Classification of Air Switches | Electricity

Air switches are classified as air-break switches and isolators (or disconnect switches).

(i) Air-Break Switches:

The air-break switch has both the blade and the contact equipped with arcing horns. Arcing horns are pieces of metal between which the arc resulting from opening a circuit carrying current is allowed to form. As the switch opens, these horns are spread farther and farther apart and the arc is lengthened until it finally breaks.

Air-break switches are of several designs. Some are operated from the ground by a hook on the end of a long insulated stick; some others through a system of linkages are opened by a crank at the foot of the pole. Where more than one conductor is opened, there may be several switches mounted on the same pole. These may be opened singly or altogether in a “gang” as this system is called. Some switches are mounted so that the blade opens downward and these may be provided with latches to keep the knife blade from jarring open.

(ii) Oil Switches:

The oil switch has both the blade and the contact mounted in an oil-filled tank. The switch is usually operated from a handle on the outside of the case. As the switch opens, the arc formed between the blade and contact is quenched by the oil. Oil switches may be remote-controlled as well as manually operated. These are used for capacitor switching, street lighting control and automatic disconnect in case of power failure.

(iii) Isolators:

Since isolators (or isolating switches) are employed only for isolating circuit when the current has already been interrupted, they are simple pieces of equipment. They ensure that the current is not switched into the circuit until everything is in order.

Isolators or disconnect switches operate under no load condition. They are not equipped with arc-quenching devices. They do not have any specified current breaking capacity or current making capacity. The isolators in some cases are used for breaking charging current of transmission line.

Isolators are employed in addition to circuit breakers, and are provided on each side of every circuit breaker to provide isolation. While opening a circuit, the circuit breaker is opened first, then isolator. If an isolator is opened carelessly, when carrying a heavy current, the resulting arc could easily cause a flash-over to ground.

This may shatter the supporting insulators and may even cause a fatal accident to the operator, particularly in hv circuits. While closing a circuit, the isolator is closed first, then circuit breaker. Isolators are necessary on the supply side of the circuit breakers in order to ensure isolation (disconnection) of the circuit breaker from the live parts for the purpose of maintenance. Automatic switching of isolators is preferred.

Isolators employed in power systems are usually 3-pole isolators, each having three identical poles. Each pole consists of two or three insulator posts mounted on a fabricated support. The fixed and moving conducting parts are of copper or aluminium rods. During the opening operation the conducting rods swing apart and isolation is obtained. The simultaneous operation of three poles is obtained by mechanical interlocking of the three poles.

To prevent the mal-operation, the isolator is provided with the following interlockings:

(a) Interlocking between three poles for simultaneous operation.

(b) Interlocking with circuit breakers – Isolator cannot be opened unless the circuit breaker is opened and circuit breaker cannot be closed unless the isolator is closed.

(iv) Load Interrupter Switches:

In addition to isolators and circuit breakers, there is one more device, called the load- interrupting switch, which combines the functions of the isolator and a switch. They are designed only for breaking and making the load currents.

These switches are designed and used to close and open high voltage circuits under normal working conditions (at normal load). The arc extinguishing device of the load interrupter is made in the form of a split, moulded plastic chute fitted with organic glass inserts. This chute surrounds the moving knife of the arc extinguishing system. The stationary arcing contact is located in the lower part of the chute.

Load interrupter switches are intended only for handling low-energy arcs resulting from the interruption of load current and altogether unsuitable for extinction of high-energy fault current arcs.

When the switch is opened, the working contacts between which the arc is drawn separate, acted upon by the high temperature of arc, the walls of the organic material insert generate gases (mainly hydrogen), which create a longitudinal blast serving to extinguish the arc. Lever-arm manually operating mechanisms are employed for closing and opening the load interrupter switches.

It is a wide practice to install load-interrupting switches in low-capacity installations like industrial-shop, urban and rural, and like distribution substations where it is possible to provide short-circuit current protection with high-voltage fuses and where the only duty of load-interrupting switch is to make and break the load current.

The installation of a load-interrupting switch, including the high-voltage fuses, is not only cheaper, but also usually requires less space than the installation of a high-voltage circuit breaker.

(v) Earthing Switch:

Earthing switch is connected between the line conductor and earth. Normally it is open and it is closed to discharge the voltage trapped on the isolated or disconnected line. When the line is disconnected from the supply end, there is some voltage on the line to which the capacitance between the line and earth is charged. This voltage is significant in hv systems. Before commencement of maintenance work it is necessary that these voltages are discharged to earth by closing the earthing switch. Normally, the earthing switches are mounted on the frame of the isolator.