Copyright Goodheart-Willcox Co., Inc. Chapter 10 How Electric Meters Work 179 10.2.2 Current Measurement Most HVACR multimeters’ in-line current measurement capacity is limited to the dc μA range. However, multimeters used for HVACR direct digital controls can measure in-line ac μA, dc μA, and mA ranges. Some multimeters can measure up to 10 A ac and dc in-line currents by using larger shunts. Currents larger than 10 A are not com- mon for portable meters. The inductive clamp (or clamp-on meter) is the safe and simple way to measure the larger current draw of motors. Figure 10-13 illustrates the meter configuration for measuring current. The meter is in series with the load, so that load current flows through the meter. The red probe connects to the power source. Some of the current flows through the fuse and feeds the analog to digital converter, while the remaining larger current flows through the shunt and to the load. When the inductive clamp is used, the small-induced current is first amplified and then sent to the A/D. It is processed the same as in-line current measurement. What is OL? OL, shown as O.L on the meter, means “over-the-limit,” “over range,” or “over load.” It means that the measurement taken is beyond the meter’s capacity limit for the specific function. For example, the highest resistance value a meter can measure is 40,000,000 Ω, so any value measured over that amount is displayed as OL. A technician should know and understand these values by reading the owner’s manual specifications. Pro Tip Load 3V Power source In-line measurement Inductive clamp Magnetic field A/D Fuse Amplifier Microprocessor 6.120 µA Shunts Goodheart-Willcox Publisher Figure 10-13. A digital multimeter configured to measure current by in-line or inductive clamp method.