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Why does a meter short out when measuring VCC and VDD at the same time

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DexterMccoy

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Why does a plug in bench DVM meter short out when measuring VCC and VDD at the same time?

VCC is +30volts and VDD is -30 volts

The Bench DVM meter shorts out the burns the circuit board, but why?

How do you prevent this from happening?

I had my red dvm meter probe on VCC +30 volts and my black meter probe on VCC -30 and it shorted out the power supply diodes and fried the board

But why?

Why would this create a short if the DVM meters negative input is connected to earth?

-30 volts VDD and + 30 volts VCC = 60 volts potential difference

Is the Bench DVM meter chassis ground connected to the meters probes?
The meter probes would have to be internally inside the bench DVM meter connected to earth or chassis ground of the DVM meter

If a circuit boards power supply earth or chassis is different then the DVM meters chassis ground or earth ground can this cause a ground loop or short when measuring from VCC to VDD rails?

When can a SHORT occur when measuring from VCC to VDD supply rails?

The grounds would have to be different how so?
 

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Does your "plug-in DVM" have a three-wire (Line, Neutral, and Ground) type of line cord? If so, likely the DVM's black lead is internally connected to the Green ground wire in the line cord...
 
Does your "plug-in DVM" have a three-wire (Line, Neutral, and Ground) type of line cord? If so, likely the DVM's black lead is internally connected to the Green ground wire in the line cord

YES it does

ARE YOU SURE tho that the black lead is connect directly to the green ground wire?

and how can i check this?

Is there any other way of having two different grounds or a ground loop that would cause a DIRECT short from VCC positive supply to VDD negative supply from using a PLUG IN DVM meter?

Would have two different EARTHS from a test fixture earth ground that is different than the circuit board earth ground cause a VCC positive supply to VDD negative supply short from using a Plug in dvm meter when measuring from VCC positive supply to VDD negative supply?
 
If you have another DMM, unplug the DVM and simply measure the resistance from Blk to the big prong...
btw-most 'scopes have this same problem: Ground clip on the probe connects to earth ground through the green wire in the line cord.

I would use a battery-powered Fluke DMM to make the measurement that shorted your supplie(s).
 
Many times a bench DVM might have a particular spec as to how far above ground the negative terminal can be. Keithley had it marked on the binding posts. 30 V is not unusual.

Isolated AC supplies to the equipment being tested is one way out, but then that equipment cannot have a ground.

Model of meter? Link to manual?
 
Isolated AC supplies to the equipment being tested is one way out

If you have isolated AC supplies with two difference earths, and you measure the VCC positive supply to the VDD negative supply using a Plug in the wall type DVM meter Or O scope, will this cause a direct short?

Or what is causing this direct short from VCC positive supply to VDD negative supply, when measuring using a Plug in the way type DVM meter?
 
If you have another DMM, unplug the DVM and simply measure the resistance from Blk to the big prong.

I didn't this, and I didn't get any continuity , unless it's a DVM meter chassis ground

Is the Negative probe connected to the chassis ground internally inside the DVM meter or to the earth ground prong?
 
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