LV switchgear functions - Electrical protection

Electrical protection assures:

• Protection of circuit elements against the thermal and mechanical stresses of short-circuit currents
• Protection of persons in the event of insulation failure
• Protection of appliances and apparatus being supplied (e.g. motors, etc.)

The aim is to avoid or to limit the destructive or dangerous consequences of excessive (short-circuit) currents, or those due to overloading and insulation failure, and to separate the defective circuit from the rest of the installation.

A distinction is made between the protection of:

• The elements of the installation (cables, wires, switchgear…)
• Persons and animals
• Equipment and appliances supplied from the installation

The protection of circuits

• Against overload; a condition of excessive current being drawn from a healthy (unfaulted) installation
• Against short-circuit currents due to complete failure of insulation between conductors of different phases or (in TN systems) between a phase and neutral (or PE) conductor

Protection in these cases is provided either by fuses or circuit-breaker, in the distribution board at the origin of the final circuit (i.e. the circuit to which the load is connected). Certain derogations to this rule are authorized in some national standards, as noted in Standards and description of circuit-breakers .

The protection of persons

According to IEC 60364-4-41, Automatic disconnection in case of fault is a protective measure permitted for safety

• Circuit breaker or fuses can be used as protective devices that "automatically interrupt the supply to the line conductor of a circuit or equipment in the event of a fault of negligible impedance between the line conductor and an exposedconductive-part or a protective conductor in the circuit or equipment within the disconnection time required " (60364-4-41 sub-clause 411)
• Against insulation failures. According to the system of earthing for the installation (TN, TT or IT) the protection will be provided by fuses or circuit-breakers, residual current devices, and/or permanent monitoring of the insulation resistance of the installation to earth

The protection of electric motors

• Against overheating, due, for example, to long term overloading, stalled rotor, single-phasing, etc. Thermal relays, specially designed to match the particular characteristics of motors are used.

Such relays may, if required, also protect the motor-circuit cable against overload.

Short-circuit protection is provided either by type aM fuses or by a circuit-breaker from which the thermal (overload) protective element has been removed, or otherwise made inoperative.