Copyright Goodheart-Willcox Co., Inc. Chapter 17 Electrical Troubleshooting Fundamentals 377 6 including many control circuits, are often measured in low dc voltage, from 1 V to 25 V. 17.1.2 Ohmmeters An ohmmeter is a meter used for measuring the electrical resistance in circuits. In addition to measuring specific resis- tances, ohmmeters may also be used to check for short cir- cuits, open circuits (circuit continuity), and shorts to ground (ground faults). An ohmmeter is connected in parallel with a load or circuit after applied voltage has been turned off and locked out, Figure 17-3. An ohmmeter applies its own voltage to the load as part of its measuring process. This known volt- age is provided by a low-voltage battery inside the meter. Voltmeter Range Switch When measuring voltage, always set a voltmeter’s range switch to a higher voltage setting than is expected from the circuit. For example, when checking a 120 V circuit, set the range switch to the 240 V setting. This prevents damage to the meter if there is a voltage surge or higher than expected voltage. Caution Ohmmeter Usage Be certain to turn off a circuit’s power before taking resistance readings with an ohmmeter. Otherwise, the instrument may be ruined. Take voltmeter readings across switches and electrical loads in series with the intended resistance reading to ensure that no voltage is present. Caution When measuring the resistance across an open switch, the reading should be either ∞ (infinity) or OL (open loop). This means that the resistance is so high that the meter cannot measure the resistance value. When checking the continuity of a conductor, the ohmmeter reading should be zero or nearly zero. If the meter reads near zero (0 Ω), it means there is no resistance, which indicates that the conductor is not broken and that all the connections are good. If the meter reads ∞ (infinity) or OL (open loop), the conductor has an open. 17.1.3 Ammeters An ammeter is an instrument that measures current in amperes in a conductor. There are two types of ammeters: clamp-on ammeters and in-line ammeters. A clamp-on ammeter makes it easy to measure alternating current because it is not necessary to disconnect wires or attach leads to obtain a reading. In-line ammeters require the circuit to be opened and then have the meter connected in series with the circuit. Most electrical work in HVACR can be performed with a clamp-on ammeter, Figure 17-4. Clamp-on Ammeters A clamp-on ammeter is an ac ammeter that senses and measures current based on the magnetic field produced by the alternating current flowing through a conductor. In the same way that a transformer uses a primary coil to induce current in a secondary coil, the current flow- ing through a conductor induces current in the coil of wire inside the clamp-on ammeter’s jaws. The jaws can be opened, slipped around a conductor, and closed again. The current generated in the jaws by the conductor’s elec- tromagnetic field is measured by the ammeter and shown on a digital display. Clamp-on ammeters often have other functions as well, such as settings for measuring voltage and resistance, Figure 17-5. Only a single wire of a circuit should be placed in the jaws of a clamp-on ammeter. Current in an ac circuit flows through the ungrounded (hot) and grounded (neutral) wires to an electrical load and back. The current measured Ohmmeter Measurements Before measuring resistance in a device, refer to the device’s specification to find the expected resistance value. Whenever possible, disconnect at least one side of the device from the rest of the circuit before measuring its resistance. This will ensure that the ohmmeter reading will be across the intended device and not through the rest of the circuit. Be familiar with a circuit’s electrical diagram to avoid accidentally taking any unintended or false readings. Pro Tip Power source Switch Ohmmeter Heating element Light Ω3 Ω1 Ω2 Ω4 50Ω OL or ∞ 0Ω 10Ω Goodheart-Willcox Publisher Figure 17-3. Connecting an ohmmeter in parallel enables a technician to check the continuity of a conductor (Ω1) or to measure the resistance across a device, such as a lamp (Ω 2 ), a heating element (Ω 3 ), or an open switch (Ω 4 ).