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these arresters, operating while across 2300 volts. It should be noted that the static spark only is seen crossing the gaps beneath the low resistance, the dynamic shunting through the resistance and appearing as bright dynamic arcs in the four remaining gaps. There are only two series gaps in this arrester, giving about the correct low-frequency arc-over for the line voltage. The 1000-volt arrester is similar to this, but has only one series
FIG. 12-35,000-VOLT GRADED-RESISTANCE MULTIGAP ARRESTER FOR GROUNDED Y SYSTEMS
gap. For voltages below 300 volts, a single protecting gap is used without any resistance. This is more in the nature of a voltage-limiting device than a true lightning arrester.
As explained before, the distribution of potential across a multigap arrester, and therefore the breakdown voltage, depends to a great extent upon the capacity of the cylinders to ground.
The greater the capacity, the more distorted will be the potential gradient and the lower the breakdown voltage of the arrester, and vice versa. It has been known to us for a long time that by
FIG. 13-35,000-VOLT GRADED-RESISTANCE MULTIGAP ARRESTER FOR DELTA AND UNGROUNDED Y SYSTEMS
ground wires called antennæ, placed parallel to the arrester (see Figure 16), the capacity of the cylinders to ground could be
increased and the arrester made more sensitive. By this arrangement, more gaps could be used for a given voltage arrester, giving a greater factor of safety in extinguishing the arc. This arrangement has the disadvantage, however, that it produces an arrester rather too sensitive to frequency. In order to prevent
high-frequency disturbances of low voltage from continually setting off the arrester, the number of gaps necessary may be excessive; too many to give protection against low-frequency surge. The antennæ can also be run in the opposite direction, from line toward ground (see Figure 17), decreasing the capacity of the
cylinders to ground and giving a more uniform voltage distribution. In this case the number of gaps in the arrester will have to be less for the same arcing voltage, thus decreasing the factor of safety of arc interruption. Our arresters this year are not
supplied with antennæ, but this is a means of adjustment that can always be used in case of necessity.
ALUMINUM CELL ARRESTER
The aluminum cell arrester consists of plates of aluminum arranged as electrodes of a battery. The electrolyte may be any one of a number of solutions. When current passes through an aluminum cell, an insulating film forms on the surface of the metal. This film has a certain dielectric strength. If voltage rises above that critical value, current can flow through the cell with very little impedance. When, however, the voltage falls below that critical voltage, the film at once re-forms and shuts
off the current. The cell is a true electric valve, and protects independently of frequency. If an alternating voltage with a maximum above the critical voltage of the cell is impressed across such an arrester, the peak of each voltage wave will be cut off by the arrester. This arrester can be used either directly across the line or it can be connected in series with a single spark-gap. The arrester was developed by Professor Creighton a number of years ago.
THE LIQUID ELECTRODE ARRESTER
An arc from a liquid cathode is unstable unless the voltage is above a certain high value, somewhat above 1500 volts. A cell partly filled with electrolyte, having two electrodes suspended near the surface (see Figure 18), will act as an arrester when
used on voltage just below that required to maintain the liquidcathode arc. That is, any rise in voltage above normal can pass current through the cell, but on normal voltage the arc can not maintain. Cells in series can be used up to any voltage desired. Another form of this arrester is shown in Figure 19. This type has electrodes dipping into the liquid. The arrester requires an additional gap in series. When the spark-gap arcs over, current flows through the arrester. As soon, however, as the liquid at the electrode point has become vaporized by the current, the liquid is repelled, forming a depression beneath each electrode, and giving an arc path for the current. One arc in each cell has, of course, a liquid cathode, and, on normal line voltage, extinguishes as explained above. The arc is not, however, chopped
LIQUID ELECTRODE CELL
off short, but, due to a cushion effect, dies out slowly enough. to cause no rise in voltage. This arrester was described for the first time by Professor Creighton in his paper before the American Institute of Electrical Engineers on March 29, 1907.
Horn arresters should not be considered as true lightning arresters, but rather as insulation intentionally weak. If-due. for instance, to direct stroke-the insulation of a line must fail, it is very much preferable that it should do so over a horn arrester. Horn arresters with resistance are usually useless, except on constant-current circuits, as the current of discharge is too limited to relieve the line. With no resistance, or with