Impulse Protective Gap Arrester:
It was pointed out that the sphere gap has an impulse ratio of unity, but suffers from the drawback that the arc between its electrodes is not self-extinguishing. The horn gap is self-extinguishing but it has a high impulse ratio of 2 or 3 unless the setting is small, as with low voltages.
The impulse protective gap arrester is designed to have a low impulse ratio, even less than unity and to extinguish the arc. In principle the impulse protective gap arrester is very simple, as illustrated in Fig. 15.25. The gap proper consists of two sphere-horn electrodes S1 and S2 which are connected respectively to the line and to the arrester, the latter being usually of the electrolytic type. An auxiliary needle electrode E is placed midway between S1 and S2 and is connected to them via (R, C1) and C2. At normal line frequency the impedance of capacitance C1 is quite large as compared to the impedance of resistance R; and if C1 = C2 the potential of the auxiliary electrode will be midway between those of S1 and S2 and the electrode has no effect on the flash-over between them.
In case of a transient occurring over the line, the impedance of capacitors C1 and C2 decreases and the impedance of the resistance R now becomes predominent. The result is that practically whole of the voltage is concentrated across the gap between E and S1. The gap at once breaks down, the rest of the gap between E and S2 immediately following. In effect, the length of the gap is halved the instant a high-frequency wave occurs.
On the other hand, a slight time lag is introduced due to the fact that, in the initial breakdown, one of the electrodes is pointed. However, an apparent impulse ratio of 0.6 to 0.7 can be achieved with the commercial forms of the gap. The electrolytic arrester on the earth side extinguishes the arc.