10 Electrical Power Systems Technology
Small Units
The measurement of a value is often less than a whole unit, for ex-
ample 0.6 V. 0.025 A, and 0.0550 W. Some of the prefixes used in such mea-
surements are shown in Table 1-4.
For example, a millivolt (mV) is 0.001 V, and a microampere (μA) is
0.000001 A. The prefixes of Table 1-4 may be used with any electrical unit
of measurement. The unit is divided by the fractional part of the unit. For
example, to change 0.6 V to millivolts, divide by the fractional part indi-
cated by the prefix. Thus, 0.6 V equals 600 mV, or 0.6 V ÷ 0.001 = 600 mV.
To change 0.0005 A to microamperes, divide by 0.000001. Thus, 0.0005 A =
500 μA. When changing a base electrical unit to a unit with a prefix, move
the decimal point of the unit to the right by the same number of places in
the fractional prefix. To change 0.8 V to millivolts, the decimal point of
0.8 V is moved three places to the right (8.↵ 0 ↵0), since the prefix milli has
three decimal places. So 0.8 V equals 800 mV. A similar method is used for
converting any electrical unit to a unit with a smaller prefix.
Table 1-4. Prefixes of Units Smaller Than 1
——————————————————————————————
Prefix Abbreviation Fractional Part of a Whole Unit
——————————————————————————————
milli m 1/1000 or 0.001
(3 decimal places)
micro μ 1/1,000,000 or 0.000001
(6 decimal places)
nano n n 1/1,000,000,000 or 0.000000001
(9 decimal places)
pico p 1/1,000,000,000,000 or 0.000000000001
(12 decimal places)
——————————————————————————————
When a unit with a prefix is converted back to a base unit, the prefix
must be multiplied by the fractional value of the prefix. For example, 68
mV is equal to 0.068 V. When 68 mV is multiplied by the fractional value
of the prefix (0.01 for the prefix milli), this gives 68 mV × 0.001 = 0.0068 V.
That is, to change a unit with a prefix into a base electrical unit, move the
decimal in the prefix unit to the left by the same number of places as the
value of the prefix. To change 225 mV to volts, move the decimal point in
225 three places to the left ( 2 2 5 ), since the value of the prefix milli has
three decimal places. Thus, 225 mV equals 0.225 V.
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