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NORA Oilheat Technicians Manual

Chapter 8 Basic Electricity Figure 8-23: Ohmmeter readings No resistance (short) To understand what your ammeter is measuring, you should know: • The type of circuit • The design ampacity (current carrying capacity) of the components • The design current draw of the loads. The design ampacity is usually marked on each component and must be higher than the amp rating of the fuse protecting that component. For example, a 15-amp fuse should protect components exposed to 120 VAC. The design current draw of a load is usually listed on the rating plate. The listed design current draw, in amps, is for steadystate normal operation. On start-up and under increased motor load, there is a higher required torque and a slower operating speed, the motor current will be higher. The surge current draw on initial startup of a motor may be four or five times its normal current draw. Sometimes the surge current draw is also listed on the rating plate. If a burner motor bearings are dry, or the oil pump has rust in its gear set, the motor Measurable resistance 8-20 Basic Electricity Infinite resistance (open) will not be able to function normally and the current draw might be less than the listed rating. Measuring resistance The ohmmeter is used to measure resistance. It measures the resistance between two points. It can measure just one load or a whole circuit. The ohmmeter has its own electric source (a battery). Since it provides its own power, disconnect the circuit or device to be tested from the power source. It measures the resistance between two points by applying a steady voltage from the meter’s battery to a de-energized circuit or device. Figure 8-23. You can use the ohmmeter to see if theres is a complete circuit. This measurement is called continuity. If you touch one ohmmeter lead to a wire disconnected from the power source going into a circuit and, at the same time, touch one of the leads to the wire coming out of the circuit and there is no current returning to the meter, it means the pathway between the two wires is broken. The ohmmeter will read infinite resistance and there is no continuity. If, on the other hand, 1 amp of current returns for every volt applied to the circuit, there is no resistance. The ohmmeter will read zero. This is a short circuit. Any reading between zero and infinite is the resistance to flow for the circuit measured. Unlike the voltmeter and ammeter, the ohmmeter must never be connected to a live circuit. It supplies its own power from its battery, and it will be damaged if outside current goes through it. To use the ohmmeter, disconnect the power supply and isolate the line or device


NORA Oilheat Technicians Manual
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