measuring voltage/current

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Anonymous

What is the meaning of measuring voltage/current in reference to ground or checking voltage/current in reference to ground.
 
Reference to ground means that the zero volts is the ground. In parctical terms, connect the negative to a voltmeter to the ground or zero point and the positive to the the live or hot point. You cannot refer to ground when measuring current sincce sou connectit in series.
 
When you test a voltage, you put the red test lead to an input or circuit terminal, obvious, but where do you touch the black common test lead?

You could touch the black lead to the metal chassis of the PLC or the metal cabinet the equipment is located in. I would call this an earth reference since the NEC code would cause this metalic cabinet to be earth grounded.

Or you could touch black lead to the negative or companion input terminal to red test lead terminal. This could be called a signal common reference since that terminal might be a different common in relation to the earth.

If you test the voltage between the signal common and the earth common, you might find they are not the same, there is a voltage on the signal common of a couple volts possibly.

What you reference your measurements to is important.
 
It’s my opinion that someone may use this term, for an ungrounded system (the transformer), you can see voltage variance or different readings from ground to line, from line to line and or line to common, so what they are saying is one leg of your meter should be on ground. Hope this helps.
 
Checking voltage to ground is as easy as putting one lead on your test point and the other on any metal object in the cabinet (assuming the circut has been wired correcly).
Checking current to ground generaly involves a sensor or probe and requires you to insert your meter in series by removing the probe wire and attaching one lead to the probe and one lead to the wire.

Ground is used as a referance in flame rectification and water level probes, your readings will be in micro amps. (may need a transducer depending on your meter.)
 
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