CHAPTER LVII

SWITCHING DEVICES

A switch is _a piece of apparatus for making, breaking, or changing the connections in an electric circuit_.

The particular form and construction of any switch is governed by the electrical conditions under which it must operate.

_Since the electric current cannot be_ ~stopped instantly~ _when the circuit in which it is flowing, is broken_, ~an arc is formed~ _as the switch contacts separate_; this tends to burn the contacts, and to short circuit, the severity of such action depending on the voltage and the proximity of the switch terminals. Accordingly in switch design, provision must be made to counteract these tendencies. Thus,

1. The contacts should separate along their entire length, rather than at a point; 2. The terminals should be far enough apart and properly protected to prevent short circuiting of the arcs; 3. The break should be quick; 4. The gap should be surrounded by the proper medium (air or oil) to meet the requirements of the electrical conditions.

A great variety of switches have been introduced to suit the different requirements. Knife switches are used for low pressure service, the multiple break form being used where it is desired to reduce the arcing distance.

~Ques. How should single throw switches be installed?~

Ans. They should open downward so gravity will keep them open.

~Ques. How should double throw switches be installed?~

Ans. Horizontally.

~Ques. What is a plug switch?~

Ans. A switch in which the current is ruptured in a tube enclosed at one end, thereby confining the arc and limiting the supply of air.

They are used on high pressure circuits of from 10,000 to 20,000 volts, for transferring live circuits and for voltmeter and synchronizing circuits where there is very little energy. The usual current capacity is from 4 to 7½ amperes.

~Forms of Break.~--On high pressure circuits there are several types of switch: they are classified with respect to the break, that is to say, according as the break takes place,

1. In open air; 2. In an enclosed air space; 3. Aided by a metal fuse; 4. Aided by a horn; 5. In oil.

~Ques. What is the objection to open air break?~

Ans. The relatively long gap required to extinguish the arc, limiting this form of switch to low or moderate pressure circuits.

The open air arc may cause very high voltage oscillations when the circuit contains inductance and capacity unless the break occur at zero value.

~Ques. What are disconnecting switches?~

Ans. Knife switches in series with other switches so that the apparatus controlled by the latter may be repaired in safety by entirely disconnecting it from the bus bars or live circuit.

Such switches are not intended to rupture the load current.

~Ques. What are the features of the enclosed air break?~

Ans. The switch is more compact than the open air break type, but pressure oscillations are caused on opening the circuit the same as with the open air break, and it is not desirable for heavy current.

~Ques. How is the fuse arranged in the metal fuse break type of switch?~

Ans. It is placed in a tube fitted with powdered carbonate of lime or some other insulating powder.

~Ques. Describe its operation.~

Ans. The moving arm of the switch draws the fuse through the tube, thus opening the circuit without much disturbance.

~Ques. What is the objection to the metal fuse switch?~

Ans. The powder is set flying by the explosion of the arc, which, as it settles, gets into the bearings of any machine that may be in the vicinity.

~Ques. What is a horn break switch?~

Ans. One provided with horn shaped extensions to the contacts, as shown in fig. 2,219.

The arc formed on breaking the circuit, as it travels toward the extremities of the horns, becomes attenuated and is finally ruptured.

~Ques. What are the objections to this type of switch?~

Ans. The considerable space required for the horns and arcs, and the line surges caused by the arc.

Horn switches were used extensively for high pressure alternating current circuits before the introduction of oil switches.

~Oil Switches.~--The extensive use of high pressure currents and alternating current motors and other devices introducing inductance make it necessary to use switches radically different from the ordinary air break types.

The opening of circuits of considerable current value with inductive loads is not possible with old style switches which were quite adequate for the service for which they were designed. These circuits are controlled with ease and certainty by the oil switch.

~Ques. What is an oil switch?~

Ans. One in which the contact is broken under oil.

This type of switch is the one almost universally used on high pressure alternating current circuits, because of the fact that the oil tends to cause the current to break when at its zero value, thus preventing the heavy arcing which would occur with an air break switch, and the consequent surges in the line which are so often the cause of breakdown of the insulation of the system.

~Ques. What is the nature of an oil break?~

Ans. It is not a quick break.

Oscillograph records show that the effect of the oil is to allow the arc to continue during several cycles and then to break the current, usually at the zero point of the wave.

~Remote Control Oil Switches.~--It is desirable in the case of switches on high pressure circuits to locate the parts which carry the high pressure current at some distance from the switchboard in order that they may be operated with safety.

With respect to the manner in which the switches are operated they may be classed, as

1. Hand operated; 2. Power operated.

~Ques. What kind of power is used?~

Ans. Electricity is used in most cases; in some installations, switches are operated by compressed air.

~Ques. For what pressures should remote control switches be used?~

Ans. For pressures above 1,100 volts.

~Ques. Describe the operating mechanism of a remote control, hand, and electrically operated switch.~

Ans. For hand operation, the mechanism between the operating lever and switch proper, consists simply of a system of links and bell cranks. Various shapes of bell crank are used, to permit change in direction or position of the force applied to operate the switch.

~Ques. Name two classes of electrically operated remote control switch.~

Ans. Those operated by solenoids, and those operated by motors.

The solenoid type are closed by the action of a plunger solenoid, and opened either by another solenoid called a "tripping coil" or by gravity. Some examples of remote control are shown in the accompanying illustrations.

~Ques. What indicating devices are used with electrically operated switches?~

Ans. Red and green lamps; ~red~ for _closed_ and ~green~ for _open_ as shown in fig. 2,231.

~Ques. For what service are motor operated switches used?~

Ans. For exceptionally heavy work where the kilowatt rupturing capacity is greater than that for which the other types are suitable.

~Rupturing Capacity of Oil Switches.~--While an oil switch may be designed for a given pressure and to carry a definite amount of current, it should not be understood that the switch will necessarily rupture the amount of normal energy equivalent to its volt ampere rating.

Oil switches are often used on systems with generator capacity of many thousand kilowatts. It is therefore essential that the switches shall be able to break not only their normal current, but also greatly increased current that would flow if a short circuit or partial short circuit occur.

Under short circuit conditions alternators develop instantaneously many times their normal load current, while the sustained short circuit current is approximately two and a half to three times normal, or even higher with turbine alternators. Hence, circuit breakers of the so called instantaneous type must be capable of rupturing the circuit when the current is at a maximum, whereas, non-automatic switches, or circuit breakers with time limit relays will be required to interrupt only the sustained short current circuit. The reason is evident, since the delay in opening the switch allows the current to approach the sustained short circuit conditions.