Chapter 8 Basic Electricity Figure 8-21: Clamp-on ammeter Figure 8-22 Clamp-on Ammeters (Each Load is 100 ohms) 120 Volts L1 L2 Chapter 8—Basic Electricity 8-19 open switch will be the applied pressure of the power source. A voltmeter measures the difference in electrical pressure or the potential difference in emf between two points in contact with the test leads. Voltmeters are used in parallel to whatever is being tested. The two things to remember when testing for voltage on a live circuit are: There has to be resistance between the two points being measured and there must be a complete circuit to make a zero reading mean something. Measuring current An ammeter measures the rate at which the electric current flows from the power source, through the wire and load, and back to the source. There are two kinds of ammeters; in-line and clamp-on. In-line ammeters are not commonly used by service technicians in our industry. They are primarily used during bench testing. The clamp-on ammeter is easy to use. It uses electromagnetic induction. Whenever electricity flows through a wire, it creates a magnetic field around that wire. The clamp-on ammeter converts the strength of the magnetic field into a current reading. To use a clamp-on ammeter, first pick the correct scale (when in doubt start high) then open the jaws of the meter, insert one line between the jaws, close the jaws and take a reading. This meter can be used safely on a live circuit without disconnecting the power since the magnetic field is not affected by the wire’s insulation. See Figures 8-21 and 8-22. Clamp over one line at a time because the magnetic fields from each wire cancel each other out. If you placed both the hot and neutral lines between the jaws you would get a zero reading even if there was current flowing in the wires. You can test either the hot or neutral line, but not both at the same time. Since AC and DC currents create different magnetic fields, be sure to set your meter for the current you are measuring. If the current you are measuring is too low for your meter to read accurately, you can loop the wire around the jaws several times to increase sensitivity. Just divide the reading by the number of loops. Ten loops is an easy number to use.
NORA Oilheat Technicians Manual
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