Voltage on surface oif Double Insulated tools

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retcec

Member
Location
Washington, D.C.
A recent issue came up with a double insulated tool during a safety inspection. An inspector tested an energized double insulated screw gun and found voltage to ground. The inspector placed a voltmeter from the tools metal surface to a nearby receptacle ground opening and got the reading of ~5V.

Seems like a few issues with this test that could have caused the voltage. First, the double insulated tool naturally has no connection to ground, and any system ground will be a different potentials, resulting in the voltage that could have been on the ground from any other circuits connected. I don't know if a test of the tools plug (prongs) to this same metal surface was done, which could have indicated a real problem.

Has anyone seen anything like this before? What could have caused the voltage?
 

kwired

Electron manager
Location
NE Nebraska
I did not go to the links Smart posted, but he is right on with the phantom or ghost voltage, I'm guessing the words capacitive coupling are used somewhere in those links.

Your safety inspector needs to understand this activity before testing with that method, a non contact voltage tester may very well light up in the vicinity of the tool as well, but that should be expected.
 

retcec

Member
Location
Washington, D.C.

I did not go to the links Smart posted, but he is right on with the phantom or ghost voltage, I'm guessing the words capacitive coupling are used somewhere in those links.

Your safety inspector needs to understand this activity before testing with that method, a non contact voltage tester may very well light up in the vicinity of the tool as well, but that should be expected.


Thanks, this information is useful, but the situation here is as follows: a double-insulated power tool is plugged-in (energized), a voltmeter was used to measure voltage from the metal section to a nearby receptacle and voltage was observed. Now, I get the phantom voltage, after powering off equipment, but this situation is different in that the equipment is energized.


Thanks,
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Thanks, this information is useful, but the situation here is as follows: a double-insulated power tool is plugged-in (energized), a voltmeter was used to measure voltage from the metal section to a nearby receptacle and voltage was observed. Now, I get the phantom voltage, after powering off equipment, but this situation is different in that the equipment is energized.


Thanks,
But you need to measure not just the voltage but the available current.
A high impedance voltmeter can read 120V AC when the source impedance (capacitive) allows less than 1ma of current to flow through a low impedance load.
 

kwired

Electron manager
Location
NE Nebraska
Thanks, this information is useful, but the situation here is as follows: a double-insulated power tool is plugged-in (energized), a voltmeter was used to measure voltage from the metal section to a nearby receptacle and voltage was observed. Now, I get the phantom voltage, after powering off equipment, but this situation is different in that the equipment is energized.


Thanks,

Sounds typical to me. Not knowing what was all in Smart's links - you have a capacitor here, (at least) two conductors with a charge between them with separation by an insulator. It is a very weak capacitor and just touching it shorts out the voltage you are seeing but with a high impedance input meter you are able to read that voltage. Use of a low impedance meter to make the measurements wouldn't show any voltage, because the meter itself would draw enough current while taking the reading to short out the " weak capacitor".
 

Smart $

Esteemed Member
Location
Ohio
... the equipment is energized.
I have to assume you mean energized but not ON, as in the trigger pulled. That means one conductor is energized, and one is not (grounded neutral)... so there will be a capacitance effect between case and the energized conductor.

Try the same while the trigger is pulled and there's current flowing both directions.

Also, as others point out, using a hi-impedance meter decreased the potential to sink the phantom voltage. Typical DVM's have an input impedance of 10kohm... so if you read ~5V, that's only a current of 0.5mA. Use a low-impedance meter and I'll bet the voltage and current drop real close to, if not 0.
 

ATSman

ATSman
Location
San Francisco Bay Area
Occupation
Electrical Engineer/ Electrical Testing & Controls
I have to assume you mean energized but not ON, as in the trigger pulled. That means one conductor is energized, and one is not (grounded neutral)... so there will be a capacitance effect between case and the energized conductor.

Try the same while the trigger is pulled and there's current flowing both directions.

Also, as others point out, using a hi-impedance meter decreased the potential to sink the phantom voltage. Typical DVM's have an input impedance of 10kohm... so if you read ~5V, that's only a current of 0.5mA. Use a low-impedance meter and I'll bet the voltage and current drop real close to, if not 0.

I agree S$ but
I would think you meant more like 10 Meg ohm. That is why I carry a Wiggy with me or even a 60W light bulb with pigtails to scare away any ghost voltages...:D
 

Smart $

Esteemed Member
Location
Ohio
I agree S$ but
I would think you meant more like 10 Meg ohm. ...
That would be the case for many... depends on make and model. Newer DMM's more so than older. I'm certain some of the first DMM's on the market only had 10kohm impedance. I stated the lowest value I thought might still give a phantom voltage reading.
 
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