Spark, gap

Spark gap These have a pair of conducting rods with an adjustable gap, depending upon the spark over voltage of the arrester. Precise protection is not possible, as the spark-over voltage varies with polarity, steepness and the shape of the wave. These arresters are also now obsolete for the same reasons. 

A CT is provided in series with the spark gap to sense its operation during a line fault. As soon as there is arcing, it provides an instantaneous command to a short-circuit relay. The relay, in turn, closes the bypass breaker, within 3 to 5 cycles, leaving only the natural line impedance in the faulty eircuit. Now = 0, which limits the fault current to the natural level of the system, as if the capacitors were not connected. The shorting device is restored to its original status as soon as the fault condition is cleared. The device must be capable of interrupting the line fault... 

Figure 26.11 Overvoltage protection with the help of a spark gap and a shorting switch...

Rapid reinsertion of the capacitors as soon as the fault coiiditions are removed is an important requirement to retain the stability of the system. This can be achieved with the use of an additional ZnO. non-linear resistance (ZnO being the latest in this field compared to SiC. which was used earlier), across the capacitor banks (Figure 26.12). Generally, the ZnO resistor will be adequate to dampen the fault current without initiating the spark gap. and w ill limit the overvoltage across the capacitors. It will also permit automatic reinsertion of the capacitors as soon as the fault conditions are removed without causing a delay. The spark gap will serve as a backup to the ZnO resistor in the event of very severe faults. 

Fig. 10-4 Insulating Joint with explosion-proof spark gap (I/sohz = 1 1/50//s = 2.2 kV surge current =100 kA). 1, insulating flange 2, hexagonal nut 3, insulator ring 4, insulator sleeve 5, insulator disc 6, steel disc 7, blue asbestos gasket 8, explosion-proof spark gap.

So that the function of the spark gap isolators is not cancelled, cables with lightning protection must have insulated sleeves installed at their ends, in addition to the spark gap isolators, so that the metal sheathing or the shield that is connected at soldered joints to the lightning reinforcement is also separated from the operational ground of the building (foundation ground) and from the ground at the remote end. 

If insulated sleeves and spark gap isolators lie directly in soil, they eonstitute danger spots. Currents ean exit and then reenter. This danger is redueed if they are installed in a dry pit and the first meter of the eable on either side is insulated. 

This section deals with overvoltage spark gaps (gas discharge spark gaps) which produce a connection between pipeline and ground only when a trigger voltage... 

D = chemical detonator, C = condenser discliarge, S = permanent spark gap. 

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