SWITCHBOARDS
=General Principles of Switchboard Connections.=--The interconnection
of generators, transformers, lines, bus bars, and switches with
their relays, in modern switchboard practice is shown by the
diagrams, figs. 2,636 to 2,645. The figures being lettered A to J
for simplicity, the generators are indicated by black discs, and the
switches by open circles, while each heavy line represents a set of
bus bars consisting of two or more bus bars according to the system
of distribution. It will be understood, also, in this connection,
that the number of pole of the switches and the type of switch will
depend upon the particular system of distribution employed.
Diagram A, shows the simplest system, or one in which a single
generator feeds directly into the line. There are no transformers
or bus bars and only one switch is sufficient.
In B, a single generator supplies two or more feeders through a
single set of bus bars, requiring a switch for each feeder, and a
single generator switch.
In C, two generators are employed and required and the addition
of a bus section switch.
D, represents a number of generators supplying two independent
circuits. The additional set of bus bars employed for this purpose
necessitates an additional bus section switch, and also additional
selector switches for both feeders and generators.
E, shows a standard system of connection for a city street
railway system having a large number of feeders.
[Illustration: FIGS. 2,645 and 2,646.--Diagrams illustrating general
principles of switchboard connections.]
This arrangement allows any group of feeders to be supplied from
any group of generators.
[Illustration: FIG. 2,646.--Fort Wayne switchboard panel for one
alternator and one transfer circuit. Diagram giving dimensions,
arrangement of instruments of board, and method of wiring. The
different forms of standard alternating current switchboard panels
for single phase circuits made by the Fort Wayne Electric Works are
designed to fulfill all the usual requirements of switchboards for
this class of work. The line includes panels equipped for a single
generator; for one generator and two circuits; one generator and one
transfer circuit; one generator, an incandescent and an arc lighting
circuit; and also feeder panels of different kinds.]
It also permits the addition of a generator switch for each
generator.
F, represents the simplest system with transformers.
It requires a single generator transformer bank, switch and
line. The arrangement as show at F is used where a number of
plants supply the same system.
G, represents a system having more than one line.
In this case a bus bar and transformer switch is used on the
high tension side.
H, shows a number of generators connected to a set of low
tension bus bars through generator switches, and employing a low
tension transformer switch.
I, shows the connections of a system having a large number of
feeders supplied by several small generators. In this case, the
plant is divided into two parts, each of which may be operated
independently.
[Illustration: FIG. 2,647.--General Electric =small plant alternating
current switchboard=, _designed for use in small central stations and
isolated plants_. They are for use with one set of bus bars, to which
all generators and feeders are connected by means of single throw
lever switches or circuit breakers, suitable provision being made for
the parallel operation of the generators.]
J, represents the arrangement usually employed in modern plants
where the generator capacity is large enough to permit of a
generator transformer unit combination with two outgoing lines. By
operating in parallel on the high tension side only, any generator
can be run with any transformer. The whole plant can be run in
parallel, or the two parts can be run separately.
[Illustration: FIG. 2,648.--Crouse-Hinds =voltmeter and ground
detector radial switch=, arranged for mounting on the switchboard.
The switch proper is placed on the rear of the board with hand wheel,
dial, and indicator only on the front side. The current carrying
parts are of hard brass, with contact surfaces machined after
assembling. The contact parts are of the plunger spring type, and the
cross bar has fuse connections. Ground detector circuits are marked
G+ and G- for two wire system, and G+, G-, GN+ and GN- for three wire
system. When the voltmeter switch is to be used as a ground detector,
two circuits are required for a two wire system, and four circuits
for a three wire system, that is, a six circuit voltmeter and ground
detector switch for use on a two wire system has two circuits for
ground detector and four circuits for voltmeter readings. A six
circuit voltmeter and ground detector switch, for use on a three wire
system, has four circuits for ground detector and two circuits for
voltmeter readings.]
=Switchboard Panels.=--The term "panel" means the slab of marble or
slate upon which is mounted the switches, and the indicating and
controlling devices. There are usually several panels comprising
switchboards of moderate or large size, these panels being classified
according to the division of the system that they control, as for
instance:
1. Generator panel;
2. Feeder panel;
3. Regulator panel, etc.
In construction, the marble or slate should be free from
metallic veins, and for pressures above, say, 600 volts, live
connections, terminals, etc., should preferably be insulated from
the panels by ebonite, mica, or removed from them altogether, as
is generally the case with the alternating gear where the switches
are of the oil type.
[Illustration: FIGS. 2,649 and 2,650,--Wiring diagrams of
Crouse-Hinds voltmeter and ground detector switches. Fig. 2,649
voltmeter switch; fig. 2,650 voltmeter and ground detector switch. A
view of the switch is shown in fig. 2,648; it is designed for use on
two or three wire systems up to 300 volts.]
The bus bars and connections should be supported by the
framework at the back of the board, or in separate cells, and the
instruments should be operated at low pressure through instrument
transformers.
The panels are generally held in position by bolting them to an
angle iron, or a strip iron framework behind them.
=Generator Panel.=--This section of a switchboard carries the
instruments and apparatus for measuring and electrically controlling
the generators. On a well designed switchboard each generator has, as
a rule, its own panel.
[Illustration: FIGS. 2,651 to 2,653.--Diagrams of connections for
generator panels. =Key to symbols=: =A=, ammeter; =A.S.=, ammeter
switch; =C.T.=, current transformer; =F.=, fuse; =F.A.=, direct
current field ammeter; =F.S.=, field switch; =G.C.S.=, governor
control switch; =L.S.=, limit switch (included with governor motor);
=O.S.=, oil switch; =P.I.W.=, polyphase indicating wattmeter;
=P.W.M.=, polyphase watthour meter; =P.R.=, pressure receptacle;
=P.P.=, pressure plug; =Rheo.=, rheostat; =S.=, shunt; =S.R.=,
synchronizing receptacle; =S.P.=, synchronizing plugs; =T.B.=,
terminal board for instrument leads; =V=, alternating current
voltmeter.]
[Illustration: FIGS. 2,654 and 2,655.--Diagrams illustrating =a
simple method of determining bus capacity= as suggested by the
General Electric Co. Fig. 2,654 relates to any panel; the method is
as follows: =1.= Make a rough plan of the _entire board_, regardless
of the number of panels to be ordered. _The order of panels_
shown is recommended, it being most economical of copper and best
adapted to future extensions. =2.= To avoid confusion keep on one
side of board everything pertaining to exciter buses, and on other
side everything pertaining to A. C. buses. =3.= With single lines
represent the exciter and A. C. buses across such panels as they
actually extend and by means of arrows indicate that portion of each
bus which is connected to feeders and that portion which is connected
to generators. _Remember that "Generator" and "Feeder" arrows must
always point toward each other_, otherwise the rules given below do
not hold. Note also that the field circuits of alternator panels are
treated as D. C. feeders for the exciter bus. =4.= On each panel
mark its ampere rating, that is, the maximum current it supplies
to or takes from the bus. For A. C. alternator panels the D. C.
rating is the excitation of the machines. =5.= Apply the following
rules _consecutively_, and note their application in fig. 2,654.
(For the sake of clearness ampere ratings are shown in light face
type and bus capacities in large type.) =A.= _Always begin with the
tail of the arrow and treat "generator" and "feeder" sections of
the bus separately._ =B.= _Bus capacity for first panel = ampere
rating of panel._ =C.= _Bus capacity for each succeeding panel =
ampere rating of panel plus bus capacity for preceding panel._ (See
sums marked above the buses in fig. 2,654.) =D.= _For a panel not
connected to a bus extending across it, use the smaller value of
the bus capacities already obtained for the two adjoining panels._
(See exciter bus for panel C.) =E.= _The bus capacity for any feeder
panel need not exceed the maximum for the generator panels_ (see A.
C. bus for panel G) _and vice versa_ (see exciter bus for panel B).
Hence the corrections made in values obtained by applying rules =B=
and =C=. The arrangement of panels shown in fig. 2,654 is the one
which is mostly used. The above method may, however, be applied to
other arrangements, one of which is shown in fig. 2,655. Here the
generators must feed both ways to the feeders at either end of the
board so that in determining A. C. bus capacities it is necessary to
first consider the generators with the feeders at one end, and then
with the feeders at the other end as shown by the dotted A. C. buses.
The required bus capacities are then obtained by taking the maximum
values for the two cases.]
[Illustration: FIG. 2,656.--End view showing =general arrangement of
switchboards= for 240, 480, and 600 volt alternating current. The cut
shows a single throw oil switch mounted on the panel.]
In the case of a dynamo, a good representative panel would have
mounted upon it a reverse current circuit breaker, an ammeter, a
double pole main switch (or perhaps a single pole switch, since
the circuit breaker could also be used as a switch) a double pole
socket into which a plug could be inserted to make connection with
a voltmeter mounted on a swinging bracket at the end of the board;
a rheostat handle, the spindle of which operates the shunt rheostat
of the machine, the rheostat being placed either directly behind
the spindle, if of small size, or lower down with chain drive from
the hand wheel spindle, if of larger size, a field discharge switch
and resistance, a lamp near the top of the panel for illuminating
purposes, a fuse for the voltmeter socket, and, if desired, a
watthour meter. If the dynamo be compound wound, the equalizing
switch will generally be mounted on the frame of the machine, and in
some cases the field rheostat will be operated from a pillar mounted
in front of the switchboard gallery. If the generator be for traction
purposes, the circuit breaker is more often of the maximum current
type, and a lightning arrester is often added, without a choke coil,
the latter as well as further lightning arresters being mounted on
the feeder panels.
[Illustration: FIGS. 2,657 and 2,658.--Two views of a =feeder panel=,
showing general arrangement of the devices assembled thereon. A,
circuit breaker; B, ammeter; C, voltmeter; D, switches.]
In the case of a high pressure alternating current plant of
considerable size, the bus bars oil switches, and the current and
pressure transformers are generally mounted either in stoneware
cells, or built on a framework in a space guarded by expanded metal
walls, and no high pressure apparatus of any sort is brought on to
the panels themselves.
[Illustration: FIGS. 2,659 to 2,666.--Diagram of connections for
three phase feeder panels. =Key to symbols=: A, ammeter; A.S.,
three way ammeter switch; B.A.S., bell alarm switch; C.T., current
transformer; F, fuse; O.S., oil switch; P.I.W., polyphase indicating
wattmeter; P.W.M., polyphase watthour meter; T.B., terminal board;
T.C., trip coils for oil switch.]
=Feeder Panel.=--The indicating and control apparatus for a feeder
circuit is assembled on a panel called the feeder panel.
The most common equipment in the case of a direct current feeder
panel comprises an ammeter, a double pole switch, and double pole
fuses or instead of the fuses, a circuit breaker on one or both
poles; in the case of a traction feeder a choke coil and a lightning
arrester are often added.
[Illustration: FIGS. 2,667 and 2,668.--Diagrams of connections for
two phase and three phase installations: A and A1, ammeter; C.C.,
constant current transformer; C.T., current transformer; D.R.,
discharge resistance; F, fuse; F.S., field switch; L.A., lightning
arrester; O.S., oil switch; P.P., pressure plug; P.R., pressure
receptacle; P.T., pressure transformer; S and S1, plug switches;
T.C., oil switch trip coil; V, voltmeter.]
The equipment of a typical high pressure three phase feeder panel is
an ammeter (sometimes three ammeters, one in each phase) operated
by a current transformer, and oil break switch with two overload
release coils, or three if the neutral of the circuit be earthed, the
releases being operated by current transformers.
[Illustration: FIG. 2,669.--Crouse-Hinds radial ammeter switch,
arranged for mounting directly on the switchboard. It is designed
for use with external shunt ammeters of any make or capacity, and in
connection with the required number of shunts, makes possible the
taking of current readings of a corresponding number of circuits by
means of one ammeter. The wiring diagram is shown in fig. 2,670.]
The switch when on a large system is often in a cell some distance
behind the panel, and is then controlled by a system of levers, or by
a small motor which is started and stopped by a throw over switch on
the panel, in which case there is generally a lamp or lamps on the
panel to show whether the switch is open or closed.
Air brake switches or links are placed between the bus bars
and the oil switch to allow of the latter being isolated for
inspection purposes, and as a general rule no apparatus carrying
high pressure current is allowed on the front of the panel. With
both direct and alternating current feeders, a watthour meter is
often added to show the total consumption of the circuit.
[Illustration: FIG. 2,670.--Wiring diagram for Crouse-Hinds radial
ammeter switch as illustrated in fig. 2,669. The switch proper is on
the rear of the switchboard, and the hand wheel dial and indicator on
the front.]
A typical three phase generator panel is provided with three
ammeters, one in each phase, operated from three current
transformers, one to each ammeter, a volt meter, a power factor
indicator, and an indicating watthour meter, all operated from
one or more pressure transformers, and the necessary current
transformers, the operating handle of the oil switch, which is
connected to the switch itself by means of rods, two maximum
releases operated by current transformers, or a reverse relay
for automatically tripping the switch, lamps for indicating when
the switch is tripped, a socket for taking the plug which makes
connection between the secondary of a pressure transformer and
the synchronizer on the synchronizing panel, and a lamp for
illuminating purposes, while on the base of the panel or on a
pillar at the front of the gallery is mounted the gear for the
field circuit. This consists of a double pole field switch and a
discharge resistance, an ammeter, a handle for the rheostat in
the generator field, and (if each alternator have its own direct
coupled exciter) possibly also a small rheostat for the exciter
field.
NOTE.--In some cases where the capacity of
the plant is not very great, the oil switch
is mounted on the back of the panel, and the
bus bars, current transformers, &c., on the
framework, also just at the back of the panel,
but under no circumstances, in good modern
practice, is high pressure apparatus permitted
on the front of the board. Where the capacity of
the plant is very large, the oil switches are
operated electrically by means of small motors,
and in this case the small switch gear for
starting and stopping this motor is mounted on
the generator panel, also the lamp or lamps to
indicate when the switch is open, and when closed.