dc voltage reading - ground vs negative

Thread Starter

naugle522

Joined Feb 3, 2018
5
When I measure dc voltage can I put one lead of my meter to ground or frame like Ac, or do I need to connect the my second lead to the actual negative in the circuit to get a proper reading. I'm working with control circuits on machines where the control voltage is stepped down using a power supply?
 

Thread Starter

naugle522

Joined Feb 3, 2018
5
sometimes when I measure dc voltage with one lead to a terminal and one lead to ground I get a false reading. Do I need to connect the second meter lead to the actual negative to get a true reading?
 

Thread Starter

naugle522

Joined Feb 3, 2018
5
what I mean is when I measure 120 ac I can put the second lead to frame and get a voltage reading to follow the circuit.. In 24 v dc I have to have one lead on negative, not ground or frame to get a proper reading? Is this correct?
 

Thread Starter

naugle522

Joined Feb 3, 2018
5
I think I'm talking about common (negative). I need a new meter I'm curios about your apion on the greenlee cmt 90, I'm debating on that or the fluke clamp on? I love the cmt because its automatic and I don't have to chage the settings. I just spent 300 on a extech and ruined it when I forgot to change the ohms setting and I gave it 600 vac. now my ohms is off and n o fuse. ill never buy an extech again. I'm an industrial mechanic or millwright.
 
Everything is not referenced to chassis ground. The 24 VDC supply that runs the PLC may be isolated from ground.

There is ground, earth and common primarily. There can be many commons.

Speaking in general terms, you would need to be aware of signal grounds, shields, earth, commons, high current ground.

It's very easy in the process world too have a 0-5V input that's driven by a 0-20 mA isolated output. A resistor is placed at the input of the 0-5V device and now the reference is one side of the resistor.

Meter with touch-hold may be useful. One of my tools was a small battery operated oscilloscope.
it was actually fun to have a "complex" multimeter because no one would borrow it. Sane with an RPN calculator.

You might have a hard time convincing management to buy you a thermal imager. They can spot problems before they occur. I'd suggest documenting any repairs where you think a thermal imager would have helped.
IR thermometers are cheap and can also help.
 

MrChips

Joined Oct 2, 2009
19,922
I think I'm talking about common (negative). I need a new meter I'm curios about your apion on the greenlee cmt 90, I'm debating on that or the fluke clamp on? I love the cmt because its automatic and I don't have to chage the settings. I just spent 300 on a extech and ruined it when I forgot to change the ohms setting and I gave it 600 vac. now my ohms is off and n o fuse. ill never buy an extech again. I'm an industrial mechanic or millwright.
The problem is not with the meter. The problem is the user.

RULE #1 when using any test instrument such as a meter.

Set the meter range to the correct range before connecting test probes.
 

MrChips

Joined Oct 2, 2009
19,922
Circuit common is what the original designer chose as common.

Presumably, if you are trying to verify a voltage reading, you are comparing with an expected reading that someone else determined to be the nominally correct reading. Hence the common reference point will be the reference point that that person used to establish the nominal reading.
 
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