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Home / Technical Articles / Purpose of Disconnect Switches In HV Substation

Types and Characteristics

Switches may be of several types, the common characteristic is that none are designed to interrupt fault currents and the insulation or insulators associated with them must coordinate with the rest of the system and a BIL (Basic insulation level) capable of withstanding voltage surges.

Purpose of Disconnect Switches In HV Substation (on photo: Siemens Goab switch)
Purpose of Disconnect Switches In HV Substation (on photo: Siemens Goab switch)

In general, they consist of a conducting blade, hinged at one end, and a stationary contact on the other, both terminals mounted on suitable insulators that conform to the common insulator requirements of BIL coordination.

Almost every major line or equipment in a substation has associated with it a means of completely isolating it from other energized elements as a prudent means of insuring safety by preventing accidental energization. These simple switches, called disconnects, or disconnecting switches, are usually installed on both sides of the equipment or line upon which work is to be done.

They should not be operated while the circuit in which they are connected is energized, but only after the circuit is deenergized. As a further precaution, they may be opened by means of an insulated stick that helps the operator keep a distance from the switch.

Locking devices are sometimes provided to keep the disconnects from being opened accidentally or from being blown open during periods of heavy fault currents passing through them.

Although not designed to be closed to energize the line or equipment with which they are associated, in certain circumstances they may be closed, using special care to close them firmly and rapidly.

Disconnects may be singleblade units or multiple units operated together.

Air break switches have characteristics similar to disconnects, but have the stationary contacts equipped with arc suppressing devices that enable them to be opened while energized, but recognizing a limitation as to the current that may be safely interrupted.

Transmission line disconnect switch
Transmission line disconnect switch (photo by Efrem Oshinsky @ Flickr)

The device may be a simple arcing horn which stretches the arc that may form until it cannot sustain itself.

Another type has a flexible “whip” attached to the stationary contact that continues the contact with the moving blade until a point is reached at which the whip snaps open very rapidly extinguishing any arc that may form.

Still another has an interrupting unit mounted at the free end of the switch blade and which suppresses the arc within the unit as the switch opens; the interrupting unit may contain a vacuum chamber, or a series of grids which cause the arc to break up into smaller ones and are more readily extinguished.

Both types function similarly as larger units do in circuit breakers.

Oil switches have the blade open from its contact under oil which suppresses any arc that may form. They have higher current breaking capacity than air break switches and are particularly suited for underground systems or where moisture or pollution make air switches impractical.

However, they (oil switches) are more expensive at first cost, to operate and to maintain.

138 kV Disconnects Switches (VIDEO)

Words from Robbie Oleksyn (author):

These 138kv disconnects switches have Joslyn current interrupters on them you can see them the grey tubes on the contact point, they didn’t work properly that was the issue. We were breaking parallel from two sources and when we were breaking it, the load transfer caused an arc and it didn’t extinguish itself the line tripped on over current unbalance.

Disconnect Switch closing in on 138 kV (VIDEO)

Resource: Power & Transmission Distribution – A. J. Pansini (get this book at Amazon)

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Dale Jennings
    Mar 04, 2020

    What does it mean when the HV crew on the 161kv are “piercing a switch”?

  2. assdasd
    Jun 06, 2015

    This article doesn’t answer the question “Purpose of Disconnect Switches In HV Substation”….

  3. pdelec
    Jan 15, 2013

    can you plz post an article regarding capacitor banks, its protection delta residual voltage transformers.
    Also suggest me how the system power factor will behave at varying loads?
    How the powerfactor will drop if the equipment is run at low load?

  4. pdelec
    Jan 15, 2013

    Hi sir,
    Can you plz tell me power factor is low at light loads?

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