In parts one and two of this series, I
have only spoken about direct current (DC), were the source of
voltage is trying to push the current in one direction.
Another form of electricity is alternating current (AC) in
which the voltage source alternates the current direction.
Everything I have covered in parts one and two apply to both
DC and AC. As I continue I will point out the differences.
Measuring voltage,
current and resistance
Over the past one hundred years various instruments have been
used to measure voltage, current and resistance. Two examples
of instruments that can measure DC voltage are shown in photo
1.
Today, the most common electrical test
instrument in use is the digital multimeter because of its
ability to accurately measure voltage, current and resistance.
We used to call them volt-ohm-meters (VOMs).
Instrument
accuracy
The accuracy of an instrument is a statement of the largest
allowable error expressed as a percentage. Seventy years ago
the most accurate instruments were ones designed to measure
one thing. For example, the Weston DC voltmeter had six
voltage ranges, 0 to 25 millivolts (0.025 volts), 0 to 50
millivolts, 0 to 500 millivolts, 0 to 5 volts, 0 to 50 volts,
and 0 to 150 volts. The accuracy of the instrument is
described on the meter in the following manner: "This
instrument indicates International Volts and is correct within
½ of 1% of full scale value at any part of the scale at
75°F." That means that the limit of inaccuracy for this
instrument connected on the 0 to 150 volt scale is 0.005 x 150
= 0.75 volts plus or minus (±).
By "plus or minus,"
I mean the reading could be off by as much as 0.75 volts high
or low. In comparison, the Fluke Model 175, digital multimeter,
set to measure DC volts has a range from 0.1 millivolts (
0.0001 volts) to 1000 volts. The limit of inaccuracy for the
instrument is ± 0.15% of the reading. That means the limit of
inaccuracy when the meter is reading 120 volts is 0.0015 x 120
= 0.18 volts. The limit of inaccuracy when the meter is
reading 1.0 milivolts is .0015 x 1 millivolts = 0.0015
milivolts (0.0000015 volts). The Fluke multimeter also
measures AC volts from 0.1 millivolts to 1000 volts, DC and AC
amps from 0.01 milliamps to 10.00 amps, and resistance from
0.1 W to 50.00 MW (50 million W). I will talk about measuring
currents greater than 10 amps in another segment.
Photo
2. Leeds & Northrup Company Type S test switch,
"Wheatstone Bridge" used to measure resistance.
Resistance
variation with temperature
In part two of this series, I calculated the resistance of the
filament of a 60-watt light bulb to be 240 ohms. That is the
resistance when the bulb is energized at 120 volts. If you
connect the terminals of a 60-watt bulb to digital multimeter
set to measure resistance, the meter will read about 17 W. The
reason the multimeter measures 17 W is because the multimeter
is measuring the resistance of the filament at room
temperature. When the bulb is energized at 120 volts, the
temperature of the filament is around 2200° F. When the bulb
is first energized, for an instant the current drawn by the
bulb is 120 V / 17 W = 7 A. In the first few milliseconds
after closing the switch, the current heats up the filament
from room temperature to 2200°F, the resistance changes from
17 W to 240 W, and the current drops from 7 A to 0.5 A. If we
wire an ammeter in the bulb circuit, the ammeter would not
measure the surge in current because it happens too quickly.
The bulb filament is a perfect example of the resistance of a
material increasing with temperature.
The multimeter can
measure the temperature of the filament at room temperature
because the voltage being applied to the bulb by the
multimeter is very low, typically 0.5 to 1.5 V. If the
multimeter applies a voltage of 1.5 volts to measure
resistance, the initial current in the bulb filament per Ohm’s
law (equation 2 of part two) is the voltage divided by the
resistance, 1.5 V / 17 W = 0.088 A (88 milliamps). That is a
very low current and yet if you continue to measure the
resistance of the bulb filament with the multimeter for a few
minutes, you will notice the resistance of the filament
gradually increasing. That is because the 88 milliamp current
is heating up the filament. The instrument measuring the
resistance of the bulb filament is changing the resistance.
Instrument affect
on the quantity being measured
When we use instruments to measure quantities like voltage,
current, and resistance, it is very important that the
instrument doesn’t affect the quantity being measured.
Voltmeters must have a very high internal resistance and
ammeters must have a very low resistance to keep the
instrument from affecting the quantity being measured. The
Weston DC voltmeter, vintage 1936, has an internal resistance
of 3000 W when connected on the 0 to 150 volt scale. In
contrast, the Fluke digital multimeter, vintage 2004, has an
internal resistance of 10 MW (10 million W) when set on the DC
voltage scale. In general, modern multimeters have very little
affect on the voltage, current, or resistance being measured
in electric power circuits. When measuring these quantities in
electronic circuits, the same is not always true.
Please send me your comments on this
series. If you have questions about basic electricity or
general questions about the NESC, please e-mail me at dave.young@conectiv.com .
National Electrical Safety Code and NESC
are registered trademarks of the Institute of Electrical and
Electronics Engineers. National Electrical Code and NEC
are registered trademarks of the National Fire Protection
Association.
Dave is a consulting engineer with Conectiv Power
Delivery of Wilmington, Delaware, where he has been working with and
teaching all aspects of the NESC for over 33 years. He is a member of
the NESC Interpretations Subcommittee and represents the Edison Electric
Institute on the NESC Overhead Line Clearances Subcommittee 4. Dave is
also an inspector member of the IAEI. |
