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 case, a dioxin residue was found which caused additional environmental problems in cleanup and
repair after the fire incident.
Askarel liquids have been classified as toxic to animals and humans by the Environmental
Protection Agency (EPA). As of 1 October 1977, the EPA no longer allows the manufacture of
PCB fluids or the sale of askarel-filled transformers. To avoid an uncontrolled discharge to the
environment, the users of askarel-insulated transformers must monitor and dispose of PCB liquid
in accordance with applicable standards. (See ANSI C107.1-1974, Guidelines for Handling and
Disposal of Capacitor- and Transformer-Grade Askarels, Containing Polychlorinated Biphenyls;
and IEEE Standard 76-1974, Guide for Acceptance and Maintenance of Transformer Askarel in
Equipment.)
3.2.3.3 High-Fire-Point Insulating Liquids. In the late 1970s, less-flammable, nontoxic,
high-fire-point (greater than 3000C) fluids were introduced to replace askarel as a transformer
dielectric (insulating) fluid. These fluids are designated as either polydimethylsiloxane (silicone)
or high-molecular-weight hydrocarbons. These liquids will burn, but they do so very slowly and
release much less heat than flammable mineral oil. They are also nontoxic, unlike askarel.
Because of these characteristics, transformers insulated with high-fire-point liquid may be
installed inside building areas not containing vaults, liquid confinement areas, or fire suppression
systems. Although it is rarely economical to do so, askarel transformers may be drained, flushed,
and refilled with a high-fire-point liquid. The kVA rating of a retro-filled transformer must be
decreased for continuous service. The National Electric Code (NEC) contains installation
information for less-flammable liquid insulated transformers. The Factory Mutual Research
Corporation lists high-fire-point qualifying liquids.
3.2.3.4 Air or Gas. The ventilated dry-type transformer has applications in distribution
systems, and may be installed indoors without a vault. The sealed or gas-filled dry-type
transformer has very limited use, due principally to the higher price. Nitrogen and air-insulated
transformers are generally limited to 15 kV and lower operating voltages.
3.2.3.5 Basic-impulse Insulation Level (BIL). It is characteristic of most insulations that
the maximum voltage which they can successfully withstand varies inversely with the duration of
the voltage. Since power systems are subject to various types of overvoltage, some of long
and some of short duration, power distribution equipment is usually required to withstand at least
two different types of dielectric tests. The first are the so-called low-frequency (60-cycle) tests,
usually of one minute duration, that establish the ability of the insulation to withstand moderate
overvoltage of relatively long duration. The others are the impulse tests designed to prove that
the insulation will not break down on voltage surges of high magnitude but short duration, such
as those produced by lightning. The impulse test commonly used consists of the application of a
very short duration full-wave voltage surge of a specified crest value to the equipment insulation.
The crest value of the wave the insulation can withstand without breakdown is the BIL. To
simplify the design and application of electrical equipment, a series of Standard BIL's have been
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