Chapter 21 Electrical Test and Measurement Equipment 409 Modern clamp-on ammeters use a Hall eff ect sensor instead of a current transformer. A Hall eff ect sensor is a semiconductor device that is sensitive to magnetic fi elds. Hall eff ect sensors allow a clamp-on ammeter to mea- sure both AC and DC current. In general, low-cost models use a current trans- former and more expensive models use a Hall eff ect sen- sor. When purchasing a clamp-on ammeter, consider whether the capability to measure DC current is needed. Many models have a built-in DMM included to save the technician time and bother switching from one test instru- ment to another in the middle of troubleshooting a problem. T E C H T I P Low Current Measurements Clamp-on ammeters suffer from poor accuracy when measuring low current values. This problem can be minimized by placing a second turn of the conductor being measured through the clamp-on core. When this is done, the reading obtained will be twice the actual current in the conductor, so you must divide the reading by two. Higher cur- rents do not present this accuracy problem. 21.3.6 Megohmmeter A megohmmeter is an ohmmeter that uses high voltage to make resistance measurements. An example of a meg- ohmmeter is shown in Figure 21-18. Megohmmeters are also called meggers or insulation testers. Most DMMs can measure resistance in the megohm range. However, DMMs use a low voltage to accomplish this task. In some cases, shorts may not be readily appar- ent when tested at a low voltage. Voltage ratings and CAT levels Input jack Test mode selector Clamp opens to be placed around conductor Ratings and CAT level for clamp-on ammeter Trigger opens clamp Goodheart-Willcox Publisher Figure 21-17. This clamp-on ammeter is capable of measuring AC or DC current. In addition, it has a DMM for measuring resistance, AC voltages, and DC voltages a diode test function a capacitor test function and a noncontact voltage detector. Select voltage to be applied Press to energize component being tested Goodheart-Willcox Publisher Figure 21-18. Megohmmeters are also known as meggers or insulation testers. The range switch is set to the proper range, test leads are connected to the device under test, and then the test button is pressed to energize the high voltage and make the measurement. Copyright Goodheart-Willcox Co., Inc.