F O R M 2
THE PATENTS ACT, 1970
(39 of 1970)
&
The Patents Rules, 2003 COMPLETE SPECIFICATION
(See section 10; rule 13)
1.Title of the invention: IMPROVED ZONE SELECTIVE INTERLOCKING ASSEMBLY AND METHOD THEREOF
2. Applicant(s):
(a) NAME : LARSEN & TOUBRO LIMITED
(b) NATIONALITY : An Indian Company
(c) ADDRESS : L & T House, Ballard Estate, Mumbai
400 001,State of Maharashtra, India
3. PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in which it is to be performed:

TECHNICAL FIELD OF THE INVENTION
This invention relates generally to circuit breakers. More particularly the present invention relates to Zone Selective Interlocking (ZSI) with Failsafe sensitivity to reduce stress on cables due failure of downstream breaker.
BACKGROUND AND THE PRIOR ART
Zone Selective Interlocking (ZSI) is a method which allows two or more circuit breakers to communicate with each other so that a short circuit or ground fault will be cleared by the breaker closest to the fault in the minimum time.
US7570471 discloses a zone-selective-interlocking system for an electrical power distribution system that has at least one upstream breaker and at least one downstream breaker is provided. Each breaker in the power distribution system has a zone-selective-interlocking circuit that includes a zone-selective-interlocking-input circuit and a zone-selective-output circuit. The zone-selective-interlocking system also includes a monitoring device to monitor the voltage sent from the zone-selective-interlocking-output circuit of the at least one downstream breaker to the zone-selective-interlocking-input circuit of the at least one upstream breaker and determine whether there is a proper or improper connection between the upstream and downstream breaker and whether the downstream breaker is sending a restraint signal to the upstream breaker.
US2012316805A1 discloses a circuit interrupter apparatus includes circuit
interrupter and a device. The circuit interrupter includes separable contacts, an
operating mechanism structured to open and close the separable contacts, and a
trip mechanism cooperating with the operating mechanism to trip open the
separable contacts. The trip mechanism includes
a zone selective interlocking input and a zone selective interlocking output. The device includes a first input electrically interconnected with the zone selective interlocking input, a second input electrically interconnected with the

zone selective interlocking output, and an indicator circuit structured to indicate that the zone selective interlocking input occurred at the first input or that the zone selective interlocking output occurred at the second input.
OBJECTS OF THE INVENTION
A basic object of the present invention is to overcome the disadvantages/drawbacks of the known art.
Another object of the present invention is to provide a failsafe sensitive delay value to reduce stress on circuit breaker.
Another object of the present invention is to trip the breaker faster under fault condition with ZSI functionality ON.
Another object of the present invention is to reduce the stress on bus bars and cabling of downstream breaker.
Yet another object of the present invention is to alter the set delay accordingly to clear the fault faster.
Yet another object of the present invention is to sense the ZSI delay in overload condition.
These and other advantages of the present invention will become readily apparent from the following detailed description read in conjunction with the accompanying drawings.
SUMMARY OF THE INVENTION
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the present invention. It is not intended to identify the key/critical elements of the invention or to delineate the scope of the invention. Its

sole purpose is to present some concept of the invention in a simplified form as a prelude to a more detailed description of the invention presented later.
In an aspect of the present invention, there is provided a zone selective interlocking assembly for clearing fault(s) in circuit breaker (s), the assembly comprising:
an upstream trip unit;
a downstream trip unit;
wherein the upstream trip unit in co-ordination with the downstream trip unit having a zone selective interlocking (ZSI) trip signal with a failsafe sensitivity value for reducing the time to clear the fault in the circuit breaker, thereby enhancing the protection to the circuit breaker.
Another aspect of the present invention, there is provided a method for clearing fault(s) in circuit breaker, the method comprising steps of:
detecting a fault in the circuit breaker,
checking a ZSI trip signal for clearing the fault,
discarding a set delay value for fault the in the downstream trip unit and loading a failsafe sensitive value of the ZSI trip signal, if the ZSI trip signal is active, therefore clearing the fault,
tripping the circuit breaker, when the fault is not cleared during elapsing failsafe sensitive value,
applying a normal fault handler routine for clearing the fault in the circuit breaker, if the ZSI trip signal is not active.
Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which,

taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The following drawings are illustrative of particular examples for enabling methods of the present invention, are descriptive of some of the methods, and are not intended to limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description.
Figure 1: Illustrates Co-ordination of upstream and downstream breaker
Figure 2: Illustrates algorithm for fault detection using ZSI discrimination
Persons skilled in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and may have not been drawn to scale. For example, the dimensions of some of the elements in the figure may be exaggerated relative to other elements to help to improve understanding of various exemplary embodiments of the present disclosure.
Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features, and structures.
DETAILED DESCRIPTION OF THE INVENTION
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition,

descriptions of well-known functions and constructions are omitted for clarity and conciseness.
The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.
By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to those of skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide.
Accordingly, present invention an improved zone selective interlocking assembly and method thereof
The present invention provides a circuit breaker in which upstream trip unit have additional settings in co-ordination to downstream trip unit for ZSI trip time. ZSI trip time have Failsafe sensitivity value which is set higher than the maximum set delay value of downstream breaker. This value of Failsafe sensitivity ensures that fault is not carried for longer time and cleared immediately upon failure of downstream breaker to clear the fault and enhanced protection setting
Under Fault condition:
Case 1: 600A breaker with downstream ZSI received signal as active. ZSI delay set at 50ms

During fault condition, this breaker send ZSI signal to 1200A breaker and trips in no time (<20ms). But if this doesn’t happen, 1200A breaker would trip after 0.3s.
With improved logic implemented, upstream breaker would wait only for set ZSI delay (50ms) and would trip the system if the fault is not cleared. This would reduce the stress by 250ms.
Case 2: 600A setting (LT: 0.4x Ir – 240A, ST: 2x Ir – 480A, STD: 100ms)
1200A setting (LT: 1 x Ir – 1200A, ST: 6 x Ir – 7200A, STD: 300ms) ZSI delay: 150ms
The bus bars/cables are selected according to 480A for the 600A section breaker. If fault current of 1000A flows, the 1200A breaker does not register this as fault. Hence it would not follow ZSI logic.
With improved logic, ZSI delay of 150ms would be activated if it receives ZSI signal from downstream breaker and would trip the breaker if the signal persists beyond ZSI set delay. Thereby protects the system. The STD delay would be applicable for fault as per setting of breaker. This is shown in Figure 1.
Under Fault Detection condition:
Once the fault is detected, trip unit checks whether ZSI is active or not. If not active, then normal fault handler routine is called and subsequent action taken.
If ZSI is active, then trip unit discards set delay value and loads ZSI failsafe sensitive value and keeps monitoring the ZSI signal being received from downstream breaker.
If the fault is not cleared and if the ZSI failsafe sensitive time is elapsed, it initiates tripping of breaker logic. Otherwise it goes to normal fault handler routine.

WE CLAIM
1. A zone selective interlocking assembly for clearing fault(s) in circuit
breaker (s), the assembly comprising:
an upstream trip unit;
a downstream trip unit;
wherein the upstream trip unit in co-ordination with the downstream trip unit having a zone selective interlocking (ZSI) trip signal with a failsafe sensitivity value for reducing the time to clear the fault in the circuit breaker, thereby enhancing the protection to the circuit breaker.
2. The assembly as claimed in claim 1, wherein the failsafe sensitivity value of the ZSI trip signal is set higher than the maximum a set delay value of the downstream trip unit for clearing the fault(s).
3. The assembly as claimed in claim 1, wherein the ZSI trip signal reduces the fault trip delay value to the failsafe sensitivity value for clearing the fault, when the downstream trip unit is failed to clear the fault.
4. The assembly as claimed in claim 1, wherein detecting the ZSI trip signal is active or not by the upstream trip unit, when a fault is detected in the circuit breaker.
5. The assembly as claimed in claim 4, wherein the upstream trip unit discarding the set delay value in the downstream trip unit and loading the failsafe sensitive value of the ZSI trip signal when the ZSI trip signal is active, therefore clearing the fault.
6. The assembly as claimed in claim 5, wherein if the ZSI failsafe sensitive value is elapsed and the fault is not cleared, the ZSI trip signal trips the circuit breaker.

7. The assembly as claimed in claim 1, wherein a normal fault handler routine is applied to the fault in the circuit breaker, when the ZSI trip signal is not active.
8. A method for clearing fault(s) in circuit breaker, the method comprising steps of:
detecting a fault in the circuit breaker,
checking a ZSI trip signal for clearing the fault,
discarding a set delay value for fault the in the downstream trip unit and loading a failsafe sensitive value of the ZSI trip signal, if the ZSI trip signal is active, therefore clearing the fault,
tripping the circuit breaker, when the fault is not cleared during elapsing failsafe sensitive value,
applying a normal fault handler routine for clearing the fault in the circuit breaker, if the ZSI trip signal is not active.
9. The method as claimed in claim 8, wherein a upstream trip unit in coordination with a downstream trip unit having a zone selective interlocking (ZSI) trip signal with a failsafe sensitivity value for reducing the time to clear the fault in the circuit breaker, thereby enhancing the protection to the circuit breaker.
10. The method as claimed in claim 8, wherein the failsafe sensitivity value of the ZSI trip signal is set higher than the maximum a set delay value of the downstream trip unit for clearing the fault(s).

11. The method as claimed in claim 8, wherein the ZSI trip signal reduces the fault trip delay value to the failsafe sensitivity value for clearing the fault, when the downstream trip unit is failed to clear the fault.