FORM 2
THE PATENTS ACT, 1970 (39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
A gas circuit breaker APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTORS
Kale Subodh, Shete Shekhar and Potnis Shrikant, all of Crompton Greaves Ltd, Switchgear (S3) Division, A-3, MIDC, Ambad, Nashik, Maharashtra, India, and Singal Vivek and Jaiswal Rajendra, both of Crompton Greaves Ltd , AMPTC, CG Global R&D Centre, Kanjurmarg (E), Mumbai - 400042, Maharashtra, India, all Indian nationals
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:

FIELD OF THE INVENTION
This invention relates to a gas circuit breaker
BACKGROUND OF THE INVENTION
Circuit breakers are protective electrical devices used in power systems in order to disconnect electrical loads from power source and protect the loads against high currents flowing in the power systems under short circuit or fault conditions. Among the various classes of circuit breakers, gas circuit breakers are very effective in breaking high currents. A gas circuit breaker includes a stationary arc contact and a moving arc contact located within an insulating material nozzle surrounding the contacts and comprising a constriction. The moving arc contact is disposed in the nozzle adapted to move along the central axis of the nozzle and make and break contact with the stationary arc contact. The nozzle is filled with an arc quenching insulating gas such as SF6 gas. During breaking or opening of the arc contacts, intense arcing takes place between the contacts and in order to quench the arc, the quenching gas is blown against the arc being generated in the arcing zone in the nozzle through the constriction in the nozzle. Because of the constriction in the nozzle, the velocity of the quenching gas emerging from the constriction suddenly increases and the quenching gas suddenly expands so as to effectively quench the arc. Arcing causes ablation or erosion of the contacts and the nozzle. In order to prevent erosion of the nozzle and maintain the arc quenching efficiency of the circuit breaker, the nozzle is made with an erosion resistant material such as polytetrafluroethylene (PTFE), preferably teflon. Further, in order to increase the amount of gas developed in the nozzle

during arcing and improve the arc quenching efficiency of the circuit breaker, the nozzle is pigmented or filled with molybdenum disulphide (M0S2). During arcing, the M0S2 burns or evaporates and increases the effective volume of the quenching gas so as to increase the quenching and performance efficiency of the circuit breaker (US 5925863), However, due to burning of M0S2 and due to the thermal effects of arcing on the surface of the insulating nozzle and the nozzle constriction during arcing, the crossection of the nozzle and the constriction still keeps on enlarging or increasing. When the increase in the crossection exceeds certain limits, it has a negative influence on the velocity and expansion of the quenching gas emerging from the constriction. As a result of erosion of the nozzle and nozzle consiiiction, the arc quenching and performance efficiency of the circuit breaker and life of the circuit breaker are thus reduced. Further PTFE is not an erosion resistant material but an insulating and thermally stable material with good chemical resistance. The nozzle is usually made by isostatic moulding followed by sintering of the nozzle at about 350°C.
OBJECTS OF THE INVENTION
An object of the invention is to provide a gas circuit breaker having improved arc
quenching and performance efficiency.
Another object of the invention is to provide a gas circuit breaker having improved life.

DETAILED DESCRIPTION OF THE INVENTION
According to the invention there is provided a gas circuit breaker comprising a stationary arc contact and a moving arc contact located within an insulating material nozzle concentrically surrounding the contacts and comprising a constriction, the moving arc contact being disposed in the nozzle adapted to move along the central axis of the nozzle and make and break contact with the stationary arc contact, the nozzle being filled with an arc quenching insulating gas such as SF6 gas, the nozzle being made with an erosion resistance material such as polytetrafluroethylene and pigmented or filled with a pigment or filler such as molybdenum disulphide, wherein the nozzle further comprises a gas flow restricting and expanding member made of a hard, erosion resistant, thermally conductive and arc reflecting material and located in the nozzle against the sidewall of the constriction thereof facing the arcing zone in the nozzle, the said member comprising a constricted portion in abutment with the sidewall of the constriction facing the arcing zone and matching with the crosssection of the constriction and a flared portion directed towards the arcing zone.
The following is a detailed description of the invention with reference to the accompanying drawings, in which :
Fig 1 is a schematic crosssectional view of a part of a conventional gas circuit breaker;

Fig 2 is a schematic crosssectional view of part of only the nozzle, moving main contact and moving arc contact of the conventional gas circuit breaker of Fig 1; and
Fig 3 is a schematic crosssectional view of part of only the nozzle., moving main contact and moving arc contact of a gas circuit breaker according to an embodiment of the invention.
The gas circuit breaker 1 as illustrated in Figs 1 and 2 of the accompanying drawings comprises a stationary arc contact 2 and a moving arc contact 3 located within an insulating material nozzle 4 surrounding the contacts and comprising a constriction 5. The moving arc contact is disposed in the nozzle adapted to move along the central axis of the nozzle and make and break contact with the stationary arc contact. The movement to the moving arc contact is given by the piston rod 6 connected to it. The nozzle is filled with an arc quenching insulating as such as SF6 gas (not shown). Moving main contact and stationary main contact of the circuit breaker are marked 7 and 8, respectively. The fixed piston, puffer cylinder and thermal puffer of the circuit breaker are marked 9, 10 and 11, respectively. The circuit breaker as illustrated in Fig 3 of the accompanying drawings comprises a gas flow restricting and expanding member 12 made of a hard, erosion resistant, thermally conductive and arc reflecting material and located in the nozzle against the sidewall of the constriction facing the arcing zone in the nozzle. The member 12 comprises a conctricted portion 12a in abutment with the sidewall of the constriction facing the arcing zone arid matching with the crosssection of the constriction. The member

12 also comprises a flared portion 12b directed towards the arcing zone. The nozzle is made of an erosion resistant material such as polytetrafluroethylene preferably, teflon by isostatic moulding alongwith a filler such as molybdenum disulphide. During breaking or opening of the arc contacts under short circuit or fault conditions, intense arcing takes place between the contacts and the arc is quenched by blowing the quenching gas into the arcing zone. Controlled ablation or erosion of the nozzle takes place so as to increase the effective volume of gas developed in the nozzle during arcing and improve the arc quenching and performance efficiency of the circuit breaker. Inspite of the controlled ablation or erosion of the nozzle, the crosssection of the nozzle and the nozzle constriction keeps on enlarging or increasing. However, ablation or erosion of the member 12 does not take place as it is made of hard, erosion resistant, thermally conductive and arc reflecting material. Due to the crosssection of the constricted portion of the member 12 in abutment with the nozzle constriction being identical to the nozzle constriction and the flared portion of the nozzle being directed towards the arcing zone, velocity and expansion rate of the quenching gas emerging from the constriction in the direction of the arcing zone continue to be maintained inspite of erosion in the nozzle constriction. Further the insert is thermally conductive and arc reflecting. Therefore, the heat developed during arcing is conducted away along the nozzle surface and dissipated. The arc is also reflected or deflected into the arcing zone without allowing it to spread. As a result of all this, the arc quenching and performance efficiency of the circuit breaker is improved and maintained over a long period of time. The life of the circuit breaker is also improved. The member 12 is formed separately with the hard, erosion resistant, thermally conductive and arc reflective

material and embedded in the nozzle during isostatic moulding of the nozzle. Alternatively the member 12 is formed during isostatic moulding of the nozzle using the hard, erosion resistant, thermally conductive and arc reflective material as filler. The nozzle is sintered as usual at about 350°C. Besides molybdenum disulphide, other ablating or evaporating material also can be used as filler in the nozzle material. The member 12 is made of a hard, erosion resistant, thermally conductive and arc reflective material such as boron nitride or diamond, preferably boron nitride.

We claim
1) A gas circuit breaker comprising a stationary arc contact and a moving arc contact located within an insulating material nozzle concentrically surrounding the contacts and comprising a constriction, the moving arc contact being disposed in the nozzle adapted to move along the central axis of the nozzle and make and break contact with the stationary arc contact, the nozzle being filled with an arc quenching insulating gas such as SF6 gas, the nozzle being made with an erosion resistance material such as polytetrafluroethylene and pigmented or filled with a pigment or filler such as molybdenum disulphide, wherein the nozzle further comprises a gas flow restricting and expanding member made of a hard, erosion resistant, thermally conductive and arc reflecting material and located in the nozzle against the sidewall of the constriction thereof facing the arcing zone in the nozzle, the said member comprising a constricted portion in abutment with the sidewall of the constriction facing the arcing zone and matching with the crosssection of the constriction and a flared portion directed towards the arcing zone.
2) The circuit breaker as claimed in claim 1, wherein the nozzle is made of teflon by isostatic moulding with molybdenum disulphide as filler.
3) The circuit breaker as claimed in claim 1 or 2, wherein the gas flow restricting and expanding member is made of boron nitride.

4) The circuit breaker as claimed in anyone of claims 1 to 3, wherein the gas flow restricting and expanding member is made separately with the hard, erosion resistant, thermally conductive and arc reflecting material and embedded in the nozzle during isostatic moulding of the nozzle.
5) The circuit breaker as claimed in anyone of claims 1 to 3, wherein the gas flow restricting and expanding member is formed during isostatic moulding of the nozzle using the hard, erosion resistant, thermally conductive and arc reflective material as filler.