The present invention relates to an interrupting chamber for a circuit breaker, the chamber comprising two arcing contacts separating from each other on opening of the circuit breaker and subsequent to prior separation of the main contacts, the interrupting chamber further comprising a blast nozzle secured to one of the main contacts and to one of the arcing contacts, the blast nozzle having a generally cylindrical shape including a neck and defining a volume of revolution including an upstream portion and a downstream portion.

French patent No. FR 2 809 531 is already known and describes an interrupting chamber for a circuit breaker of that type (see Figure 1). It includes a first contact 1, known as the movable arcing contact that is connected to the first main contacts 2, known as movable main contacts. It further includes a second arcing contact 3, known as the stationary contact and second main contacts 4, known as stationary main contacts. A blast nozzle 5 is connected to the movable main contact 2. The contacts 2 and 4 are known as main contacts because current passes mainly therethrough when the circuit breaker is in the closed position. During opening of the circuit breaker, as shown in Figure 1, those main contacts 2 and 4 separate first before separation of the arcing contacts 1 and 3, with an electric arc being formed between them when they separate and remaining until said arc is definitively broken. Contacts 1 and 2 are known as "movable" and contacts 3 and 4 are known as "stationary", because on opening, only contacts 1 and 2 move towards the right of the figure.

The interrupting chamber includes a blast nozzle 5. As can be seen in Figure 1, the nozzle 5 presents the general shape of a body of revolution. The chamber includes a neck 12 that separates the volume of revolution into an upstream portion 7 and a downstream portion 8. At the location of the neck 12, the inside volume of the blast nozzle is cylindrical and it is blocked by the stationary arcing contact 3 while the circuit breaker is in the closed position and also for a few milliseconds after separation of the contacts 1 and 3. In its upstream portion, the nozzle 5 includes a portion 9 surrounding the movable arcing contact 1.

The downstream portion of the inside volume defined by the nozzle 5 includes a first conical portion 8a immediately following the neck, having an angle lying in the range 8° to 17°. This last portion 8a is followed by a second portion 8b that is also conical and that has a cone angle that lies in the range 12° to 25°. Given the speed with which the contacts separate, the lengths of the various portions, namely the neck 12, the first conical portion 8a, and the second conical portion 8b are calculated so that the end of the second arcing contact 3 is located inside the first conical portion 8a, as shown in Figure 1, over a time period that lies in the range the minimum arcing duration to the mean arcing duration after separation of the contacts. Provision is also made for said same end of the stationary arcing contact 3 to be located inside the second conical portion 8b over the time period that lies in the range the mean arcing duration and the maximum arcing duration after separation of the contacts.

However, a device of this type presents the following drawbacks:

• The rise in pressure is insufficient with low currents, namely currents that are less than or equal to 30% of the rated breaking capacity of the circuit breaker, the rated breaking capacity being the maximum current that the circuit breaker is able to break. Consequently, the rise in pressure solely as a result of thermal effect is insufficient. It is necessary to compress the gas. The breaking operation requires more energy.

An object of the present invention is specifically to provide an interrupting chamber for a circuit breaker that overcomes those drawbacks.

These objects are achieved in accordance with the invention, by the fact that the downstream portion of the blast nozzle includes a single conical portion, by the fact that the stationary arcing contact is hollow, and by the fact that the length of the neck of the blast nozzle is sufficient for the stationary contact to remain in contact with the neck while the blast nozzle is moving during opening of the circuit breaker. The sole purpose of the conical portion is to prevent hot gas from arriving between the main contacts and giving rise to arcing between said contacts during the voltage recovery stage after the circuit has been broken.

By means of these characteristics, the arc is completely enclosed inside the blast nozzle. The arc vaporizes the material, which makes it possible to use this property to increase the pressure inside the blast nozzle and then inside the blast volume V_th to which this rise in pressure is communicated when the current has an amplitude that is sufficient for generating said rise in pressure. When the current crosses zero said pressure escapes and blasts the arc. The cone is very long and continues until it covers the stationary contact in the open position of the circuit breaker. Blasting makes it possible to cool the arc and to eliminate the conductance of the medium between the arcing contacts. Voltage recovery can then take place after the current has been interrupted.

In comparison with the prior art, the rise in pressure due solely to the thermal effect is sufficient for low short-circuit currents that are less than 30% of the short-circuit breaking power of the circuit breaker, e.g. 10% of the rated short-circuit breaking capacity. As a result, there is no need to compress the gas for currents that are greater than about 10% of the rated short-circuit breaking capacity. There is therefore less need for energy for the breaking operation.

Preferably, the arcing contact includes side openings so as to make it easier for the blast gas to escape.

More preferably, the interrupting chamber includes a separation wall arranged in the movable main arcing contact, the separation wall co-operating with the upstream portion of the blast nozzle to define a thermal volume V_th, valves being provided in the separation wall and a stationary piston situated upstream from said wall, the piston co-operating with the separation wall to define a compression volume V_c.

This device operates when currents are low. When the arc is too weak to generate a pressure that is sufficient, the compression of the gas inside the volume V_c makes it possible to produce the rise in pressure necessary for breaking the circuit. When the separation wall moves, the gas is compressed inside the volume V_c. The valve(s) 20 open(s) and the high pressure in V_c is exhausted into the volume V_th inside the blast nozzle, until the arc is extinguished.

Additional and/or alternative characteristics are listed below:

• The arcing contacts are of the arcing rod and tulip contact type;
• The two contacts may be: one stationary and the other movable, or both movable, the movements separating them from each other during opening of the circuit breaker as shown in patent EP 1 032 009;
• The interrupting chamber presents arcing contacts that are of the butt contact type and include a spring behind the movable arcing contact so as to produce the contact force necessary in the closed position.
• The interrupting chamber is designed for a circuit breaker that is insulated externally in air;
• The interrupting chamber is designed for a gas-insulated metal-enclosed circuit breaker.
• The interrupting chamber is designed for a circuit breaker of the "dead tank" type, i.e. a circuit breaker having its active portion contained in a gas-insulated metal enclosure, and with current inlets and outlets via air-insulated overhead bushings.

Other characteristics and advantages of the present invention also appear on reading the following description of an example given by way of illustration, with reference to the accompanying figures, in which:

• Figure 1, described above, shows an interrupting chamber of the prior art.
• Figure 2 shows an interrupting chamber of the present invention.

This interrupting chamber is shown with the circuit breaker in the open position. This interrupting chamber includes the first movable main contact 2 and stationary main contacts 4. When the circuit breaker is in the closed position, the movable contacts 2 are inside the main contacts 4. Current passes mainly through said contacts when the circuit breaker is in the closed position. During opening of the circuit breaker, the stationary 4 and movable 2 main contacts separate by moving towards the right of the set of movable contacts.

The interrupting chamber also includes a stationary arcing contact 3. As can be seen in Figure 2, this stationary contact is hollow. It includes one or more side blast openings 15. These openings 15 facilitate evacuation of blast gas. The circuit breaker also includes a movable arcing contact 1. This movable arcing contact is housed behind an insulating cap 11.

The blast nozzle 5 includes an inside channel 12 that is cylindrical in shape, of constant section, and of great length. The cylindrical channel 12 is sufficiently long that, even when the circuit breaker is fully in the open position, the end of the stationary arcing contact 3 that is to the right in the figure is still inside the blast nozzle 5. At its downstream portion, the blast nozzle includes a portion of conical shape. However, unlike in the prior art the exit cone 14 of the nozzle has only one cone angle. As mentioned above, the prior art device includes a first conical portion 8a and a second conical portion 8b having a different cone angle and immediately following the conical portion 8a. On the contrary, in the invention, as shown in Figure 2, the blast nozzle includes a conical portion 14 having only one cone angle and that does not contribute to blasting between arcing contacts 1 and 3. The sole purpose of the conical portion is to prevent the hot gas passing through the openings 15 from arriving between the main contacts 2 and 4 and giving rise to arcing between said contacts during the voltage recovery stage after the current has been interrupted.

In its upstream portion, the conical nozzle includes a cylindrical portion 9 of larger diameter, that surrounds the stationary contact. This upstream portion 9 is connected to the movable main contact 2 by means of a ring 17 attached to the movable main contact 2. A through channel 16 is provided between the cylindrical volume 12 and a thermal chamber V_th. A separation wall 18 separates the chamber V_th from a chamber V_c. One or more valves 20 are disposed in the separation wall 18. When said valves are open, they enable the high pressure that is being established in the chamber V_c to pass into the chamber V_th. In Figure 2, the chamber V_c is terminated on the right by a stationary piston 22.

The device operates as follows. During opening of the circuit breaker, the movable unit constituted by the blast nozzle 5, the ring 17, the stationary arcing contacts 1, the cover cap 11, the chamber V_th and the movable main contact 2 moves towards the right, as shown in Figure 2. When the stationary arcing contact 3 separates from the movable arcing contact 1, an arc appears between said two parts. This electric arc is completely enclosed inside the cylindrical portion 12 of the part 5, including in the maximum opening position of the circuit breaker. The cylindrical portion 12 of the nozzle 5 goes as far as covering the end of the stationary arcing contact 3. The arc is completely enclosed inside said insulating cylinder and it vaporizes the material constituting the walls of the cylindrical volume 12, thereby creating a high pressure. When the alternating current crosses zero, said high pressure escapes and blasts the arc. Unlike the prior art, a particularity of the invention is that the neck 12 of the nozzle 5 is very long and it goes as far as covering the end of the stationary arcing contact 3. The blast chamber V_th includes a volume that increases in pressure with increasing energy of the arc. The hot gas moves up the inside of the channel 16 and the pressure of the chamber V_th increases. When the alternating current crosses zero, the movement reverses and the gas escapes into the movable arcing contact 1 and the stationary arcing contact 3. For interrupting low currents, the operating member (not shown) pulls the movable portion, in particular, the movable arcing contact 1, the main arcing contact 2, and the nozzle 5 towards the right, and that compresses the gas contained in the volume V_c, the volume of said chamber being reduced when the separation wall 18 moves towards the stationary piston 22. The high pressure inside the chamber V_c opens the valve(s) 20 that communicate with the thermal volume V_th, which has substantially the same high pressure. When the current crosses zero, the high pressure exhausts through the orifices of the hollow contacts 1 and 3, cooling the arc and interrupting the current.

When interrupting a high current, operation is similar, but in this event the energy of the electric arc vaporizes the inside surface of the nozzle 5 and of the insulating cap 11 situated in front of the movable arcing contact 1. The high rise in pressure inside the volume V_th closes the valve(s) 20 between the volumes V_th and V_c. Arc blasting is carried out by exhausting the volume V_th through the insides of the stationary contact 3 and of the movable contact 1.

The conical portion 14 situated in downstream side of the blast nozzle 5 makes it possible to prevent the hot gas that passes through the openings 15 from flowing between the main contacts 2 and 4. The arcing contacts may be of the arcing rod and tulip contact type, or of the butt contact type. One of the arcing contacts may be stationary and the other may be movable, or both may be movable.

The advantages of the present invention are as follows:

• The invention requires less operating energy in comparison with automatic blast circuit breaker chambers of the conical type.

Its design is simpler in comparison with dual motion contact chambers.

For interrupting capacitive currents (low currents) the necessary opening speed may be obtained with a single movement of the contacts since the compression energy is low and the rise in pressure is low during the first 8 to 10 milliseconds after contact separation. A more sophisticated design, such as for example with dual motion of the contacts, is not always necessary.