7.2.2

1. 7.2.2 Identifying Procedures for Measuring Voltage Between Two Given Points in an Electrical Circuit A professional service technician uses established routines, knowledge and experience to get the customer’s appliance problem identified and repaired in the shortest time consistent with safety and quality work. In this module, you will learn to: • Establish a troubleshooting routine • Identify voltage or electromotive force • Connect the meter to measure electrical current • Read voltage values

2. Establishing a troubleshooting routine Experienced appliance technicians typically apply a quick checklist of simple problems that may cause the gas appliance to malfunction or not operate: • How does the customer describe the problem? • Is the gas supplied at the proper volume and pressure? • On water boilers, is the supply water valve open? • Is the appliance plugged in, or otherwise supplied with electrical power? • Are appliance access doors that hold in electrical interlock switches properly installed and seated? • Are air filters in place and clean?

3. Are required appliance circuits and components properly electrically grounded? • Is the electrical wiring to fan motors, as well as their start and run capacitors, apparently in proper order? Are any associated pulleys and belts in proper working order? • Is there low voltage power to the thermostat? Has the thermostat been properly operated to establish a call for heat? Listening to the customer leads the list, followed by a quick check of simple problem conditions often reveals the cause of the service call. If these items are apparently OK, more detailed troubleshooting is called for. The first step is to locate the appliance manufacturer’s operating instructions and electrical diagrams.

4. Identifying voltage or electromotive force Electromotive force (EMF) is a term used to describe the electrical pressure which causes electrical current to move through a conductor (circuit). The unit of measurement for electromotive force is the volt. The symbols used for specifying volts are the letters "E" or "V." The most common devices used for generating a source voltage in electrical power circuits are: (1) Chemical (battery ‑ the schematic symbol for the battery is: (2) Mechanical (generator ‑ the schematic symbol for the generator is:

5. Figure 1. Electrical Circuits Any EMF introduced into a circuit by a voltage source is called a voltage rise. When electrons flow through a load they give up their energy to the load. Notice that the energy provided to the circuit by the voltage source is released to the circuit. Since the energy introduced into the circuit is called voltage rise, the energy removed from the circuit by the load is called a voltage drop.

6. Figure 2. Symbol for Voltmeter Connecting the meter to measure electrical current Voltage is always measured between two points. The schematic symbol for the voltmeter is illustrated in Figure 2. Notice that one of the leads is marked negative and the other is marked positive. As with ammeters, polarity must be observed when measuring voltage, especially when an analog meter is used.

7. Figure 4a. One Lead Clipped to Ground Figure 4b. Using One Hand to Probe When measuring voltage, the circuit being tested is not broken or disturbed in any way. The meter is connected directly across the load. Figure 3. Measuring Voltage

8. Precautions to be taken when measuring voltages include the following practices: • Check to be sure the voltage to be measured is not higher than the meter can measure — (check meter and probe lead ratings). • Be certain the meter is set on the proper voltage range. If the value of the voltage is unknown, set the range switch on the highest range. • Be sure to hold the meter leads only by the insulated portions. • When working on electronic equipment, it is a good practice to connect one meter lead to ground and leave it there. This way only one hand is required to make voltage checks. This reduces the chance of receiving an electrical shock, since there is no complete path for current to flow through the body.

9. Figure 5b. Digital Multimeter Set to Measure DC Volts Reading voltage values Figure 5a shows the front of an analog multimeter including the graduated scales, selector switch, and range switch. The meter is set up to measure DC voltages. Notice the selector switch is set to measure DC circuits, and the range switch is set to measure 0‑10 volts. Figure 5a. Analog Multimeter Set to Measure DC Volts

10. When using an analog meter, the technician must remember that in some cases it is necessary to apply the mathematical operation of increasing the scale reading by the correct “power of 10s” to read the measurement. For example: in Figure 5a the meter is set on the 10 V AC scale and readings are made on the lowest line (0-10). If the meter were set on the 250 V AC scale, the reading would be made on the top AC scale (0-2.5); however, the pointer reading must be multiplied by 100.

11. The following examples illustrate how to read AC or DC volts on analog meters with the range switch at various settings. Example 1: If the selector switch is set on DC, the range switch set on 50 volts, and the pointer is positioned as shown, then the actual number of volts measured is 7 volts DC.

12. Example 2: If the selector switch is set on DC, the range switch set on 250 volts, and the pointer is positioned as shown, then the actual number of volts measured is 185 volts DC.

13. Example 3: If the selector switch is set on AC, the range switch set on 50 volts, and the pointer is positioned as shown, then the actual number of volts is 27 volts AC.

14. Example 4: If the selector switch is set on AC, the range switch is set on 10 volts, and the pointer is positioned as shown, then the actual number of volts measured is 3.7 volts AC.

15. Remember: • Voltage measurements are always made across a load or power source. • Voltage measurements are always made parallel to the circuit. • Voltage measurements are made on “live” circuits—use protective measures. • Do not attempt voltage measurements of the output of a spark or hot surface ignition module. The voltages generated by these devices are in excess of the measuring ranges of typical multimeters and can damage your meter and may cause death or injury.

16. Time to See If You Got the Key Points of This Module… • Complete the Review on pages 8 & 9. • See if you are ready for the Certification Exam by checking off the performance criteria on page 10.