Discharge-dependent breakdown, electrical

Discharge-Dependent Breakdown. Electrical discharges (voltage breakdown in gas) may occur on the surface of or in voids or discontinuities of electrical insulation under a wide variety of conditions. Such discharges are a source of ionization and r-f interference but, more importantly, may cause slow degradation of the plastic surface or within cracks or voids. Plastics vary greatly in their resistance to such degradation. For example, polytetrafiuoroethylene (PTFE) is susceptible, whereas silicone elastomers are resistant. 

The theoretical basis for electric breakdown in solids is still limited. Several types of breakdown in solids are recognized intrinsic breakdown, breakdown dependent on physical defect, thermal breakdown, discharge-dependent breakdown, failure by electrical treeing, and failure by water (electrochemical) treeing. Conditions and material characteristics determine the type of breakdown which takes place. Actual electrical breakdown may involve two or more types simultaneously or sequentially. 

Electrostatic Discharges An electrostatic discharge takes place when a gas or vapor-air mixture is stressed, electrically, to its breakdown value. Depending upon the specific circumstances, the breakdown appears as one of four types of discharges, which vary greatly in origin, appearance, duration, and incendivity. 

The concept of effective electric field strength was originally developed (6, 42) to take into account the observed frequency dependence of the electric field strength required for gas breakdown. However, this concept is of great general utility for reasonably homogeneous discharges since it allows one to compare the effect on the electrons of an applied... 

Finally, discharge breakdown may occur when gas discharges occur in the pores of a dielectric material. The electric strength of the ceramics depends upon the specimen and pore sizes in other words, larger pores will increase the probabUily of failure as the electric field in a pore, Ep, is greater than that in the surrounding... 

The storability will depend on the discharge load and also on the cell structure. Failure in storage is usually due to the breakdown of cellulosic compounds within the cell which, at first, results in a reduction of the Umiting-current density at the anode. Further breakdown produces low-drain internal electrical paths and a real loss of capacity due to self-discharge. Eventually, complete self-discharge can occur, but at 20°C and below these processes take many years. 

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