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Old 02-19-2012, 06:47 PM   #1
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Volt Meter Testing Question


I have a question about testing with a volt meter. While I was at work I was watching someone test with their meter inside of a panel. This panel had AC and DC circuits. Obviously with one lead you would have to go to the positive wire, but for the other wire he would put his lead on either the ground or sometimes the negative wire. On a DC system, would you test the + to ground or would you test from + to -? I guess I have the same queston from AC also.

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Old 02-19-2012, 07:00 PM   #2
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Volt Meter Testing Question


For DC he would be checking the Negative - and the Positive + For AC he would be checking between the Hot wires and neutral or ground.

Could have also been checking for resistance or load with amp meter.

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Old 02-19-2012, 07:16 PM   #3
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I have a question about testing with a volt meter. While I was at work I was watching someone test with their meter inside of a panel. This panel had AC and DC circuits. Obviously with one lead you would have to go to the positive wire, but for the other wire he would put his lead on either the ground or sometimes the negative wire. On a DC system, would you test the + to ground or would you test from + to -? I guess I have the same queston from AC also.
I'm not aware of any way that one panel could have both AC and DC circuits. I think you may have misinterpreted something. Regardless, voltage measurements are always between two points, because voltage cannot be defined without a reference. When we refer to a "120 volt" system, that means 120 volts between hot and neutral/ground. Most (but not all) building power systems are referenced to ground, but also have more than one hot wire. The voltage between hot wires is different from the hot-to-ground voltage. In most homes, it is double (240V). In many commercial buildings, it is multiplied by the square root of three (120/208V or 277/480V). In a DC circuit, voltage could be measured from + to -, from + to ground (if the system has a ground reference), or from - to ground (sometimes - and ground are not the same).
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Old 02-19-2012, 07:19 PM   #4
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Volt Meter Testing Question


I work in panels all the time with both DC and AC, the DC is only 24V and is used for analog control circuits. The DC has no reference to ground and must be measured + to -.

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Old 02-19-2012, 07:36 PM   #5
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Thanks for the replies guys. I work in a factory. All the machines have AC and DC in the panels with multiple voltages. I guess I am confused on how the components would have no reference point to ground since the components are mounted on strips of aluminum called din rail. The din rail is mounted on to the backplane of the panel which in essence the panel must be grounded. So if I have a good grounding point for DC could you check off of that?
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Old 02-19-2012, 07:47 PM   #6
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There are 'common mode' and 'differential mode' methods of supplying power, as well as signals. There are also balanced and unbalanced setups. There might be a Web tutorial somewhere.
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Old 02-19-2012, 08:08 PM   #7
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Typically the panels have a power supply with a 120v coming in and it gets knocked down to 24v. Acts as a transformer or is. I know what everything is in the panels, just dont understand the testing methods.
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Old 02-19-2012, 08:14 PM   #8
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Volt Meter Testing Question


I also work in a lot of panels that have both AC and DC but there are different configurations. What you are describing sounds like the DC negative is connected to the chassis (panel) ground. Sometimes the DC negative is not connected to chassis ground and in that case you would have to measure directly to a + and - bus.
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Old 02-19-2012, 08:52 PM   #9
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Thank u
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Old 02-19-2012, 08:56 PM   #10
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When working with small electronics I work with testing many different voltages AC and DC.
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Old 02-20-2012, 01:28 AM   #11
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I see: when you say "panel" you're not referring to a "panel board" containing buses with circuit breakers mounted on them, but to a control cabinet containing contactors, PLCs, etc. Those certainly can contain AC and DC wiring. Some industrial control protocols do not use ground-referenced signals to avoid interference. They use two (or more) wires isolated from ground to carry information.

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