Vacuum Switching Tubes Introduction
1. Basic structure and sort for vacuum switching tubes
The following drawing shows the Basic structure of vacuum switching tubes. Out can, which is cylinder made by inorganic material such as glass, ceramic or crystallitic glass, and two metal covers seal a container. There are two contact heads in container, a stationary head fixed on the end of the stationary conductive pole and a moveable head fixed on the end of the moveable conductive pole. The moveable conductive pole pass through the center hole of the bellow and the metal cover. The bellow, which is brazed to the middle of the moveable conductive pole on one end and brazed to the metal cover on the other end, is a stretchy part can flex axially as its wavy side. Due to the bellow between moveable conductive pole and the metal cover, vacuum switching tubes can keep vacuum seal when the moveable conductive pole move axially. The stationary and moveable conductive head always in high vacuum, the gas pressure should lower than 1.33*10-3 Pa, normally around 10-4 Pa.Around the contact heads and bellow are shield, the shield around contact heads called main shield and the one around bellow called accessorial shield or bellow shield.
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If a vacuum switch is used in the circuit as the figure, when the operating machine makes the moveable conductive pole move up, the moveable contact head and the stationary head contact and turn on the power, let current pass through the load. Contrarily, when the operating machine makes the moveable conductive pole move down, the moveable contact head leaves the stationary head and brings vacuum arc, which is kept by the metal vapour from contact head until the current approach the AC zero point, at that moment the almost inactive vapour and the vacuum arc plasma diffusion make the arc go out. The space between contact heads become insulated then the current is interrupted.
Vacuum switching tubes can be sorted by utilization, for breaker, for load switches, for contactor and for special utilization. Vacuum switching tubes for breaker should be capable to break short circuit current, which is used in circuit and electric equipment for over loading and short circuit protection, normally needn’t frequently operate. It can break rated short circuit current more than 30 times.
Vacuum switching tubes for load switches only need to break load current, or rated working current. It is usually used with fuse breaker that can break short circuit current, can break load current 200 to 300 times.
Vacuum switching tubes for contactor need frequently operate under certain load current. It can operate 500,000 to 1,000,000 time or even more. It is usually used with fuse breaker too.
Vacuum switching tubes for special utilization is used in special condition, such as that can break high voltage DC current, can trigger spark or can break high frequency current.
Vacuum switching tubes can be sorted by its contact head structure, such as flat contact head, transverse magnetic field contact head, axial magnetic field contact head, crossed magnetic field contact head (cup-shaped contact head) and other special contact head structure.
Vacuum switching tubes can be sorted into vacuum glass switching tubes (the insulated envelope made by glass), vacuum ceramic switching tubes (the insulated envelope made by ceramic) and vacuum crystallite glass switching tubes (the insulated envelope made by crystallite glass) by the material for the envelope.