7.1.3 student book © 2005 propane education & research councilpage 1 7.1.3 applying a systems...

7.1.3 Student Book © 2005 Propane Education & Research Council

7.1.3Applying a Systems Approach

to Gas Appliance Troubleshooting

Troubleshooting gas appliance problems is easier when technicians understand the concept that the appliance operates as a system and its electrical circuits act to make the system function. Understanding how circuits operate in the system requires a basic knowledge of the types of electrical circuits used and how components work in the different types of circuits.

7.1.3 Student Book © 2005 Propane Education & Research Council Page 1

In this module, you will learn to:

1. Identify series circuits

2. Identify parallel circuits

3. Identify the significance of voltage drop at points in a circuit

4. Perform basic electrical circuit troubleshooting skills

5. Isolate the fault in an electrical circuit


Identifying series circuits

If load devices, switches, or other components are connected in the circuit in such a way the total current passes through each component, they are referred to as series-connected components.

Figure 1. Series Circuit

The circuit diagram illustrated in Figure 1 is a fuse, heater, and switch connected in series.


A series circuit provides only one path for the current to flow. If the path is broken, there is no current flow, and the circuit becomes an open circuit, or if the resistance in the circuit suddenly becomes zero and the voltage remains the same, the result will be a high current flow.

The condition of no resistance and high current flow is referred to as a short circuit.


Open circuits— Electrical circuits may be opened deliberately, such as by switches, or they may be opened as a result of a defect or problem, such as a broken wire or burned-out coil. Since too much current flowing in a circuit can damage the power source and the load device, fuses are used in the circuits to limit the amount of current.

Figure 2. Open Electrical Circuits


Short circuits — A short circuit exists when current can flow from the negative terminal of the power source through the connecting wires, and back to the positive terminal of the power source without going through the load device(s).

Figure 3. Short Circuit

The high current caused by the short circuit can damage power sources, burn the insulation on connecting wires, and start fires from the intense heat it produces in conductors. Fuses and other circuit breakers are the major means of protecting against the dangers of short circuits.


Fundamental characteristics of series circuits include:

• A series circuit has only one path for the current to flow.

• If the current path in a series connected circuit is broken, the circuit is open and no current flows.

• Series connected load devices allow the current to pass through each load device.


Identifying parallel circuits

If load devices, switches, or other components are connected in the circuit in such a way they provide different current paths, the components are referred to as parallel connected components.

Figure 4. Parallel Circuit

Each load provides a separate path for current flow. The separate paths are called branches, and the current flowing in each branch is called branch current.

Since the current from the power source is divided between or among the branches, the current in any one branch is less than the current leaving the power source.


Open circuits— If the open or break in a parallel circuit is at a point where total circuit current flows, the entire circuit is open and all current flow stops.

Figure 5. Break (Open) at a Point of Total Current Flow

If the circuit is open at a point where only a branch current flows, then only that branch is open and current continues to flow in the remainder of the circuit.

In order for a fuse to protect a parallel-connected circuit, it must be connected at a point in the circuit where the circuit current flows, or each branch must be fused.


Figure 6. Break (Open) at a Point Where Branch Current is Flowing

L 1 L 2

C o i l

C o i l

H E AT R E L AY

FA N R E L AY

F U S E


Short circuits— When a parallel-connected circuit is shorted, the same effects occur as when a series connected circuit is shorted. The effects of a short are:

• A sudden and very large increase in circuit current

• Heating of connecting wires

• Possible burning of the insulation on the connecting wire

• The possible damage to the power source caused by excessive current drain

The branches of a parallel-connected circuit are connected directly between the terminals (L1 & L2) of the power source.

Therefore, damaging short circuits are more likely to occur in parallel-connected circuits than in series-connected circuits.


Identifying the significance of voltage drop a points in a circuit

Every electron at the negative battery terminal in Figure 7 has been given energy by the battery. When the electron moves around the circuit it gives up the energy, so when it arrives at the positive terminal it has lost all the energy the battery has given it. The electron loses its energy by giving it to the circuit resistance, usually in the form of light, heat, or a magnetic force.

If a load is connected across the terminals of a 10-volt battery, 10 volts would be lost, or dropped, by the current flowing through the load.

If two or more loads are connected in series across the terminals of a voltage source, some voltage would be dropped across each load, but the total voltage dropped would be 10 volts.

Figure 7. Circuit ContainingOne Load Device


If the resistance of each light is the same, the voltage drop across each light will be 5 volts. Therefore, the total voltage drop in a circuit always equals the source voltage.

Figure 8. Circuit Containing Two Load Devices


Series-connected loads— With series-connected loads the voltage drop across all the loads is equal to the source voltage, which is 24 volts in the low voltage circuit of Figure 9. This is true whether there is one load device or 50 load devices. If the source voltage remains constant, the more load devices there are the less voltage drop across each load device.

Figure 9. Series-Connected Load Devices

Since the voltage drop across any load device is the energy given to the load, the voltage dropped depends on the current flowing through the load device and the resistance of the load device.

The total resistance in the circuit illustrated in Figure 9 is calculated by adding the resistances. In this circuit the total resistance is 12 ohms (2 + 4 + 6). The source voltage is 24 volts (E). The current in the circuit is 2 amperes.


Parallel-connected loads— In a circuit composed of parallel-connected load devices, the source voltage is dropped across each load. The reason for this is that all parallel load devices are connected together across the power source.

Figure 10. Parallel-Connect Load Devices

7.1.3 Student Book © 2005 Propane Education & Research Council Pages 8 & 9

Performing basic electrical circuit troubleshooting skills

Control circuit problems may be classified into four basic types:

• No Power. If there is no power, the circuit will not operate.

• Open Circuits. Any problem causing interference with the normal flow of electrical current through the coil is considered to be an open circuit fault. Such problems include a blown fuse, broken wire, loose connection at terminals, and dirty contacts in push button switches, limit switches, or relays.

• Short Circuits. Under certain conditions the flow of electrical current may take a path different from that intended by the circuit design. A short circuit may operate the coil at the wrong time or prevent the coil from operating at the right time. In many cases the first indication of a possible short circuit is a blown fuse in the control circuit.

• Defective Load. For example, if the coils (load) will not operate with the required voltage measured directly across the terminals, the load is defective.


Circuit elements— To illustrate basic circuit faults consider the electrical circuit of a flashlight. As illustrated in Figure 11, each of the four circuit elements is capable of introducing a fault in the circuits.

Figure 11. Flashlight Electrical Circuit


Figure 12. Simple Control Circuit

The circuit in Figure 12 illustrates that this circuit has eight possible "normal" faults. These are called "normal" faults because they occur most frequently and relate to open circuit problems. This circuit also has four "abnormal" faults caused by short circuits.


Isolating the fault in an electrical circuit

Figure 14. Control Circuit With Switch Closed


Troubleshooting tip: Start testing procedures in the middle of the circuit. Since the circuit does not operate when the switch is closed, open the switch and start troubleshooting by testing the voltage at the terminals of the power source, then the control fuse, etc. With eight possible faults, it may take eight tests to locate the problem.

Figure 15. Control Circuit With Switch Open

Voltage is usually present on the power source side of the switch and on the power source side of the coil, as illustrated in Figure 15.


The purpose of making the first test in the middle of the circuit is to reduce the number of tests needed to isolate a fault.

As an example, if the meter indicates 24 volts when connected as indicated in the diagram, it proves the voltage is present at these two points. Therefore, possible faults 1, 2, 3, 4, and 8, as illustrated in Figure 12 (page 10), are eliminated.

In other words, in this particular circuit the first test provides as much information as making five individual tests.

Each test can produce only one of two possible results. They will indicate a "yes" voltage or "no" voltage. When this first test is made in the middle of the circuit, the reading on the meter will indicate which direction to go to find the fault.


Figure 16. Control Circuit - Second Test


Figure 17. Control Circuit Test Showing Broken Connection


Troubleshooting Tip: Move only one test lead at a time. In the above tests only one test lead was moved from one test to the next test. This is very important when isolating a fault. In this particular series of tests, voltage was present on the first test. Locating a point where zero voltage was indicated isolated the fault.

Second Test PositionFirst Test Position

Third Test Position

7.1.3 Student Book © 2005 Propane Education & Research Council Pages 15 - 21

Time to See If You Got the Key Points of This Module…

• Complete the Review on pages 15 - 20.

• See if you are ready for the Certification Exam by checking off the performance criteria on page 21.

Problem #1


The customer's complaint is the motor does not run when all the switches are in operative position. Which component is defective?

Problem #2


The customer's complaint is the motor does not run when all the switches are in operative position. Which component is defective?

Problem #3


The solenoid valve in circuit #2 does not operate when the switches are in the operative position. Which component is defective?

Problem #4


The motor in circuit #2 does not run when the switches are in the operative position. Which

component is causing the problem?

Problem #5


The solenoid valve in circuit #3 will not operate when the switches are in the operative position. The problem is an open interlock switch. Fill in the Test Check Table indicating the presence of a voltage between each test point.