High- and medium-voltage gas blast circuit breaker

A gas blast circuit breaker comprises a cylindrical metal or insulative enclosure filled with pressurized insulative gas, a fixed arc contact and a mobile assembly coupled to an operating member comprising a gas blast cylinder associated with a gas blast nozzle and cooperating with a piston, and a mobile arc contact carried by a tubular member inside the gas blast cylinder. The inside wall of the tubular member comprises a bottleneck whose cross-section is less than the inside cross-section of the tubular member and coaxial therewith. The minimum cross-section of the bottleneck has a diameter less than or equal to the diameter of the inside cross-section of the mobile arc contact and equal to at least 0.75 times the diameter of the inside cross-section of the mobile arc contact. The operation of the circuit breaker is improved by increasing the gas pressure near the mobile arc contact by virtue of a particularly simple and low-cost arrangement.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention concerns a high- and medium-voltage gas blast circuit 
breaker comprising a cylindrical metal or insulative enclosure filled with 
pressurized insulative gas, a fixed arc contact and a mobile assembly 
coupled to an operating member comprising a gas blast cylinder associated 
with a gas blast nozzle and cooperating with a piston, and a mobile arc 
contact carried by a tubular member inside the gas blast cylinder. 
2. Description of the Prior Art 
Circuit breakers of the above type are known. 
The tubular member carrying the mobile arc contact often has lateral 
openings to obtain a double gas blast effect in the sense that the 
pressurized gas in the annular space between the gas blast cylinder and 
the tubular member supporting the mobile arc contact is blown inside the 
gas blast nozzle and also towards the interior of the mobile arc contact 
and therefore towards the interior of the tubular member supporting it. 
An object of the present invention is to improve the operation of a circuit 
breaker of this kind and in particular of thermal expansion circuit 
breakers using the energy of the arc by increasing the gas pressure near 
the mobile arc contact by means of a particularly simple and low-cost 
arrangement. 
SUMMARY OF THE INVENTION 
The present invention consists in a gas blast circuit breaker comprising a 
cylindrical metal or insulative enclosure filled with pressurized 
insulative gas, a fixed arc contact and a mobile assembly coupled to an 
operating member comprising a gas blast cylinder associated with a gas 
blast nozzle and cooperating with a piston, and a mobile arc contact 
carried by a tubular member inside the gas blast cylinder, in which 
circuit breaker the inside wall of said tubular member comprises a 
bottleneck whose cross-section is less than the inside cross-section of 
the tubular member and coaxial therewith, the minimum cross-section of the 
bottleneck having a diameter less than or equal to the diameter of the 
inside cross-section of the mobile arc contact and equal to at least 0.75 
times the diameter of the inside cross-section of the mobile arc contact. 
By virtue of this arrangement, the pressure of the insulative gas entering 
the tubular member is increased and the gas blast is improved. In this way 
it is possible to increase the insulative gas pressure up to 25%. 
To enable a choice of bottleneck cross-section the distance between the 
mobile arc contact and the bottleneck is preferably slightly greater than 
the length of the fixed arc contact inside the tubular member in the 
tripped position. 
The bottleneck may be in the form of an attached separate part. 
In the case of a circuit breaker comprising a protective insulative or 
tubular metal lining on the inside wall of the tubular member, the 
bottleneck is preferably a deformed part of said lining. 
The invention is described in more detail hereinafter with reference to a 
drawing showing one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
The cylindrical insulative enclosure 1 filled with insulative gas, usually 
SF.sub.6, accommodates: 
a fixed permanent contact 2, 
a fixed arc contact 3, 
a mobile assembly comprising a gas blast cylinder 4 carrying a mobile 
permanent contact 5, a gas blast nozzle 6, a gas blast piston 7 and a 
coaxial tubular member 8 inside the cylinder 4 and carrying a mobile arc 
contact 9. 
The mobile assembly is shown in the open position and the gas is blown by 
the piston 7 in the direction of the arrows into the gas blast nozzle 6 
and into the tubular member 8 carrying the mobile arc contact 9. 
The gas is then discharged through at least one aperture provided near the 
end of the tubular member 8 that is not shown. 
The tubular member 8 is provided near the contact 9 with a bottleneck 
formed by a deformed portion 10 of the metal tube 11, usually a steel 
tube, protecting the tubular member 8 against the high temperature. 
This portion 10 is deformed symmetrically with respect to the longitudinal 
axis of the tubular member 11 is at a distance d from the mobile arc 
contact 9 which is preferably slightly greater than the length of the 
fixed arc contact 3 inserted in the mobile arc contact 9 in the tripped 
condition of the circuit breaker. 
Preferably: 
EQU 0.75 d.sub.0 .ltoreq.d.sub.1 .ltoreq.d.sub.0 
d.sub.0 being the diameter of the inside cross-section of the mobile arc 
contact 9 and d.sub.1 being the minimum diameter of the bottleneck flow 
cross-section. 
This structural arrangement has the advantage of producing a pressure rise. 
To be more precise, it is in this way possible to increase the density of 
the gas around the mobile contact 9 up to 25%, providing improved 
dielectric strength when interrupting low capacitive currents at this 
contact 9. If the diameter of the bottleneck is less than 0.75 d.sub.0, 
the bottleneck is deleterious with high currents which require a 
sufficient flow cross-section in at least the tubular member 8 to ensure 
the gas blast effect and de-ionization of the space between the contacts 3 
and 9. 
The FIGURE shows a preferred shape of the bottleneck providing optimum gas 
flow conditions. 
The bottleneck can be of any biconical or other shape. 
In an alternative embodiment the bottleneck may be obtained by attaching a 
separate part to the inside wall of the member 8 supporting the mobile arc 
contact 9. This part may be made from metal, for example steel, or an 
insulative material, for example PTFE. 
The invention applies to conventional type high-voltage circuit breakers of 
the insulative or grounded metal enclosure type.