Page 4 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

FUNDAMENTALS OF ELECTRIC SYSTEMS

FUNDAMENTALS OF ELECTRIC SYSTEMS 1.3

FIGURE 1.2 A parallel-plate capacitor with conductors (plates) of area A.

TABLE 1.1 Properties of Some Dielectrics*
Dielectric
Dielectric strength, †
Material constant kV/mm
Vacuum 1.00000 ∞
Air 1.00054 0.8
Water 78 —
Paper 3.5 14
Ruby mica 5.4 160
Porcelain 6.5 4
Fused quartz 3.8 8
Pyrex glass 4.5 13
Bakelite 4.8 12
Polyethylene 2.3 50
Amber 2.7 90
Polystyrene 2.6 25
Teflon 2.1 60
Neoprene 6.9 12
Transformer oil 4.5 12
Titanium dioxide 100 6
*These properties are at approximately room temperature
and for conditions such that the electric field E in the dielectric
does not vary with time.
†
This is the maximum potential gradient that may exist
in the dielectric without the occurrence of electrical break-
down. Dielectrics are often placed between conducting plates to
permit a higher potential difference to be applied between them
than would be possible with air as the dielectric.

dielectric to that without dielectric. Table 1.1 illustrates the dielectric constant and dielec-
tric strength of various materials.
The high dielectric strength of vacuum (∞, infinity) should be noted. It indicates that if
two plates are separated by vacuum, the voltage difference between them can reach infin-
ity without having flashover (arcing) between the plates. This important characteristic of
vacuum has led to the development of vacuum circuit breakers, which have proved to have
excellent performance in modern industry.

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