Surge arresters may be of the valve or expulsion type. They are rated not only on their normal voltage classification in kV, but also on their crest voltage capability in kV at a standard 1.5 × 40-μs wave (or other specified wave), and their discharge-current capability in amperes or thousands of amperes (kA).

For high-voltage application, surge arresters may consist of a number of unit-value valve arresters connected in series in one overall unit, as shown in below for a 69-kV arrester.

Cross section of a 69-kV Thyrite (valve) surge arrester.

Lightning or surge arrester elements are enclosed in an insulated casing. Under severe operating conditions, or as a result of multiple operations, the pressure generated within the casing may rise to the point where pressure relief ratings are exceeded.

The arrester then may fail, with or without external flashover, exploding and violently expelling fragments of the casing as well as the internal components, causing possible injury to personnel and damage to surrounding structures. The action represents a race between pressures building up within the casing and an arcing or flashover outside the casing.

The ‘length’ of the casing of the arrester limits its ability to vent safely. The use of polymer insulation for the casing permits puncturing to occur, without the fragmentation that may accompany breakdown and failure of porcelain.


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