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Main short circuit faults occurring in networks and machines
Main short circuit faults occurring in networks and machines (photo credit: pieperpower.com)

Short-circuit characteristics

Short-circuits can be defined according to three main characteristics:

1. Their origin

  1. They may be mechanical: breakdown of conductors or accidental electrical connection between two conductors via a foreign body such as a tool or animal;
  2. They may be electrical: following the degradation of the insulation between phases, or between phase and frame or earth, or resulting from internal overvoltages (switching surges) or atmospheric overvoltages (stroke of lightning);
  3. They may be due to an operating error: earthing of a phase, connection between two different voltage supplies or different phases or closing of a switching device by mistake.

2. Their location

  1. The short-circuit may be generated inside equipment (cable, motor, transformer, switchboard, etc.), and it generally leads to deterioration;
  2. The short-circuit may be generated outside equipment (cable, motor, transformer, switchboard, etc.). The consequences are limited to disturbances which may, in the course of time, lead to deterioration of the equipment in question and thereby cause an internal fault.

3. Their duration

  1. Self-extinguishing: the fault disappears on its own;
  2. Fugitive: the fault disappears due to the action of protective devices and does not reappear when the equipment is started up again (the fault is “burnt out” after reenergization);
  3. Permanent: these faults require de-energization of a cable, machine, etc., and intervention by the operating personnel.

Different types of short-circuits

Phase-to-earth: 80% of cases *

Phase-to-earth short-circuit
Figure 1 – Phase-to-earth short-circuit

Phase-to-phase: 15% of cases *

These faults often degenerate into three-phase faults.

Phase-to-phase short-circuit clear of earth
Figure 2 – Phase-to-phase short-circuit clear of earth

Two-phase-to-earth short-circuit
Figure 3 – Two-phase-to-earth short-circuit

Three-phase: 5% of cases (originating as such) *

Three-phase short-circuits
Figure 4 – Three-phase short-circuits


Causes of short-circuits

Degradation of insulating materials due to:

  • Degradation in surface quality (pollution);
  • Excessive temperature;
  • Partial discharge in the vacuoles (micropockets) inside the insulating materials.
  • Accidental reduction in electrical insulation (presence of animals, tree branches, tools left by carelessness on a busbar, etc.).
  • Destruction due to external causes (hit by a shovel, etc.).
  • Overvoltages causing a breakdown in equipment insulation (switching surges or lightning strike).
NOTE * The percentages given are for this article only. They come from fault statistics on public distribution networks. The order of magnitudes must be approximately equivalent in industrial networks. It may be observed therefore that phase-to-earth short circuits are the ones most frequently occurring and that three-phase faults are fairly rare.

Other types of faults

On motors:

  1. Too many successive start-ups leading to overheating and mechanical shocks on couplings;
  2. Excessive start-up time or rotor locking leading to the same result;

On generators:

  • Loss of excitation due to a fault in the rotor circuit (cut, short-circuit, etc.), leading to overheating of the rotor and stator and loss of synchronism with the network;
  • Variations in frequency due to an overload or faulty operation of a frequency regulator (for a generator operating cut-off from the utility network).
  • Phase opposition connection of a generator with the network or of two parts of a network coming from different sources.
  • Overvoltages due to a lightning strike.
  • Switching surges (of a switch or circuit-breaker).
  • Overloads on cables, transformers, motors or generators.
  • Reversal of the direction of energy flow in the absence of an electrical fault. In the event of a power cut or a drop in voltage caused by the utility, an internal generation plant may supply energy to the utility.
  • Variations in voltage due to faulty operation of the on-load tap changers of a transformer, or network under or overload.
  • The presence of a negative-phase component due to a non-symmetrical voltage source, a large single-phase consumer, a connection error or phase cutting leads to overheating of the motors or generators, and a loss in generator synchronism.

IEC 61439 – Short-circuit withstand tests (VIDEO)


Reference: Protection of Electrical Networks – Christophe Prévé

About Author //

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

One Comment


  1. Larrie Ruel D. Quilantang
    Aug 06, 2014

    Hi. Thank you for sharing your knowledge to others. God Bless you.

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