Fault Currents in Electrical Distribution

The Fault Current Types

There are four types of fault current, divided into two categories:

• The overcurrent category:

  • Overload currents

  • Short-circuit currents

• The ground fault category:

  • Low intensity ground faults

  • High intensity ground faults

The Overcurrent Category

The main characteristics and associated risks are described below:

• Overload currents:

  These are mainly due to problems with excessive loads on receivers. For example, operating too many consumers in a workshop at the same time (lighting, heating, power) can bring about an electrical distribution overload.

  The main risks from overload currents are gradual deterioration in equipment, or a fire.

• Short-circuit currents:

  These can be due to deterioration in the plant or within a receiver. For example, a short-circuit between two phases in the winding of a motor being operated in severe conditions (vibration, damp and/or corrosive atmosphere).

  The risks associated with short-circuit currents are the instantaneous deterioration of equipment, a fire, or even an explosion due to the high energy level at the site of the short-circuit.

The Ground Fault Category

Ground faults can be due to abnormal aging of the plant, of a load or conductors, that have deteriorated in damp conditions.

The intensity of such fault currents depends on the ground connection diagram used. These currents can be:

• Very low in value, that is, well below the rated feed current in the TT system (these are known as leakage currents or residual ground fault currents).

• High in value, that is, identical to a short-circuit current in the TN-S system (these are known as ground fault currents).

Whatever the value of ground leakage currents, they present a very grave risk of electrocution or fire.