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.
[Illustration: FIGS. 2,199 and 2,200.--General Electric triple pole
solenoid operated, single throw remote control switch, and push button
switch for operating same. Switch is a self-contained unit with two
sets of contacts, main laminated copper brushes, and carbon auxiliary
contacts to take the arc on breaking the circuit. The main brushes are
so made that each lamination makes an end on contact with the switch
blade without any tendency to force the laminations apart. A wiping
effect, given to the contacts every time the switch is closed, keeps
the contact surfaces clean and insures good contact at all times.
The carbon auxiliary contacts are made of blocks of carbon fastened
without screws. ~In operation~, the switch is actuated by a double coil
solenoid, one coil for closing and one for opening, controlled by the
single pole double throw push button switch shown in fig. 2,200, which
is normally in the open position and remains closed only when held by
the operator. One of these switches is furnished with each control
switch and must always be used, as the solenoid coils are not intended
for continuous service. The power required to operate the remote
control switch is small, being approximately 1.6 amperes at 110 volts,
0.81 amperes at 220 volts direct current, and 10 amperes at 110 volts,
and 6 amperes at 220 volts alternating current 60 cycles. The main
switch can be closed and opened by hand, and the push button located at
any point.]
~Ques. How should double throw switches be installed?~
Ans. Horizontally.
[Illustration: FIGS. 2,201 and 2,202.--Palmer service switch and
fuse box, for either plug, cartridge or open link fuses. Fig. 2,201
illustrates the box in open position for the inspection of fuses, etc.
The cover is held open by a simple lock so that the switch cannot
fall closed by gravity, the box may be mounted so that the service
wires lead directly into a sealed terminal chamber from any direction,
and all current carrying parts made accessible by the opening of the
switch are dead. Fig. 2,202 illustrates the device with side of box
and cover cut away to show interior and the normally sealed cover
of terminal chamber removed. The switch contacts do not enter their
contact clips until the flanged cover of the box has closed the switch
opening, no current connections being made to line or load until the
box is completely closed, and in consequence there is no opportunity
to make improper connections to any live parts of switch, when conduit
connections are used to the service and meter wires.]
~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.
[Illustration: FIG. 2,203 and 2,204.--~Bus transfer plug switch.~ The
method of supporting the contact farthest from the panel consists
of a porcelain pillar of the same height as the receptacle, clamped
to a brass connecting or bus bar which in turn is fastened to the
receptacle.]
[Illustration: FIG. 2,205.--~Ammeter jack.~ This plug switch is
insulated for high pressure and ~consists of two parts~: _the ammeter
jack, and the ammeter jack plug, cable, and bushing_. The receptacle,
which is simple in construction, consists of a brass bushing well
insulated from the panel and protected on the front of the panel by a
porcelain bushing. ~On the end of this tube~ and insulated from it,
is a phosphor bronze spring which, when the plug is out, rests on the
brass tube and keeps the circuit closed. ~The plug~ consists of a
brass rod well insulated and set in a brass tube, both being fastened
in a handle which is stained black and polished. Inside the handle
is run a twin conductor cable, one side being soldered in the brass
tube and the other to the brass rod. The other end of the cable is run
through a bushing set in the panel and thence to the ammeter or current
transformer. Where it is desired to remove the plug and cable from the
board, or to plug both ends of the cable in different receptacles, a
plug instead of a bushing should be used. In this case a cable should
be provided with a plug on each end.]
~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.
[Illustration: FIG. 2,206.--Westinghouse fused starting switch for
squirrel cage motors. It is arranged for National Electric Code fuses
on one end only and has springs on the other end to open the switch
automatically if left closed at this end. The corresponding terminals
at both ends of the switch are connected in grooves in the back of the
slate base so that the wiring need be connected to one set of these
terminals only, thus decreasing the number of connections necessary,
as shown in fig. 2,207. In starting an induction motor, the switch is
thrown to the end that is not fused and held there until the motor is
up to running speed; then it is quickly thrown to the fused position,
thus protecting the circuit under running conditions.]
[Illustration: FIG. 2,207.--Diagram of connections of Westinghouse
fused starting switch for squirrel cage motors. The starting current of
induction motors is several times the normal running current and, when
the controlling switch is fused to carry the running load only, the
fuses are apt to blow when the motor is started. The fuses must be of a
capacity to prevent overloads under running conditions. These switches
are designed to meet this difficulty and are used without auto-starters
to control motors up to 5 horse power rating.]
~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.
[Illustration: FIG. 2,208.--Westinghouse single pole disconnecting
switch. Disconnecting switches are used primarily for isolating
apparatus from the circuit for purposes of inspection and repair; also
for sectionalizing feeders. They are not designed for opening under
load, and therefore no attempt should be made to open them with current
in the circuit. In connection with lightning arrester installations,
disconnecting switches are particularly useful, providing a simple
and effective means for isolating the arresters while cleaning and
inspecting. The switch is opened and closed with a hook on the end of a
wooden pole, which hook engages in a hole provided in the switch blade.
This type of disconnecting switch is intended for wall mounting. The
live parts are mounted on porcelain insulators carried on a cast iron
yoke or base, forming a simple and substantial construction.]
~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.
[Illustration: FIGS. 2,209 and 2,210.--Westinghouse disconnecting
switches for pressures over 3,300 volts.]
[Illustration: FIGS. 2,211 and 2,212.--Westinghouse selector type
disconnecting switch. Fig. 2,211, view showing both sides closed; fig.
2,212, view with one side open. The selector type of disconnecting
switch is a transfer switch which does not require the circuit to be
interrupted while making the change. It can also be used to connect two
independent circuits in parallel. In construction, it is in effect two
single throw, single pole disconnecting switches with the hinge jaws
connected together and mounted on the same insulator. The hinge jaw is
also provided with dummy jaws to hold either blade of the switch in
the open position. Except for these differences in the hinge jaws, the
construction is similar to the switch shown in fig. 2,209. It should
not be used to open the circuit when loaded.]
[Illustration: FIG. 2,213.--Hook stick for operating a disconnecting
switch.]
~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.
[Illustration: FIG. 2,214.--Baum 35,000 volt, 200 ampere, double break
pole type switch. While designed for disconnecting purposes only, it
can break considerable amperage. The levers and couplings are fastened
with tape pins. The control shaft coupling is adjustable to any
angle, and the switch can be locked in the open or closed position. A
removable wooden handle is supplied and the switch can be handled in
any weather. The arms can be extended to hold fuse fittings, or dead
end insulators in the event of a heavy strain, but it is preferable
to have fuses on another structure as a precaution against coming
in contact with the energized portion of the switch, and it is also
preferable to take the strain of the line on a pole a few feet from the
switch, rather than on the switch structure, particularly in the larger
sizes. An insulating wood section in the control shaft separates the
control handle from the remainder of the switch. Discharging horns can
be fitted to this type of switch and when so equipped they have been
found capable of breaking considerable loads.]
~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.
[Illustration: FIG. 2,215.--Pacific swivel type blade for Baum pole
top switches. The ~twist type~ _of blade_, here shown, is especially
adapted to switches operating in freezing or sleety weather. ~It
will be seen~ that the first few degrees through which the rotating
insulator is moved _have the effect of twisting the blade between the
shoes of the contact_, which breaks any seal through freezing, or
corrosion.]
[Illustration: FIG. 2,216.--Pacific 22,000 volt, 100 ampere, ~pole
top switch~ equipped with fuse tubes; designed to meet the need for a
small group controlled disconnecting switch, having several features
making it suitable for use with service transformer installations and
line branches. ~The switch is made~ with clamped pipe arms permitting
adjustment. It is equipped with fuse tubes and fittings, but should
the fuses be not desired, the arm may be shortened. Provision is made
for fitting insulator pins to the top of the arms, when the switch is
mounted vertically, which will hold insulators at right angles to the
switch, making it possible to end a line on the top of these arms and
then drop down through the switch to the bank of transformers. _The
switch is so constructed that_ ~gravity~ _tends to hold it in either
the open or the closed position_. Provision can be made for locking.]
[Illustration: FIG. 2,217.--Horn break switch. ~In operation~, the
arc formed at break, will travel toward the extremities of the horns
because of the fact that a circuit will tend to move so as to embrace
the largest possible number of lines of force set up by it. Hence, the
arc that starts between the horns where they are near together rises
between them until it becomes so attenuated that it is extinguished.]
[Illustration: FIG. 2,218.--Westinghouse rear connected motor starting
switch, for pressures up to 600 volts. It is used for starting rotary
converters and direct current motors of large capacity having starting
torque small enough to permit cutting out the starting resistance in
few steps. The clips can be connected to any type of resistor, the
steps of which are successively short circuited as the switch closes;
the amount of resistance in the armature circuit is thus gradually
reduced. A pause should be made after each step of resistance is thrown
in to allow the motor speed to accelerate. If the starting switch do
not have to carry the full load current and can be short circuited by
another switch, a starting switch of smaller capacity equivalent to 50
per cent of running current of the machine can be used. The switch is
of the single pole, single throw, rear connected, four point, knife
blade type.]
~Ques. Describe its operation.~
Ans. The moving arm of the switch draws the fuse through the tube, thus
opening the circuit without much disturbance.
[Illustration: FIG. 2,219.--Baum disconnecting switch with horns and
auxiliary contacts (Pacific Mfg. Co.). This switch is for use on
systems operating at 100,000 volts or over. It has a spacing of five
feet between outer insulators, is equipped with auxiliary shoes that
break the circuit between the horns, diverting it from the current
carrying contacts so that they are not attacked by the arc.]
~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.
[Illustration: FIG. 2,220.--Kelman switching mechanism. The pantograph
arrangement of the contact blades gives a double horizontal break deep
down in the oil. This gives over the break a heavy head of oil which
immediately closes in around the thin blades as they leave the contacts
in opening, thus effectually extinguishing the arc. The opening spring
acts within the pantograph itself without any intervening mechanism,
and the light weight of the few moving parts enables the spring to
accelerate the blades rapidly, thus obtaining a quick break. The
contacts are of the return bend type, which makes a flexible contact,
to obtain alignment with the blades at all times. The pantograph and
contacts are supported on corrugated porcelain insulators on a hardwood
base or insulator board. The insulators are fitted with iron ends for
securing the different parts. At each end of the insulator board is an
upright or lifting board which serves to lift the switching mechanism
out of the tank. The leads are heavily insulated.]
~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.
[Illustration: FIG. 2,221.--Sectional view of Pacific weatherproof oil
switch for use in places exposed to the weather. All moving and contact
parts are supported from the cast iron top and are readily removable
for inspection or repair.]
~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.
[Illustration: FIG. 2,222.--General Electric central station triple
pole single throw oil switch; view of switch in tank. This type is for
pressures up to 110,000 volts, being adapted for stations employing
open wiring, since the connections are made at the top of the switch
and its construction obviates the need for isolating it in a cell. One
tank with two breaks in series are used for each phase.]
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.
[Illustration: FIGS. 2,223 to 2,226.--Westinghouse indoor, two pole
double throw oil switch for pressures not over 6,600 volts. Fig. 2,223,
open position; fig. 2,225, closed position. This type of switch is
suited for a wide range of application, being made in both switchboard
and wall mounting styles; also for remote mechanical control by the
use of bell cranks and connecting rods. The wall mounting style is
adaptable to motor installations on account of the facility with which
it may be mounted on any support, convenient to the motor operator. The
lever and handle extend outward over the oil tank, so that the switch
may readily be mounted against a wall, post or any vertical support.
The characteristic features of this type of switch are: knife blade
contacts submerged in oil; live parts carried on a porcelain base
affording a permanent insulation between adjacent poles, and between
the frame and live parts; compactness and accessibility; enclosure of
all live metal parts; and low first cost. Each contact jaw has attached
to it an arcing piece which takes the final break, thus preventing any
burning of the jaws. These arcing pieces are inexpensive and readily
replaced when worn or burnt away. The contact making parts are enclosed
in a sheet metal oil tank which has an insulating lining. The leads are
brought out at the top. Connections to the outside circuit are made
inside the switch and a porcelain insulator is slipped over the joint,
thus providing a straight continuous connection from the line with
maximum insulation. On the 6,600 volt switch, insulation is obtained
by the use of porcelain bases for supporting the live parts. In the
3,300 volt switch specially treated wooden bases are used, suitable
barriers being provided between the poles where necessary to prevent
arcs communicating.]
~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.
[Illustration: FIG. 2,227.--Kelman electric control unit for oil
switch. It consists of an iron frame which contains the opening and
closing coils and the bearings for the operating bell crank. A small
switch on the frame automatically opens the coil circuit at the end of
the stroke in either direction and operates signal lamps to indicate
the open or closed position. The automatic overload release opens the
switch by closing the opening coil circuit. This electrical operating
unit gives satisfactory service through a wide variation of voltage.
It requires a momentary expenditure of energy of from 1,500 to 4,000
watts, depending on the size.]
~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.
[Illustration: FIGS. 2,228 and 2,229.--Views showing mechanism of hand
operated remote control switches. Fig. 2,228, straight mechanism; fig.
2,229, angular mechanism.]
~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.
[Illustration: FIG. 2,230.--Pacific ~oil switch with solenoid control~,
designed for 60,000 and 70,000 volt installations; it is capable of
handling a 25,000 kw. generating station. The break is horizontal,
made by the rotation of a flat member edgewise through the oil. The
solenoid, at its extreme outer position, has a free start before
commencing to move the control parts of the switch. As it approaches
the extreme inner position, where the opening spring and the contacts
begin to offer the greatest resistance, the magnetic action is, of
course, most powerful, and the leverage by which it is applied moves
to an increasing radius, by means of rollers working in the curved
slots of the control shaft levers. These curved slots and rollers have
the additional advantage of making the opening action very free and
smooth. The tripping coil does not act on the latch directly, but gives
a hammer blow that is positive. The latch proper is a roller having a
powerful hold and easy release. Current can not be left on either the
closing or opening coils, as they are automatically cut out by the
movement of 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.
[Illustration: FIG. 2,231.--Diagram of connections of motor operated
remote control switch. The motor which operates the switch is
controlled by a small lever generally mounted on the panel with the
instruments which are in the circuit controlled by the switch. The
standard pressure for operating the motors is 125 volts.]
~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.
[Illustration: FIG. 2,232.--General Electric motor operated three
phase oil switch. The operation of the oil switch is accomplished by
a small hand controlling switch, generally mounted on the panel, with
the instruments which are in the circuit controlled by the oil switch.
The standard pressure for the operating motor is 125 volts. The switch
has six breaks, each break being a separate tank. In addition to this
isolation of the breaks, each phase is enclosed in a fireproof brick
compartment, making it impossible for trouble in one phase to be
communicated to another. The cells are constructed of brick with top
and bottom slabs of slate. The capacities of such switches, range from
2,500 to 60,000 volts, and from 100 to 1,000 amperes.]
~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.
[Illustration: FIGS. 2,233 to 2,235.--Diagrams showing connections for
General Electric single, double, and triple pole, solenoid operated
remote control switches. The operating coils are shown connected to
main switch circuit, but may be connected to an entirely separate
control circuit. Connections are the same for either alternating or
direct current.]
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.
[Illustration: FIG. 2,236.--Westinghouse three pole hand operated
remote control oil switch, adapted for the control of alternating
current circuits of small and moderate capacities, the pressures of
which do not exceed 25,000 volts. Each unit is installed in a separate
masonry compartment. The open position of contacts is maintained by
gravity. Up to and including the 600 ampere capacity, the contacts
are cone shaped with an arcing tip, as shown for capacities in excess
of 600 amperes, brush contacts are furnished with auxiliary arcing
contacts of the butt type. Each pole has two sets of contacts, thus
providing a double break in each line. With both types of contact, the
final break of the arc is taken and the main contacts protected by
auxiliary arcing contacts which are inexpensive and readily renewable.
The upper or stationary contacts are mounted on porcelain insulators
secured in the soapstone base. The lower or movable contacts are
carried by a wooden rod connected to and moved vertically by the
operating mechanism. The operating mechanism of the hand operated
breaker consists of a simple system of levers, bell cranks, and rods.
The necessary energy for making a positive contact is small owing to
the use of a toggle mechanism. The leads are brought out of the top
of the breaker through heavy porcelain insulators. On breakers above
3,500 volts, the connections to the line wires are made by means of
a union which can be tightened with a socket wrench fitting inside
the insulator. As the leads coming into the switch are necessarily
insulated wire or cable, this arrangement eliminates all exposed live
parts and is well adapted to making connections readily to bus bars
located above or in the rear of the circuit breakers.]
[Illustration: FIG. 2,237.--Cutler-Hammer ~enclosed float switch~,
designed for the automatic control of alternating current motors
operating pumps used to fill or empty tanks, sumps or other reservoirs.
~The switch is operated~ _by the rise and fall of a copper float which
is connected to the switch lever by a brass rod or copper chain_. As
the water level rises and falls, the float moves up and down. This
movement is transmitted to the switch lever and the switch (if the
movement be sufficient) is tripped to make or break the motor circuit.
~To insure the best operation~ it is necessary that the float rod be
provided with a guide so that the float will move up or down in a
vertical line, as shown. The minimum difference in water level at which
the switch will operate is approximately 10 to 12 inches. When the
float is placed in a closed tank, the minimum height inside from the
bottom of the tank to the top should be at least 6 inches greater than
the difference in water level to provide sufficient clearance for the
float. ~When this type switch is used as a tank switch~, the contacts
are closed when the water level is low, putting the motor, driving the
pump, in motion. When the water in the tank reaches a predetermined
high level the float arm opens the switch contacts, and the motor is
disconnected from the line. ~For sump pump purposes~, the contacts open
on low level and close on high level, the lever being reversed for this
purpose. Two pole, three pole and four pole switches of this type are
made, all arranged to completely disconnect single phase, two phase and
three phase motors from their circuits. When used with small motors
which may be thrown across the line to start, the switch may be used
without a self starter if desired.]
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.