Sunday, January 4, 2015

Electrical quantity measurement : electrical meter connection and summary of electrical quantity measurement

6–4 ELECTRICAL METER CONNECTION

The proper techniques of connecting electrical meters should be demonstrated and practiced in a laboratory setting. It is assumed that the reader has access to such facilities. However, the following guidelines are offered for your review and consideration:

1. When using multifunction test instruments, be sure that the test leads are correctly inserted into the proper jacks (terminal openings) designated for the task.

2. Always connect voltmeters parallel to a load or source; see Figure 6–11.

3. Always connect ammeters in series. To establish such a connection, break the circuit and then insert the meter into the break; see Figure 6–12.

4. DC meters are polarized: positive (+) and negative (–). Take care to connect the positive (red-colored) test lead toward the positive end of the source. If mistakes are made in the polarization, the pointer will deflect downhill, that is, to the left of 0.

5. When ohmmeters are applied to a circuit, be sure that the power is turned off. Better yet, disconnect one side of the resistance to be measured.

6. When meters with multiple ranges are employed, protect the meter by switching to the highest range first to prevent overloading.

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SUMMARY

• Analog meters are instruments in which numerical data are represented by the position of a moving pointer over a scale.

• Multimeters, such as VOMs, serve different functions with multiple ranges.

• The range of a meter at maximum deflection must not be exceeded.

• DC meters must be correctly polarized.

• Voltmeters are connected in parallel.

• Ammeters are connected in series.

• Resistors must be disconnected from the circuit before an ohmmeter is attached.

Achievement Review
PART I DC VOLTMETER READING

Look at the scale of a DC voltmeter shown on page 75. Then assume that the pointer deflects to the individual settings as shown in the chart on page 74 and that the range selector is in the position indicated. Your instructor will give further instruction as necessary.

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PART II DC AMMETER READING

Look at the scale of a DC ammeter shown on page 77. Next, assume that the pointer deflects to the individual settings shown in the chart on page 76 and that the range selector is in the position indicated. Your instructor will give further explanation if necessary.

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PART III CIRCUIT CHECKS

Following are schematics of a number of circuits with instruments attached. Some of these schematics have mistakes incorporated. Your task is to check each circuit for mistakes. Circle the mistake(s) as you see them, and briefly explain what is wrong. If no mistakes are found, label the drawing OK and state what is being measured, for example, RT, I2, E1.

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