The alternating current (AC) clamp-on ammeter is used to measure the total AC current being drawn by a system or by the individual loads in a system, such as the compressor, fan motors, or heaters. In three-phase systems, it is used to measure the current drawn by each phase to determine the percent of current imbalance.
The main advantage of the clamp-on ammeter is that it allows current measurements to be made without disconnecting any of the circuit wiring. Figure 1-22 shows a clamp-on ammeter connected to measure current. C lamp-on ammeters have a movable set of jaws that can be opened and placed around each of the wires to be measured, one wire at a time. The clamp-on ammeter works like a transformer. The wire being measured acts like the primary of the transformer and the jaws of the ammeter as the secondary. Current flowing through the wire creates a magnetic field that induces a current in the jaws of the clamp-on ammeter. This induced current passes through the meter movement, providing an indication of how much current is passing through the wire . Even though most clamp-on ammeters can also measure voltage and low-level values of resistance, their primary use is for current measurement.
Both analog and digital clamp-on ammeters are available, and are usually capable of measuring up to 300 amperes. Digital meters (Figure 1-23) are popular because of their accuracy and ease of use. Generally, the accuracy of the ammeter increases if it h as a number of selectable current ranges. When servicing three-phase equipment where the current imbalance must not exceed 10 percent, the accuracy of the ammeter is extremely important. Analog mete rs with a full-scale accuracy of ±3% and dig ital meters with an accuracy of ±2% are typical Other useful features include rugged construction, the ability to capture and h old surge currents, and protection against meter damage caused by over-ranging.