Short circuit currents. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
Free library / Electrician

Short circuit currents. Encyclopedia of radio electronics and electrical engineering

Free technical library

Encyclopedia of radio electronics and electrical engineering / Electrician's Handbook

Short circuit (short circuit) is the connection of current-carrying parts of different phases or potentials to each other or to the equipment case connected to the ground, in power supply networks or in power receivers.

A short circuit can happen for a variety of reasons.:

deterioration of insulation resistance in a humid or chemically active environment;
in case of unacceptable overheating of the insulation;
mechanical stress;
erroneous influences of personnel during maintenance and repair, etc.

As can be seen from the very name of the process, during a short circuit, the current path is shortened, i.e., it goes bypassing the load resistance, so it can increase to unacceptable values if the voltage is not turned off under the action of protection.

But the voltage may not turn off even in the presence of protection, if a short circuit occurs at a remote point, and due to the high resistance to the short circuit, the current is insufficient to trigger the protection. But this current can be enough to set fire to the wires, which can lead to a fire.

Hence arises the need to calculate the short circuit current - TKZ. The value of TKZ may change if other power receivers in more remote places are connected to the power supply network. In such cases, the TKZ is again calculated at the installation site of new electrical receivers.

TKZ also produces an electrodynamic effect on devices and conductors, when their parts can be deformed under the action of mechanical forces that arise at high currents.

The thermal effect of TKZ is to overheat devices and wires. Therefore, when choosing devices, they must be checked according to the conditions of the short circuit, so that they can withstand the short circuit at the place of their installation.

As you know, along with networks with a solidly grounded neutral, there are networks with an isolated neutral. Consider characteristic differences these networks during short circuit.

In practice, in most cases, single-phase short circuits occur. In networks with an isolated neutral, when one phase is connected to the ground, the mode is not a short circuit and the uninterrupted power supply is not disturbed, but it must be turned off, as it corresponds to an emergency condition. When one phase is shorted to earth in this network, the voltages on the other two phases increase by 1,73 times, and the voltage at the zero point becomes equal to the phase voltage relative to earth.

In networks with a dead-earthed neutral, when the wire is connected to the ground, the fuse burns out or the circuit breaker trips, while the power supply is interrupted, and when the fuse burns out, the motor windings can be damaged when operating in two phases.

Author: Koryakin-Chernyak S.L.

See other articles Section Electrician's Handbook.

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