FORM 2
THE PATENT ACT 1970
&
The Patents Rules, 2003
COMPLETE SPECIFICATION (See section 10 and rule 13)
1. TITLE OF THE INVENTION:
"An Analytical Concept for Current Measurement in Short-Circuit Testing"
2. APPLICANT:
(a) NAME: Larsen & Toubro Limited
(b) NATIONALITY: Indian Company registered under the
provisions of the Companies Act-1956.
(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.

An Analytical Concept for Current Measurement in Short-Circuit Testing
FIELD OF INVENTION
The present invention relates to a measuring highest current during short-circuit current on low voltage switchgear testing using coils.
BACKGROUND OF THE INVENTION
Conventional way for measurement of short-circuit current is to use ohms law with a high precise value shunts along with the AC test voltage over the circuit breaker. This high precise value shunts have many disadvantage, like they are less accurate for low current measurement, if for this current viewing resistors the coaxial connector is damaged the whole shunt is then out of service, this resistors are easily susceptible to unwanted noise and spike disturbances, also the value of the resistances change along with their life span, the shunts are bulky hence they have limited flexibility, and they have comparatively high cost as compared to the use of coils in the electrical circuit for measuring this high currents.
Also, the current transformers can be used for measurement of this currents, but the use of traditional iron core transformers (CTs), solid or split, may be limited by both electrical characteristics and mechanical design. Iron core CTs have typically been used in permanent applications while iron core clamp-on CTs have long been the standard in portable test and measurement applications. The mechanical design of these sensors generates number of problems in applications, thereby requiring measurement to be done in tight spaces. The rigid jaw of clamp-on CTs can hinder or prevent installation in crowded panels and may only allow measurement of a single conductor. In some applications the user may be required to carry several different CTs to cover the ranges that may be encountered. This can be a significant problem if the user must measure high currents that require large, heavy CT.s. Accurate current measurement when using a typical split core

or clamp on CT will depend on the alignment of the jaw. The split in the iron core requires the mating surfaces to be properly aligned for accurate measurement. Some applications may prevent these surfaces from aligning properly or allowing the user to determine if they are completely closed. Installation and removal of iron core CTs on a live conductor can result in an inductive kick. This voltage surge can represent a significant hazard to both equipment and personnel. These hazards can also exist if the secondary becomes open circuited while installed on a live conductor. Circuit loading can also be a problem when using iron core CTs in a low impedance circuit. The added load of the CT can actually change the characteristics of the measured circuit.
If DC current is present on the conductor being measured the iron core can become magnetically saturated. The CT must then be degaussed to remove any residual field before it can be used again.
Further, Early applications of the technology were limited because the low output voltage was inadequate to drive the measuring equipment of the day. Similar Patents have been filed namely Device for measuring an electrical current in a conductor using a rogowski coil having US Patent No.: 5,442,280 which describes a PCB mount across a conductor for measuring current. Also other patent for Current Measuring Device with US Patent no.: 6,614,218 Bl have been filled which depicts various integrator arrangements for measuring current across the conductor for low power applications.
Due to all the above disadvantages there is a need to provide low cost, highly accurate measurement techniques, which can cover entire current ranges for testing right from few amperes to thousands of kilo Amperes of testing.
Objects of the invention

Object of the present invention is to reduce set up making time, human efforts & to bring more precise results economically.
Another objective of this invention is to contribute towards viability in green technology by limiting the heat dissipated to the environment by use of conventional methods.
Yet another object of the invention is to minimizes the electromagnetic interference from nearby conductors, thus providing accurate results.
Summary of the invention
According to the present invention there is provided a test circuit for testing short-circuit current in a electric device. The test circuit having a a plurality of resistor R, a plurality of reactor X, a closing device A, a plurality of sensing devices and a fusible element. The plurality of resistor R and the plurality of reactor X are connected between the electric device and a supply source S. The closing device A is connected to the electric device. The plurality of sensing devices connected to the electric device. The fusible element F is connected with the device and the supply source for detecting fault current and earthing.
Brief Description of Drawings:
Other features as well as the advantages of the invention will be clear from the following description.
In the appended drawings:
Fig.l depicts construction of a rogowski coil constructed in general;

Fig.2 illustrates an arrangement for measuring high current testing in switchgear products as stated in international standards;
Fig.3 illustrates Timing Comparison between traditional and use of coils;
Fig.4 illustrates general comparison between traditional method and integrated output of the coils;
Fig.5 depicts the linear relation between the output voltage and input current in coils; and
Fig.6 depicts the effect of the coils from nearby conductors having high magnetic field.
Detail Description of the Invention
For a thorough understanding of the present invention, reference is to be made to the following detailed description, including the appended claims, in connection with the above-described drawings. Although the present invention is described in connection with exemplary embodiments, the present invention is not intended to be limited to the specific forms set forth herein. It is understood that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient, but these are intended to cover the application or implementation without departing from the spirit or scope of the claims of the present invention. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.
The present invention provides a test circuit that reduces set up making time, human efforts & to bring more precise results economically. Further, the test circuit contributes towards viability in green technology by limiting the heat

dissipated to the environment by use of conventional methods. Moreover, the test circuit minimizes the electromagnetic interference from nearby conductors, thus providing accurate results.
Figure 2 shows a test circuit for testing short-circuit current in a electric device. The test circuit having a plurality of resistors, for the purpose of explanation three resistors R(1) are depicted, a plurality of reactor, for the purpose of explanation three reactors X(1) are depicted, a plurality of sensing devices, for the purpose of explanation three sensing devices I1, I2 and I3 are depicted , a closing device A and a fusible element F. Each of the sensing devices I1, I2 and I3 consists of a helical coil of wire with the lead from one end returning through the centre of the coil to the other end, so that both terminals are at the same end of the coil. The whole assembly of the test circuit is then wrapped around the straight conductor whose current is to be measured. Since the voltage that is induced in the coil is proportional to the rate of change (derivative) of current in the straight conductor, the output of the Rogowski coil is usually connected to an electrical (or electronic) integrator circuit to provide an output signal that is proportional to the current.
As shown in figure 2, the test circuit arrangement with supply source S feeds a circuit including resistors Rl, reactors X and the equipment D under test. The equipment D is connected in series with the resistors Rl, and their value should be obtained by series coupling of individual reactors. Since the transient recovery voltage characteristics of test circuits including large air-cored reactors are not representative of usual service conditions, the air-cored reactor in each phase is shunted by a resistor taking approximately 0.6 % of the current through the reactor. In each test circuit (the resistors and reactors are inserted between the supply source S and the equipment D under test. The positions of the closing device A and the current sensing devices (I1, I2,I3) may be different. There is one and only one point of the test circuit which is earthed. This may be the short-circuit link of the test circuit or the neutral point of the supply or any other convenient point, but the method of earthing is stated in the test report. All parts

of the test circuit are normally earthed in service, including the enclosure or the screens is insulated from earth and connected to a point. This connection comprises a fusible element F consisting of a copper wire 0,8 mm in diameter and at least 50 mm long, or of an equivalent fusible element for the detection of the fault current.
As shown in figure 1 the sensing devices are generally mounted at the position II, 12,13. This current represents the amount of current actually flowing through the circuit and the product in terms of voltage. Each of the sensing device is capable of detecting change in flux in both DC and AC components. In another embodiment, the sensing device is a coil, preferable rogowski coils. A Rogowski coil, named after Walter Rogowski, is an electrical device for measuring alternating current (AC) or high speed current pulses. It consists of a helical coil of wire with the lead from one end returning through the centre of the coil to the other end, so that both terminals are at the same end of the coil. The whole assembly is then wrapped around the straight conductor whose current is to be measured. Since the voltage that is induced in the coil is proportional to the rate of change (derivative) of current in the straight conductor, the output of the Rogowski coil is usually connected to an electrical (or electronic) integrator circuit to provide an output signal that is proportional to the current.
This voltage can then be multiplied by some multiplication factor 'x' to get the corresponding output current.
The induced voltage in the Rogowski coil is defined by the following general equation:
Where,
M is the mutual inductance
di/ dt is the rate of change of current flowing through the circuit.

Generally the output obtained from the circuit is non-integrated and with some disturbances as shown in figure 3.
Figure 3 shows an comparison between the conventional method and the output of the Rogowski coils. The Rogowski coil output are generally distorted generally as compared to use of traditional resistances in circuit. The voltage outputs are generally time derivative with respect to input voltage from supply's' as shown in figurel. To cancel this time derivative effect generally an normal integrator is connected to output circuit of this Rogowski coils.
The output voltage when measured across a normal integrator it can be depicted as:

Where,
Vo(t) = Actual output voltage of the Rogowski coil.
RC = Integrator time constant
M = mutual inductance of the special Rogowski coils.
Now as shown in above equation as the M and RC are constant the output voltage is actually proportional to current flowindg int circuit as hown in figure 1.
Figure 4 depicts the correlation between traditional method and use of the Rogowski coil.
The output current through this Rogowski coils are less distorted and more accurate as compared to use of traditional resistances and current transformers.

Figure 4 describes waveform at high current of approximately about 80000A. It describes the similarity between high output current of the Rogowski coil and the use of conventional resistors. The output voltage obtained from the above comparison where actually accurate for output from the Rogowski coils as compared to resistances when matched with their calculated ideal theoretical value.
Figure 5 actually depicts the linearity relation between the output voltage and input current.
It represents that as the input current is varied from few amperes to thousands of kilo amperes the ouput voltage also varies. The only problem with this output was that it was of low impedance. But with the present development in microprocessor and microcontroller technology this low driven output voltage can also be measured very accurately on measuring systems.
Also the effects of the Rogowski coils from nearby conductors are negligible when placed in magnetic field of conductors placed nearby. This gives us the advantage that we can simultaneously measure the output current in the three phase system.
Figure 6 depicts the effect of the Rogowski coils from nearby conductors having
high magnetic field.
This can generally be termed as the suppression of electromagnetic interference
effect.
Also when we compared the output from precise low value resistances and the Rogowski coil the output coil has accuracy of ±0.5% as compared to ±3% of the shunts. Also the considerably low cost of the Rogowski coils and the integrator can replace the high cost of these precise value resistances.

Advantages of the invention
It ensures the safety point of perspective for human beings as it does not involve the hazards for the CT secondary being kept open. Also does not involve the saturation and cross interference problem.
The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, and to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood . that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient, but such omissions and substitutions are intended to cover the application or implementation without departing from the spirit or scope of the claims of the present invention.

We Claim:
1. A test circuit for testing short-circuit current in a electric device, the test
circuit comprising:
a plurality of resistors R and a plurality of reactors X connected between the electric device and a supply source S;
a closing device A connected to the electric device;
a plurality of sensing devices connected to the electric device, and a
a fusible element connected with the device and the supply source for detecting fault current and earthing.
2. The test circuit as claimed in claim 1, wherein each of the sensing devices capable of detecting changes in flux in both DC and AC components.
3. The test circuit as claimed in claim 1, wherein each of the sensing devices is rogowski coil capable of sensing change in flux in both DC and AC components.