SWITCHES AND FUSES

By D . M . Karkare

Isolating Switch

Isolator ( Disconnecting switch) operate under no load condition. It does not have any specified current breaking

capacity or current making capacity. Isolator is not even used for breaking load

currents. In some cases isolators are used for breaking

charging currents of transmission line.

Contd…

Circuit breaker can make and break electric circuit under normal current or short circuit condition.

Isolators are used in addition to circuit breakers. While opening a circuit, the circuit breaker is

opened first, then isolator. While closing a circuit, the isolator is closed

first, then the circuit breaker.

Contd..

Isolators are necessary on supply side of circuit –breakers in order to ensure isolation (disconnection) of the circuit-breaker from live parts for the purpose of maintenance. To prevent the mal-operation, the isolator is

provided with the following interlockings: Interlocking between three poles for simultaneous operation. Interlocking with circuit-breakers.

Contd…

Isolators cannot be opened unless the circuit breaker is opened, Circuit breakers cannot be closed unless the

isolator is closed.

Load Break Switches

In addition to isolators and circuit-breakers, there is one more device called Load Interrupting switch, which combines functions of the isolator and a switch. These are used for breaking load current.

Earthing Switch

Earthing switch is connected between the line conductor and earth. Normally it is open. When the line is disconnected, the earthing

switch is closed so as to discharge the voltage trapped on the line. Though the line is disconnected, there is some

voltage on the line to which the capacitance between the line and earth is charged.

Contd..

This voltage is significant in high voltage system. Before proceeding with the maintenance work

these voltages are discharged to earth, by closing the earth switch. Normally, the earthing switches are mounted

on the frame of isolator.

Sequence of Operation

While Opening: 1) Open Circuit Breaker 2) Open Isolator 3) Close earthing switching While Closing: 1) Open Earthing Switch 2) Close Isolator 3) Close Circuit-breaker.

Use of Load Break Switches

In distribution systems, voltages upto 33kV are used. The fault levels may not be high enough to

justify the use of circuit breakers economically. In such cases, the load break switches are used

in conjunction with H.R.C fuses and circuit-breakers.

Contd..

Load break switches are capable of making, breaking currents under normal conditions. They can carry the specified current of

specified values for specific time. They are capable of making but not breaking,

short circuit currents. Switch isolators or switch disconnectors

combine the functions of switch and isolators.

Load break switches serve following:

- Breaking rated currents - Making rated currents - Making specified S.C currents - Carrying specified short-circuit currents. - Interrupt small inductive, capacitive currents. While selecting the schemes with load break

switches, circuit breakers or H.R.C. fuses should be provided at strategic locations so as to interrupt fault currents, since load break switches cant do so…

Fuse

It is essentially a short piece of metal inserted in a circuit which melts when a predetermined value of current flows through it and thus breaks the circuit.

Rated Carrying current of a fuse is the maximum current, which it can carry without any undue heating and melting.

It depends upon the permissible temperature rise of the contacts of the fuse holder, fuse and upon the deterioration of fuse caused by oxidation.

Contd… • Fusing current is the minimum current at which a

fuse element shall melt. • Schwartz and James have defined the fusing

current as “ the minimum current to fuse the wire in such a time interval as shall be necessary for the wire to have attained its steady temperature”

For a round wire the approximate value of fusing current is given by

I= kd3 Where K is a constant depending upon the metal of the wire and d is the diameter of the wire

Fusing current depends on : Material Length- the shorter the fuse the greater the current Diameter Shape of cross-section of the fuse element The previous history Size and location of the terminals The type of enclosure employed Whether the fuse is stranded or not. The fusing current for a stranded fuse will be less than

the product of the fusing current for one strand and the number of strands

Fusing factor

It is defined as the ratio between the minimum fusing current and the rated carrying current.

Its value is always more than 1 Fusing factor of a fuse can be determined from

the time-current characteristics of the fuse. This characteristics defines the operating time

as a function of the fusing current.