Charging of Electrolytic Arresters.—In electrolytic arresters all electrolytes dissolve the film when the arrester is on open circuit, the extent of the dissolution depending upon the length of time the film is in the electrolyte, and upon its temperature. It is therefore necessary to charge the cells from time to time and thus prevent the dissolution and consequent rush of current which would otherwise occur when the arrester discharges.
Ques. Describe the charging operation for arresters with grounded circuits.
Ans. It consists in simply closing simultaneously the three horn gaps so that the full pressure across the cells causes a small charging current to flow and form the films to their normal condition.
Fig. 2,396.—Sectional view of General Electric vacuum tube arrester for railway signal circuits. The arrester is essentially a gap in a vacuum. In construction, the gap is formed between the inner wall of a drawn metal shell and a disc electrode mounted concentric with it. The electrode is supported on a brass rod which serves as the lead in connection, and has ample current carrying capacity. The electrode system is insulated from the tube and rigidly supported in position by a bushing made of vitreous material. The bushing does not form the vacuum seal, that being made by a special compound. The open end of the tube is finally closed by a porcelain bushing. The tube is exhausted in a special machine which solders a small hole in the end after the vacuum has been established. The possibility of solder entering the active part of the vacuum space is prevented by a diaphragm punching, and both the electrode and the lining of the tube are of non-arcing metal. The arrester has a spark pressure of from 350 to 600 volts direct current, and an equivalent needle gap of about .005 inch. The arrester will not stand a continuous flow of current due to excessive heating, hence if there be a possibility of this due to high pressure crosses, fuses should be used. R.R.S.A. standard terminals are used.
Ques. Describe the charging operation for arresters for non-grounded circuits. Ans. First, the horn gaps are closed for five seconds and opened again to normal position, thus charging the cells of the three line stacks. Second, with the horn gaps still in normal position, the position of the transfer device is reversed and the horn gaps are again closed for five seconds and returned to the normal position.
The complete charging operation takes but a few moments and should be performed daily. The operation is valuable, not only in keeping the films in good condition, but also in giving the operator some idea of the condition of the arrester by enabling him to observe the size and color of the charging spark.
Fig. 2,397.—Highland Park sub-station, Charlotte, N.C., showing old lightning arrester tower on the left and General Electric aluminum (electrolytic) cell lightning arrester and horn gaps in foreground.
Grounded and Non-grounded Neutral Circuits.—It is important to avoid the mistake of choosing an arrester for a thoroughly grounded neutral when the neutral is only partially grounded, that is, grounded through an appreciable resistance. Careful consideration of this condition will make the above statement clear.