Internal Arc-Proof Switchgear Power Systems

In low-voltage switchgear power systems there are generally two types of fault to which the development of currents of remarkable intensity is associated: bolted fault and arc fault.

Bolted Faults and switchgear power systems

The term “bolted” is a reference made to a fault within a switchgear power system in which two or more live parts at different potential get in touch. This is an example of phase-to-phase or phase-to-earth short-circuits to which the circulation of anomalous current within the ring developed at the fault moment.

Within the switchgear power systems, a bolted fault’s most harmful effects are prevalently of the electrodynamic, proportional to I2, due to the high intensity of the current and to the low fault resistance involved.

Arc Faults and switchgear power systems

An arc fault, within a switchgear power system, occurs when there is a reduction in the dielectric strength of the insulating means (air, in low voltage switchgear power systems) interposed between two or more conducting elements at different potential.

The arc fault is generated at the moment when, due to high ionization of the air, there is a breakdown of the dielectric of the medium and the consequent flow of the current through it.

Within the switchgear power systems, the arc fault’s highest stresses are of the thermal type and are proportional to RaI2 owing to the high value taken by the arc resistance Ra; this is because the fault current flows in a medium which is always insulating, even if extremely ionized.

These type of arc stresses, within a switchgear power system, will generally manifest themselves as either: 1) high thermal gradients caused by the quick and intense rise in air temperature, 2) high pressure gradients in the form of a pressure wave, or 3) high ionization of the air with consequent reduction of its insulating strength.

Low-Voltage switchgear power systems and the Consequences of an Arc Fault

Mostly, in a low-voltage switchgear power system designed and tested according to the Standard IEC 60439-1, an arc fault is not very likely to occur. However, should it occur, the consequences would be extremely harmful to both the equipment as well as the personnel.

Causes Of An Arc Fault in a Switchgear Power System

The causes of an arc fault within a switchgear power system can be both technical as well as nontechnical. Among the nontechnical are the following:

    1. Personnel errors: mostly during maintenance operations.

    2. Installation operations not performed accurately.

    3. Inadequate maintenance, exposed especially during severe environmental conditions.

Among the technical causes of an arc fault in a low voltage switchgear power systems are as follows:

    1. Breakdown of the insulation – essentially in the proximity of the supports of the busbars and of the plug-in contacts of the withdrawable units (this accounts for 75% of all technical causes).

    2. Overvoltages – generating disruptive discharges between the points at minimum clearances (15% of all technical causes).

    3. Constructional defects of the apparatus (10% of the technical causes).