AMMETERS A meter used to measure the flow of electric current is a current meter. Current meters that measure current in amperes are called ammeters. The ammeter is connected in series with the circuit source and load. Panel-mounted ammeters, such as those used in power plants, are permanently wired into the circuit. Figure 7-17 shows two typical panel-mounted ammeters. Portable ammeters are temporarily connected into a wiring system at whatever point in the system a current reading is desired; for example, feeder current is measured by opening the feeder and wiring the meter in series with the feeder source and load. Circuits branching off the feeder may be opened and an ammeter inserted into the branch. Using a clamp-on ammeter (fig. 7-18) is an exception to the rule previously stated requiring ammeters to be series-connected. The clamp-on ammeter consists in part of clamp-on transformer jaws that can be opened and placed around a conductor. The

Figure 7-17.- Typical panel ammeters.

Figure 7-18.- Clamp-on ammeter. jaws are actually part of a laminated iron core. Around this core, inside the instrument enclosure, is a coil winding that connects to the meter circuit. The complete core (including the jaws) and the coil winding are the core and secondary of a transformer. The conductor, carrying the current to be measured, is like a primary winding of a transformer. The transformer secondary is the source of power that drives the meter movement. The strength of the magnetic field surrounding the conductor determines the amount of secondary current. The amount of secondary current determines the indication of current being measured by the meter.

All ammeters will have an adjustable scale. The function and range of the meter are changed as the scale is changed. To take a current measurement, turn the selector until the AMP scale you wish to use appears in the window. To take measurements of unknown amounts of current, you should rotate the scale to the highest amperage range. After taking the reading at the highest range, you may see that the amount of current is within the limits of a lower range. If so, change the scale to that lower range for a more accurate reading.

After choosing the scale you want, depress the handle to open the transformer jaws. Clamp the jaws around only one conductor. The split core must be free of any debris because it must close completely for an accurate reading. To measure very low currents in a small flexible conductor, you may wrap the conductor one or more times around the clamp-on jaws of the meter. One loop will double the reading. Several loops will increase the reading even more. After taking the measurement, divide the reading by the appropriate number of loops to determine what the actual current value is.

The clamp-on ammeter is convenient and easy to use. To measure the current of a single-phase motor, for example, simply rotate the selector until the desired amp scale appears, clamp the jaws around one of the two motor conductors, and take the reading.

Some clamp-on instruments are capable of more than one function; for example, they are designed for use as an ohmmeter or a voltmeter when used with the appropriate adapter or test leads.