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: EARTHING ASSEMBLY FOR AIR CIRCUIT BREAKER PROVIDING LOW IMPEDANCE

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

The present invention relates generally to an earthing assembly for circuit breakers and more particularly, to an earthing assembly for air circuit breaker providing low impedance.

BACKGROUND AND THE PRIOR ART

Conventional earthing assemblies for draw-out type circuit breakers consist of finger–rails arrangement in which fingers assembly (mounted on breaker) slides over fixed rail (mounted on cradle). Conventional design has restricted contact overlap between finger & rail, which put limitations on contact resistance. It uses multiple components for assembly, which increases cost of earthing system.

Every electrical installations needs effective earthing system in order to fulfill following functions:
1. To provide a sufficiently low impedance to facilitate satisfactory protection under fault conditions.
2. To ensure that human beings in the vicinity of installation are not exposed to unsafe voltages under steady state or fault conditions.
3. To maintain system voltages within limits under fault conditions (such as lightning, switching surges or inadvertent contact with higher voltage systems), and ensure that insulation breakdown voltages are not exceeded.

Typical earthing system for fixed installation consists of earthing bars or cable connected to metal frames of installation. Low impedance requirement of earthing system in fixed installation is easily achievable due of bolted joints.

The present invention relates to an earthing assembly for draw-out type Air Circuit Breaker. Earthing assembly design is based on cantilever spring arrangement for fixed contact part of assembly, on which moving contacts slides at different operating positions.
Cantilever spring arrangement provides necessary flexibility for movement of sliding contacts over fixed contact. Cantilever spring arrangement of earthing assembly offers low frictional resistance during the rack-in operation of breaker. Additional retaining springs at end of cantilever provides necessary contact force between fixed & sliding contacts sufficient to withstand against blow open forces due to high fault currents.

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 an earthing assembly for air circuit breaker providing low impedance.

Another object of the present invention is to provide flexibility in the movement of contacts.

Another object of the present invention is to provide low frictional resistance during rack-in operation.

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

There is provided an earthing assembly for circuit breakers.

According to one embodiment of the present invention, there is provided an earthing assembly for air circuit breaker providing low impedance.

Other embodiment of the present invention provides flexibility in the movement of contacts.

Other embodiment of the present invention provides for low frictional resistance during rack-in operation.

Additionally, this earthing assembly is simple and cost effective.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Fig 1 illustrates the schematic view of circuit breaker and cradle assembly.

Fig. 2 illustrates sliding earthing contact assembly.

Fig. 3 illustrates sliding contacts breaker in isolated position.

Fig. 4 illustrates contact breaker in Service/Test position.

DETAILED 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.

Reference is first invited to Fig 1 where the fixed part of circuit breaker assembly i.e. cradle and withdrawable part of circuit breaker assembly i.e. breaker (8) is shown. It also show fixed earthing contacts mounted on cradle (9) and sliding earthing contacts mounted on breaker.

Fig. 2 shows the arrangement of different components for sliding earthing contact assembly. It shows the arrangement for breaker right supporting plate (7), cradle front support (6), cradle earthing support (3), sliding earthing contacts (2), Restraining springs (4) and cradle rear support (5). It also shows direction of Restraining Spring Force, Frs; Blow Open Force (Fbo), Cantilever Spring Force (Fcs).

Fig. 3 shows different components like fixed earthing contacts, sliding earthing contacts and breaker right supporting plates in isolated condition.

Fig. 4 shows the arrangement of different components during service/test position.

The invented system is thus an earthing assembly for air circuit breaker providing low impedance.

DETAILED DESCRIPTION OF THE INVENTION

Rotating or sliding sub-assemblies of electrical installation needs special kind of arrangement in order to provide electrical connectivity between fixed & rotating/sliding subassemblies. A draw-out type circuit breaker mainly comprises two parts, one is breaker (8) assembly, which consists of the switching contact assemblies and mechanism, & the other one is a fixed external frame (cradle) assembly (9). The circuit breaker (8) mounted on cradle (9) can be connected or disconnected from main power circuit by axial withdrawal motion of the breaker. Draw-out type circuit breakers need special kind of arrangement in order to provide electrical continuity of earthing circuit between breaker (8) & cradle (9).

Accordingly the present invention for earthing assembly consists of earthing support (3) & fixed earthing contact (1) which act as a cantilever spring arrangement. As shown in figure 4 sliding earthing contact(2) of breaker(8) slides on fixed earthing contact(1) while racking operation of air circuit breaker(8). Cantilever spring arrangement of fixed earthing contact(1) causes low frictional resistance during the rack-in operation of breaker(8),as sliding earthing contact (2) press the fixed earthing contact(1) in down ward direction.

Use of restraining springs (4) at the cantilever end of fixed earthing contact (1) provides necessary contact pressure between sliding earthing contact (2) & fixed earthing contact (1).
As shown in figure 2 blow open force (Fbo) generated due to earth fault current is counter balanced by restraining spring force (Frs) & cantilever spring force (Fcs).
1. Fixed Earthing Contacts (Mounted On Cradle)
2. Sliding Earthing Contacts(Mounted On Breaker)
3. Cradle Earthing Support
4. Restraining Springs
5. Cradle Rear Support
6. Cradle Front Support
7. Breaker Right Supporting Plate
8. Breaker (Withdrawable Part Of Circuit Breaker Assembly )
9. Cradle (Fixed Part Of Circuit Breaker Assembly)

Terminology:
Frs : Restraining Spring Force
Fbo : Blow Open Force
Fcs : Cantilever Spring Force
Although the embodiments herein are described with various specific embodiments, it will be obvious for a person skilled in the art to practice the embodiments herein with modifications. However, all such modifications are deemed to be within the scope of the claims.
It is also to be understood that the following claims are intended to cover all of the generic and specific features of the embodiments described herein and all the statements of the scope of the embodiments which as a matter of language might be said to fall there between.

WE CLAIM:

1. An earthing assembly for air circuit breakers providing low impedance having a fixed earthing contact (1) mounted on a cradle, a sliding/moving earthing contact (2) mounted on a breaker and a breaker right supporting plate, said earthing assembly comprising:
cantilever spring means that include earthing support (3) and fixed earthing contact (1); and a restraining element placed in a substantially downward direction at the cantilever end of said fixed earthing contact.

2. System as claimed in claim 1 wherein said restraining element comprising spring means (4) adapted to provide necessary contact pressure between sliding earthing contact (2) & fixed earthing contact (1).
3. System as claimed in claim 1 wherein said circuit breaker is a draw-out type air circuit braker.

4. System as claimed in claim 1 wherein said moving earth contacts (2) slides on said fixed earthing contacts (1) at different operating positions.

5. System as claimed in claim 1 wherein said restraining element provide contact force between said fixed and sliding contacts to withstand blow open forces during fault.

6. System as claimed in claim 1 wherein said cantilever spring means causes low frictional resistance during the rack-in operation.

7. System as claimed in claim 1 wherein blow open force generated due to earth fault current is counter balanced by restraining spring force produced by said restraining spring means and cantilever spring force produced by said cantilever spring means.

8. An earthing assembly for air circuit breakers providing low impedance having a fixed earthing contact mounted on a cradle, a sliding earthing contact mounted on a breaker and a breaker right supporting plate as herein substantially described and illustrated with reference to the accompanying drawings.

Dated this 2nd day of January 2012

Abhishek Sen
Of S. Majumdar & Co.
(Applicant’s Agent)