408 Electrical Systems Th e RMS value is important because a VOM mea- sures in RMS. A DMM can measure in peak voltage if it does not convert to RMS. Without any type of conver- sion, a DMM would read a voltage much higher than would a VOM. It is not diffi cult to convert the peak value of a sine wave to RMS. You simply multiply the peak voltage value by 0.707 to obtain the RMS value. Th e problem is that with modern industrial electronics, the voltage may not be in a sine wave pattern. Th ree-phase variable frequency motor drives do not produce a sine wave output. Switching power supplies and other electronic devices distort the normal sine wave of the AC line power. For this reason, true RMS algo- rithms (a part of a computer program) are used. With true RMS, the DMM is capable of measuring the RMS value of most any type of waveform. T E C H T I P True RMS DMMs If you will be working with industrial electronics, be sure to have a DMM that reads true RMS in order to ensure your readings are correct. Phantom Voltages Phantom voltages, also called ghost voltages, are DMM readings of electrical potential or voltage between con- ductors that have no actual voltage diff erence. Th ese readings are a result of induction or IR drop (resistance) across conductors that are connected to the same point but have diff erent resistances. In addition, the amplifi ers inside the DMM are very sensitive, and the DMM will auto-range until it can obtain some sort of reading, even if that reading is so small as to be negligible. Phantom voltages can range from a few millivolts to hundreds of volts depending on the situation. Th e DMM inputs have an extremely high impedance (oppo- sition to the fl ow of alternating current). Th is high impedance means that, although the phantom voltage might be quite high, its current is extremely low, pos- sibly in the microamp realm. Th e high impedance of the DMM does not present much of a load, so even these extremely small currents will cause the DMM to display a phantom voltage reading. An example of a phantom voltage reading is shown in Figure 21-16. T E C H T I P Phantom Voltage Readings Be careful when interpreting voltage readings and pay close attention to the range indication. Does the range show mV or does it indicate V? False interpreta- tion might lead you to believe there is a real voltage present when it is only a result of a phantom voltage. 21.3.5 Clamp-On Ammeter DMMs have a limited range for measuring AC current. Most DMMs are capable of safely measuring a maximum current of 10 A. Another drawback of using a DMM to measure current is that the DMM must be series- connected. Th is requires disconnecting a current-carrying conductor and placing the DMM into the circuit. A clamp-on ammeter eliminates these shortcomings by providing a noncontact method of measuring current. See Figure 21-17. To accomplish this, some clamp-on ammeters use a current transformer that can be opened to insert the conductor to be measured. No disconnection or physical contact with a live connection is required. Th e magnetic fi eld around the conductor to be measured is coupled into the clamp-on ammeter’s current transformer. Because older clamp-on ammeters use a current trans- former, they are limited to measuring only AC currents. Display indicates millivolts Goodheart-Willcox Publisher Figure 21-16. A DMM displaying a phantom voltage. Be careful in interpreting the reading. Notice the mV indication in the display. The reading is 6.3 mV, not 6.3 V. Copyright Goodheart-Willcox Co., Inc.