LV switchgear functions - Switchgear control
Switchgear-control functions allow system operating personnel to modify a loaded system at any moment, according to requirements,and include:
- Functional control (routine switching, etc.)
- Emergency switching
- Maintenance operations on the power system
In broad terms “control” signifies any facility for safely modifying a load-carrying power system at all levels of an installation. The operation of switchgear is an important part of power-system control.
This control relates to all switching operations in normal service conditions for energizing or de-energizing a part of a system or installation, or an individual piece of equipment, item of plant, etc.
Switchgear intended for such duty must be installed at least:
- At the origin of any installation
- At the final load circuit or circuits (one switch may control several loads)
Marking (of the circuits being controlled) must be clear and unambiguous.
In order to provide the maximum flexibility and continuity of operation, particularly where the switching device also constitutes the protection (e.g. a circuit-breaker or switch-fuse) it is preferable to include a switch at each level of distribution, i.e. on each outgoing way of all distribution and subdistribution boards.
The operation may be:
- Either manual (by means of an operating lever on the switch) or
- Electric, by push-button on the switch or at a remote location (load-shedding and reconnection, for example)
These switches operate instantaneously (i.e. with no deliberate delay), and those that provide protection are invariably omni-polar.
The main circuit-breaker for the entire installation, as well as any circuit-breakers used for change-over (from one source to another) must be omni-polar units.
Emergency switching - emergency stop
An emergency switching is intended to de-energize a live circuit which is, or could become, dangerous (electric shock or fire.
An emergency stop is intended to halt a movement which has become dangerous.
In the two cases:
- The emergency control device or its means of operation (local or at remote location(s)) such as a large red mushroom-headed emergency-stop pushbutton must be recognizable and readily accessible, in proximity to any position at which danger could arise or be seen
- A single action must result in a complete switching-off of all live conductors
- A “break glass” emergency switching initiation device is authorized, but in unmanned installations the re-energizing of the circuit can only be achieved by means of a key held by an authorized person
It should be noted that in certain cases, an emergency system of braking, may require that the auxiliary supply to the braking-system circuits be maintained until final stoppage of the machinery.
Switching-off for mechanical maintenance work
This operation assures the stopping of a machine and its impossibility to be inadvertently restarted while mechanical maintenance work is being carried out on the driven machinery. The shutdown is generally carried out at the functional switching device, with the use of a suitable safety lock and warning notice at the switch mechanism.
- One break in each phase and (where appropriate) one break in the neutral.
- Taking into account stalled motors.
- In a TN schema the PEN conductor must never be opened, since it functions as a protective earthing wire as well as the system neutral conductor.