| 摘要 |
A surge protection device which provides multiple back-up and tends to fail closed circuit or shorted is disclosed. The primary surge protection is provided by a gas break-down tube set to operate at one voltage level. A secondary or back-up surge protection is provided to operate at a higher voltage by means of a flat ceramic member sandwiched between the top of the tube and a cap that is affixed over the tube and ceramic member. The cap is sealed to the tube and the spacer end encloses a volume of air which is thus protected from moisture and dust. Upon failure of the gas tube, the back-up unit shorts surges across the shortest path between the cap and tube top, which path is adjacent a dielectric wall formed by the member. Discharge via this path tends to vaporize metal in the enclosed volume which upon redeposit on the ceramic surface tends to permanently short across that surface so that the device tends to fail closed circuit. In a second embodiment, the ceramic wall is formed in a stairstep arrangement of a ceramic member having zones of high and relatively low dielectric material. A third, still higher voltage, failsafe discharge air gap is provided in both embodiments, between the cap and a post atop the tube through ambient air.
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