Short-circuits can be defined according to three main characteristics:
1. Their origin
- They may be mechanical: breakdown of conductors or accidental electrical connection between two conductors via a foreign body such as a tool or animal;
- 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);
- 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
- The short-circuit may be generated inside equipment (cable, motor, transformer, switchboard, etc.), and it generally leads to deterioration;
- 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
- Self-extinguishing: the fault disappears on its own;
- 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);
- Permanent: these faults require de-energization of a cable, machine, etc., and intervention by the operating personnel.
Short Circuit Current Calculation at Various Point of Electrical Circuits (Isc)?
Different types of short-circuits
Phase-to-earth: 80% of cases *
Phase-to-phase: 15% of cases *
These faults often degenerate into three-phase faults.
Three-phase: 5% of cases (originating as such) *
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).
Other types of faults
- Too many successive start-ups leading to overheating and mechanical shocks on couplings;
- Excessive start-up time or rotor locking leading to the same result;
- 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