Curiosity on a work bench

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Dsg319

Dsg319

Senior Member
Location
West Virginia
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Wv Master “lectrician”
So in my dads garage he has wooden bench and cabinets and on the top bench a big aluminum plate to work on. And just out of curiosity I wondered if it was bonded with the egc. So I got a meter out and tested a nearby outlet egc and couldn’t get it to ring out or show any sign of continuity. BUT when I was checking voltage of the Outlet I could get around 113volts from ungrounded conductor to the work bench. Also from the equip grounding conductor to the work bench would read around 3 volts. Would anybody have any idea what would cause this ? Work bench is in wooden with aluminum top to work on and sets upon a concrete floor in a garage which is attached to the house.
 
The line-voltage reading is because the volt-meter uses so little current that it's not enough to bring the aluminum up to the line voltage, so it sees a voltage difference. If you test from hot to table again, and while reading voltage, connect a small 120v bulb also between hot and the aluminum, the voltage will collapse, making the aluminum hot. You can then read line voltage between the aluminum and the EGC.
 
Concrete's a fair conductor of electricity, especially when it contains moisture or metal rebar. Wood's not a great insulator unless it's perfectly dry. The low side of the electrical service is connected to the Earth.
www.ecmag.com

What Is A 'Ufer' Ground? Concrete-Encased Grounding Electrodes


At a recent seminar, one of the attendees asked, “What is a ‘Ufer’ ground?” This is a common question. A “Ufer” ground is slang for what the National Electrical Code (NEC) addresses as a concrete-encased grounding electrode. The term “Ufer” does not appear in the Code, but many in the industry...
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LarryFine said:
The line-voltage reading is because the volt-meter uses so little current that it's not enough to bring the aluminum up to the line voltage, so it sees a voltage difference. If you test from hot to table again, and while reading voltage, connect a small 120v bulb also between hot and the aluminum, the voltage will collapse, making the aluminum hot. You can then read line voltage between the aluminum and the EGC.
Click to expand...
You lost me here.
 
Dsg319 said:
So in my dads garage he has wooden bench and cabinets and on the top bench a big aluminum plate to work on. And just out of curiosity I wondered if it was bonded with the egc. So I got a meter out and tested a nearby outlet egc and couldn’t get it to ring out or show any sign of continuity. BUT when I was checking voltage of the Outlet I could get around 113volts from ungrounded conductor to the work bench. Also from the equip grounding conductor to the work bench would read around 3 volts. Would anybody have any idea what would cause this ? Work bench is in wooden with aluminum top to work on and sets upon a concrete floor in a garage which is attached to the house.
Click to expand...
Make model and year of your meter please. Mine is a Fluke 12 circa nineteen ninetysomething.
 
A large metal plate, with no metallic or even high resistance connection to earth still acts as the plate of a physically large (but low capacitance) capacitor to ground. So when you connect one lead of a high resistance meter to the hot and the other lead to the plate, you are connecting the negative meter lead in series with the capacitance to ground.
As long as the impedance of the meter is high enough, most of the voltage in the resulting voltage divider will show up across the meter.

When you measure between EGC and the plate, you are measuring any difference between the EGC voltage and local ground as well as any small voltage capacitively coupled to the plate from its environment.
 
LarryFine said:
The table top is floating, so a connection to a hot conductor, say through a solenoid tester, would energize it.
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don_resqcapt19 said:
The load will energize the aluminum to line voltage. Since there is no complete circuit, there is no voltage drop across the load.
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I see what you mean. I always just thought about a load making phantom voltage disappear. I wasn't thinking about putting the load in series.
 
ActionDave said:
I see what you mean. I always just thought about a load making phantom voltage disappear.
Click to expand...
That's exactly the issue here: phantom voltage. The difference is that the OP was using hot, rather than ground, as the reference.

So, instead of a floating conductor picking up induced voltage, he was reading phantom voltage to a surface that wasn't.
 
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