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Refrigeration Service
Engineers Society
1666 Rand Road
Des Plaines, Illinois 60016
UNDERSTANDING ELECTRICAL SCHEMATICS
Part 2
by Howard L. Pemper, CMS
INTRODUCTION
for example, shows both a normally open and a normally closed single-pole, single-throw (SPST) switch.
This type of switch either opens or closes one circuit.
Figure 1B shows a single-pole, double-throw (SPDT)
switch. Again, only one circuit can be controlled at
any given time, but in this case the switch has two different “connected” positions, which means that it can
direct current to either of two paths.
In this section, you will take a look at a packaged
gas/electric system, which is a relatively complex
heating and cooling unit. The wiring diagram for this
equipment is more difficult than those you studied in
Part 1—but again, the schematic as a whole can be
simplified by breaking it down into its basic circuits.
As you examine individual control circuits and the
associated components that they operate, the overall
diagram becomes easier to understand, as do the
various machine functions. The schematics in this
section may include some symbols with which you
are not familiar. For your convenience, many of the
schematic symbols currently used and recognized by
the HVAC/R industry are collected in Figure 16 at the
end of this chapter.
A. Single-pole, single-throw (SPST)
Normally closed (N/C)
Normally open (N/O)
SWITCH SYMBOLS
B. Single-pole, double-throw (SPDT)
Generally speaking, a wiring schematic shows the
condition of a piece of equipment when there is no
power being applied to the unit. Therefore, if a switch
is depicted as being normally open (N/O) or normally
closed (N/C), remember that the position of the
switch is shown as it appears when there is no power
applied to that circuit. If there is any deviation from
this practice, there will be an explanatory note on the
schematic.
C. Double-pole, double-throw (DPDT)
As you may know, a switch is characterized by the
number of contacts (or poles) and the number of
positions (or throws) it has. Think of the number of
poles as the number of circuits that the switch can
control at one time, and the number of throws as the
number of paths a single circuit can take. Figure 1A,
© 2003 by the Refrigeration Service Engineers Society, Des Plaines, IL
Supplement to the Refrigeration Service Engineers Society.
Figure 1. Switch symbols
1
630-141
Section 4A n
Double-pole, single-throw (DPST)
operation of the control. In Figure 3, for example, the
temperature switch (RS-2) is shown with the arm
above the contacts. This signifies that the switch
opens on a rise in temperature and closes on a drop
in temperature. The pressure switch (AFS-2) is
shown with the arm below the contacts. This signifies
that the switch opens on a drop in pressure and
closes on a rise in pressure.
Double-pole, double-throw (DPDT)
An example of an SPDT limit switch (LS) is shown in
Figure 4. When there is an increase in temperature,
the contacts “C” to “N/C” move to the “N/O” position.
When the temperature decreases, the contacts “C” to
“N/O” move back to the “N/C” position.
Relays
Relays are electrically operated control switches. The
schematic symbols used to represent relays are the
same as those for manually operated switches,
except that relay symbols often include a solenoid
coil. There are several possible ways of depicting the
solenoid coil. Figure 5 shows two different schematic
representations of a DPDT relay. Note that multiplepole relays, like multiple-pole switches, are connected
mechanically but not electrically.
Three-pole, single-throw (3PST)
Contactors
Figure 2. Multiple-pole switches
A contactor is a type of heavy-duty relay that handles
higher voltages and higher currents than a control
A switch that can control more than one circuit at a
time is shown schematically as having more than one
set of contacts. Look back at Figure 1C on the previous page. It shows an example of a double-pole, double-throw (DPDT) switch, which can control two
circuits at the same time. The dashed line represents
the mechanical connection, and tells you that the
contacts move together, but are not connected electrically. Figure 2 above shows a few of the many other
variations that are possible in depicting multiple-pole
switches.
RS-2
1
AFS-2
3
1
2
Figure 3. Temperature and pressure controls
N/O
Controls
LS
C
Pressure and temperature controls are switches, too,
and they also may be configured with various combinations of poles and throws. The position of the
switch “arm” in the schematic symbol indicates the
N/C
Figure 4. SPDT limit switch
2
Coil
Coil
Figure 5. DPDT relays
relay. Contactors appear nearly identical to relays on
schematic diagrams. Some manufacturers employ
contactors that use a single set of contacts. A “bus
bar” is placed over the connection where the other
set would be, as shown in Figure 6. Figure 16 at the
end of this chapter includes many other symbols for
switches and relays.
L1
L2
T1
T2
Coil
Figure 6. Two-pole contactor with bus bar
THE BASIC DIAGRAM
problems that you may encounter with a particular
type of equipment.
Let’s take a look at a “generic” schematic of a packaged heating/cooling unit. In order to illustrate the
various options that may be possible with a unit of this
kind, the schematic has been put together by taking
parts from several different manufacturers. Because
of its complexity, the schematic is broken into three
parts. Figure 7 (spread across pages 4 and 5) shows
the high-voltage components, and Figure 8 (pages 6
and 7) shows the low-voltage or control circuits. There
normally is no “point-to-point” or line diagram with this
type of schematic, but a component layout is often
provided. This is shown in Figure 9 (pages 8 and 9).
RELAYS
As you saw earlier, relays can and do have many
contacts. A single relay may have a function in two,
three, or sometimes four different circuits. Its contacts
may be located in various parts of the schematic, and
you must know how to find them if you are to know
how the unit works. In our generic diagram, several
relays provide lockouts for the cooling and heating
sections, which means that the two sections cannot
come on at the same time.
EQUIPMENT
Locate control relay CR-1 at the bottom of Figure 8
(line 183). Now look at the detail shown in Figure 10A
(found on page 10). As you can see, control relay
CR-1 has five sets of contacts that are operated by
one coil. The first two sets of contacts, CR-1a and
CR-1b, are found on lines 42 and 48, respectively
(see Figure 10B). The third set, CR-1c, is found on
line 77 (see Figure 10C). The fourth set, CR-1d, is
found on line 149 in the low-voltage section of the
schematic (see Figure 10E). The last set, CR-1e, is
found on line 90 (see Figure 10D). Remember that
when a number in the right-hand margin of Figure 10A
is underlined, it designates a set of normally closed
(N/C) contacts.
In order to service any piece of equipment, you first
must know what you are working with. Even before
you begin a visual inspection of the equipment itself,
a quick look at the schematic will give you a general
idea of the type of equipment and its components. In
Figure 7, for example, it’s easy to spot the two compressors—therefore, you can assume that this is a
two-stage cooling system. Likewise, in Figure 8 you
can see two ignition systems—again, you can conclude that this is a two-stage heating system. With
just a quick glance at the schematic, you have determined what the unit is. As you become more experienced, you also may have a good idea of the kinds of
3
Figure 7. High-voltage circuits
4
46
45
44
43
42
41
40
39
38
37
36
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
YEL
115 V
208 V
CB-1
110 A
BLU
TRSF-1
230 V
YEL
BLK
BLU
YEL
BLK
YEL
CR-1a
1
3
FU-1
8A
GRY
BLK
BLK
BLK
GND
208/230-3Ø
TB-1
BLU
YEL
BLK
YEL
BLU
YEL
BLK
BLU
YEL
BLK
IFMC
CC-1
T3
T2
T1
T3
T2
T1
T2
L2
T2
OFMC-2
L1
T1
L2
OFMC-1
L1
T1
L3
L2
L1
L3
L2
L1
BLU
YEL
BLK
YEL
BLU
YEL
BLK
1
ATS
3
T3
L3
BLU
T1
T2
CC-2
L2
L1
YEL
BLK
BLU
YEL
BLK
R
C
S
R
C
S
C1 OFMC-2 C2
C1 OFMC-1 C2
CAP
CAP
IFM
BLU
YEL
BLK
BRN
BRN
COMP-1
BRN
C2 OFMC-2
C2 OFMC-1
TRSF-1
OFM-2
OFM-1
29, 31
22, 24
COMP-2
Figure 7. High-voltage circuits (continued)
5
94
93
92
91
90
89
88
87
86
85
84
83
82
81
80
79
78
77
76
75
74
73
72
71
70
69
68
67
66
65
64
63
62
61
60
59
58
57
56
55
54
53
52
51
50
49
48
47
GRY
GRY
CR-7
IFRC
IFRH
3
3
3
IDFMR
1
3
CR-1e
13
15
1
1
1
CR-2b
1
2
CR-1c
1
2
BLK
3
ASR-2
1
3
CR-2a
1
1
BLU
BLU/RED
3
ASR-1
1
6
CR-1b
4
CCPS
L
2
GRY
GRY
M
OFC-2
M
OFC-1
L
HPS-1
2
HPS-2
2
120 V
220 V
120 V
2 ASR-2 3
1
220 V
2 ASR-1 3
1
C
3
C2
C1 IDFM C2
C1
USLV
C1 IFMC C2
CC HTR-2
OFC-2
C1 TDR-2 C2
C1 CC-2 C2
OFC-1
4 TDR-1 6
C1 CC-1 C2
CC HTR-1
Heater
TDR-2
LPS-2
Heater
TDR-1
LPS-1
WHT
C
1
BRN
BRN
BRN
BRN
BRN
BRN
BRN
BRN
BRN
BRN
OFC-2
TDR-1
C2 IDFM
C2 USLV-1
C2 IFM
C2 IFMC-2
CC HTR-2
CC HTR-1
OFC-2
C2 TDR-2
C2 CC-2
C2 CC-2
BRN
6
C2 CC-1
C2 CC-1
15, 17, 18
63
63, 68
9, 10, 12
48
48, 53
1, 3, 4
Figure 8. Low-voltage circuits
6
143
142
141
140
139
138
137
136
135
134
133
132
131
130
129
128
127
126
125
124
123
122
121
120
119
118
117
116
115
114
113
112
111
110
109
108
107
106
105
104
103
102
101
100
99
98
97
96
95
IGN-2
IGN-1
C
CB-2
3A
LS
24 V
115 V
1
1
GVR-2
IGN PCB
GVR-1
N/C
N/O
BLU
BLK
RS-2
RS-1
YEL
3
2
TRSF-2
WHT
1
1
IGN-2
AFS-2
IGN-1
AFS-1
2
3
1
CSb
CSa
3
T1
T1
FS
IGN-2
GVR-2
FS
IGN-1
GVR-1
IGN
GV-2
IGN
GV-1
T2
T2
C1 CR-7 C2
VIO
VIO
VIO
VIO
VIO
VIO
T2
IGN PCB
IGN-2
IGN-1
C2 TRSF-2
T2 GV-1
T2 CR-7
TRSF-2
Figure 8. Low-voltage circuits (continued)
7
188
187
186
185
184
183
182
181
180
179
178
177
176
175
174
173
172
171
170
169
168
167
166
165
164
163
162
161
160
159
158
157
156
155
154
153
152
151
150
149
148
147
146
145
144
6
9
6
3
3
HR-1a
1
HR-2a
1
HR-1b
4
HR-1c
7
HR-2b
4
5
CR-1d
12
CR-2c
10
4
2
LOGIC
TDR-3
1
3
R
G
W2 W1 Y2 Y1
C
C1 CR-1 C2
C1 CR-2 C2
C1 HR-1 C2
C1 HR-2 C2
C1 IFRC C2
C1 IFRH C2
C1 IDFMR C2
VIO
VIO
VIO
VIO
VIO
VIO
VIO
VIO
LOGIC
C2 USLV-1
C2 CR-1
C2 CR-2
C2 CR-2
C2 CR-2
C2 HR-2
C2 HR-1
C2 IFRC
3
IDFMR
C2 IFRH
C2 IDFMR
42, 48, 77, 149, 90
63, 80, 146
161, 152, 149
158, 146
85
82
92
8
7
9
6
3
5
4
6
3
C2
C2
Figure 9. Component layout
8
TRSF-2
C1
C1
4
1
5
2
IFRH
5
2
6
3
6
3
C1
C1
5
2
6
3
4
1
TRSF-1
5
2
6
3
IDFMR
1 CCPS 2
C2
C2
4
1
HR-1
C2
C2
1
1
4
1
5
2
CR-7
5
2
AFS-2 2
AFS-1 2
C1
C1
4
1
HR-2
6
3
6
3
C2
C2
3
1
IGN-1
3
RS-2
1
RS-1
N/O
C
N/C
LS
T1
T2
IGN-2
GV-1
T1
GV-2
W2 W1 Y2 Y1
GVR-2
IGN PCB
2
1
IFRC
13 14 15
4
1
CR-2
G
GVR-1
C1
C1
5
4
10 11 12
2
1
CR-1
R
C
T2
Figure 9. Component layout (continued)
9
8
5
7
4
6
9
3
TDR-2
L
L
208/230/3-phase
2
1
COMP-1&2
TDR-1
220 V 120 V
OFC-2
220 V 120 V
OFC-1
C2
T2
T1
C1
L2
L1
OFMC-1
M
M
C1
T1
L1
1
3
1
3
1
C2
T2
L2
3
TDR-3
ASR-2
ASR-1
OFMC-2
2
4
2
4
2
CAP-2
CAP-1
C1
T1
L1
T2
L2
CC-1
C1
T1
L1
C2
T3
L3
T2
L2
IFMC
T2
L2
CC-2
C2
T3
L3
C1
T1
L1
C2
T3
L3
Detail A
178
179
180
C2 CR-1
181
182
C1 CR-1 C2
183
C2 USLV-1
VIO
42, 48, 77, 90, 149
184
185
186
187
Detail B
37
38
39
40
FU-1
8A
CR-1a
1
3
41
42
BRN
C2 OFMC-1
C1 OFMC-2 C2
BRN
C2 OFMC-2
ATS
1
43
44
C1 OFMC-1 C2
3
GRY
BRN
45
46
47
48
CR-1b
4
ASR-1
6
1
OFC-1
3
L
M
1
2
1
3
C1 CC-1 C2
BRN
C2 CC-1
49
50
HPS-1
LPS-1
51
Detail C
75
76
77
CR-1c
1
2
CC HTR-1
GRY
BRN
CC HTR-1
78
79
87
88
89
Detail D
CR-1e
13
15
USLV
CCPS
1
2
C1
90
91
Detail E
147
148
149
150
7
9
HR-1c
10
12
CR-1d
151
152
Figure 10. Locating control relay contacts
10
C2
C2 USLV-1
CC-1
CR-1b
ASR-1
OFC-1
HPS-1
LPS-1
Figure 11. Series circuit
CIRCUITS
flow is indicated by the arrows. Notice that the current
must pass through all of the switches before it can
energize the coil. If any of the switches is open, the
coil cannot be energized.
A schematic diagram can look complicated when
viewed as a whole. However, as stated previously, it
becomes much easier to analyze a problem if you
break down the large diagram into smaller individual
circuits. All electric circuits conform to one of three
basic arrangements.
Parallel circuits
In a parallel circuit, there are two or more separate
paths or branches for current flow. A parallel circuit
arrangement allows any one of a number of controls
or switches to energize a load. Look at Figure 12A,
for example. When either of relay contacts IFRH or
IFRC is closed, current will pass through the circuit
and energize coil IFMC. However, note that both
IFRH or IFRC must be open in order for the coil to be
de-energized.
Series circuits
In a series circuit, components are arranged one after
another, so that the same current flows through all of
the components in one continuous path. Figure 11
shows an example of a number of switches and a
relay coil connected in series. The direction of current
A. Multiple switches, single load
IFMC
IFRH
IFRC
B. Single switch, multiple loads
OFMC-1
CR-1a
ATS
OFMC-2
Figure 12. Parallel circuits
11
CC-1
CR-1b
ASR-1
OFC-1
HPS-1
LPS-1
TDR-1
ASR-1
220 V
TDR-1
120 V
Heater
OFC-1
Figure 13. Series-parallel circuit
A single switch can energize several loads at the
same time in a parallel circuit. All of the loads will get
the same amount of current when the switch closes.
In Figure 12B, for example, when switch CR-1a is
closed, current will pass through the circuit and energize coil OFMC-1. And as long as thermostat ATS is
closed, coil OFMC-2 also will be energized.
multiple-path) circuits. Such a circuit allows some
operations to proceed while stopping others. Seriesparallel circuits are primarily used in control and
safety applications. The detail shown in Figure 13
above illustrates a series-parallel circuit. When switch
CR-1b closes, current flows through both branches of
the circuit. As long as all of the switches ASR-1,
OFC-1, HPS-1, and LPS-1 are closed, current
passes through the upper branch and energizes coil
CC-1. If any of the switches is open, however, the
current will be allowed to pass only through the lower
branch to coil ASR-1.
Series-parallel circuits
As the name implies, a series-parallel circuit is a
combination of series (or single-path) and parallel (or
C1 IDFMR C2
IFRH
C2 IDFMR
IDFMR
1
2
3
1
2
3
4
5
6
4
5
6
C2 IFRH
C1 IFRH C2
C1
IDFMR
A. Schematic diagram
C2
C1
B. Component diagram
Figure 14. Terminal connections
12
C2
With all of the switches in the upper branch
closed and coil CC-1 energized, current
simultaneously passes through the TDR-1
contacts to the TDR-1 relay. This is a
“delay-on-make” time-delay relay, which
means that after a specified period of time,
the TDR-1 contacts will open. This type of
control is commonly used for low-pressure
bypass during low ambient conditions.
When coil CC-1 is energized, note that current also passes through the 120-V resistor, through the resistor marked “Heater,”
and through differential pressure control
OFC-1. This is an oil failure control. When
oil pressure is sufficient, differential pressure control OFC-1 will open, thus taking
the heater out of the circuit and preventing
the OFC-1 contacts from opening.
IGN-1
GVR-2
IGN-1
IGN PCB
IGN-2
CONNECTIONS AND WIRING
GVR-1
GVR-2
Figure 15. Relay connections on igniter printed
circuit board
In order to conserve wire and space, some
manufacturers terminate more than one
coil connection at the same point. This practice is
depicted schematically as shown in Figure 14A,
where the terminals from the IDFMR and the IFRH
are taken off the same terminal connection. Note that
the schematic shows two connections or wires at that
point (the arrows point to the connection points).
Because line diagrams are not used in complex
schematics, the component diagram will show the
terminal connections on the relays and other devices
(see Figure 14B).
contacts for the igniters are on the printed circuit
board, but they cannot be physically replaced. The
contacts themselves are usually shown enclosed in a
“box” drawn with dashed or dotted lines. This same
approach is used for the gas valve relay shown in
Figure 15, and in some instances for fan relays as
well.
LEGENDS
There normally isn’t enough space on a schematic to
accommodate the full spelling of each component. A
legend listing many of the abbreviations used in the
schematic that you have been studying is shown on
the next page.
In some cases, a control or relay may not be shown
as a replaceable item, or even as a component that
can be tested. In Figure 15, for example, the timing
13
LEGEND
AFS . . . . . Air flow switch
IDFM . . . . . Inducer fan motor
ASR . . . . . Anti short-cycle relay
IDFMR . . . . . Inducer fan motor relay
ATS . . . . . Air temperature switch
IFM . . . . . Indoor fan motor
CAP . . . . . Capacitor
IFMC . . . . . Indoor fan motor contactor
CB . . . . . Circuit breaker
IFRC . . . . . Indoor fan relay (cooling)
CC . . . . . Compressor contactor
IFRH . . . . . Indoor fan relay (heating)
CC HTR . . . . . Crankcase heater
IGN PCB . . . . . Igniter printed circuit board
CCPS . . . . . Capacity control pressure
IGN . . . . . Igniter
switch
LPS . . . . . Low-pressure switch
COMP . . . . . Compressor
LS . . . . . Limit switch
CR . . . . . Cooling relay
OFC . . . . . Oil failure control
CS . . . . . Centrifugal switch
OFM . . . . . Outdoor fan motor
FS . . . . . Flame sensor
OFMC . . . . . Outdoor fan motor contactor
FU . . . . . Fuse
RS . . . . . Rollout switch
GND . . . . . Ground
TB . . . . . Terminal board
GV . . . . . Gas valve
TDR . . . . . Time-delay relay
GVR . . . . . Gas valve relay
TRSF . . . . . Transformer
HPS . . . . . High-pressure switch
ULSV . . . . . Unloader solenoid valve
HR . . . . . Heating relay
Note: Figure 16 on the following three pages shows
many of the schematic symbols used in the HVAC/R
industry today.
14
SWITCHES
N/O
or
N/C
or
Closes
on fall
Closes
on rise
Pushbutton switches
N/O
Pressure
SPST
N/C
Two-position
Temperature
H-O-A switch
HAND
SPDT
OFF
Liquid level
AUTO
Foot switches
Flow
DPST
N/O
N/C
Limit
Time-activated switches
DPDT
Differential
pressure
De-energized
Energized
RELAYS
Coil
SPST
(N/O)
Coil
DPST
Coil
Coil
Coil
SPST (N/O)
SPST (N/C)
Coil
Coil
SPST
(N/C)
Coil
SPDT
DPST
SPDT
Coil
Coil
DPDT
DPDT
L1
L1
L2
Coil
T1
2-pole contactor
T2
L1
L2
Coil
L2
L3
Coil
T1
T2
T1
2-pole contactor
with bus bar
T2
3-pole contactor
Figure 16. Schematic symbols
15
L1
L2
L3
T1
T2
T3
Coil
T3
3-pole contactor
with bus bar
TRANSFORMERS
Primary
115 V
Current transformer
Boost-and-buck transformer
Secondary
24 V
“Boosting”
24 V
460 V
230 V
208 V
120 V
“Bucking”
2.5 V
Common
Multivoltage control transformers
Common
Tapped
460-V primary
460-V primary
H1
H2
H3
H4
H1
H2
H3
H4
X1
X2
X3
X4
X1
X2
X3
X4
Variable
230-V secondary
120-V
secondary
230-V
primary
Air core
115 V
10 kV
230-V
primary
H1
H2
H3
H4
H1
H2
H3
H4
X1
X2
X3
X4
X1
X2
X3
X4
460-V secondary
Always consult the manufacturer’s
nameplate for proper connections.
High-voltage transformer
Circuit breakers
Fuses
Cartridge or
plug fuse
120-V
secondary
Thermal overload
switch
Fusible link
Singlepole
Doublepole
Overload safety
switch
Triplepole
Mechanical
interlock
Thermal overload
Figure 16. Schematic symbols (continued)
16
Magnetic overload
switch
Ground connections
Conductors
Chassis
ground
Wires
connected
Wires crossing
but not connected
Thermocouple
Earth
ground
Lamp
Capacitor (fixed)
Battery
Terminal
nearest ground
Shielded cable
ELECTRONIC COMPONENTS
Transistors
Multiple-conductor
cable
Diode
C
AND gate
Number of conductors
in conduit or cable
B
SCR
Connectors
E
NPN
NAND gate
Male
C
B
Female
LED
OR gate
E
PNP
Engaged
(plug and receptacle)
Diac
NOR gate
D
G
Resistors
S
N-channel JFET
Fixed
Triac
NOT gate
D
G
Potentiometer
Zener diode
Variable
S
Buffer amp
P-channel JFET
Rheostat
Tunnel diode
Op amp
Tapped
UJT
Photodiode
Resistance heater
Solenoid
Thermistor
MOV
T
Cad cell
Figure 16. Schematic symbols (continued)
17
IGT
Refrigeration Service Engineers Society
1666 Rand Road
Des Plaines, IL 60016
847-297-6464