Chapter 9 Electrical Wiring Diagram Basics 207
Copyright Goodheart-Willcox Co., Inc.
Sequence of Operations
Consider the ladder diagram shown in Figure 9-9. This
ladder diagram is a representation of the schematic
diagram shown in Figure 9-4. This system has a relay
with its coil marked COIL and two sets of normally
open contacts, 1-3 and 4-6. These two sets of contacts
close when the relay coil is energized. This system also
has a set of normally open vent damper contacts, VD,
that closes once the vent damper, controlled by the
vent damper motor, VDM, has fully opened.
Determining the sequence of operations for this
system can be broken down into a series of individual,
logically organized steps:
1. Identify the components with no switches in series
with them that are energized all the time. For
instance, the primary size of the transformer is
powered all the time, so 24 V is always supplied
to the control circuit. Also, line voltage power is
always supplied to the top portion of the circuit
from L1 and L2, Figure 9-10A.
2. Identify the action that starts system operation.
The action that typically starts the system is often
an external event, such as the fl ipping of a switch
or the closing of the thermostat. In this circuit,
the action that starts the system is the thermostat,
identifi ed as STAT.
B E T W E E N T H E L I N E S
Determining the Start Action
The action that starts a system can be easily found by the process of elimination. For example, the circulator pump,
CP, circuit at the top of the diagram cannot initiate system operation because that circuit is waiting for the closing of
contacts 1-3 for the circulator pump to turn on. Similarly, the VDM and GV circuits do not become energized until other
system events occur.
3. Once the STAT closes, the relay coil, COIL, becomes energized, Figure 9-10B.
4. The energizing of the COIL causes the 1-3 and 4-6 sets of contacts to close.
5. The closed 1-3 contacts energize the circulator pump, CP, and the closed 4-6 contacts energize the vent damper
motor, VDM, Figure 9-10C.
6. VDM operation opens the vent damper and closes the vent damper contacts, VD.
7. As long as the limit switch, LIM, is closed, the gas valve, GV, becomes energized and opens, starting the
heating cycle, Figure 9-10D.
8. Under normal system operation, the system remains in heating mode until the space thermostat no longer
calls for heat. The heat cycle can also terminate in the event there is an unsafe temperature condition in the
appliance that causes the limit switch to open, de-energizing the heat source.
Procedure
Goodheart-Willcox Publisher
Figure 9-9. A ladder diagram that indicates 24 V is always
being supplied to the control circuit.
L1 L2
X-FRMR
1
CP
COIL
VDM
VD
GV
STAT
LIM
24 V
3
4 6
Copyright Goodheart-Willcox Co., Inc.
Sequence of Operations
Consider the ladder diagram shown in Figure 9-9. This
ladder diagram is a representation of the schematic
diagram shown in Figure 9-4. This system has a relay
with its coil marked COIL and two sets of normally
open contacts, 1-3 and 4-6. These two sets of contacts
close when the relay coil is energized. This system also
has a set of normally open vent damper contacts, VD,
that closes once the vent damper, controlled by the
vent damper motor, VDM, has fully opened.
Determining the sequence of operations for this
system can be broken down into a series of individual,
logically organized steps:
1. Identify the components with no switches in series
with them that are energized all the time. For
instance, the primary size of the transformer is
powered all the time, so 24 V is always supplied
to the control circuit. Also, line voltage power is
always supplied to the top portion of the circuit
from L1 and L2, Figure 9-10A.
2. Identify the action that starts system operation.
The action that typically starts the system is often
an external event, such as the fl ipping of a switch
or the closing of the thermostat. In this circuit,
the action that starts the system is the thermostat,
identifi ed as STAT.
B E T W E E N T H E L I N E S
Determining the Start Action
The action that starts a system can be easily found by the process of elimination. For example, the circulator pump,
CP, circuit at the top of the diagram cannot initiate system operation because that circuit is waiting for the closing of
contacts 1-3 for the circulator pump to turn on. Similarly, the VDM and GV circuits do not become energized until other
system events occur.
3. Once the STAT closes, the relay coil, COIL, becomes energized, Figure 9-10B.
4. The energizing of the COIL causes the 1-3 and 4-6 sets of contacts to close.
5. The closed 1-3 contacts energize the circulator pump, CP, and the closed 4-6 contacts energize the vent damper
motor, VDM, Figure 9-10C.
6. VDM operation opens the vent damper and closes the vent damper contacts, VD.
7. As long as the limit switch, LIM, is closed, the gas valve, GV, becomes energized and opens, starting the
heating cycle, Figure 9-10D.
8. Under normal system operation, the system remains in heating mode until the space thermostat no longer
calls for heat. The heat cycle can also terminate in the event there is an unsafe temperature condition in the
appliance that causes the limit switch to open, de-energizing the heat source.
Procedure
Goodheart-Willcox Publisher
Figure 9-9. A ladder diagram that indicates 24 V is always
being supplied to the control circuit.
L1 L2
X-FRMR
1
CP
COIL
VDM
VD
GV
STAT
LIM
24 V
3
4 6