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 or circuits (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 between a phase and neutral conductor or between a phase an PE or earth.
Protection in these cases is provided by overcurrent protective devices. Such devices could be circuit-breaker (IEC 60898, IEC 60947-2, IEC 60947-6-2, IEC 61009), or fuses (IEC 60269-2, IEC 60269-3, IEC 60269-4 or IEC 60947-3).
The protection of persons
According to IEC 60364-4-41, Automatic disconnection in case of fault is a protective measure permitted for protection against electric shock.
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 exposed conductive-part or a protective conductor in the circuit or equipment within the disconnection time required « (IEC 60364-4-41 sub-clause 411).
The protection of appliances (e.g electric motors)
Switchgear such as circuit breakers or fuses can also provide protection of appliance alone or with additional relay. For induction motor for example protection against long start , stalled rotor, single-phasing could be required.
Against overheating, due, for example, to long term overloading, stalled rotor, single-phasing, etc. Overload 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.