Measuring Circuit Quantities (Mod.3 Sect.1 Part 2)

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Measuring Resistance in Series Circuits

Caption: : In this illustration, the resistance of a resistor is being measured with a multimeter. The multimeter probes are connected in parallel with the resistor’s leads.

In order to measure resistance in a circuit, you must connect the probes of a multimeter in parallel with (across) the component or device. In Figure 8, a simple resistor is being tested. Note how the multimeter probes are connected in parallel with the resistor’s leads. Resistance readings must be taken on a deenergized circuit! Whenever you’re making resistance measurements, be sure that the power has been disconnected from the circuit.

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Measuring Resistance in Parallel Circuits

When you’re using a multimeter to test a resistor in a circuit, it’s important to remember the series and parallel circuit connections we discussed earlier. For example, look at Figure 9. In the figure, a multimeter is being used to measure the resistance value of two heaters that are connected in parallel. Note that the probes of the multimeter are connected in parallel across the resistors, which is correct. However, because the two resistors are connected in parallel, the reading you see on the multimeter will indicate the combined resistance of both resistors.

Caption: : In this figure, a multimeter is being used to make a resistance measurement. Because these two heaters are connected in parallel, the meter is actually measuring the combined resistance of both heaters. To measure the resistance of just one heater, the parallel connection would have to be broken.

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Measuring Voltage Phase-to-phase

Caption: : Shown here is a typical three-phase motor-starter circuit.

On some AC systems, you’ll be measuring not only voltage-to-ground but also voltage phase-to-phase. Look at the motor-starter circuit shown in Figure 15. In this circuit, 440 VAC, three-phase power is provided by a source to a disconnect (not shown). The motor-starter circuit taps power from the disconnect and places the three-phase power on three fuses. Power from the fuses goes to a motor starter or contactor that consists of a coil, three pairs of contacts, and a thermal overload relay system. This is a three-phase wye connected circuit. The voltage between any two phases will be 440 VAC. The voltage between any one phase and the grounded neutral will be the phase-to-phase voltage divided by 1.73; which in this case will be 254 VAC. Power from the thermal overloads is then fed to the motor. When the control circuit powers the coil, the contacts close and the motor turns.

Measuring Voltage Phase-to-phase

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If there’s a problem with such a circuit, the best place to perform a voltage test is at the fuses as shown in Figure 16. This voltage test consists of three voltage measurements. The first voltage measurement is made from the load side of fuse A to the load side of fuse B. (The “load side” is the side closest to the load, which is the motor in this circuit.) Note the position of the meter probes in the figure. At this test point, you should measure 440 VAC. Next, measure the voltage from the load side of fuse B to the load side of fuse C. At this test point, you should again read 440 VAC. Finally, measure the voltage from the load side of fuse A to the load side of fuse C. At this test point, you should again read 440 VAC. If a fuse is open, the voltage tests will produce readings of only 254 VAC (or slightly less). So, for example, if fuse A is open, the readings from fuse A to B or A to C will be about 254 VAC. The output of the contactors can also be tested using a similar series of voltage measurements when the contactor’s coil is energized. The first voltage measurement is made from the load side of contact A to the load side of contact B. Next, measure the voltage from the load side of contact B to the load side of contact C. Finally, measure the voltage from the load side of contact A to the load side of contact C. At all three test points, you should read 440 VAC. If you measure 254 VAC at any point, a contact inside the contactor may be bad or a thermal overload element may be open.

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Measuring Voltage Phase-to-ground

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Caption: : A phase-to-ground test should reveal full-line voltage at the base of each fuse or at the output terminals of the motor starter.

Voltage tests may also be made from phase to ground. The black meter lead would be placed at ground and the white lead would be moved to the other circuit points (the load side of each fuse and the load side of each contactor). The voltage measured at the load side of each fuse or at the output terminal of a contactor should be approximately 254 VAC. (If this were a single-phase 220 VAC system, like the electrical system in most residential installations, you should measure 110 VAC from either fused hot line to ground.)

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Measuring Current

Caption: : A current measurement will quickly locate an open heater in a three-phase system, such as the one shown here.

Measuring Current

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Caption: : When measuring current, you must break open the circuit and connect the meter leads in series with the other circuit components.

Current measurements are quite different than resistance or voltage measurements. When you’re making resistance and voltage measurements, you connect the meter’s test leads in parallel with a component. In contrast, when you’re making current measurements, you connect the meter’s test leads in series with the components or components under test. A typical ammeter connection is shown in this figure. As you can see, this circuit has been opened and the meter’s leads have been connected in series with the other circuit components to complete the circuit. When taking current measurements, like voltage measurements, you must work around an energized circuit. Therefore, always pay attention to what you’re doing and avoid contact with energized circuit components. These energized components can create a lethal shock hazard! A much safer way to measure high currents in AC and DC circuits is to use clamp-type probes. These probes measure the electromagnetic field around conductors rather than the actual current flow through them. By using these probes, you don’t need to open the circuit and connect the meter leads in series with the other circuit components.