Arc resistance

The arc resistance of a polymer is the length of time for which we can apply an electrical discharge to its surface before it breaks down and begins to conduct electricity. Breakdo vn typically takes the form of conductive carbonaceous tracks that are caused by oxidation due to locally high temperatures. Polymers mth low polarity typically fare better in this test than polar polymers. Arc resistance is reduced by dirt, moisture, and other surface contaminants. 

We can measure arc resistance on dry surfaces or when they are wetted by an electrolyte dripping on the surface between pairs of sharp electrodes,. We report arc resistance as the time it takes to form a conductive track into which the arc disappears. 

This test shows the ability of a material to resist the action of an arc of high voltage and low current close to the surface of the insulation in tending to form a conducting path therein. The arc resistance data are of relative value only for distinguishing materials of nearly identical composition, such as for quality control, development, or identification. 

Specimens are thin sheets or plates having parallel plane surfaces and are of a size sufficient to prevent flashing over. Dielectric strength varies with thickness and, therefore, specimen thickness must be reported. The dielectric strength varies inversely with the thickness of the specimen. The dielectric strength of plastics will drop sharply if holes, bubbles, or contaminants are present in the specimen being tested. 

Since temperature and humidity affect results, it is necessary to condition each type of material as directed in the specification for that material. The test for dielectric strength must be run in the conditioning chamber or immediately after removing the specimen from the chamber. 

The specimen is placed between heavy cylindrical brass electrodes, which carry electrical current during the test. There are two ways of running this test for dielectric strength. In the short-time test the voltage is increased from zero to breakdown at a uniform rate. The precise rate of voltage rise is specified in the governing material specifications. In the step-by-step test the initial 

---

Electrical Resistance—Conductivity. Most fillers are composed of nonconducting substances that should, therefore, provide electrical resistance properties comparable to the plastics in which they are used. However, some fillers contain adsorbed water or other conductive species that can gready reduce their electrical resistance. Standard tests for electrical resistance of filled plastics include dielectric strength, dielectric constant, arc resistance, and d-c resistance. 

Due to thek similarity to a-c furnaces, d-c furnaces can be substituted for neatly any a-c furnace including the open-arc, submerged-arc, and arc-resistance furnaces, provided that design criteria, particularly electrical parameters, are properly chosen. Currently, steel and ferrochrome is being made commercially in d-c furnaces and a siUcon metal pilot plant is being built. 

Nylons have excellent arc and tracking resistance. Arc resistance is not affected by moisture or temperature up to about 100°C. Comparative... 

Electrical and electronics are the largest end uses for PPS. Principal advantages of PPS are strength at high temperature, dimensional stabiUty, flame and arc resistance, and precision moldabiUty. Products are coimectors, cod bobbins, relays, and switches. The growth of electrical and electronic end uses is 10%. Apphances are the second largest ends uses for PPS resins as a replacement for phenoHc, but also for metal parts. 

Bcctiuse of the low conttict gap. low arc resistance tind fast clearance, the tire energy dissipated in vacuum for a particular current is 1/10 that of oil tintl 1/4 that of SF,. and is illustrated in Figure 19.6,... 

Arc Resistance ASTM D495. When an electric current is allowed to travel across the surface of an insulator the surface will become damaged over time and become more conductive. The arc resistance is a measure of the time (in seconds) required to make an insulating surface conductive under a high-voltage, low-current arc. 

The cycloaliphatic resins also are clearly superior in arc resistance and arc track resistance. This has led to applications in the tension insulators, rocket motor cases and transformer encapsulation. 

Where plastics are to be used for electrical applications, then electrical properties as well as mechanical and other properties need to be considered. Whilst properties such as resistivity, power factor and dielectric constant are important, they may not be all-important. For example, although polyamides and many thermosetting plastics may show only moderate values for the above properties, they have frequently been used successfully in low-frequency applications. Perhaps more important for many purposes are the tracking and arcing resistance, which are frequently poor with aromatic polymers. 

The electneal properties of PTFE are dominated by its extremely tow dielectric constant (2.1) This value is invanant over a broad range ot temperatures (- 40 to 250 °C) and frequencies (5 Hzto 10 GHz). Smularly, PTFE has an unusually low dissipation factor, which is also quite mdependent of temperature and frequency fhis behavior results from the high degree of dipolar symmetry of the perfluonnated and unbranched chains The dielectnc strength, resistivity, and arc resistance are very high... 

Cold forming Machining Fusion welding Inert gas xy gos arc Resistance spot welding Protective anodising... 

Good arc resistance to prevent permanent damage to the surface of the unit in case of an accidental arc over. 

Power Factor, 60 Cycles Arc Resistance, sec Water Absorption, 24 hr, % Rockwell Hardness... 

Phenolic resins Good acid resistance, good electrical properties (except arc resistance), high heat resistance Compression molding, continuous lamination... 

Silicones Highest heat resistance, low water absorption, excellent dielectric properties, high arc resistance Compression molding, injection molding, encapsulation... 

---