Fire hazard of short circuits, overloads, large transient resistances. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Fire hazard of short circuits, overloads, large transient resistances. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electrical safety, fire safety

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Short circuit may occur due to a violation of the insulation between the phase wires, the phase wire and the neutral wire of the network, or when the phase wire is shorted to metal structures that are connected to the ground, short to ground.

A short circuit can be complete (metal) and incomplete. With a full circuit, the transient resistance at the place of the wire closure is insignificant, and it can be neglected. More often, an incomplete short circuit occurs, since in the places where the wires are shorted, a transition resistance appears due to loose contacts, the formation of an oxide film, and charring of the insulation.

In the event of a metal short circuit, protection is triggered, which turns off the network. In case of an incomplete short circuit, the protection devices cannot work, resulting in a fire. A fire is also possible with the wrong choice of protection devices.

The greatest fire hazard have wires and cables in polyethylene insulation.

A fire can occur due to an overload in the network when connecting electrical receivers, the rated current consumption of which exceeds the allowable current for a given section of the network wire or if the section of the conductor is incorrectly selected. In case of overload, the protection may not work until the wires are heated to ignition temperature, then a short circuit occurs between them. Especially dangerous is overload in the electrical networks of private residential buildings, garden buildings, since they often do not have overload protection devices, but only devices that turn off the network in case of a short circuit. In terms of fire, significant transient resistances that occur at the junction of wires, their connection to switches, sockets, shields, and household appliances are dangerous.

Reliability of contact is ensured by crimping, soldering or special clamps equipped with spring washers.

It should be borne in mind that even with proper connection in special clamps, after 3-3,5 years, the resistance of the contacts increases by 2 times. To a large extent, the transition resistance increases during short-term short circuits, which are switched off when the protection is working properly. With an increase in transient resistance and non-emergency operation of the power network, protection devices do not turn off, since the current is insufficient for their operation. At the same time, the wire at the point of contact with a large contact resistance can heat up to the ignition temperature of the insulation.

Even with a slight overload in the network, but with a large transient resistance at the point of contact and a long failure of the protection, the wire can catch fire.

Author: Korshevr N.G.

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