The significance of True RMS

The rapid changes in technology have meant that methods previously used for testing equipment are no longer accurate in many cases. Because an electrician comes across new technology on a regular basis, not to have a True RMS (root mean square) meter can be compared to a boxer going into a fight with one arm tied behind his back. In this article, Phil Collins, Engineering Support Manager, Techtronics Australia, explains what True RMS (TRMS) is and how to find it.

True RMS means that the meter’s voltage and current readings that you are observing are not dependent on the wave form. Other meters such as averaging and peak meters do rely on the wave form being sinusoidal. The TRMS meter takes into account the wave form and therefore gives the correct reading for the current and voltage.

TRMS readings are the DC equivalent reading you would obtain if the equivalent load were being supplied with DC. By DC equivalent we mean the voltage and current readings result in the same power consumed in the AC circuit. Using a non-TRMS meter to test these non-sinusoidal waveform currents could result in an error of up to 40%.

When I was a youngster there were no electronic calculators, so we all used slide rules. In fact, in those days there was very little in the way of digital electronic equipment. Times have changed. Nowadays, in order to get more efficiency, all sorts of technology is being used. An example of this is the use of inverters to supply motors. Inverters are more efficient, but the down side is that they often distort the wave forms quite radically, which means that if you do not use a TRMS meter, the readings you take will probably be wrong - up to 40% wrong.

If you are taking a reading of the current draw on a motor that is being driven through an inverter, and it is running at less than full speed, the wave form is being distorted but you don’t know it. The current ratings shown on the motors rating plate say one thing and the non-TRMS meter would be reading that the motor is not overloaded, when in fact it could be.

All systems have circuit breakers and/or other protective devices, but if the meter is not reading the current correctly, the fuses could be underrated and could rupture, or the circuit breaker trip. As stated, you could have an overloaded motor and not know it. In the case of three-phase systems, you would normally check the current of all three legs and if they are in balance you would think that’s okay. With the non-TRMS meter they could be balanced and have low readings. However, this is inaccurate and they may be tripping the circuit breaker, but you might not know that.

Are TRMS meters readily available?

A number of electricians are using the wrong meters. While TRMS meters are readily available, you have to ask for them. If the meter is TRMS, that information will be clearly indicated on the packaging and on the meter itself.

If it is not TRMS, the information brochure and packaging will not alert you to the fact.

So an electrician should ask the question. “Is this meter True RMS?”

Importance of having TRMS

It is essential to have TRMS meters now because, with the added emphasis on carbon reduction schemes, all sorts of new technology is being tried to reduce power consumption.

The use of electronics has increased dramatically and we are seeing more and more distortion of wave forms. This may not be a problem with smaller equipment, but larger equipment will definitely distort waveforms and introduce harmonics. These harmonics affect everything and only TRMS meters, unlike averaging meters, are capable of reading these. Why is this important? Because if an electrician is making calculations for protection sizing and they are actually under-reading the current (which causes heat), they will be wrong in all of their measurements and they will be left wondering why it is all not working out.

Do computers and other equipment affect machinery?

Yes, when you are dealing with digital electronics, any external load on the circuit will make the circuit change. We saw that in the good old days when we used analog meters with the needle. Just the way they were built meant that they would ‘load up’ circuits.

Even placing a small adhesive label on the tracks on a circuit board made it appear as another circuit and that would throw the whole thing out. This is particularly the case with C-Mos and even more advanced technology with extremely high resistance. The modern digital meters have extremely high input impedance. In the old days, the analog meters had around 10,000 Ω per volt, whereas these days it is more like 10m Ω. The input impedance just has to be this high to prevent the meter from loading up the digital electronic circuits, changing the way they operate.

Examples of where TRMS meters should be used

TRMS meters should be used in any area or situation where the sine waves could be affected by other technologies and could be distorted: for example, with HVAC systems, as well as industrial motors with inverters or heat pumps.

In fact, they should be used wherever a lot of high tech units are being employed. When I first started working, nearly everything was electro-mechanically controlled, whereas nowadays, just about everything interfaces and is computer controlled. Even your toaster has built in RCD.

I have tested so many meters and I can recommend the Milwaukee TRMS meter because of its nine key features. Externally, the meter has a rubber protective covering and is also dust- and waterproof which is not common among meters. Other features include: IP rating of IP54 (requiring only one hand to change the settings); a backlight to help in reading the display - on most models the display background becomes black and the numerals remain white for clearer reading particularly in cabinets or darkened areas; it has a recess on the back so that you can clip a voltage detector to it; you can clip a magnet onto it to allow for hands-free use; there is a work light for ease-of-use in darkened areas; and frequency measurement, important because of the increased use of microprocessors.

Low Z voltage measurement is another feature. Because these meters have such high input impedance they don’t ‘drain’ any voltage being produced by capacitive coupling.

This is important because in the case of a cable that is open circuit, but running in the same conduit or cable tray as live wires, if you use a meter with no low Z, it will show that voltage is present when in fact there is no voltage. The low Z function will show that the cable is actually ‘dead’ whereas, due to capacitor coupling, it would normally show it as being live. What you do in this Milwaukee meter is select low Z and the meter drops the input impedance and any voltage that is induced in the non-connected cable will be drained to give a zero reading.