Copyright Goodheart-Willcox Co., Inc. Chapter 10 How Electric Meters Work 183 10.3.3 Capacitor Test The meter is configured to charge the capacitor under test and record the charging time. Recall that current stops flowing when the capacitor is charged. The meter then discharges the capacitor and that time is recorded. The microprocessor uses the charge and discharge times to calculate capacity, which is displayed as a value in microfarads. An open or shorted capacitor is displayed by OL. The capacitor test takes several seconds to complete. Larger capacitors take longer. Some meters dis- play dashes while the test is in progress. The displayed value should be compared to the capacitor nameplate value. 10.3.4 Frequency and Duty Cycle While the meter is set in voltage mode, the A/D and processor analyze the mea- sured signal to derive the frequency of the ac voltage. Most meters require press- ing a switch to engage this feature. Duty cycle is displayed as a percentage of on-versus-off time for a square waveform used in pulse width modulation. This means the power is turned on and turned off for specific time periods to control the operation of a load. This topic will be explored in Chapter 12, Switches, Electronic Components, and Sensors. 10.3.5 Noncontact Voltage (NCV) The noncontact voltage (NCV) feature checks for the presence of voltage without using the test probes. Instead, it uses magnetic induction to sense voltage. An antenna is typically placed at the inductive clamp jaw, Figure 10-18. A live wire induces a cur- rent in the antenna. The meter annunciates the live wire by sounding a buzzer and a lamp. Most meters use a dedicated push button for this function. 10.4 Test Leads Test leads are the conductors connected to the meter to make measurements. The test leads contain plugs to connect to the meter and probes to connect to the test points. Test leads must be very flexible since they are bent and twisted during normal use. While test leads typically use 18 AWG stranded wire, it is made of about 190–40 AWG strands. Properly Discharge Capacitors Always properly discharge capacitors with a 20,000 Ω 2 W resistor before testing. Safety Note Checking Prefixes Check the prefix displayed. Be aware of displayed values in the nanofarad (nF) range when the measured capacitor value should be in the microfarad range. Pro Tip Always Check Actual Voltage The NCV is for quick reference only. Always check actual voltage with the voltmeter function. Ensure that power is off before working on the circuit. Safety Note Goodheart-Willcox Publisher Figure 10-18. NCV – noncontact voltage antenna NCV antenna NCV function