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Appendix B: Equipment and Instrument Circuits

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Figure B.1: Equivalent circuit for non-

isolated 120 V (rms), 60 Hz power supply.

Appendix B

Equipment and Instrument Circuits

Electrical engineers measure and use a wide variety of electrical circuit variables, such as volt-

age, current, frequency, power, and energy, as well as electrical circuit parameters, such as resis-

tance, capacitance, and inductance. Many instruments can be used to make such measurements,

but the proper use of the instruments and interpretation of the measurements depend on a funda-

mental understanding of how the instruments work, their capabilities, and their limitations.

This appendix provides a brief overview of the fundamentals of the electrical equipment and in-

struments that you will use in this laboratory course. As you encounter more and varied types of

electrical equipment and instruments in this and subsequent courses, you will find several books,

in addition to your textbook, useful in developing your understanding and measurement skills. In

addition, many commercial instrument manufacturers publish handbooks and application notes

that provide more information on specific measurement techniques.

AC Electrical Sources

In this laboratory you will use several different types of sources for electrical voltage and cur-

rent. All of those sources and all of the measurement equipment, except for a few battery-

powered devices, get their power from electrical outlets on the workbench. The workbench is

equipped with two duplex outlets (receptacles with two outlets) for 110 V power, but these out-

lets are not the same. One duplex outlet provides NON-ISOLATED, 120V, 60 Hz electrical

power, such as you would find in your home. All of the power strips along the back of the bench

also use this same power. The second duplex outlet provides ISOLATED, 120V, 60 Hz electrical

power; the power goes through an isolation transformer, so its voltage is floating relative to earth

ground. The isolation transformer has a turns ratio of one and is mounted behind the lab bench.

Both sets of outlets are powered through one GFCI (Ground Fault Circuit Interrupter; a.k.a., GFI

– Ground Fault Interrupter) circuit breaker in the bench. The third power source on the bench

uses a separate circuit breaker that provides 3-phase, 208V, 60 Hz power via banana-plug jacks.

Figure B.1 shows an equivalent circuit for the

workbench’s NON-ISOLATED 120 V, 60 Hz supply

receptacle. The neutral (N) line carries current

from the load back to the source and is connected

to ground (G) at the source for purposes of

personnel and equipment safety. With respect to

ground, the high or “hot” (H) line varies sinu-

soidally at a potential that ranges from a negative

maximum of about -169 Volts to a positive

maximum of about +169 Volts.

Why have a separate ground and a neutral

wire? Suppose we had the scenario shown in

Figure B.2, in which an electrical load is only

supplied by two wires, and the supply voltage is

grounded at the source. The ungrounded “hot” wire is colored black and the grounded return

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