7.2.1 Student Book © 2005 Propane Education & Research CouncilPage 1 7.2.1 Identifying Basic Meter and Hazard Reducing Guidelines Used in Measuring Electrical

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

7.2.1

Identifying Basic Meter and Hazard Reducing Guidelines Used in Measuring Electrical

Quantities

The multimeter is a primary tool for the appliance service technician. In this module, you will learn to:

1. Identify the operating principle of the basic analog meter movement

2. Identify how to read a typical analog multimeter scale

3. Identify the operating principle and functions of a voltmeter

4. Identify the operating principle and functions of an ammeter


5. Identify the operating principle and functions of an ohmmeter

6. Identify the operating principle and functions of digital multimeters (DMM)

7. Identify the operating principle and functions of clamp meters

8. Identify principles of meter safety and meter categories


There are many types and kinds of meters used by technicians doing electrical work. The most widely used meters are the:

(1) Voltmeter ‑ used to measure voltage

(2) Ammeter ‑ used to measure electrical current

(3) Ohmmeter ‑ used to measure electrical resistance Sometimes these three meters are combined into one case and called a multimeter, or V.O.M, (volt, ohm, milliampere) meter.

Figure 1a. Analog Multimeter Figure 1b. Digital Multimeter


Identifying the operating principle of the basic analog meter movement

The basic analog meter movement used in an analog electrical measuring device consists of a fixed permanent magnet and a moving coil.

The meter movement depends on the interaction between a fixed magnetic field created by the permanent magnet and the varying magnetic field around the coil.

Electrical current flowing through the moving coil causes he magnetic field around the moving coil.

Figure 2. Basic Analog MeterMovement

7.2.1 Student Book © 2005 Propane Education & Research Council Pages 2& 3

Identifying how to read a typical analog multimeter scale

Figure 3. Typical Analog Multimeter Face

7.2.1 Student Book © 2005 Propane Education & Research Council Pages 2& 3

Figure 3 shows the face of a meter showing various scales.

The top scale, (read from right to left) is the ohms scale. The ohms scale is calibrated to read from 0‑3000 ohms. Reading on the ohms scale can be increased by adjusting the range switch on the meter.

The scale marked DC is used for measuring direct current and voltage values. Notice the scale is calibrated to read values of 0‑2.5, 0‑5, and 0‑10.

The scale marked AC is used for measuring alternating current (AC) voltage only. The AC scale is also calibrated to read values of 0‑2.5, 0‑5, and 0‑10. Like the ohms scale, adjusting the range switch on the meter increases (or changes the range) for the reading for both DC and AC measurements.

Figure 3. Typical Analog Multimeter Face


Identifying the operating principle and functions of a voltmeter

If 110 or more volts were applied directly to the moving coil of an analog voltmeter, the wire forming the coil would be burned out immediately. To prevent this, a very high resistance is placed in series with the moving coil resulting in only a very small current flowing through the coil of the meter.

In addition to protecting the voltmeter's moving coil, the high resistance in series with the moving coil serves two other purposes:

1. With practically no current flow through the moving coil, placing the voltmeter in a circuit or across a single component in the circuit will not affect the current flow in the circuit or the component.

2. By switching to one of several multiplier resistors in a voltmeter, one meter can be used to measure several ranges of voltage (for example, 0‑2.5V, 0‑10V, 0‑50V, 0‑250V, 0‑1000V and 0‑5000V volts) using an appropriate scale on the meter dial.


Figure 4. Jacks and Switch Positions for Measuring DC Volts


Figure 5. Jacks and Switch Positions for Measuring AC Volts


Figure 6 illustrates a scale calibrated to read the value of both DC and AC volts. When DC voltage values are to be read, the selector switch on the front of the meter is set on DC. If AC voltage values are to be read, the selector switch is set on AC.

Figure 6. Meter Scale Calibrated to Measure AC or DC Volts


Identifying the operating principle and functions of an ammeter

Analog ammeters involve the same principles of operation as voltmeters. The scale is calibrated from 0‑10, 0‑5, and 0‑2.5 amperes. The move-ment of the pointer is proportional to the amount of current flowing through the meter (moving coil).

Figure 7. Meter Scale

7.2.1 Student Book © 2005 Propane Education & Research Council Pages 5 & 6

Current measurements made with a standard analog or digital ammeter (not a clamp type) requires the placing of the meter in series with the part of the circuit being measured.

The ammeter is actually a sensitive voltmeter measuring the voltage drop across an internal meter resistance placed in series with the circuit.

Figure 8. Measuring Current

Figure 9. Milliampere Range


Identifying the operating principle and functions of an ohmmeter

The basic meter movement of a voltmeter is also used for measuring the resistance of a component. The ohmmeter has a built‑in set voltage source (a battery). The ohmmeter should never be connected to any outside voltage source.

Illustrated in Figure 10 is a simple ohmmeter circuit. The circuit consists of a basic meter movement, (Rm) a variable resistance, (Rn) and a fixed resistance (Rf). The resistance to be measured is placed between the terminals A and B.

Figure 10.Simple Ohmmeter Circuit


Note the scale of an ohmmeter is the reverse of the voltmeter scale. With the ohmmeter scale full deflection of the needle to the right end of the scale is a reading of zero ohms, while the left end of the scale indicates an infinite resistance, shown by the symbol:


Ohmmeters are designed to measure resistance on several ranges of resistance values. The ohmmeter illustrated in Figure 11 is designed to read on a scale of multipliers of 1,100, and 10,000 ohms.

Figure 11. Ohm Range


Identifying the operating principle and functions of digital multimeters (DMM)

Digital multimeters do not use the electromechanical movement method to measure electrical quantities as described for analog meters. Instead, digital meters use electronic components, semi-conductors and integrated circuit technology.

Figure 12. Voltage Measurement withDigital Multimeter

The values obtained are displayed as positive or negative numbers (with decimal point if appropriate) on a display screen.


Figure 13. Bar Graph and Digital Display

The display in Figure 13 shows that the instrument is measuring in the 0 to 30 amps range.

The wave symbol, ~, indicates that the current is AC. The bar graph confirms that the actual reading is at least 7 on the 0 to 30 scale, with the decimal in the digital display indicating that the actual reading is 7.25 amps AC. A bar graph shows changes in trends in a signal or electrical quantity reading just like an analog needle, but is more durable and less prone to damage.

To fully understand and use all the features and capabilities of a digital multimeter, it is important to study the manufacturer’s instructions, and consult any technical bulletins that may be available from the manufacturer.


Identifying the operating principle and functions of clamp meters Technicians in the propane industry are frequently required to determine if motors are starting and running properly. Determining starting and running amp draw is the first step in making that determination. Using a clamp meter makes the measuring of starting and running amps a fairly simple process.

Figure 14. Analog and Digital Clamp Multimeters

By comparing actual starting and running amps measured with the starting and running amps design ratings stamped on motor data plates, a technician can get a good start on diagnosing problems with motors.

Often the problem may be in the wiring supplying the current to the motor, and excessively high amp readings are a strong indication of possible wiring under sizing.


Identifying principles of meter safety and meter categories

Electric Shock

While most people are aware of the danger from electric shock, few realize how little current and how low a voltage is required for a fatal shock. Current flows as low as 30 mA can be fatal (1 mA = 1/1000A). Let's look at the effects of current flow through a "typical" 150 pound male:

• At about 10 mA, muscular paralysis of the arms occurs, so that he cannot release his grip.

• At about 75-250 mA, for exposure exceeding 5 seconds, ventricular fibrillation occurs, causing disruption of the coordination of the heart muscles—the heart can no longer function. Higher currents cause fibrillation at less than 5 seconds. The results are often fatal.

Fluke Corporation, ABC's of Multimeter Safety


Electric Shock

Now let's calculate the threshold for "hazardous" voltage. The approximate body resistance under the skin from hand to hand across the body is 1000, under dry conditions. A voltage of only 30V across 1000 will cause a current flow of 30 mA. Under wet conditions, or if there is a cut in the skin, resistance drops radically. The threshold of hazardous voltage is cut in half to 15V.

For multimeter manufacturers and users, the object is to prevent accidental contact with live circuits at all costs. Look for:

• Meters and test leads with double insulation

• Meters with recessed input jacks and test leads with shrouded input connectors

• Test leads with finger guards and a non-slip surface

• Meter and test leads made of high-quality, durable non-conductive materials

Fluke Corporation, ABC's of Multimeter Safety


Meter Safety

Two factors regarding the selection and use of electrical meters must be considered before using a meter.

• Be sure that your test meters are rated to protect you in the locations you will be working.

• Be sure that the ratings of test leads and accessories provide equal or greater protection.

Figure 15. Meter Categories by Location Fluke Corporation, ABC's of Multimeter Safety

As a general rule the instrument selected for use should provide the highest protection for the user for the location and type of equipment being tested. The closer the location to primary (high voltage) sources, the higher the category the meter and test leads should have.


Over Voltage Installation Categories


Work Safely: Meter Precautions

• Work on de-energized circuits whenever possible. Use proper lock-out/tag-out procedures. If these procedures are not in place or not enforced, assume that the circuit is live.

• On live circuits, use protective gear:

Use insulated tools. Wear safety glasses or a face shield. Wear insulated gloves; remove watches or other jewelry. Stand on an insulated mat. Wear flame resistant clothes, not ordinary work clothes.


• When making measurements on live circuits:

Hook on the ground clip first then make contact with the hot lead. Remove the hot lead first, the ground lead last.

Hang or rest the meter if possible. Try to avoid holding it in your hands, to minimize personal exposure to the effects of transients. (A transient is a momentary high voltage spike which "rides" into circuits from an exterior source such as a nearby lightning strike.)

Use the three-point test method, especially when checking to see if a circuit is dead. First, test a known live circuit. Second, test the target circuit. Third, test the live circuit again. This verifies that your meter worked properly before and after the measurement.

Use the old electricians' trick of keeping one hand in your pocket. This lessens the chance of a closed circuit across your chest and through your heart.

7.2.1 Student Book © 2005 Propane Education & Research Council Pages 13 - 17

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

• Complete the Review on pages 13 - 16.

• See if you are ready for the Certification Exam by checking off the performance criteria on page 17.

Documents

7.2.1 Student Book © 2005 Propane Education & Research CouncilPage 1 7.2.1 Identifying Basic Meter and Hazard Reducing Guidelines Used in Measuring Electrical