The present invention relates to vacuum-type circuit interrupters which are used in electrical distribution networks. The vacuum interrupter is an evacuated device in which electrical contacts are closed when the device is carrying fault or load current in a normally operating distribution network. The interrupter acts as a switch when the contacts are moved apart, with an arc forming during the initial seconds of contact opening. This arc will extinguish for an AC system at a natural current zero of the alternating current wave form. The arcing that takes place during opening of the vacuum interrupter is a high power arc, which must be dissipated effectively within the device without damaging the device. A variety of contact structures has evolved to prevent localized burning of the arc upon the contact structures, and to thereby ensure reliable, repetitive operation of the vacuum interrupter. A commonly used contact structure has spiral arms to provide current paths which produce a magnetic field which interacts with the arc current to drive the arc in a rotating fashion about the contact surface.
In order to prevent localized destructive arc burning upon the contact surfaces during interruption, the use of arc transfer-type contacts is disclosed in U.S. Pat. No. 3,244,843, and U.S. Pat. No. 4,081,640. The initial arc is struck between the normal load current-carrying conductors or contacts of the interrupter, with this initial arc which forms being transferred to an auxiliary electrode or set of electrodes.
It is well known that a magnetic field directed transverse to the arcing current will tend to produce arc movement and preferably rotary arc movement about the electrode structures. It is also known that an axial magnetic field directed parallel to the arc will tend to produce a diffuse arc condition which prevents overheating of the contacts which might otherwise lead to re-ignition of the arc following initial interruption. Such axial field vacuum interrupters are taught in U.S. Pat. No. 4,117,288 and in copending application Ser. No. 965,012, filed Nov. 30, 1978 and entitled "Vacuum Type Circuit Interrupter". The aforementioned U.S. Pat. No. 3,244,843 describes a device in which a pair of central butt contacts are employed as the load current-carrying contacts, with annular auxiliary arcing contacts about the butt contacts. The annular auxiliary contacts have multiple turn coils connected from the back surface of the auxiliary annular arcing contacts to the conductive support rod for the butt-type contacts. After transfer of the arc, these coil turns generate an axial magnetic field. The annular contacts are said to be operative to carry load current as well as the butt contacts and when both are fully closed, parallel load current-carrying paths are set up. The effect of this structure is to have current flowing through the coil conductor associated with the annular auxiliary contact, with the inherent resistive losses that this entails during normal load current-carrying operation.
The interrupter structure taught in aforementioned U.S. Pat. No. 4,117,288 utilizes annular cup-shaped electrodes, which serve as the normal load current-carrying conductors with a recessed arcing electrode pair within the cup-shaped main electrodes. The arc, upon separation of the cup-shaped contacts, transfers to the central disc-like contacts within the cup-shaped contact, and axial magnetic field generating coil turns are associated with the disc-like arcing contacts. In this structure, load current does not flow through the axial magnetic field generating coil turns until the interrupter is opened, and the arcing current flows between these recessed disc-like contacts. In the above-mentioned copending application, the contacts are essentially disc- or butt-type contacts with axial magnetic field generating coil turns extending from the back perimeter surfaces of the disc- or butt-type contacts, and connected and supported from the conductive support rod for the butt- or disc-like contacts. In such an embodiment, the load current will normally flow through the axial field generating coil portions. It has been found that for high power interruption it is difficult to generate sufficient axial magnetic field without introducing undesirable resistive load from the coil turn portions into the interrupter structure. Such additional load resistance may cause excessive heating of the conductive support stems for the contacts.