Disconnector switch

Isolating Switches are normally Switched only when not on load but they may also interrupt the no load current of small Transformers as well as disconnect short pieces of overhead lines or cables.



Disconnector Switch.

1 - Support insulator

2 - Fixed contact

3 - Moving contact

4 - Coupling contact

5 - Moving earthing contact

6 - Drive insulator

7 - Arcing contact

The BS: 3078-1959 on isolators distinguishes between “off load” and “on   load” isolator as under:


1 - Off Load Isolator is an isolator which is operated in a circuit either when the isolator is already disconnected from all
      sources of supply or when the isolator is already disconnected from the supply and the current may be due to
      capacitance currents of bushings, Bus-Bar connections, and very short lengths of cable.

2 - On Load Isolator is an isolator which is operated in a circuit where there is a parallel path of
     low impedance so that no significant change in the voltage across the terminals of each pole
     occurs when it is operated.




1-     supporting insulator

2-     fixed contact

3-     moving contact

4-     earthing Switch

5-     driving insulator

To ensure that the off load isolators are not operated inadvertently under load it is necessary that the isolators are suitably interlocked with the connected breakers.

Isolating Switches can broadly be divided into the three categories given ahead.

a)     Bus isolator.

b)    Line isolator.

c)     Transformer isolating.


An isolator may be constructed single pole or three poles and shall be rated in terms of:

        1 - Voltage.

The rated voltage of an isolator or an earthing Switch shall be one of the highest system voltages, given below:

3, 6, 7.2, 12, 24, 36, 72.5, 123, 145, 245, 300 and 420 kV.

2 - Insulation level.

  The rated insulation level should be selected from standard Tables according

  To IS: 9921

3 - Frequency

             Rated frequency should be 50 Hz in Kuwait.

4 - Normal current (for Disconnector only)

The rated normal current of an isolator or an earthing Switch should have one   of the following

Standard values:

200 A, 400 A, 630 A (alternatively 800 Amps), 1250 A, 1600 A, 2000 A, 2500 A, 3150 A, 5000 A and 4000 A.

5 - Short time withstand current

The rated short-time withstands current of a Disconnector or earthing Switch should have one of the following values:

8, 10, 12.5, 16, 20, 25, 31.5, 40, 50, 63, 80 or 100 KA.

 6 - Duration of short circuit

The short time current rating of an isolator, unless directly associated with and protected by a fuse or by a Circuit Breaker fitted with series releases or current Transformer operated releases when it need not be assigned a short time rating, should not be less than the short circuit current at the point of installation or the corresponding ratings of the associated Circuit Breaker.

The rated maximum duration of short circuit is one second.

For short circuit duration greater than one second, the relation between current (I) and time (t), unless otherwise specified, shall be assumed to be in accordance with the formula:

                                  t = constant

7 - Peak withstand current

The rated peak withstand current of a Disconnector or earthing Switch is that peak current which it shall be able to carry in the closed position without material deterioration. It shall have a value 2.5 times the rated short time withstand current.


8 - Short circuit making current (for earthing Switches only)

The earthing Switches to which a rated short circuit making current has been assigned shall be capable of making at any applied voltage, upto and including that corresponding to their rated voltage, any current upto and including their short circuit making current.

 9 - Contact zone

Divided frame Disconnector and earthing Switches shall be able to operate within the limits of their rated contact zone. For examples of rated contact zones, the reader may refer to IS: 9921 (Part II)-1982.

10 - Mechanical terminal load

Disconnector and Earthing Switches should be able to close and open whilst subject to their rated mechanical terminal loads, where assigned, plus wind loads acting on the equipment itself.

11 - Supply voltage

 closing and opening devices (where these operating devices are supplied separately) of auxiliary circuits, peak power And total duration of operations.


The rated supply voltage shall preferably be one of the standard values given below:

DC. Volts                             AC. volts

    24                                    110 Single phase

    48                                    240 Single phase                                     

    110                                  240 /415 three phases                                  


The operating device shall be capable of closing and opening the isolator at any value of the supply voltage between 85 percent and 110 percent of the rated voltage.

12 - Supply frequency of closing and opening devices and of auxiliary circuits.

The rated supply frequency of an operating device or an auxiliary circuit is the frequency at which the conditions of operation and heating are determined.

13 - Pressure of compressed gas supply for operation.

The rated pressure should correspond to the operating pressure of the associated air blast Circuit Breakers, if installed and preferably have one of the following standard values:

500, 1000, 1600, 2000 or 3900 kPa.

The pneumatic operating device shall be capable of closing and opening the isolator when the air pressure is between 85 percent and 105 percent of the rated supply pressure.



1. Type Tests laid down in IS: 9921(part 4)-1985

  A) Normal Type Tests

      - Dielectric Tests Comprising Of.

1 - Lightning Impulse Voltage Tests

2 - Switching Impulse Voltage Tests for rated

3 - Power Frequency Voltage Tests.

4 - Artificial Pollution Tests.

5 - Partial Discharge Tests.

6- Tests on Auxiliary and Control Circuits.

B) Routine Tests

The following shall comprise routine tests:

1- Power Frequency Voltage Test,

2-Voltage Test on Auxiliary Equipment.

3- Operation test

4- Measurement of the Resistance of the Main Circuit.