said user settable circuit wherein under fault condition said ac/dc generator generates an ac signal upon sensing any leakage current value greater than a threshold value and said ac signal is used in a trip system, to isolate any device from the mains or/and is used in remote indications.

Full Text	FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENS RULES, 2003 COMPLETE SPECIFICATION [See section 10, Rule 13] ANALOG CIRCUITRY TO MONITOR EARTH LEAKAGE CURRENT; LARSEN & TOUBRO LIMITED, A COMPANY INCORPORATED UNDER THE COMPANIES ACT, 1956, WHOSE ADDRESS IS L&T HOUSE, BALLARD ESTATE, MUMBAI - 400 001, MAHARASHTRA, INDIA THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED. 1 FIIELD OF INVENTION: This invention relates to earth leakage current detection devices. BACKGROUND OF THE INVENTION Earth leakage current is a fault current that is generally generated by any malfunction of the power lines or the electrical loads to which the power line is connected. In other words, a fault current occurs when a live conductor part makes electrical contact with ground. The earth leakage current flows towards earth when a live part of an electrical system is in contact with a human body or if an insulation fault occurs in the system. The fault current then flows via the person in physical contact, as a body current, and in the system as a fault current, to earth. This fault current can be dangerous. To ensure protection against such dangerous fault current in an electrical system, a residual current device is used. The residual current device immediately isolates the electrical system from the mains power supply in the event of a fault current, which is greater than the rated fault current. The rated fault current is normally 30 mA for human protection and 200 mA for system protection. It is known in the art that the residual current devices detect any imbalance in current flow through live and neutral conductors by passing the conductors through a CBCT (Core Balanced Current Transformer) comprising a sensing winding. The CBCT basically detects an imbalance between the current flowing in the live conductor with respect to the current flowing in the neutral conductor, and this imbalance results in a current being induced into the 2 sensing winding. This winding is then connected to suitable electronic circuitry for the necessary action. Patent No.US 6 025 980 (Earth leakage protective relay) provides a load resistor connected to the transformer for detecting earth leakage current. Such designs detect only earth leakage current and not the earth fault current. The output of such relays is given to a dedicated switching transistor in order to activate the electric relay to interrupt said circuit current. However, as soon as the system is interrupted, the electronic circuit gets de-energized and the output toggles back to original state. Another US patent bearing number US 6 483 681 (Fault-current protective switchgear) discloses a design that uses an amplifier circuit producing a fault voltage derived from fault current and a comparison circuit for comparing the fault voltage with a reference voltage. The output of this design is a switching transistor for operating a relay which is not Isolated from the circuit and hence not used further to activate any circuit such as a trip circuit. Yet another patent US 6 882 511 entitled "Electronic Earth Leakage current device", discloses an invention that uses a circuit comprising a moving contact and a fixed contact which can be mutually coupled/ uncoupled. The actuation means of this design is a part of the unit itself i.e. it is a dedicated output, and is not used in any further circuit. Thus, there is a continuing need for efficient and cost effective circuit for earth leakage current detection device, which can provide an isolated output to be further used in other necessary circuits as well. 3 SUMMARY OF THE INVENTION: The present invention provides a simple and inexpensive residual current device for detecting the earth leakage current. The present invention provides an electronic current detection circuit comprising: a multiphase power supply.logic circuitry having a user settable circuit electrically connected to said multiphase power supply; an ac/dc generator electrically connected to said user settable circuit; wherein under fault condition said ac/dc generator generates an ac signal upon sensing any leakage current value greater than a threshold value and said ac signal is used in a trip system, to isolate any device from the mains and is used in remote indications. A trip unit including circuit breakers is used as a trip system for isolating the device from the mains whereas a potential free contact is used for producing remote indications. As per the present invention the multiphase logic circuitry detects the leakage current or the- fault current. The user settable circuit can be used as an operational amplifier or a voltage follower or an attenuator for determining the threshold value of the leakage current. The invention also allows a user to change the threshold value in the same circuit. The AC/DC generator generates a dc signal under normal condition and an ac signal under fault condition. The dc signal generated by the AC/DC generator under normal condition is blocked by a filter circuit. However, under the fault condition, the ac signal is generated that is passed through the filter circuit to a DC or averaging circuit. Thus a DC or average value of said ac signal generated by the AC/DC generator under fault condition. The device of 4 the invention also includes a comparator to prevent the occurrence of false tripping and a time delay circuit for setting a time delay by the user. The output of the electronic current detection circuit design consists of an optocoupler with a DC input and an AC driven output. An advantage of this design is that, once the output is triggered the output remains in that state (irrespective of the system) till it is reset. Another advantage of this circuit is that the optically isolated output can be used in any other circuit as per the customers need. The invention uses multiple phases in power supply hence the circuit works, efficiently with any single phase. Other features, advantages and objects of the invention will be apparent from the following description. BRIEF DESCRIPTION OF THE DRAWINGS: Figure 1 shows the electronic earth leakage current detection circuit. Figure 2 shows the user settable operational amplifier circuit. Figure 3 shows the schmitt trigger along with averaging circuit and high pass filter. Figure 4 shows the comparator circuit and the time delay circuit. Figure 5 shows the trip unit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS: The present invention provides a device for detecting the earth leakage current, which produces an output to trip the system and isolate the device downstream from the mains. The invention also provides a potential free 5 contact that can be used for remote indication. The output of the device consists of an optocoupler with a DC input and an AC driven output, which can be used as per customers need. Figure 1 shows block diagram of the device as per the present invention. The device comprises a Core Balanced Current Transformer for measuring the vector sum of current passing through all the phases and neutral thereby detecting the presence of leakage current in the system. This Leakage current or the resultant current is converted to an earth leakage voltage or an earth fault voltage. This is accomplished by using a switch 4 positioned in a desired manner in order to select resistor R1 or R2 as a burden resistor. This earth leakage voltage or an earth fault voltage is given as an input to a user settable circuit 6, which is basically used to set the threshold value of the leakage current or the tripping current value. The output of user settable circuit 6 is further given to an AC/DC generator 7 through a conductor 13 which produces an ac signal when its input exceeds the said threshold value. This AC/DC generator may be in the form of a Schmitt Trigger and the ac signal may be in the form of a square wave of 50 Hertz. This signal is given to a filter circuit such as a High Pass Filter (HPF) 8 through conductor 14 and the HPF in turn gets connected to DC or Averaging Circuit 9 through conductor 15.The HPF 8 blocks the DC output of the Schmitt Trigger 7 and allows ac signal i.e. 50 Hertz square wave to pass to the DC or Averaging Circuit 9 through conductor 15. The average output is further compared to a threshold by a comparator 10 to avoid false tripping. Consequently, a Time Delay circuit 11 gets its input from conductor 17 wherein a particular time delay is added by the user through a selector switch (not shown). The output gets connected to 6 the Tripping Unit 12 through conductor 22b, which consists of two parts. First part is an optically isolated output for an alarm annunciation or as a trip signal to a circuit breaker and second part is a MOSFET used to drive a FSD, which in turn drives a potential free contact for remote indication. As explained earlier, the output of CBCT (1) is given to a user settable circuit, which may act as an operational amplifier, a voltage follower or an attenuator, depending upon the leakage value of the current set by the user at which he/she desires tripping. The output of this user settable circuit is given to the Schmitt trigger. When the input of Schmitt Trigger is greater than the threshold value, it generates an ac signal in form of a square wave. This square wave basically represents the leakage current which suggests that if the leakage current is above the threshold value that the user has set, an immediate action needs to be taken for the same. Under normal conditions the output of Schmitt Trigger is a DC signal, which is blocked by the high pass filter. When a leakage current is above the set point of user, the output of the Schmitt Trigger generates an ac signal of a 50 Hz square wave, which goes to the DC or the Average through the high pass filter. The output of the averaging circuit is given to the comparator where the average value is compared with one more threshold to avoid false tripping. This output goes to a capacitors bank that is set according to the time delay set by the user. Finally, the output drives an Opto-Coupler having an input as a DC signal and the output is driven by an ac voltage signal ranging from 110V to 415V. This voltage can be used as an alarm annunciation or for tripping a Circuit Breaker. Further, the output signal is given to a MOSFET, which is used to drive an 7 FSD. This FSD is associated with a mechanism, which provides a potential free contact for the user. The present invention further comprises multiphase power supply, which powers the internal circuitry and provides the working voltage for the associated electronic circuitry. This power supply provides an in-built surge and impulse protection. Fig.2 shows the user settable circuit 6, which can be used as an Amplifier, or a Voltage Follower or an Attenuator according to the switch 24 position to select resistors R4-R30. The circuit comprises resistors R3-R30 and a capacitor C1 that gets connected between conductor 13 and 26 and the operational amplifier 19. The resistor values are selected to provide an output signal G1 to the Schmitt Trigger 7 of Fig.1 having the output in accordance with the following equation.: Where , Ri = R3 C = C1 Rf = (R4 + R28) when switch 24 position is at point. 1 such that (R4 + R28) > R3 hence used as an Amplifier. Hence , G1={(R4 + R28)/ R3}/sqrt (1 + square(w[R4 + R28JC1)) 8 Rf = (R5 + R29) when switch 24 position is at point 2 such that (R5 + R29) = R3 hence used as a Voltage Follower Hence , G1={(R5 + R29)/ R3}/sqrt (1 + square(u)[R5 + R29JC1)) Rf = (R6 + R30) when switch 24 position is at pt 3 such that (R6 + R30) The schmitt trigger 7 along with high pass filter 8 and DC or averaging circuit 9 is shown in Fig.3. The threshold value for the Schmitt Trigger is set by resistors R13 and R14 given by the following equation: G2(pt.14)=R14 (Vsat) / (R13 R14) Equation2 When input to Schmitt Trigger 7 at conductor 13 is less than the threshold given by Equation 2, the output is a DC value, which is blocked by the HPF 13 or RC circuit consisting of C2 and R15 and hence output of circuit 14 of fig.1 is zero or less than the next stage threshold determined by resistors R26 and R27. When input to Schmitt Trigger 7 as shown in figure 1 is more than threshold given by equation 2, the output is a square wave of 50 Hertz, which passes the HPF circuit 8 of fig.1 and the output of DC or averaging circuit is DC, which is greater than the threshold of the next stage. The output of DC or averaging circuit 9 is further connected to the comparator 10 by conductor 16 as shown in fig.4 with a threshold value decided by resistors R26 and R27. The output goes to the Time delay circuit 11 through conductor 17 as shown in Fig.4a. Figure 4b shows the time delay being set by 9 the switch 25 selecting a capacitor from the capacitor bank C4 to C9. This charge in the capacitor is compared to another threshold decided by register divider R19 and R20 and Operational Amplifier 22a. As explained earlier, the output signal from the time delay circuit 11 gets connected to the Tripping Unit 12 through conductor 22b, which consists of two parts namely an optically isolated output for alarm annunciation or as a trip signal to circuit breaker, and a MOSFET that can be used to drive an FSD for potential free contact. This Trip Unit is as shown in Fig.5. In Fig.5 the optocoupler 23 has a DC input and AC output. The resistor R22 is used to control the input current in the optocoupler where as resistor R23 is used to prevent the current in the output stage of the optocoupler. Once the Trip signal is given to optocoupler through conductor 22b it triggers its output thus providing a low impedance path to the signal at point 2. This triggers the switching triac Q2 into conduction. The triac in turn provides current to input coil of associated shunt coil or load of the output relay causing the circuit breaker to trip capacitor C10 is provided for noise immunity in the supply and avoid false tripping. Further, the output of the operational amplifier 22a is given to voltage divider network R24 and R25, which is connected to the gate point 4 of the switching MOSFET Q1, which is source connected to ground. The drain point 5 of Q1 connects to voltage supply (Vcc) through coil of the associated FSD and diode D3 to-provide bias to the FET and operate FSD to give the 'TRIP' signal at a remote location or to drive a potential free contact which changes state when it receives a trip signal. 10 Thus the present invention is designed such that the device has an optically isolated output, which can be used as per the customers need. The design may also have another output, which is potential free for remote indication. Further, the invention is designed to use all the three phases in power supply, which enables the circuit to work properly with any single phase. The design of the present invention also allows the output to remain in ON condition once the output is triggered ON. Therefore, irrespective of the input failure, the output is continuously active till the system is reset. The invention provides the usage of same circuit for any rating by simply changing the CBCT of that value. Furthermore, the inbuilt power supply provides surge and impulse protection. The foregoing description of the invention has been described for purposes of clarity and understanding. It is not intended to limit the invention to the precise form disclosed. Various modifications may be possible within the scope and equivalence of the appended claims. 11 WE CLAIM 1. An analog circuitry to monitor earth leakage current comprising: a multiphase power supply; a logic circuitry having a user settable circuit electrically connected to said multiphase power supply; an AC/DC generator electrically connected to said user settable circuit; wherein under fault condition said AC/DC generator generates an AC signal upon sensing any leakage current value greater than a threshold value and said AC signal is used in a trip system to isolate any device from the mains or said AC signal is used in remote indications. 2. An analog circuitry to monitor earth leakage current as claimed in claim 1 wherein the logic circuitry detects the leakage current or the fault current and converts it to leakage voltage or fault voltage using two burden resistors. 3. An analog circuitry to monitor earth leakage current as claimed in claim 1 wherein the user settable circuit act as an operational amplifier or a voltage follower or an attenuator. 4. An analog circuitry to monitor earth leakage current as claimed in claim 1, wherein said AC/DC generator is a Schmitt trigger. 5. An analog circuitry to monitor earth leakage current as claimed in claim 1 wherein thethreshold value of the leakage current is set in the user settable circuit. 12 6. An analog circuitry to monitor earth leakage current as claimed in claim 1 further comprises a filter circuit for blocking the DC signal generated by the AC/DC generator under normal' condition. 7. An analog circuitry to monitor earth leakage current as claimed in claim 1 further comprises a DC or averaging circuit for obtaining a DC or average value of said AC signal generated by the AC/DC generator under fault condition. 8. An analog circuitry to monitor earth leakage current as claimed in claim 1 further comprises a comparator to prevent the occurrence of false tripping. 9. An analog circuitry to monitor earth leakage current as claimed in claim 1 further comprises a time delay circuit for setting a time delay by the user. 10. An analog circuitry to monitor earth leakage current as claimed in claim 1 wherein a trip unit including circuit breakers is used for isolating the device from the mains. 11. An analog circuitry to monitor earth leakage current as claimed in claim 1 wherein a trip unit including potential free contact is used for producing remote indications. Dated this 17th day of March, 2006. FOR LARSEN & TOUBRO LIMITED By their Agent (GIRISH VIJAYANAND SHETH) KRISHNA &SAURASTRI 13 ABSTRACT An electronic current detection circuit is disclosed that comprises a multiphase power supply and a logic circuitry having a user settable circuit electrically connected to said multiphase power supply, an ac/dc generator electrically connected to said user settable circuit wherein under fault condition said ac/dc generator generates an ac signal upon sensing any leakage current value greater than a threshold value and said ac signal is used in a trip system, to isolate any device from the mains or/and is used in remote indications. 14

Full Text

FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENS RULES, 2003
COMPLETE SPECIFICATION
[See section 10, Rule 13]
ANALOG CIRCUITRY TO MONITOR EARTH LEAKAGE CURRENT;
LARSEN & TOUBRO LIMITED, A COMPANY INCORPORATED UNDER THE COMPANIES ACT, 1956, WHOSE ADDRESS IS L&T HOUSE, BALLARD ESTATE, MUMBAI - 400 001, MAHARASHTRA, INDIA
THE FOLLOWING SPECIFICATION
PARTICULARLY DESCRIBES THE
INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED.
1

FIIELD OF INVENTION:
This invention relates to earth leakage current detection devices.
BACKGROUND OF THE INVENTION
Earth leakage current is a fault current that is generally generated by any malfunction of the power lines or the electrical loads to which the power line is connected. In other words, a fault current occurs when a live conductor part makes electrical contact with ground. The earth leakage current flows towards earth when a live part of an electrical system is in contact with a human body or if an insulation fault occurs in the system. The fault current then flows via the person in physical contact, as a body current, and in the system as a fault current, to earth. This fault current can be dangerous.
To ensure protection against such dangerous fault current in an electrical system, a residual current device is used. The residual current device immediately isolates the electrical system from the mains power supply in the event of a fault current, which is greater than the rated fault current. The rated fault current is normally 30 mA for human protection and 200 mA for system protection.
It is known in the art that the residual current devices detect any imbalance in current flow through live and neutral conductors by passing the conductors through a CBCT (Core Balanced Current Transformer) comprising a sensing winding. The CBCT basically detects an imbalance between the current flowing in the live conductor with respect to the current flowing in the neutral conductor, and this imbalance results in a current being induced into the
2

sensing winding. This winding is then connected to suitable electronic circuitry for the necessary action.
Patent No.US 6 025 980 (Earth leakage protective relay) provides a load resistor connected to the transformer for detecting earth leakage current. Such designs detect only earth leakage current and not the earth fault current. The output of such relays is given to a dedicated switching transistor in order to activate the electric relay to interrupt said circuit current. However, as soon as the system is interrupted, the electronic circuit gets de-energized and the output toggles back to original state.
Another US patent bearing number US 6 483 681 (Fault-current protective
switchgear) discloses a design that uses an amplifier circuit producing a fault voltage derived from fault current and a comparison circuit for comparing the fault voltage with a reference voltage. The output of this design is a switching transistor for operating a relay which is not Isolated from the circuit and hence not used further to activate any circuit such as a trip circuit.
Yet another patent US 6 882 511 entitled "Electronic Earth Leakage current
device", discloses an invention that uses a circuit comprising a moving contact and a fixed contact which can be mutually coupled/ uncoupled. The actuation means of this design is a part of the unit itself i.e. it is a dedicated output, and is not used in any further circuit.
Thus, there is a continuing need for efficient and cost effective circuit for earth leakage current detection device, which can provide an isolated output to be further used in other necessary circuits as well.
3

SUMMARY OF THE INVENTION:
The present invention provides a simple and inexpensive residual current device for detecting the earth leakage current.
The present invention provides an electronic current detection circuit comprising: a multiphase power supply.logic circuitry having a user settable circuit electrically connected to said multiphase power supply; an ac/dc generator electrically connected to said user settable circuit; wherein under fault condition said ac/dc generator generates an ac signal upon sensing any leakage current value greater than a threshold value and said ac signal is used in a trip system, to isolate any device from the mains and is used in remote indications. A trip unit including circuit breakers is used as a trip system for isolating the device from the mains whereas a potential free contact is used for producing remote indications.
As per the present invention the multiphase logic circuitry detects the leakage current or the- fault current. The user settable circuit can be used as an operational amplifier or a voltage follower or an attenuator for determining the threshold value of the leakage current. The invention also allows a user to change the threshold value in the same circuit.
The AC/DC generator generates a dc signal under normal condition and an ac signal under fault condition. The dc signal generated by the AC/DC generator under normal condition is blocked by a filter circuit. However, under the fault condition, the ac signal is generated that is passed through the filter circuit to a DC or averaging circuit. Thus a DC or average value of said ac signal generated by the AC/DC generator under fault condition. The device of
4

the invention also includes a comparator to prevent the occurrence of false tripping and a time delay circuit for setting a time delay by the user.
The output of the electronic current detection circuit design consists of an optocoupler with a DC input and an AC driven output. An advantage of this design is that, once the output is triggered the output remains in that state (irrespective of the system) till it is reset. Another advantage of this circuit is that the optically isolated output can be used in any other circuit as per the customers need.
The invention uses multiple phases in power supply hence the circuit works, efficiently with any single phase.
Other features, advantages and objects of the invention will be apparent from the following description.
BRIEF DESCRIPTION OF THE DRAWINGS:
Figure 1 shows the electronic earth leakage current detection circuit.
Figure 2 shows the user settable operational amplifier circuit.
Figure 3 shows the schmitt trigger along with averaging circuit and high pass filter.
Figure 4 shows the comparator circuit and the time delay circuit.
Figure 5 shows the trip unit.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:
The present invention provides a device for detecting the earth leakage current, which produces an output to trip the system and isolate the device downstream from the mains. The invention also provides a potential free
5

contact that can be used for remote indication. The output of the device consists of an optocoupler with a DC input and an AC driven output, which can be used as per customers need.
Figure 1 shows block diagram of the device as per the present invention. The device comprises a Core Balanced Current Transformer for measuring the vector sum of current passing through all the phases and neutral thereby detecting the presence of leakage current in the system. This Leakage current or the resultant current is converted to an earth leakage voltage or an earth fault voltage. This is accomplished by using a switch 4 positioned in a desired manner in order to select resistor R1 or R2 as a burden resistor. This earth leakage voltage or an earth fault voltage is given as an input to a user settable circuit 6, which is basically used to set the threshold value of the leakage current or the tripping current value. The output of user settable circuit 6 is further given to an AC/DC generator 7 through a conductor 13 which produces an ac signal when its input exceeds the said threshold value. This AC/DC generator may be in the form of a Schmitt Trigger and the ac signal may be in the form of a square wave of 50 Hertz. This signal is given to a filter circuit such as a High Pass Filter (HPF) 8 through conductor 14 and the HPF in turn gets connected to DC or Averaging Circuit 9 through conductor 15.The HPF 8 blocks the DC output of the Schmitt Trigger 7 and allows ac signal i.e. 50 Hertz square wave to pass to the DC or Averaging Circuit 9 through conductor 15. The average output is further compared to a threshold by a comparator 10 to avoid false tripping. Consequently, a Time Delay circuit 11 gets its input from conductor 17 wherein a particular time delay is added by the user through a selector switch (not shown). The output gets connected to
6

the Tripping Unit 12 through conductor 22b, which consists of two parts. First part is an optically isolated output for an alarm annunciation or as a trip signal to a circuit breaker and second part is a MOSFET used to drive a FSD, which in turn drives a potential free contact for remote indication.
As explained earlier, the output of CBCT (1) is given to a user settable circuit, which may act as an operational amplifier, a voltage follower or an attenuator, depending upon the leakage value of the current set by the user at which he/she desires tripping. The output of this user settable circuit is given to the Schmitt trigger. When the input of Schmitt Trigger is greater than the threshold value, it generates an ac signal in form of a square wave. This square wave basically represents the leakage current which suggests that if the leakage current is above the threshold value that the user has set, an immediate action needs to be taken for the same. Under normal conditions the output of Schmitt Trigger is a DC signal, which is blocked by the high pass filter. When a leakage current is above the set point of user, the output of the Schmitt Trigger generates an ac signal of a 50 Hz square wave, which goes to the DC or the Average through the high pass filter. The output of the averaging circuit is given to the comparator where the average value is compared with one more threshold to avoid false tripping. This output goes to a capacitors bank that is set according to the time delay set by the user.
Finally, the output drives an Opto-Coupler having an input as a DC signal and the output is driven by an ac voltage signal ranging from 110V to 415V. This voltage can be used as an alarm annunciation or for tripping a Circuit Breaker. Further, the output signal is given to a MOSFET, which is used to drive an
7

FSD. This FSD is associated with a mechanism, which provides a potential free contact for the user.
The present invention further comprises multiphase power supply, which powers the internal circuitry and provides the working voltage for the associated electronic circuitry. This power supply provides an in-built surge and impulse protection.
Fig.2 shows the user settable circuit 6, which can be used as an Amplifier, or a Voltage Follower or an Attenuator according to the switch 24 position to select resistors R4-R30. The circuit comprises resistors R3-R30 and a capacitor C1 that gets connected between conductor 13 and 26 and the operational amplifier 19. The resistor values are selected to provide an output signal G1 to the Schmitt Trigger 7 of Fig.1 having the output in accordance with the following equation.:

Where ,
Ri = R3 C = C1
Rf = (R4 + R28) when switch 24 position is at point. 1 such that (R4 + R28) > R3 hence used as an Amplifier.
Hence , G1={(R4 + R28)/ R3}/sqrt (1 + square(w[R4 + R28JC1))
8

Rf = (R5 + R29) when switch 24 position is at point 2 such that (R5 + R29) = R3 hence used as a Voltage Follower
Hence , G1={(R5 + R29)/ R3}/sqrt (1 + square(u)[R5 + R29JC1))
Rf = (R6 + R30) when switch 24 position is at pt 3 such that (R6 + R30) The schmitt trigger 7 along with high pass filter 8 and DC or averaging circuit
9 is shown in Fig.3. The threshold value for the Schmitt Trigger is set by
resistors R13 and R14 given by the following equation:
G2(pt.14)=R14 (Vsat) / (R13
R14) Equation2
When input to Schmitt Trigger 7 at conductor 13 is less than the threshold
given by Equation 2, the output is a DC value, which is blocked by the HPF 13
or RC circuit consisting of C2 and R15 and hence output of circuit 14 of fig.1
is zero or less than the next stage threshold determined by resistors R26 and
R27. When input to Schmitt Trigger 7 as shown in figure 1 is more than
threshold given by equation 2, the output is a square wave of 50 Hertz, which
passes the HPF circuit 8 of fig.1 and the output of DC or averaging circuit is
DC, which is greater than the threshold of the next stage.
The output of DC or averaging circuit 9 is further connected to the comparator
10 by conductor 16 as shown in fig.4 with a threshold value decided by
resistors R26 and R27. The output goes to the Time delay circuit 11 through
conductor 17 as shown in Fig.4a. Figure 4b shows the time delay being set by
9

the switch 25 selecting a capacitor from the capacitor bank C4 to C9. This charge in the capacitor is compared to another threshold decided by register divider R19 and R20 and Operational Amplifier 22a.
As explained earlier, the output signal from the time delay circuit 11 gets connected to the Tripping Unit 12 through conductor 22b, which consists of two parts namely an optically isolated output for alarm annunciation or as a trip signal to circuit breaker, and a MOSFET that can be used to drive an FSD for potential free contact.
This Trip Unit is as shown in Fig.5. In Fig.5 the optocoupler 23 has a DC input and AC output. The resistor R22 is used to control the input current in the optocoupler where as resistor R23 is used to prevent the current in the output stage of the optocoupler. Once the Trip signal is given to optocoupler through conductor 22b it triggers its output thus providing a low impedance path to the signal at point 2. This triggers the switching triac Q2 into conduction. The triac in turn provides current to input coil of associated shunt coil or load of the output relay causing the circuit breaker to trip capacitor C10 is provided for noise immunity in the supply and avoid false tripping.
Further, the output of the operational amplifier 22a is given to voltage divider network R24 and R25, which is connected to the gate point 4 of the switching MOSFET Q1, which is source connected to ground. The drain point 5 of Q1 connects to voltage supply (Vcc) through coil of the associated FSD and diode D3 to-provide bias to the FET and operate FSD to give the 'TRIP' signal at a remote location or to drive a potential free contact which changes state when it receives a trip signal.
10

Thus the present invention is designed such that the device has an optically isolated output, which can be used as per the customers need. The design may also have another output, which is potential free for remote indication. Further, the invention is designed to use all the three phases in power supply, which enables the circuit to work properly with any single phase. The design of the present invention also allows the output to remain in ON condition once the output is triggered ON. Therefore, irrespective of the input failure, the output is continuously active till the system is reset. The invention provides the usage of same circuit for any rating by simply changing the CBCT of that value. Furthermore, the inbuilt power supply provides surge and impulse protection.
The foregoing description of the invention has been described for purposes of clarity and understanding. It is not intended to limit the invention to the precise form disclosed. Various modifications may be possible within the scope and equivalence of the appended claims.
11

WE CLAIM
1. An analog circuitry to monitor earth leakage current comprising:
a multiphase power supply;
a logic circuitry having a user settable circuit electrically connected to said multiphase power supply;
an AC/DC generator electrically connected to said user settable circuit;
wherein under fault condition said AC/DC generator generates an AC signal upon sensing any leakage current value greater than a threshold value and said AC signal is used in a trip system to isolate any device from the mains or said AC signal is used in remote indications.
2. An analog circuitry to monitor earth leakage current as claimed in claim 1 wherein the logic circuitry detects the leakage current or the fault current and converts it to leakage voltage or fault voltage using two burden resistors.
3. An analog circuitry to monitor earth leakage current as claimed in claim 1 wherein the user settable circuit act as an operational amplifier or a voltage follower or an attenuator.
4. An analog circuitry to monitor earth leakage current as claimed in claim 1, wherein said AC/DC generator is a Schmitt trigger.
5. An analog circuitry to monitor earth leakage current as claimed in claim 1 wherein thethreshold value of the leakage current is set in the user settable circuit.
12

6. An analog circuitry to monitor earth leakage current as claimed in claim 1 further comprises a filter circuit for blocking the DC signal generated by the AC/DC generator under normal' condition.
7. An analog circuitry to monitor earth leakage current as claimed in claim 1 further comprises a DC or averaging circuit for obtaining a DC or average value of said AC signal generated by the AC/DC generator under fault condition.
8. An analog circuitry to monitor earth leakage current as claimed in claim 1 further comprises a comparator to prevent the occurrence of false tripping.
9. An analog circuitry to monitor earth leakage current as claimed in claim 1 further comprises a time delay circuit for setting a time delay by the user.
10. An analog circuitry to monitor earth leakage current as claimed in claim 1 wherein a trip unit including circuit breakers is used for isolating the device from the mains.
11. An analog circuitry to monitor earth leakage current as claimed in claim 1 wherein a trip unit including potential free contact is used for producing remote indications.
Dated this 17th day of March, 2006.
FOR LARSEN & TOUBRO LIMITED By their Agent
(GIRISH VIJAYANAND SHETH) KRISHNA &SAURASTRI
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ABSTRACT
An electronic current detection circuit is disclosed that comprises a multiphase power supply and a logic circuitry having a user settable circuit electrically connected to said multiphase power supply, an ac/dc generator electrically connected to said user settable circuit wherein under fault condition said ac/dc generator generates an ac signal upon sensing any leakage current value greater than a threshold value and said ac signal is used in a trip system, to isolate any device from the mains or/and is used in remote indications.
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Documents:

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Patent Number

270128

Indian Patent Application Number

385/MUM/2006

PG Journal Number

49/2015

Publication Date

04-Dec-2015

Grant Date

30-Nov-2015

Date of Filing

17-Mar-2006

Name of Patentee

LARSEN & TOUBRO LIMITED

Applicant Address

L & T HOUSE, BALLARD ESTATE, MUMBAI-400 001,

Inventors:

#	Inventor's Name	Inventor's Address
1	GALA MAYUR SHAMJI	LARSEN & TOUBRO LIMITED, ELECTRICAL SECTOR-(EBG), POWAI WORKS(E) GATE NO.7, SAKI VIHAR ROAD, MUMBAI-400 072,

PCT International Classification Number

H02H3/26

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA