The higher the ohms/volt rating of a voltmeter, the less the voltmeter will upset circuit conditions. And the less circuit conditions are upset, the more accurate the reading will be. Most of the higher-end voltmeters and multimeters available now are rated at about 20,000 ohms/volt; more accurate voltmeters are rated at 100,000 ohms/volt. In some of the high-resistance circuits found in some present-day equipment, however, even a meter rated at 20,000 ohms /volt will greatly upset circuit conditions, and result in an incorrect reading. While a 100,000 ohms/voltmeter will give more accurate readings, even more accuracy is needed with some circuits. To overcome this problem, a device with a high ohms/volt rating called an electronic voltmeter was developed.
Because many insulation breakdowns occur during operation under the stress of high voltages, more dependable tests are performed with a megger (megohm-meter) and with hi-pot (high potential or high-voltage) equipment. The megger is a tiny, portable instrument that can be battery operated or powered by a hand-cranked generator. Meggers supply from 500 to 5000 volts for the insulation test. They measure leakage current, but the readout is calibrated in insulation resistance (ohms), in ranges to 100 and often 2000 megohms (MQ). The megger can measure insulation resistance between wires, between wires and ground, and between wiring and casings. Normal insulation resistance depends on the wire, insulation, length, and rating by the manufacturer. The greater the leakage, the more defective is the insulation. The wire manufacturer’s rating should be the guide.
With voltage measurements, the total resistance of the voltmeter must be considered. Reading an analog voltmeter scale is like reading an analog ammeter scale. Some multirange voltmeters have only one range marked on the scale and the scale reading must be multiplied by the range switch setting to acquire the accurate voltage reading. Other voltmeters have individual ranges on the scale for every setting of the range switch. When utilizing these meters, ensure that you read the set of values that corresponds to the range switch setting. Many digital meters also have range switches, but they may additionally have a special feature known as autoranging. This means you can either utilize the range switch or let the digital meter set the proper range by itself.
Current-measuring meters are called ammeters. There are two ways of measuring the current flowing in a circuit. The first way is to open the circuit being tested and link the meter test leads into the circuit so the current flowing in the circuit flows in series through the meter as well. This is the in-circuit-type of ammeter sometimes referred to as an in-line meter. An external type of ammeter, known as a clamp-on ammeter, can be used without opening the circuit. The clamp-on ammeter has jaws that open and clamp around the sides of one wire in the circuit and measures the magnetic field that the current in the wire produces, to determine the current flow. The clamp-on ammeter is widely used and simplifies the measurement of current from one to hundreds of amperes. It is especially helpful for field testing and troubleshooting AC power lines.
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Few things are more irritating than cumbersome stacks of differing electronic equipment that must be realigned and reconnected in order to perform routine tasks. The endless racks of boxes and miles of winding cable can be like navigating the Amazon jungle. It’s a small wonder that such common testing scenarios are prompting company engineers to search for better solutions.