Switching devices people are likely to spot in every medium voltage switchgear
Starting this article with a few basic terms used later. After, the seven most common MV switching devices will be explained in detail, referring to the ABB’s equipment as an example.
Basic terms (reminder)
Term № 1 – Disconnectors
Disconnectors are mechanical switching devices which provide an isolating distance in the open position. They are capable to open or close a circuit if either a negligible current is switched or if there is no significant change in voltage between the terminals of the poles.
Term № 2 – Isolating distances
Isolating distances are gaps of specified dielectric strength in gases or liquids in the open current paths of switching devices. They must comply with special conditions for the protection of personnel and installations and their existence must be clearly perceptible when the switching device is open.
Term № 3 – Switches
Switches are mechanical switching devices, which not only make, carry and interrupt currents under normal conditions in the network but also must carry for a specific time and possibly make currents under specified abnormal conditions in the network (e.g. short circuit).
Term № 4 – Switch disconnectors
Switch disconnectors are switches which satisfy the requirements for an isolating distance specified for a disconnector in their open position.
Term № 5 – Circuit breakers
Circuit-breakers are mechanical switching devices able to make, carry and interrupt currents occurring in the circuit under normal conditions, and can make, carry for a specified time and break currents occurring in the circuit (e.g. short circuit) under specified abnormal conditions.
Term № 6 – Earthing switches
Earthing switches are mechanical switching devices for earthing and short-circuiting circuits. They capable of carrying currents for a specified time under abnormal conditions (e.g. short circuit).
Earthing switches are not required to carry normal operating currents.
Term № 7 – Fuses
Good old fuses are switching devices that open the circuits in which they be installed by the melting of one or more parts specified and designed for the purpose of breaking the current when it exceeds a given value for a sufficiently long period.
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