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

I turned off the switch and removed both wire nuts. There are six wires entering 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 some 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 out 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 present, finished removing the wires.

But I stayed curious about those readings. Any ideas?

There is capacitance between any two wires that are in close proximity to each 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 longer the distance the wires are coupled, the better the coupling and the higher the induced voltage. If you measure with an electronic type votmeter, that 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 as the meter impedance. A moving coil voltmeter almost always has a lower impedance than an electronic voltmeter, and therefore almost always reads a lower voltage than an electronic meter. If the meter impedance is quite low, it will most likely read 0 volts.

On 04/12/2014 01:37 PM, TimR wrote:
I was removing one of two light fixtures in my shed. It's an old 8 foot fluorescent that hasn't worked since I've lived here. I intended to unwire it from the junction box.

I turned off the switch and removed both wire nuts. There are six wires entering 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 some 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 out 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 present, finished removing the wires.

But I stayed curious about those readings. Any ideas?




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

On Saturday, April 12, 2014 3:01:50 PM UTC-4, philo* wrote:



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



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


I agree and disagree.

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

On Saturday, April 12, 2014 3:24:27 PM UTC-4, TimR wrote:
On Saturday, April 12, 2014 3:01:50 PM UTC-4, philo* wrote:







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







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





I agree and disagree.



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



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.

"TimR" wrote in message
...
On Saturday, April 12, 2014 3:01:50 PM UTC-4, philo wrote:

I may go back with the ladder and both meters and measure from some live
and 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.

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.



---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

On 04/12/2014 02:24 PM, TimR wrote:
On Saturday, April 12, 2014 3:01:50 PM UTC-4, philo wrote:



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



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


I agree and disagree.

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

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 and 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.






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

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.

wrote:
On Sat, 12 Apr 2014 11:37:31 -0700 (PDT), TimR
wrote:

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

I turned off the switch and removed both wire nuts. There are six wires entering 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 some 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 out 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 present, finished removing the wires.

But I stayed curious about those readings. Any ideas?

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.

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

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

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

TimR wrote:

That's with a radio shack digital multimeter, and they're notorious for
some 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 out of the basement and untangle the leads and I had other chores
after this one.

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.

"rbowman" wrote in message
...
The cheap lightup circuit testers are handy. If the lamp loghts, the
voltage
os real.



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.





---
This email is free from viruses and malware because avast! Antivirus protection is active.
http://www.avast.com

philo* posted for all of us...

And I know how to SNIP


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


I am enamored that the OP would think this is a good idea, but then states
he owns a 260...

--
Tekkie

TimR posted for all of us...

And I know how to SNIP

(assuming an aluminum ladder on a concrete floor is close to ground)

I am still trying to get past the original tenet that one uses an aluminum
ladder while working with electricity! Please explain how this is safe.

I will not go into a diatribe about this but think of your heirs...

does anyone else have comments on this?
--
Tekkie

Tekkie® was thinking very hard :
TimR posted for all of us...

And I know how to SNIP

(assuming an aluminum ladder on a concrete floor is close to ground)

I am still trying to get past the original tenet that one uses an aluminum
ladder while working with electricity! Please explain how this is safe.

I will not go into a diatribe about this but think of your heirs...

does anyone else have comments on this?

Whilst I agree with you that one should not use metal ladders for
electrical work, they are not much worse than standing on the floor
where most work is done anyway.

--
John G

On Sat, 12 Apr 2014 13:12:39 -0700 (PDT), TimR
wrote:

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.

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.



So probably the ladder is not at ground, though I wouldn't test it across my body.

Darn right. Even a bad ground can kill you.

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

On 4/12/2014 2:21 PM, Tony Hwang wrote:
wrote:
On Sat, 12 Apr 2014 11:37:31 -0700 (PDT), TimR
wrote:

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

I turned off the switch and removed both wire nuts. There are six
wires entering 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 some 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 out 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
present, finished removing the wires.

But I stayed curious about those readings. Any ideas?

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.

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

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.

On Saturday, April 12, 2014 10:22:57 PM UTC-4, John G wrote:
Tekkie® was thinking very hard :

TimR posted for all of us...



And I know how to SNIP



(assuming an aluminum ladder on a concrete floor is close to ground)



I am still trying to get past the original tenet that one uses an aluminum

ladder while working with electricity! Please explain how this is safe..



I will not go into a diatribe about this but think of your heirs...



does anyone else have comments on this?



Whilst I agree with you that one should not use metal ladders for

electrical work, they are not much worse than standing on the floor

where most work is done anyway.



--

John G

I have been told that one should always consider a concrete floor to be at ground. That may or may not be true, but it seems a conservative approach. Setting an aluminum ladder on it would be exactly as risky as standing on the ground working on the wires. (electrically anyway, not considering falling)

A fiberglass ladder would be safer if you are working with energized equipment, as may be necessary in a production environment. As a DIYer, speed is not important, so I cut power before working even for jobs where theoretically you can do it live.

And, of course, falling off either ladder is a bad idea, and probably causes more injuries than shock.

Now, if I were right about the concrete floor being ground, AND if i were right about the ladder being grounded, then it would be safer to use the aluminum ladder, because I can quickly check if I've really cut the power, or am getting an unexpected backfeed from somewhere and about to work on a live circuit unintentionally.

But at least in my shed it isn't true, so there's no advantage.

Well, shoot, now I've thought of another experiment. I need to drag the meters back out and see what the reading is to the floor.

Same shed, same concrete floor, digital multimeter.

Voltage from slot to slot in an outlet, 121 VAC.

Voltage from hot slot to floor, 112 to 119 depending on where. Voltage to wet spot on concrete, 120.

Seems the floor is a ground even if the ladder is not.

I didn't drag out the analog meter. I don't see 120 V being phantom.

bud-- wrote:
On 4/12/2014 2:21 PM, Tony Hwang wrote:
wrote:
On Sat, 12 Apr 2014 11:37:31 -0700 (PDT), TimR
wrote:

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

I turned off the switch and removed both wire nuts. There are six
wires entering 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 some 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 out 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
present, finished removing the wires.

But I stayed curious about those readings. Any ideas?

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.

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

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.

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

Greg

Cured concrete is slightly conductive, in the millions of ohm range for a small spot. But if you were to wet a large area, then you are making contact over a large number of "points", each of whgich is slightly conductive, but 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.

On 4/13/14 4:27 PM, TimR wrote:
Same shed, same concrete floor, digital multimeter.

Voltage from slot to slot in an outlet, 121 VAC.

Voltage from hot slot to floor, 112 to 119 depending on where. Voltage to wet spot on concrete, 120.

Seems the floor is a ground even if the ladder is not.

I didn't drag out the analog meter. I don't see 120 V being phantom.


You might look up Ufer ground if you're a bit bored. The code
calls it a concrete encased electrode. They're a lot more effective
than ground rounds for lightning protection.
The short version is 20'feet or more of 1/2" rebar in concrete was
what Mr. Ufer chose to protect bomb shelters during WWII.
Livestock buildings have extra requirements for grounding due to
animals' sensitivity to current. Those involve rebar or mesh in
concrete if my memory is working right now.

On Sunday, April 13, 2014 11:14:34 PM UTC-4, Dean Hoffman wrote:
You might look up Ufer ground if you're a bit bored. The code

calls it a concrete encased electrode. They're a lot more effective

than ground rounds for lightning protection.

The short version is 20'feet or more of 1/2" rebar in concrete was

what Mr. Ufer chose to protect bomb shelters during WWII.

I am familiar with Ufer grounds, we specify them on construction projects at work.

But I'm skeptical about their value for lightning protection.

I can see why they do really well for static protection and for protection of electronics. They offer a low ohm path to ground that can be measured and can meet protection requirements. They do well on low current high voltage spikes.

But I've been unable to confirm they can actually dissipate the amount of current from lightning strikes. There seems to be no evidence out there at all.

On Monday, April 14, 2014 2:47:35 PM UTC-4, bud-- wrote:
http://www.psihq.com/iread/ufergrnd.htm

has some information on the lightning sinking capability of a Ufer ground..

Twenty ft of 1/2" rebar, properly placed, has a sinking capacity of

90,000A. That would be a very strong strike. I think lightning

protection could be designed into a Ufer ground when a house is built.

That's an interesting link, but with no bibliography and no references, it is just advertising copy.

There is a huge advantage to Ufer grounding. It allows you to beat the test. There are a number of applications where code requires you to prove that you have a sufficiently low resistance path to ground on a regular basis (usually tests at 7 month intervals). Often local soil conditions make it difficult or impossible to pass. Ufer passes - but I have yet to see evidence that passing the test equates to real protection. I'm not saying it doesn't, I'm just asking for some data.

On 4/14/2014 6:41 AM, TimR wrote:
On Sunday, April 13, 2014 11:14:34 PM UTC-4, Dean Hoffman wrote:
You might look up Ufer ground if you're a bit bored. The code
calls it a concrete encased electrode. They're a lot more effective
than ground rounds for lightning protection.

The short version is 20'feet or more of 1/2" rebar in concrete was
what Mr. Ufer chose to protect bomb shelters during WWII.

I think the original was ammunition or explosives storage. And I think
Ufer tied all the steel in the concrete in contact with the earth
together. If you have a metal structure and that is also tied in it
would be a protective cage (kinda like a Faraday cage). But it had a
very low resistance to earth in an area where that was difficult to
achieve.

A Ufer ground for a house is more limited, but is very good. and I agree
it is much better than a ground rod.

A Ufer ground has generally been required for some time to earth an
electrical service. It has, hopefully, made ground rods obsolete for
that purpose.


I am familiar with Ufer grounds, we specify them on construction projects at work.

But I'm skeptical about their value for lightning protection.

I can see why they do really well for static protection and for protection of electronics. They offer a low ohm path to ground that can be measured and can meet protection requirements. They do well on low current high voltage spikes.

But I've been unable to confirm they can actually dissipate the amount of current from lightning strikes. There seems to be no evidence out there at all.

IMHO you could easily design a building to use a Ufer ground for
lightning protection. A small steel frame warehouse should be quite
effective for lightning protection if the multiple structural columns
are tied into the rebar, and the rebar is tie-wired together.

http://www.psihq.com/iread/ufergrnd.htm
has some information on the lightning sinking capability of a Ufer ground.
Twenty ft of 1/2" rebar, properly placed, has a sinking capacity of
90,000A. That would be a very strong strike. I think lightning
protection could be designed into a Ufer ground when a house is built.

I think lightning protection added to a house would use at least 2
down-conductors with a field of ground rods for each. That must be
bonded to the electrical earthing system.

If lightning strikes, the earth potential at the building can rise far
above 'absolute' earth potential. As with any protection scheme, much of
the protection is that all wiring rises together.