curious about a volt meter reading

[TimR]

I was removing one of two light fixtures in my shed. It's an old 8 foot fl uorescent that hasn't worked since I've lived here. I intended to unwire i t from the junction box.

I turned off the switch and removed both wire nuts. There are six wires en tering the box: two from the source, two from the fluorescent fixture, two from the other light. Three black, three white.

Checking them with a meter, I got roughly 20 volts from white to black, 40 volts from white to ground (assuming an aluminum ladder on a concrete floor is close to ground), 20 volts black to ground.

That's with a radio shack digital multimeter, and they're notorious for som e phantom voltages, especially with me checking. So i thought about coming down the ladder and getting out the Simpson. But it's a pain to dig it ou t of the basement and untangle the leads and I had other chores after this one.

So I turned off main power to the shed and checked again. No voltage prese nt, finished removing the wires.

But I stayed curious about those readings. Any ideas?

[hrhofmann]

There is capacitance between any two wires that are in close proximity to e ach other, or to/between several other wires. If one is carrying a voltage of 120V AC, a voltage will be coupled to the other nearby wires. The long er the distance the wires are coupled, the better the coupling and the high er the induced voltage. If you measure with an electronic type votmeter, t hat has a high resistance/impedance between the two leads to the voltmeter, (ground and the hot lead) there will be some voltage measured, because the source impedance (the coupling) is very approximately the same impedance a s the meter impedance. A moving coil voltmeter almost always has a lower i mpedance than an electronic voltmeter, and therefore almost always reads a lower voltage than an electronic meter. If the meter impedance is quite lo w, it will most likely read 0 volts.

[gfretwell]

Any open circuit will display weird stuff on a digital meter. If you put a 1 meg resistor across the leads it will be enough to load down the antenna effect you have in those floating wires and not do anything to the real reading on power circuits.. Your Simpson (260?) is probably around 20k/volt so on the 250v scale it puts 5meg load on. Cheaper analog meters will have a higher circuit loading. Digital meters use a CMOS front end with virtually zero load. That is great for electronics, not so much for power circuits.

[philoÂÂ]

Nope, you cannot assume the aluminum ladder is a good ground

you need to check that at the box (assuming metal) or conduit.

The voltage you read was simply "stray" voltage

If there you measured 20 - 40 volts or so, the breaker much be off or the black wire(s) are disconnected

[TimR]

I agree and disagree.

I would not assume it is a good enough ground to make an accurate measureme nt.

But for safety's sake, since I'm going to be standing on it while working, it seems prudent to consider it a ground. At least, before touching a hot.

I may go back with the ladder and both meters and measure from some live an d dead outlets, just out of curiosity.

Yes, it's a Simpson 260 I saved from a discard pile. Every once in a while it comes in handy, and I like having it around even if I don't use it.

[TimR]

, it seems prudent to consider it a ground. At least, before touching a ho t.

and dead outlets, just out of curiosity.

le it comes in handy, and I like having it around even if I don't use it.

Oh, missed something in your post. Metal box, no conduit, no ground wire. It's a shed, and who knows how much DIY went into it over the years before I moved in.

[Ralph Mowery]

One thing you can do with the Simpson when you suspect 'phanton' voltage it to change the range. Start with the highest range and note the position of the meter. Don't worry about the actual voltage. If it is not near the voltage you think it should be (say less than 3/4 the voltage or less than

75 volts on a 120 volt circuit) then go to a lower range. If the meter stays near the same physical position then you can go to a still lower range and see if the meter stays near the same physical position. If it does, the voltage is the 'phantom' voltage.

While not normally very dangerous, that voltage can make you hirt yourself.

I worked as an industrical electrician. We had many wires next to each other in conduit. Often 480 volt circuits. I knew I had the power off one set of wires while changing out about a 1/2 horse power motor. When I touched the wires I got a very bad shock. I had one of the neon bulb testers with me and it lit up, but not to then normal brightness. It takes around 60 volts or beter to light that tester up. I put a Simpson meter on it and the voltage was less than 25 volts. I was in a hot area and very sweaty. That made it worse on the shock factor. That ammount of phantom voltage may have been able to generate enough current to cause major problems. It is not the voltage that maters, but the ammount of current that goes through the body, mainly the heart area for relatively low current.

[philoÂÂ]

For safety sake you should assume the ladder to be grounded...but it will actually be a poor ground if sitting on dry concrete.

[TimR]

I was curious enough to carry the ladder and both meters back out to the shed.

The digital meter consistently read about 120 V at the outlet, 50 V from the hot side of the outlet to the aluminum ladder on concrete.

The Simpson 270 consistently read 120 V at the outlet, and barely a twitch of the needle from the hot to the ladder, on any scale. (thanks for the tip about the scale, I hadn't know that one)

Same results for a 240 outlet nearby - the Radio shack reads phantom voltage from both legs, the Simpson does not. So probably the ladder is not at ground, though I wouldn't test it across my body.

I wonder if the ladder is an antenna. I'm about a block from a huge radio tower.

[Tony Hwang]

Hi, That's reason I still use an old Simpson a lot even tho 3 DVMs are kicking around. Fluke, Wavetek.

[Tony Hwang]

Hi, I don't use metal ladder when working on electrical wires. I have fiber glass or wooden ladder and step ladders for that. Just a habit from active working days.

[rbowman]

Is the Radio Shack meter digital? They have a high input impedance compared to the older Simpsons and put almost no load on the circuit. In theory, this is a got thing, but can lead to the 'phantom' voltages.

The cheap lightup circuit testers are handy. If the lamp loghts, the voltage os real.

btw, an aluminum ladder on a concrete floor isn't a reliable ground.

[Ralph Mowery]

If it is one of the neon bulb testers, that is not always true. Sometimes you can make a guess due to the brightness of the bulb.

There are also some of the no contact testers that wil light up with the phantom voltage.

[micky]

You make it sound like it's the brand of the meter that makes the difference. It is that one has high input impedance and one has low impedance.

With one excepton that I know of, this corresponds to the fact that one is digital and the other is analog. The exception is FET meters. I think they were only sold for a few years, and I think they must have all been analog (with a moving needle). Nonetheless they had high input impedance, and may have shown the ghost voltages, Even though I have one, I don't know.

The input impedance is usually written on the face plate of an analog meter, and iirc somewhere on a digital meter. Typical values are

50,000 ohms per volt** for analog (not FET)., and 11 megohms/volt for digital. **Sounds high, but it's relatively low.

Darn right. Even a bad ground can kill you.

[bud--]

Fluke makes a small cube that plugs into the meter and the leads plug into the cube. It puts a much lower resistance across the leads. The Fluke works like the Simpson.

[gregz]

I never saw that. I would just use my triplet analog. I forget, but the HV range is handy, something like 5 KV.

Greg

[hrhofmann]

Cured concrete is slightly conductive, in the millions of ohm range for a s mall spot. But if you were to wet a large area, then you are making contac t over a large number of "points", each of whgich is slightly conductive, b ut altogether those millions of tiny points have a low enough resistance to allow current levels high enough to kill youif they go through your heart. Ir, you may just fall down and hit your head and knock yourself silly.