Download Every electrical technician knows the difference between DC (Direct

Every electrical technician knows the difference between DC (Direct Current) and AC
(Alternating Current). Every electrical technician also realizes the importance of taking
accurate current measurements to protect conductors from exceeding their insulators’
ability to withstand heat or assuring devices under power work properly. However, does
every electrical technician realize that electrical current measurements aren’t always what
they appear to be?
Direct Current (DC) is straightforward. When we use a multimeter to measure direct
current, it is what it is. However, the plot thickens when we are dealing with Alternating
Current (AC). AC current travels back and forth down a conductor and can best be
described in graphical format. The most common graphical description of AC current is a
sine wave. Because the amplitude of the sine wave continuously changes over the wave
period (one complete cycle), at any given point in time, a current measurement would not
be the same. Therefore, how do we accurately measure AC Current?
One method to measure AC current would be take current measurements at increments
across one complete cycle and average them together. This would give us an average
value of the current. If the current is a perfect sine wave, mathematically, the average
value is always 0.636 times the value of the peak amplitude.
Another method to measure current is based on the current’s ability to perform work
when applied to a resistive load. The laws of physics tell us that when current passes
through a resistive load, it dissipates energy in the form of heat, mechanical motion,
radiation or other forms of energy. If the resistive load is a heating element and the
resistive load stays constant, then the laws of physics tell us that the heat produced is
directly proportionate to the current passing through the load. Therefore, if we measure
the heat, we will know the current.
Mathematically, the relationship between heat and current is such that the heat produced
is proportional to the square of the current applied to a resistance.
(Power or Heat) = (Current) ^2 * (Resistance)
If the current is continuously changing, as in AC current, the heat produced is
proportional to the average (or mean) of the square of the current applied to a resistance:
(Power or Heat) = Average [ (Current) ^2 * (Resistance) ]
Using algebra, the above formula can be rewritten to read:
Current = Square Root [ (Power or Heat) / (Resistance) ]
AND this is called the Root Mean Square Current or RMS Current.
For AC currents that are graphically represented by a sine wave, the RMS current will
always be 0.707 times the peak current. With that said, we can calculate current by
multiplying peak measurements by 0.707 if the current is a perfect sine wave. However,
perfect sine waves are rare in most commercial and industrial applications. This is
because resistive loads in commercial applications are not linear which results in
unpredictable or variable current requirements.
In order to get a True RMS measurement, we can measure the heat dissipated by a
constant resistive load and perform the above calculations. The result is a True RMS
measurement.
Now that we got all the technical discussion out of the way, which is the best method to
calculate current? Should we 1.) measure a current average 2.) multiply current peaks by
0.707 to get an RMS current, or 3.) measure the heat from a resistor and calculate a True
RMS current value?
The most accurate way to calculate current in my opinion is a True RMS method.
Average current values often are 40% less than True RMS values and that could mean the
difference between blown circuit breakers, malfunctioning motors, or worst case,
potential fire hazards. True RMS multimeters only cost about 20-30% more than the
alternative. How much is an accurate current reading worth to you?