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Question about measuring ground resistance. Proper Method?
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<blockquote data-quote="maylar" data-source="post: 8260528" data-attributes="member: 541144"><p>An ohmmeter measures resistance by putting a controlled constant current through the circuit and measuring the voltage across it. Then the processor makes the calculation to resistance. This current is very small - milliamps - and any other current flowing through the wire will throw the measurement off. The circuit MUST be powered down.</p><p></p><p>Making low resistance measurements with a 2 wire DMM is inherently inaccurate anyway, because the resistance of the meter's leads is a couple tenths of an ohm and meters are usually +/- 1 digit and +/- some percentage so that last tenth of an ohm is meaningless. And unfortunately, a tenth of an ohm at 20 amps is 2 volts drop, which we care about.</p><p></p><p>There are fancy 4 wire ohmmeters that compensate for lead resistance, but for our purposes a voltage measurement under actual load is more useful than making resistance measurements of grounds. Pick a good ground point for the neg meter lead and touch the bolts and terminals along the ground wire connections. You should never see more than about a tenth of a volt difference if everything is tight.</p></blockquote><p></p>
[QUOTE="maylar, post: 8260528, member: 541144"] An ohmmeter measures resistance by putting a controlled constant current through the circuit and measuring the voltage across it. Then the processor makes the calculation to resistance. This current is very small - milliamps - and any other current flowing through the wire will throw the measurement off. The circuit MUST be powered down. Making low resistance measurements with a 2 wire DMM is inherently inaccurate anyway, because the resistance of the meter's leads is a couple tenths of an ohm and meters are usually +/- 1 digit and +/- some percentage so that last tenth of an ohm is meaningless. And unfortunately, a tenth of an ohm at 20 amps is 2 volts drop, which we care about. There are fancy 4 wire ohmmeters that compensate for lead resistance, but for our purposes a voltage measurement under actual load is more useful than making resistance measurements of grounds. Pick a good ground point for the neg meter lead and touch the bolts and terminals along the ground wire connections. You should never see more than about a tenth of a volt difference if everything is tight. [/QUOTE]
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Question about measuring ground resistance. Proper Method?
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