EXPULSION TYPE ARRESTER
This type of arrester is also called ‘protector tube’ and is commonly used on system operating at voltages up to 33kV. Fig 11(i) shows the essential parts of an expulsion type lightning arrester. It essentially consists of a rod gap AA’ in series with a second gap enclosed within the fiber tube. The gap in the fiber tube is formed by two electrodes. The upper electrode is connected to rod gap and the lower electrode to the earth. One expulsion arrester is placed under each line conductor. Fig11 (ii) shows the installation of expulsion arrester on an overhead line.
On the occurrence of an over voltage on the line, the series gap AA’ spanned and an arc is stuck between the electrodes in the tube. The heat of the arc vaporizes some of the fiber of tube walls resulting in the production of neutral gas. In an extremely short time, the gas builds up high pressure and is expelled through the lower electrode, which is hollow. As the gas leaves the tube violently it carries away ionized air around the arc. This de ionizing effect is generally so strong that the arc goes out at a current zero and will not be re-established.
(i) They are not very expensive.
(ii)They are improved form of rod gap arresters as they block the flow of power frequency follow currents
(iii)They can be easily installed.
(i)An expulsion type arrester can perform only limited number of operations as during each operation some of the fiber material is used up.
(iii) This type of arrester cannot be mounted on enclosed equipment due to discharge of gases during operation.
(i) Due to the poor volt/am characteristic of the arrester, it is not suitable for protection of expensive equipment.