LV switchgear functions - Electrical protection: Difference between revisions
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Revision as of 13:35, 7 June 2012
Electrical protection assures:
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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 chapter H1.
The protection of persons
- 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.
