Measure open-circuited cables using a multimeter
Edited by Bill Travis
Ron Duffy, Agilent Technology, Colorado Springs, CO -- EDN, September 26, 2002
You can use a multimeter with capacitance-measurement capability to measure the length of wire or cable to an open circuit. The capacitance of a pair of wires (or a wire to a shield) is directly proportional to the length of the wire. If you know the capacitance per foot of wire, then you can calculate how far it is to the open circuit. To determine the capacitance per foot, take a trip to the hardware store, use a known length of wire, and measure the capacitance with the multimeter. You must zero the multimeter's capacitance function before making any measurements. To zero the meter, select the capacitance function and separate the leads. Then, make the measurement and simply divide the total capacitance of the wire or cable by the total length to determine the capacitance per foot. Once you have this figure, and you're sure that the suspect wire or cable carries no power or signals, you can measure the capacitance, C. The wire length in feet to the open circuit is C divided by the capacitance per foot.
The resolution depends on the range and resolution of the multimeter. The lowest range of a multimeter is usually 10 nF. The resolution in that range is 10 pF. The multimeter steps up by a factor of 10 each time it changes range. The higher ranges allow you to measure longer wires, but, as Table 1 shows, the resolution decreases by a factor of 10 for each higher range. (Note the measurements on wires longer than 1000 ft.) All but the last three entries in Table 1 measure the lengths of wire or cable and then calculate. The last three entries are from a coaxial-cable chart.
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