FORM 2
THE PATENT ACT 1970
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)
1. TITLE OF THE INVENTION:
"ELECTRONIC TRIP UNIT (ETU) FOR A CIRCUIT BREAKER"
2. APPLICANT:
(a) NAME: Larsen & Toubro Limited
(b) NATIONALITY: Indian Company registered under the
provisions of the Companies Act-195 6.
(c) ADDRESS: LARSEN & TOUBRO LIMITED,
L&T House, Ballard Estate, P. O. Box: 278, Mumbai 400 001, India
3. PREAMBLE TO THE DESCRIPTION:
COMPLETE
The following specification particularly describes the invention and the manner in which it is to be performed.

ELECTRONIC TRIP UNIT (ETU) FOR A CIRCUIT BREAKER
FIELD OF INVENTION
The present invention relates to the field of Electronic Trip Unit (ETU) for circuit breakers. More particularly, the present invention relates to a computation system for an Electronic Trip Unit (ETU) for circuit breakers.
BACKGROUND OF THE INVENTION
Generally, electrical faults such as short circuiting, overloading and earth faults in an electrical circuit or a power distribution network of a power distribution system may cause irreparable damage to the power distribution system. Further, such electrical faults may also cause fire hazards. To protect the power distribution system against such damages, a variety of safety devices such as fuses and circuit breakers are available. A circuit breaker is an automatically operated electrical switch that cuts off electrical flow to the remaining power distribution network or isolates the faulty power distribution network in an event of a possible electrical fault, such as short-circuit, overload, and earth faults, thereby protecting the power distribution system against damages caused by short-circuiting or overloading. The circuit breaker includes a trip unit, that may be an electro-mechanical trip unit or an electronic trip unit. The electronic trip unit is a programmable device which measures and times current flowing through the circuit breaker and initiates a trip signal to the circuit breaker when appropriate. More specifically, the electronic trip unit detects fault conditions based on current and voltage measurements and issues a trip command to the circuit breaker, the electronic trip unit also facilitate in recording and displaying the trip time, i.e. the time at which circuit breaker tripping occurred. The circuit breaker in-turn, by interrupting the electrical continuity immediately discontinues electrical flow to the remaining power distribution network, thereby isolating the faulty power distribution network and protecting the

remaining power distribution network against any further damage. The electronic trip unit is basically used to provide protection and control for circuit breaker under electrical fault conditions. The electronic trip unit causes discontinuation of service of the circuit breaker in presence of a short circuit, or when the circuit breaker starts to operate in any abnormal manner that might cause damage or otherwise interfere with the operation of the rest of the electrical system of which the circuit breaker is a part of. The electronic trip unit may include an interface for facilitating the programming the electronic trip unit.
The conventional Electronic Trip Unit (ETU) uses a computation system and an AC to DC Converter (ADC), particularly, an unipolar AC to DC converter (ADC) for metering current and voltage in an electrical system. The conventional Electronic Trip Unit (ETU) issues a trip command to the circuit breaker based on the current and voltage measurements. In case of the Electronic Trip Unit (ETU) using the unipolar ADC, a signal with positive cycle and negative cycle will lead to clipping of negative cycle during analog to digital conversion. In order to prevent negative cycle clipping, a DC offset of Vdd/2 (where Vdd is the supply voltage to the microcontroller) is given to the signal that enables conversion of the positive cycle and negative cycle to digital value by the ADC. In the firmware loaded in the microcontroller, this digital value of DC offset has to be subtracted to get actual AC signal sample value to enable use of these AC signal sample values for Root Mean Square (RMS) computation which is used for metering current and voltage. The current and voltage measurement done by the Electronic Trip Unit (ETU) is in turn used by other modules such as circuit breakers for providing protection against electrical faults such as short circuit, overload and so on.
More specifically, the computation system used in conventional Electronic Trip Unit (ETU) adds the DC offset to the AC signal so that it can be converted to digital value using unipolar ADC. Traditionally, offset calibration procedure is carried out in which DC offset given to each phase is saved by a microcontroller

of the computation system in a memory and during every power up, the DC offset value is accessed from the memory and used to subtract from the sample value to get actual AC signal sample value. The main drawback of the traditional method is that it does not considers fluctuation in DC offset, wherein the DC offset usually fluctuates due to change in temperature, component tolerance and fluctuation in voltage regulator output. The DC offset further fluctuates with use of different rating Rogowski coil in the electrical system. Even if the DC offset value fluctuates, the firmware uses constant DC offset value which is saved in the memory, thereby causing error in metering the current and voltage values by the conventional Electronic Trip Unit and also causes error in recording the trip time.
Another drawback of the traditional method is that it does not take into consideration, the impedance introduced due to connection of Rogowski coil. More specifically, the DC offset in the phase with Rogowski connection will be less than DC offset in phase without Rogowski connection. If the trip setting starts from very low value, it might lead to nuisance tripping due to incorrect RMS computation in the phase where Rogowski is not connected. Also, the Rogowski coil with different rating will introduce different impedance. Hence, the DC offset given to microcontroller will vary with use of different rating Rogowski coil. However, the conventional computation system of using constant DC offset value saved in the memory does not consider fluctuation in DC offset with use of different rating Rogowski coil.
Such errors in metering current and voltage values by the Electronic Trip Unit (ETU) may cause failure of the Electronic Trip Unit (ETU) to effectively control and regulate operation of the circuit breaker, thereby leading to inefficient and inaccurate operation of the circuit breaker and failure of the circuit breaker to prevent electrical faults such as short-circuit and' overload. Further, such inaccurate current and voltage measurements may lead to nuisance tripping. Further, as the Electronic Trip Unit (ETU) is also functionally coupled to a

variety of display units for providing information/data regarding circuit breaker tripping, such as current and voltage values at the time of circuit breaker tripping to maintenance personnel, wherein the display units display the current and voltage values based on erroneous metering by the Electronic Trip Unit, such erroneous information displayed at the display unit may be misleading for the maintenance personnel and may cause further confusion and chaos.
Accordingly, the conventionally used Electronic Trip Unit (ETU) has numerous disadvantages associated with using constant DC offset value saved in the memory for metering current and voltage, for example, the conventionally used Electronic Trip Unit (ETU) fails to accurately meter the current and voltage. More specifically, the conventionally used Electronic Trip Unit (ETU) fail to consider fluctuation in DC offset due to change in temperature, component tolerance and fluctuation in voltage regulator output while computing current and voltage and thereby provide erroneous voltage and current readings. Further, the conventionally used Electronic Trip Unit (ETU) fail to consider fluctuation in DC offset with use of different rating Rogowski coil and thereby provide erroneous voltage and current readings. Furthermore, the conventionally used Electronic Trip Unit (ETU) fails to accurately meter the trip time, i.e. the time at which circuit breaker tripping occurred. As the conventional Electronic Trip Unit fail to accurately meter current and voltage, the conventional Electronic Trip Unit also fail to accurately control and regulate operation of the circuit breaker, thereby deterring the circuit breakers from serving its purpose of preventing electrical faults such as short circuit and overload. Further, erroneous voltage and current readings by conventional Electronic Trip Unit (ETU) may lead to nuisance tripping. Further, the conventional Electronic Trip Unit (ETU) causes display of inaccurate information regarding tripping of the circuit breaker, thereby misleading the maintenance personnel and causing further confusion and chaos.

Accordingly, there is a need for an Electronic Trip Unit (ETU) that accurately meters the current and voltage in an electrical system. More specifically, there is a need for Electronic Trip Unit (ETU) that eliminates the draw-back associated with using constant DC offset value saved in the memory for metering current and voltage and considers fluctuation in DC offset due to change in temperature, component tolerance, fluctuation. in voltage regulator output and with use of different rating Rogowski coil. Further, there is a need for an Electronic Trip Unit (ETU) that accurately meters the trip time, i.e. the time at which circuit breaker tripping occurred. There is a need for an Electronic Trip Unit that accurately controls and regulates operation of the circuit breaker, thereby enabling the circuit breaker in preventing electrical faults such as short circuit and overload and also prevents nuisance tripping. Further, there is a need for Electronic Trip Unit (ETU) that facilitates display of accurate trip related information on display screens used for diagnosis of circuit breaker tripping, thereby assisting maintenance personal in analysing various parameters associated with the electrical circuit and the circuit breaker and accurately identifying the exact cause of the electrical fault that caused the circuit breaker tripping based on accurate information/data measured by the Electronic Trip Unit (ETU), before proceeding with the rectification of the electrical fault. Furthermore, there is a need for an Electronic Trip Unit (ETU) that assists maintenance personal in timely rectification of the electrical fault.
Further, there is a need for an Electronic Trip Unit (ETU) that is simple in construction, convenient to use and inexpensive. Still further, there is a need for an Electronic Trip Unit (ETU) that displays accurate and precise data required for identifying the nature of the electrical fault. Still further, there is a need for an Electronic Trip Unit (ETU) that is sturdy, does not fail and has longer service life. Further, there is a need for an Electronic Trip Unit (ETU) that may be easily retrofitted and used with any of the conventionally known circuit breakers.
OBJECTS OF THE INVENTION

Some of the objects of the present invention are described herein below:
It is an object of the present invention to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
An object of the present invention is to eliminate the drawbacks associated with using constant DC offset value saved in the memory for metering current and voltage by dynamically computing DC offset value per cycle and using the dynamically computed DC offset value per cycle for metering current and voltage.
Another object of the present invention is to provide an Electronic Trip Unit (ETU) that accurately meters the current and voltage in an electrical system.
Yet another object of the present invention is to provide an Electronic Trip Unit (ETU) that accurately meters the trip time, i.e. the time at which circuit breaker tripping occurred.
Still another object of the present invention is to provide an Electronic Trip Unit (ETU) that accurately controls and regulates operation of the circuit breaker based on accurate current and voltage measurements performed by the Electronic Trip Unit (ETU), thereby enabling the circuit breaker in effectively preventing electrical faults such as short circuit and overload.
SUMMARY OF THE INVENTION
An Electronic Trip Unit (ETU) for a circuit breaker is disclosed in accordance with an embodiment of the present disclosure. The Electronic Trip Unit (ETU) for a circuit breaker includes a timer, a storage memory, a computation system and an alarm system. The timer is initialized to a sampling period and facilitates

sampling of input signal value for each phase. The storage memory stores input signal sample for each cycle. The computation system meters current and Voltage and includes an AC to DC converter (ADC), a Direct Memory Access Controller (DMAC), a dynamic computation module. The AC to DC converter (ADC) is triggered with interruption of the timer. The Direct Memory Access Controller (DMAC) triggered by the AC to DC converter (ADC) and transfers the ADC conversion value to the storage memory. The dynamic computation module dynamically computes DC offset per cycle. The dynamic computation module further accesses sample value from the storage memory and utilizes the dynamically computed DC offset value per cycle for processing the sample value and computing Root Mean Square (RMS) value used for current and voltage measurement. The current and voltage measurements are in turn used by safety modules for providing protection against electrical faults. The alarm system raises an alarm if dynamic DC offset computed by the computation system is beyond a pre-determined range.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 illustrates a graph depicting variation of Rogowsksi output sample values (Digital) with different sample number;
Figure 2 illustrates a graph depicting variation of AC to DC Converter (ADC) input signal considering DC offset Digital value 512 with different sample number;
Figure 3 illustrates a graph depicting variation of AC to DC Converter (ADC) input signal considering DC offset Digital value 505 with different sample number;
Figure 4 illustrates a block diagram of an Electronic Trip Unit (ETU) in accordance with an embodiment of the present invention;

Figure 5 illustrates a flow diagram depicting the logic used in the main program of the Electronic Trip Unit of Figure 4; and
Figure 6 illustrates a flow diagram depicting the logic used in the Direct Memory Access Controller (DMAC) Interrupt Service Routine of the Electronic Trip Unit of Figure 4.
DETAILED DESCRIPTION OF THE INVENTION
The-invention will now be described with reference to the accompanying drawing which does not limit the scope and ambit of the invention. The description provided is purely by way of example and illustration.
The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the

purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.
Generally, electrical faults such as short circuiting, overloading and earth faults in an electrical circuit or a power distribution network of a power distribution system may cause irreparable damage to the power distribution system. Further, such electrical faults may also cause fire hazards. To protect the power distribution system against such damages, a variety of safety devices such as fuses and circuit breakers are available. A circuit breaker is an automatically operated electrical switch that cuts off electrical flow to the remaining power distribution network or isolates the faulty power distribution network in an event of a possible electrical fault, such as short-circuit, overload, and earth faults, thereby protecting the power distribution system against damages caused by short-circuiting or overloading. The circuit breaker includes a trip unit, that may be an electro-mechanical trip unit or an electronic trip unit. The electronic trip unit is a programmable device which measures and times current flowing through the circuit breaker and initiates a trip signal to the circuit breaker when appropriate.
The conventional Electronic Trip Unit (ETU) uses a computation system and an AC to DC Converter (ADC), particularly, an unipolar AC to DC converter (ADC) for metering current and voltage in an electrical system. The conventional Electronic Trip Unit (ETU) issues a, trip command to the circuit breaker based on the current and voltage measurements. In case of the Electronic Trip Unit (ETU) using the unipolar ADC, a signal with positive cycle and negative cycle will lead to clipping of negative cycle during analog to digital conversion. In order to prevent negative cycle clipping, a DC offset of Vdd/2 (where Vdd is the supply voltage to the microcontroller) is given to the signal that enables conversion of the positive cycle and negative cycle to digital

value by the ADC. In the firmware loaded in the microcontroller, this digital value of DC offset has to be subtracted to get actual AC signal sample value to enable use of these AC signal sample values for Root Mean Square (RMS) computation which is used for metering current and voltage. The current and voltage measurement done by the Electronic Trip Unit (ETU) is in turn used by other modules such as circuit breakers for providing protection against electrical faults such as short circuit, overload and so on.
More specifically, the computation system used in conventional Electronic Trip Unit (ETU) adds the DC offset to the AC signal so that it can be converted to digital value using unipolar ADC. Traditionally, offset calibration procedure is carried out in which DC offset given to each phase is saved by a microcontroller of the computation system in a memory and during every power up, the DC offset value is accessed from the memory and used to subtract from the sample value to get- actual AC signal sample value. The main drawback of the traditional method is that it does not considers fluctuation in DC offset, wherein the DC offset usually fluctuates due to change in temperature, component tolerance and fluctuation in voltage regulator output. The DC offset further fluctuates with use of different rating Rogowski coil in the electrical system. Even if the DC offset value fluctuates, the firmware uses constant DC offset value which is saved in the memory, thereby causing error in metering the current and voltage values by the conventional Electronic Trip Unit and also causes error in recording the trip time.
Another drawback of the traditional method is that it does not take into consideration, the impedance introduced due to connection of Rogowski coil. More specifically, the DC offset in the phase with Rogowski connection will be less than DC offset in phase without Rogowski connection. If the trip setting starts from very low value, it might lead to nuisance tripping due to incorrect RMS computation in the phase where Rogowski is not connected. Also, the Rogowski coil with different rating will introduce different impedance. Hence,

the DC offset given to microcontroller will vary with use of different rating Rogowski coil. However, the conventional computation system of using constant DC offset value saved in the memory does not consider fluctuation in DC offset with use of different rating Rogowski coil.
Such errors in metering current and voltage values by the Electronic Trip Unit (ETU) may cause failure of the Electronic Trip Unit (ETU) to effectively control and regulate operation of the circuit breaker, thereby leading to inefficient and inaccurate operation of the circuit breaker and failure of the circuit breaker to prevent electrical faults such as short-circuit and overload. Further, such inaccurate current and voltage measurements may lead to nuisance tripping. Further, as the Electronic Trip Unit (ETU) is also functionally coupled to a variety of display units for providing information/data regarding circuit breaker tripping, such as current and voltage values at the time of circuit breaker tripping to maintenance personnel, wherein the display units display the current and voltage values based on erroneous metering by the Electronic Trip Unit, such erroneous information displayed at the display unit may be misleading for the maintenance personnel and may cause further confusion and chaos.
The present disclosure envisages an Electronic Trip Unit (ETU) for circuit breakers that ameliorates one or more problems of the prior art, particularly, the Electronic Trip Unit (ETU) of the present invention eliminates drawbacks associated with conventional Electronic Trip Unit (ETU) that use constant DC offset value saved in the memory for metering current and voltage. The Electronic Trip Unit of the present invention dynamically computes DC offset value per cycle and uses the dynamically computed DC offset value per cycle for metering current and voltage. More Specifically, the Electronic Trip Unit (ETU) of the present invention utilizes a computation.system for computing the DC offset per cycle, the dynamically calculated DC offset per cycle is input to the AC to DC converter (ADC) and the output of the ADC that varies with the fluctuation in the DC offset given to ADC is used for metering the current and

voltage. Accordingly, the Electronic Trip Unit (ETU) of the present invention overcomes the drawback associated with using constant DC offset value saved in the memory for computing Root Mean Square (RMS) used for metering current and voltage, and dynamically computes DC offset value per cycle for accurately computing Root Mean Square (RMS) value for the system accurately, which in-turn is used for measuring current and voltage.
The present invention includes a firmware design for dynamic offset computation in electronic trip unit using unipolar ADC in circuit breakers. In Electronic Trip Unit, the DC offset is added to AC signal so that unipolar ADC can convert the samples to digital value. The DC offset value is subtracted from sample values to get actual value of the sample. These sample values are used in computation of RMS (Root Mean Square) value in firmware and the RMS value is used for metering current and voltage which in turn is used by other protection modules like Overload, Short Circuit, Earth Fault and so on for computation of trip time. Accordingly, the computation of the DC offset value plays a vital role in deciding the metering accuracy and trip time accuracy. The dynamic offset computation method in accordance with the present invention is highly accurate as it takes care of fluctuation in DC offset voltage and it gives indication when the dynamic Offset is out of the predefined range.
For example, considering a 10-bit ADC, it will give digital value 1023 for Vdd (Supply Voltage) considering Vdd is 3.3V. Assuming number of samples per cycle is 16 and the AC signal input is 100mV. As shown in Figure 1, the sample values in this case is 0, 17, 31, 41, 44, 41, 31, 17 ,0, -17, -31, -41, -44, -41, -31, -17
Case 1 : DC Offset is Vdd/2 (Digital Value 512) shown in Figure no. 2, the sample values are 512, 529, 543, 553, 556, 553, 543, 529, 512, 495, 481, 471, 468,471,481,495

When these sample values are added and dynamic offset is computed by dividing by 16. Dynamic Offset computed is 512. And this value is same as the DC Offset given by signal conditioning circuit
Case 2 : Digital Value of DC Offset is 505 shown in Figure no. 3 Sample Values are 505, 522, 536, 546, 549, 546, 536, 522, 505, 488, 474, 464, 461, 464, 474, 488. When these sample values are added and dynamic offset is computed by dividing by 16. Dynamic Offset computed is 505 and this value is same as the DC Offset given by signal conditioning circuit.
The Electronic Trip Unit of the present invention enhances accuracy of Data Acquisition firmware of Electronic trip unit using unipolar ADC. The dynamic offset computation method increases accuracy of metering and since the RMS value computed by Data Acquisition module is used by other protection module like Overload, Short Circuit and so on, it also increase trip time accuracy. More specifically, the Electronic Trip Unit (ETU) of the present invention eliminates DC offset calibration procedure completely and overcomes error caused by the DC offset fluctuation due to change in temperature, component tolerance and fluctuation in voltage regulator output. Also, the computation technique takes care of presence and absence of Rogowski in particular phase. Further, In case of the Electronic Trip Unit of the present invention, a range is defined for DC offset value and if the dynamic offset is outside this range then alarm indication is given. In the nut-shell, the Electronic Trip Unit of the present invention utilizes improved firmware that leads to better accuracy in metering current, voltage and trip time.
The Electronic Trip Unit (ETU) for a circuit breaker includes a timer, a storage memory, a computation system and an alarm system. The computation system includes an AC to DC Converter (ADC), a Direct Memory Access Controller (DMAC) and a dynamic computation module. The timer is initialized to sampling period. The interruption of the timer triggers the ADC which in turn

triggers the DMAC on completion of ADC conversion. The sample for each phase is accessed from the storage memory and dynamic offset value is subtracted. The obtained sample value after subtraction is squared and added to Sum Of Square (SOS) Value for that phase. When N samples are squared and added up, the Root Mean Square (RMS) is computed by dividing by number of samples and taking square root. The default value of dynamic offset is used in first cycle for computation of the'RMS. In second cycle, the dynamic offset calculated on the basis of sample values of first cycle is used for subtracting offset from the sample value saved from each phase. The Root Mean Square Value so calculated is accurate and is utilized for metering current and voltage, which is more accurate.
In the firmware for computation of RMS, input signal value on each phase is sampled and N samples are saved per cycle. This process is started by the timer whose time period is initialized to that of sampling period. When the timer interrupt is issued. It triggers ADC (Analog to Digital Converter) which in turn triggers DMAC (Direct Memory Access Controller). DMAC transfer the ADC conversion value to memory. After DMAC transfer is complete, the sample value from the memory is accessed and dynamic offset is subtracted from each sample for a cycle. The N consecutive sample values after subtracting Dynamic Offset is squared and the sum of these value is divided by N where N is number of samples taken per cycle. The square root gives the RMS value which will be used for metering.
For dynamic offset computation, a default offset value is used in first cycle for computation of RMS. During this first cycle, N sample values are summed up and average is taken by dividing by N. This gives us Dynamic Offset to be used in next cycle.
Figure 4, explains the working of the Electronic Trip Unit. Referring to Figure 4, the current passing through the circuit breaker is step down by current

transformer (2) to give current output and step down by Rogowski coil (1) to give voltage output. The out put current from the current transformer (2) is delivered to the power supply (4). The current output is given to power supply circuit (4) which gives supply voltage to trip circuit (5) and signal conditioning circuit (3) and regulated supply to the microcontroller (8), wherein power supply to the microcontroller (8) is regulated by a regulator 7. The ADC used in microcontroller (8) is unipolar and so there is a need to shift the Rogowski output signal by Vdd/2 for analog to digital conversion of positive as well as negative cycles.
Figure 5 and Figure 6 illustrates the flowchart which explains the flow of the program. The timer is initialized to sampling period, for example, if operating frequency is 50Hz, the time period of one cycle is 20ms and assuming 16 samples are taken per cycle then the sampling period is 1.25ms. The program execution starts with starting the timer and program waits till timer interrupt occurs. The timer interrupt will trigger ADC conversion as shown in 11. The completion of ADC conversion will generate ADC interrupt which will start DMAC transfer as shown in 13. Then it is checked whether ADC conversion is done for all the channels. This condition checking is done in 14 and if this condition is not satisfied then ADC conversion and DMAC transfer will take place till all channels are scanned. On completion of scanning of all channels and transferring to memory, DMAC interrupt occurs.
The program enters DMA Interrupt Service Routine (ISR) and the sample number is incremented as we have already captured the samples from all the channels. The condition checking is done in 16 to check whether the sample number exceeds predefined samples per cycles (N). If it exceeds then sample number is reinitialized to 1 or else skip the re-initialization part. Then channel number is initialized to 1. For that particular channel, sample value is accessed from the memory and this value is added to Sum of Samples variable in 20 which is used for Dynamic Offset computation. Then actual sample value is

computed by subtracting dynamic offset for that channel from the sample value obtained for that channel. This actual sample value is squared and added to Partial Sum Of Squares (SOSp). If sample number is equal to N, then mean of SOSp is taken and square root of the result give RMS value for that channel and Dynamic Offset is calculated by dividing SumOfSamp by N.
And SOSp and Sum Of Samples is reinitialized to zero. After the computation of Dynamic Offset, it is checked whether it lies in the tolerance band. If it does not then alarm indication is given. If sample number is not equal to N then 24, 25, 26, 27, 28 and 29 is skipped. It is checked whether sample from each channel is saved. If this condition is not satisfied then Sum Of Samples and SOSp is computed for other channels. Hence, dynamic offset c.omputation is done for all the channels.
Dynamic Offset Algorithm computes offset for current cycle and it is used as Dynamic Offset for next cycle which is subtracted from each sample value for computation of RMS Value. Hence, it increases accuracy of RMS which in turn which gives highly accurate trip time. Hence, the main feature of the invention is that it increases metering accuracy and trip time accuracy.
The benefits of the invention include elimination of DC Offset Calibration process, run time computation of offset will take care of fluctuation in DC Offset due to change in temperature, component tolerance and voltage regulator output. This invention will also take into account change in DC Offset due to connection and disconnection of Rogowski coils. In case the DC Offset is out of range, alarm indication is given. The Electronic Trip Unit (ETU) in accordance with the present invention considers fluctuation in DC offset and effectively increases accuracy of RMS computation for metering. RMS value is used by protection module like Overload, Short Circuit and so on. Further, the highly accurate RMS Value will give accurate trip time.

TECHNICAL ADVANCEMENTS
The Electronic Trip Unit (ETU) for circuit breakers that accurately meters current and voltage and initiates a trip signal to a circuit breaker based on measured current and voltage values, the Electronic Trip Unit (ETU) has several technical advantages including but not limited to the realization of:
• an Electronic Trip Unit (ETU) for circuit breakers that eliminates the drawbacks associated with using constant DC offset value saved in the memory for metering current and voltage by dynamically computing DC offset value per cycle and using the dynamically computed DC offset value per cycle for metering current and voltage;
• an Electronic Trip Unit (ETU) for circuit breakers that eliminates drawbacks associated with conventional Electronic Trip Unit (ETU) using computation system that fail to consider fluctuation in DC offset value while computing current and voltage and thereby provide erroneous voltage and current readings;
• an Electronic Trip Unit (ETU) for circuit breakers that accurately meters the current and voltage in an electrical system;
• an Electronic Trip Unit (ETU) for circuit breakers that accurately meters the trip time, i.e. the time at which circuit breaker tripping occurred;
• an Electronic Trip Unit (ETU) for circuit breakers that accurately controls and regulates operation of the circuit breaker, thereby enabling the circuit breaker in preventing electrical faults such as short circuit and overload;
• an Electronic Trip Unit (ETU) for circuit breakers that facilitates display of accurate trip related information on display screens used for diagnosis of circuit breaker tripping, thereby assisting maintenance personal in analysing various parameters associated with the electrical circuit and the circuit breaker;

• an Electronic Trip Unit (ETU) for circuit breakers that facilitates a maintenance personnel in accurately identifying the exact cause of the electrical fault that caused the circuit breaker tripping, before proceeding with the rectification of the electrical fault;
• an Electronic Trip Unit (ETU) for circuit breakers that is simple in construction;
• an Electronic Trip Unit (ETU) for circuit breakers that assists the maintenance personnel to timely rectify the electrical fault by providing accurate information regarding circuit breaker tripping;
• an Electronic Trip Unit (ETU) for circuit breakers that is reliable;
• an Electronic Trip Unit (ETU) for circuit breakers that is convenient to use;
• an Electronic Trip Unit (ETU) for circuit breakers that is inexpensive;
• an Electronic Trip Unit (ETU) for circuit breakers that is sturdy and that does not fail frequently and has longer service life;
• an Electronic Trip Unit (ETU) for circuit breakers that may be easily retrofitted and used with any of the conventionally known circuit breakers;
• an Electronic Trip Unit that is equipped with an alarm system that raises an alarm in case the dynamic offset is not within pre-determined range; and
• an Electronic Trip Unit (ETU) for circuit breakers that facilitates display of accurate data related to circuit breaker tripping, thereby ensuring proper maintenance and safety of the power distribution system.
Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group, of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

The use of the expression "at least" or "at least one" suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the invention to achieve one or more of the desired objects or results.
Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the invention. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the invention as it existed anywhere before the priority dale of this application.
The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the invention, unless there is a statement in the specification specific to the contrary.
While considerable emphasis has been placed herein on the specific features of the preferred embodiment, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiment without departing from the principles of the invention. These and other changes in the preferred embodiment of the invention will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

We Claim:
1. An Electronic Trip Unit (ETU) for a circuit breaker comprising: a timer initialized to a sampling period and adapted to facilitate sampling of input signal value for each phase;
a storage memory adapted to store input signal sample for each cycle; computation system adapted to meter current and voltage, said computation system comprising-.
an AC to DC converter (ADC) triggered with interruption of said timer;
a Direct Memory Access Controller (DMAC) triggered by said AC to DC converter (ADC) and adapted to transfer the ADC conversion value to said storage memory; and
a dynamic computation module adapted to dynamically compute DC offset per cycle, said dynamic computation module further adapted to access sample value from said storage memory and utilize said dynamically computed DC offset value per cycle for processing said sample value and computing Root Mean Square (RMS) value used for current and voltage measurement, said current and voltage measurements are in turn used by safety modules for providing protection against electrical faults; an alarm system adapted to raise an alarm if dynamic DC offset computed by said computation system is beyond a pre-determined range.
2. The Electronic Trip Unit (ETU) as claimed in claim 1, wherein said safety module is a circuit breaker.

3. The Electronic Trip Unit (ETU) as claimed in claim 1, wherein said alarm system is adapted to raise a visual alarm.
4. The Electronic Trip Unit (ETU) as claimed in claim 1, wherein said alarm system is adapted to raise an audio alarm.
5. The Electronic Trip Unit (ETU) as claimed in claim 1, further comprising a display system for displaying current and voltage readings metered by said Electronic Trip Unit.