Expulsion type lightning arrester is also known as a protector tube. It consists of – (i) a tube made of fibre which is very effective gas-evolving material (ii) an isolating spark gap (or external series gap) and (iii) an interrupting spark gap inside the fibre tube.
During operation arc due to the impulse spark-over inside the fibrous tube causes some fibrous material of the tube volatized in the form of gas, which is expelled through a vent from the bottom of the tube, thus extinguishing the arc just like in circuit breakers. Since the gases generated have to be expelled, one of the electrode is hollow and the diverter is open at its lower end.
Expulsion type arrester is an improvement over the rod gap in that it seals the flow of power frequency follow current. Its use is normally restricted to circuits in which the possible range of power current magnitude following a spark-over is not more than about three to one, since an excessive current may cause bursting of tube while a small current may not produce sufficient pressure to extinguish the arc.
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Though expulsion gaps offer a comparatively cheap form of surge protection but they possess the following disadvantages:
(i) They are totally unsuitable for protection of expensive station equipment on account of their poor volt-time characteristics.
(ii) There is a limit of the number of operations to which they can be safely subjected as some gas producing material is destroyed in each operation.
(iii) They are unsuitable for mounting within equipment enclosure or in close proximity to other electrical apparatus.
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The expulsion type arresters can be considered as economical means of surge protection for small rural transformers, where the valve type arresters may prove expensive and the application of air-gaps yield inadequate protection. These arresters can be also used on special transmission towers of extra height on river crossings where the possibility of lightning strokes is relatively high. The expulsion type arrester can be considered very favourably for use on system operating at voltages not exceeding 33 kV.