Dielectric strengths

Dielectric strength indicates the voltage gradient at which dielectric failure occurs is generally measured (ASTM D-149) at commercial power frequencies, and is 

Polymer Grade Volume resistivity (ohm.cm) Dielectric strength (mV/m) Dielectric constant at 1 kHz Dissipation factor at 1 kHz Surface arc resistance Tracking resistance 

Polymethyl pentene GP 16 28 2.12 0.0002 Very good Very good 

Ethylene-propylene copolymer GP 15 30 2.3 0.0005 Good Very good 

Styrene-ethylene-styrene copolymer GP 16 16 2.2 0.006 Good Good 

Phenol-formaldehyde GP 10 12 8 0.05 Very poor Very poor 

Before embarking on a detailed consideration of the application of dielectrics and insulators, it is opportune to focus attention briefly on dielectric strength and thermal shock resistance . Both properties demand careful consideration in certain applications of dielectrics and insulators. They are by no means simple to define and, generally speaking, it is necessary only to develop some appreciation of how component and operational parameters determine them. 

In the industrial situation thermal breakdown is the most significant mode of failure and is avoided through experience rather than by application of theory. Nevertheless it is important to appreciate the mechanisms leading to thermal breakdown. A third mode of failure, referred to as discharge breakdown, is of importance in ceramics because it has its origins in porosity. 

This inconsistency has already been mentioned by Bikerman ) who suggested that the tow value of the measured electroconductivity (half the value that follows from Manegold s formula) is due to an unfavorable arrai ement of the Ikjuid phase. 

If one imagines the structure of a liquid foam as a system of cubical gas bubbles (Fig. 19) and that the electrical current is directed upward, the horizontal walls of bubbles (perpendicular to the current direction) do not participate in electroconductivity and 2 of die 6 walls of each cube do not contribute to conduction. Then we have kIk = 4/6 K = 2/3 K, i.e. Manegold s formula is applicable. On the other hand, this equation coincides with Wagner s and Odolevsky s equations, which is to be expected since both these relations are also based on a cubical model of the disperse system. 

The actual structure of highly foamed systems is polyhedral therefore, the models proposed by Bikerman (Fig. 19b) and Chistyakov and Chemina (Fig. 19c) are more frequently used in case of high voltages, for example for lining high-voltage transformers. 

According to the data obtained by Domkin the value of dielectric breakdown voltage Ebr is primarily determined by the dielectric strength of the gas in the cells and is independent of the foam density at least up to 7 = 600 kg/m (Fig. 20). 

The tedmological skin at foam block edges approximately doubles Ebr (Table 8). 

Details of the equipment needed for carrying out this measurement are in ASTM D140-97a [17]. 

The dielectric strength of various polymers under various categories covering the lower (10-20 Mv/m), intermediate (1-40 MV/m), and high (40-60 MV/m) ranges is shown in Table 5.5. Dielectric constants and arc and tracking resistances are also 

Electrical Applications of Reinforced Plastics (% glass fiber in parentheses unless otherwise stated) 

Polyprt ylene Talc (20%) Electrical system housings 

Alkyl resins Short glass fiber Automotive, ignition, switches, relay bases 

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Melting temperature, °C Crystalline Amorphous Specific gravity Water absorption (24 h), % Dielectric strength, kV mm ... 

Dielectric strength, kV mm Electrical Volume (dc) resistivity, ohm-cm Dielectric constant (60 Hz) Dielectric constant (10 Hz) Dissipation (power) factor (60 Hz) Dissipation factor (10 Hz) Mechanical Compressive modulus, 10Mb in-2 9.8-12 24-31 16-24 1014-1016 4.5-6.0 19 335-600 14 ... 

Electrica.1 Properties. The bulk electrical properties of the parylenes make them excellent candidates for use in electronic constmction. The dielectric constants and dielectric losses are low and unaffected by absorption of atmospheric water. The dielectric strength is quoted for specimens of 25 p.m thickness because substantially thicker specimens cannot be prepared by VDP. If the value appears to be high in comparison with other materials, however, it should be noted that the usual thickness for such a measurement is 3.18 mm. Dielectric strength declines with the square root of increasing... 

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. 

Sulfur hexafluoride was first prepared in 1902 (1). The discovery in 1937 that its dielectric strength is much higher than that of air (2) led to its use as an insulating material for cables, capacitors (3), and transformers (4) (see Insulation,electric). Sulfur hexafluoride has been commercially available as AccuDri, SF (AUiedSignal Inc.) since 1948. It is also produced by Air Products and Chemicals in the United States and by others in Germany, Italy, Japan, and Russia. 

Sulfur hexafluoride is more stable in arcs (27) than fluorocarbons such as C2F, or refrigerants such as CCI2F2, but less stable than CF, BCl, or SiF. Exposed to 1000°C temperatures, SF decomposes to SOF2 and SF to the extent of 10 mol %. In spite of its decomposition, the dielectric strength... 

High Frequency Dielectric Strength. Dielectric strength at high frequency is important in microwave power uses such as radar (see Microwave technology). Because SF has zero dipole moment, its dielectric strength is substantially constant as frequency increases. At 1.2 MHz, SF has... 

Particle Contamination. In assembling large, high voltage equipment such as coaxial lines, contamination by metal particles may occur which may decrease the dielectric strength under various conditions by 5 to 10-fold (44—45). Metal needles are the worst contaminants and electrostatic traps or adhesive areas have been designed to cope with them (46). 

Electrical Properties. The low polarizabiHty of perfluorinated Hquids makes them exceUent insulators. Theh dielectric strengths are about 40 kV (ASTM D877) dissipation factors are about 0.0001 at 1 MH2 dielectric constants are about 1.8 volume resistivities are about 1 x 10 ohm-cm (ASTM D257) (17). 

In air, PTFE has a damage threshold of 200—700 Gy (2 x 10 — 7 x 10 rad) and retains 50% of initial tensile strength after a dose of 10" Gy (1 Mrad), 40% of initial tensile strength after a dose of 10 Gy (10 lad), and ultimate elongation of 100% or more for doses up to 2—5 kGy (2 X 10 — 5 X 10 rad). During irradiation, resistivity decreases, whereas the dielectric constant and the dissipation factor increase. After irradiation, these properties tend to return to their preexposure values. Dielectric properties at high frequency are less sensitive to radiation than are properties at low frequency. Radiation has veryHtde effect on dielectric strength (86). 

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