TRUE RMS METERS

     Basically, there are two types of current clamps commonly available: "Average Responding" and "True RMS."  "RMS" stands for "Root-Mean-Squared", which is the way of describing the mathematics necessary to figure out "True" voltage (or current.) 

If you want to understand the details, go here for the math.  Otherwise...please take our word for it.  All you really need to know is that you need a "TRMS" or a "True RMS" meter to accurately take voltage and current readings.

The average responding meters are widely used and are lower cost.  They give a correct reading for linear loads such as standard  induction motors, resistance heating, and incandescent lights. But when loads are non-linear -- those containing semiconductors -- the "average responding meters" typically read low.

     Troubleshooting branch circuits with electronic components can be difficult if you do not have the right tools. New solid-state components often conduct nonsinusoidal (distorted) or non-linear loads. In other words, the current occurs in short pulses rather than the smooth sine wave drawn by a standard induction motor. The current wave shape can have a drastic effect on an amperage reading.

     When troubleshooting a branch circuit that suffers from circuit breaker tripping (or fuse blowing), the cause of the trouble can usually be separated into one of three categories:

1. Too much current (overloaded circuit).
2. Too much heat in the electrical enclosure.
3. Faulty circuit breaker or fuse.

     Your first instinct will probably be to measure the current with an amp meter while the load is on. If the current is within the circuit rating, you may be tempted to replace the circuit breaker. Before you replace the breaker, make two other observations.

     First, analyze the load. If the load contains power semiconductors, rectifiers, SCRs, etc., be suspicious of the amp meter reading.

     Second, look at the front panel of the amp meter. Does it say "True RMS"? If you can’t find the words "True RMS" on the meter, you probably have an average responding amp meter.

     If you are trying to measure current draw by a non-linear load (containing semiconductors) without using a "True RMS" meter, you are likely to reach the wrong conclusion -- that the problem is a faulty circuit breaker. Replacing the breaker won’t help.

POWER IN AC CIRCUITS

     The power in an electric circuit does not depend on the current alone. Power is the product of both current and voltage. In direct current, or DC, circuits, power is simple to measure, since both voltage and current are constant and in the same direction. However, in an AC circuit, voltage and current are continually varying, from zero up to the maximum, then back to zero again. Therefore, the effective
voltage and current will be less than their maximum values. Values such as 120 volts and 10 amps are the Effective Values (also called Root-Mean-Square, or RMS values), which are equal to .707 times the maximum values.

     A few years ago most electrical loads were linear. Electrical loads used power on all parts of the sine wave. With the advent of electronics in lighting, computing, and variable-speed motors to name a few, things have changed. A computer power supply draws current in short pulses, often referred to as a non-linear load.

     Non-linear loads draw high peak currents, causing harmonics in the load current.  This may result in unexplained circuit breaker tripping or dangerous overheating of neutral conductors and transformers.

     Currents containing harmonics can only be measured accurately with a "True-RMS" (TRMS) instrument. A TRMS instrument will read accurately under all load conditions. 

An average responding multimeter or clamp-on meter will read a sine wave accurately as long as there are linear loads without harmonics.

     Until a few years ago, TRMS meters were very expensive. Today, they are not only more accurate and reliable, but also are also more affordable.

     An average responding multimeter or clamp-on meter will indicate amperage of a non-linear load below the actual load, except for square waveforms where it reads high. The degree of error can be as much as 45%.

CIRCUIT LOADING

     For dependability and long life of electrical equipment, most electricians prefer a circuit to be loaded to only .8 of its rated capacity. If you had a 20-amp circuit and multiplied it by .8 you would get 16, or the highest amp draw the electrician would like the circuit to handle. Industrial multipliers are lower, somewhere between .70 and .75 amps.

  &nbs p;  Why these limits? By keeping the load low in each circuit, the panel, the wiring, and connections do not overheat. They expand when heated and then contract when cooled down. This expansion and contraction loosens connections, resulting in a higher resistance connection causing still more heat.  Keeping the steady state amp draw low will also allow for inrush amp draw values associated with startup. In the case of electric motors, these values can be as high as six times the steady state or run amps for a few seconds during
startup.

CASE STUDY:

     A case study conducted at an Eddie Bauer store in St. Louis, Missouri can readily provide information to support the case for "True RMS Meters." The fluorescent lighting throughout this particular store was flicking in an intense fashion. It was annoying to the store personnel as well as the shoppers. When an individual electronic ballast was replaced upon failure, the lamps associated with the ballast seemed to operate normally again without the intense flickering. The problem appeared to be in the electronic ballast.

     Checking the electrical panel with a standard amp meter revealed no apparent overload. Was there a defective ballast, since the flickering occurred throughout most of the store? However, checking the same circuits with an Amprobe True RMS amp-meter indicated that the neutrals were overloaded. Additional neutrals were run and the loads balanced, solving the problem entirely. The cost of this solution, verses replacing all the ballasts throughout the store, represented a more than 800% savings to the customer. It should also be noted that replacing all ballasts would have only solved the problem temporarily, and the new ballast would have been damaged in time as well.

Stedi-Power, Inc
5044 B U Bowman Drive #102
Buford, Georgia 30518
PHONE: (678) 546-6780

Last Updated: 07 Jun 2004
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