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Electricity and magnetism (Part 2)

By Grant Laidlaw

Understanding the electrical aspects of refrigeration is essential. Read the first part of this article in the September/October edition of Cold Link Africa.


Although many of the clamp-on ammeters have connections to measure the voltage and the resistance, the voltage and the resistance are usually measured with a multimeter. This traditional type of instrument is illustrated in Figure 1, and has a needle that points at the voltage measured. It’s called an analogue type meter.

Figure 1: The clamp-on ammeter and the method of measuring. Modern clamp-on ammeters are also available with digital read-out.

Modern multimeters are often of the digital type, indicating the measured values in digits. Two-metre leads are required to connect the voltage to be measured to the meter. In the case of a single-phase power supply, these are the phase and the neutral or the phase and the earth. When the power supply is three-phase, the voltage can be measured from phase to phase or from phase to neutral. The normal voltages measured were discussed previously.

By using a switch on the meter, different voltage ranges can be selected. Always select a range that will put the reading towards the middle and upper third of the scale, if possible. When in doubt of the expected reading, start with the highest range and progress downward to obtain the most accurate reading.


The ohmmeter is usually incorporated in the ammeter or the multimeter. The ohmmeter is different from the ammeter and the voltmeter in that it obtains current from a little battery inside the instrument or in the ohm probe as used with the clamp-on ammeter. The meter current source is connected through the meter probes to the component or circuit being tested.

The resistance of the component or the circuit being tested will determine the amount of current flow through the meter movement. The higher the resistance, the lower the current or the meter deflection. Consequently, the meter needle doesn’t swing much for high resistance, but it does swing a lot for low resistance. In other words, the needle action is opposite to that of the voltmeter and the ammeter.

When connecting the ohmmeter it must be remembered that it operates from the small current of the battery, and connecting the ohmmeter to an external voltage will destroy the meter. So, always turn the power off before measuring resistance.

Some more expensive meters have fuses that will prevent the meter from being destroyed, and some digital meters even automatically switch to voltmeter mode when connected to an external power source.

Figure 2: An ohmmeter showing the component or circuit resistance.

Figure 2 illustrates the three readings that can be obtained when checking a circuit or a component with an ohmmeter:

  1. Shows a closed switch or contact — this will give a full-scale deflection on the meter, indicating zero ohms.
  2. Shows a good coil that will have a measurable resistance, which provides less needle deflection and a specific ohm reading on the meter.
  3. Shows a relay coil with a broken wire. In this instance, there is no path for the ohmmeter current to flow and there will be no deflection of the needle. Since there is no path, it could be said that the resistance is so high that it cannot be measured. The term ‘infinity’ (meaning too high to measure) is often used to describe an open circuit.

The digital meter will, of course, show the actual resistance of the circuit, including the resistance of the wires connecting the meter to the component or the circuit. This means that when the connection is not 100% this will show up on the meter.

When the operator’s hands touch both wires, the meter will show the resistance of the human body.

To measure the resistance of the insulation on the components, the use of an insulation tester is preferable. This instrument also measures the resistance, but instead of applying only 1.5V to the circuit, it applies approximately 500–1 000V to the component. As this resembles the actual voltage applied when the component is in a circuit the measurement is more accurate. Very often, an earth fault that the ohmmeter did not detect will show up when the test is repeated with an insulation tester.

Queries or questions?

Should you have any queries or require assistance, please contact ACRA’s Anria Pieterse (SDF) on +27 (0) 11 609 1118.




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