I would have tested for 120V using one test lead
on the light side of the switch and the other held
to ground. I'd use ground because that's usually
there and available without taking anything apart.
Or he could have tested from the light side of
the switch to neutral.
Often in a light switch there is no neutral to have a wire nut to take off.
If there is no neutral or I suspect the ground is not connected, I use what
some may call a 'hot stick'. It is a device about the size of a cigar that
lights up and/or beebs when held next to a wire that has voltage on it.
Cutting the breaker off and on while using the hot stick usually confirms
there is power or no power at the switch. Then with the power off, I check
the switch using the ohms fuction.
I often use a Simpson 260 because I do not like the digital meters for
general testing due to the 'phantom' voltages. Another favorite is a Fluke
T2. I think that is the number. It has about 10 LEDs in it. It will check
from about 6 volts to 600 volts AC/DC and also low resistances. Just hook
up the test leads and it will show if you have power or a short.
I do admitt that I have several differant testers that most home owners do
not normally have or really need.
I also have a Fluke 87 digital meter, but seldom use it for electrical test
due to the 'phantom ' voltages. I work in industry where I have to deal
with anything from low voltage control and instruments to 480 volt 3 phase
circuits. That is the reason for having those plus several other testing
Round these parts, the vast majority of them do
have neutrals. Usually the power feeds from the
panel to the switch via a romex that has both
conductors. Current NEC code now requires that
they have neutrals.
But that is why I said to just test it by holding one
lead to ground. And again, around here the vast
majority have grounds.
Yes, he could use one of those if he happened to
Agree, he could do that too.
The phantom voltage issue is more of a problem
of people having a lack of understanding of circuit
fundamentals. The OP's test procedure being an
example. I don't think either of us would have
tested it the way he did. If you understand how circuits
work, it may cause you to do a second test, etc, but it's
not really confusing.
On 5/16/2012 11:33 AM, firstname.lastname@example.org wrote:
I thought having many test devices was about having the most toys when
I am a lot more paranoid about possible failures and arc flash after
seeing some videos. Measuring on high capacity circuits, the meters
should be "category" rated. OSHA is likely to take a dim view of using
not-cat rated meters on high capacity circuits, particularly if there is
an injury. Fluke is likely to be cat rated. Not sure any Simpson 260s
are. Digital is also nice because it is more compact. (I coveted a 260
when I was about jr high school and finally got one at a garage sale
If using a Fluke, or similar, you can substantially eliminate phantom
voltage with an accessory:
On the other hand, the test would have been fine if there was an
incandescent light bulb in the socket.
I agree about circuit fundamentals, and part of that is understanding
the limitations of measuring tools (like phantom voltage).
I remember the first time I saw the effect described by Ralph where the
voltage reads about the same point on the scale on different analog
ranges. I figured it out pretty fast, but it is weird.
One of the smartest electricians I know wanted to measure the current
for a motor in a food processing plant. It was a possibly explosive
atmosphere so there was a motor control center in a purged room. He
defeated the interlock on the module door and amp-clamped a motor wire -
an absolutely standard procedure. No one knows what happened - maybe a
loose screw faulted the busbars, but there was an arc flash. He was
badly injured (including condensed copper on him) and was in the
hospital quite a while. The facility was buying distribution voltage
power from the utility and one of the fuse holders was destroyed in the
event - I have no idea how that could happen. (Arc flash was not an
issue back then.)
At a trade show a manufacturer field engineer had a seminar. As an
aside, he was working for a major company like Westinghouse and the
client wanted a module for a motor control center or switchgear. The
space was open so he was measuring the size with a steel tape. The next
thing he knew he was on his back on the other side of the room. If he
hadn't been thrown he might be dead. At least in that case he did
Some of the required inservice classes have covered arc flash. They had
an arcflash suit - pants, top, hood and gloves. (And the protection
includes the natural fiber clothes inside.) Don't know how you do useful
work in something like that. Someone brought in copies of the labels at
his facility that are required to evaluate what protection is required.
One of them said something like "No safe approach is possible" (with
protective panels removed).
The default standard for worker electrical safety is NFPA 70E. Your
plant must have a copy and I wouldn't be surprised if you have read it.
Interesting information on what protection is required for different
voltages and available fault currents.
I just retired as of yesterday. We have all the protective gear. Gloves,
coat, face shield. While we seldom use it, we have a couple of special
outfits that are rated for 100 cal. The coats we normally have are rated
for 40 cal. I was almost tempted to bring my 40 caliber Glock on the last
day and shoot at my arc flash coat and tell them it would not protect
against a 40 caliber... Almost impossiable to do useful work with that junk
on. Most of the time we can cut the power off and do a test and then take
all the stuff off.
We have lables like that all over the plant. It is a large plant and covers
many acers. A small building ( maybe 50 x 100 feet) has a motor control
center and it had a sign on it that mainly stated we could not get within
about 500 feet of it. That ment if any breaker on it tripped, it would have
to stay off. Someone finally wised up and replaced that sign with one that
Most of the arc flash junk started when a company came up with some high
dollar main breakers that were suspose to trip under certain conditions. As
many companies did not want to replace them, the arc flash hazard was
At his day job, one of the instructors of our nightly apprentice
program at the International Brotherhood of Electrical Workers put a
wiggy on the line terminals of a 4160V switch gear once. As a
reminder, a wiggy is good for 600V.
This did not end well. I don't remember the details, but I have heard
the story a thousand different times.
When I was an apprentice, my roommate was also an apprentice. He
worked for a very small company. The company was dad, son and my
roommate. The son was changing a buss fuse under load. When the two
maintenance workers that were in the area because of the outage became
aware of what the son was doing, they ran for the door. I am told
that both maintenance workers were burned on their back half of each's
bodies as they had made it to the door and were each half shielded by
the door jam as they left the room. The son was killed. He was an
experienced electrician. The rush to restore power to the building
cause him to make the error.
* wiggy = voltage tester (solenoid)
On Wed, 16 May 2012 11:57:44 -0400, "Ralph Mowery"
In which case you will still have a "capacitive ground" which will
give an (inaccurate but more or less consistent) reading. The test
will still tell you if the switch is working or not. A simple neon
tester will do the same, using your pinkie as ground.
OK Bub - I'm make it REAL simple. And I'll type slowly so you can
understand. A switch is in series with the load. The switch has 2
contacts. Both contacts are on the "live" wire, because the neutral is
not switched. The OP measured the voltage from one terminal of the
switch to the other. IF the bulb was not in the socket, the voltage
would be ZERO, with the switch on or off. If there was an incandescent
bulb in the socket, the voltage would be line voltage - roughly 120
volts, with the switch OFF, and ZERO with the switch on. - if the
switch was any good.
However, the OP had a CFL in the socket, so thecapacitance etc. of the
electronic ballast was dropping about 40 volts across it with the
extremely low current flowing through the meter - and because the
switch was shot, the reading was the same both ways. If the switch had
been good, he would have found 80 volts across the open switch, and
ZERO across the closed switch, with the CFL lit.
The CORRECT way to test the line voltage is line to neutral, or line
to ground. If there is any ground at all the digital meter will read
the same either way, within a tenth of a volt,
I happen to have made my living using test meters for quite a few
years, so I knowwhat the OP did, what he should have done, and exactly
what the meter readings he gor mean.
You obviously do not.
80 volts terminal to terminal on the switch means 80 volt drop across
the bulb. Same with switch o or off means the switch was no good. Feed
to ground would read approxemately 120 volts. With bad switch load to
ground would read zero. Good switch, load to ground 120.
By definition, with a functioning switch energized, both sides MUST read the
same and that "same" must be zero. With the switch in the OFF position, and
using a digital voltmeter, you can get any reading from zero to 120.
You don't read to good, Bub. I said from one terminal of the switch to
GROUND or neutral. Live side th ground will always be aprox 120.
Switched side to ground will be zero with the switch off, and 120 with
the switch on, with a functioning switch, reguardless of load.
Terminal to terminal on the switch will read zero with the switch
turned on, and non-zero with it turned off - the non-zero value
depending on the combination of load impedence and meter
impedence(sensitivity) - regardless if it is a digital or analog
meter, and if the nanalog is a symple d.arsenval movement or a VTVM.
The reeding from LINE to NEUTRAL or GROUND will NOT show your phantom
voltage, and the reading across the open switch is NOT phantom.
As for meter connections, you NEVER connect a voltmeter in series with
a load. You ALWAYS connect it ACROSS the load. The OP conected
his/her meter IN SERIES with the load, so it was acting as an ameter
but reading in volts, which didn't really mean anything (without
knowing the exact sharacteristics of the meter in question)
Sometimes, with no load, a digital meter will pick up a
"ghost load" reading. Or ghost voltage, can't remember.
The two wires next to each other have a very slight
transformer effect. Not enough to light a bulb, but
enough to read with a DMM.
Christopher A. Young
Learn more about Jesus
The back porch light stopped working, a new bulb didn't help.
So. I put a meter on the socket. 0 V.
I pulled the switch plate off (just a normal single pole). 80 V
terminal to terminal. Hmmm?
Killed the power, put an ohmeter terminal to terminal, infinite
resistance at both switch positions. Diagnosis bad switch, supported
by the fact that before it died completely, flipping the switch
several times made it work. Besides I've always had a CFL in that one
and I suspect the capacitor shortens the switch life due to arcing.
Okay, a new switch is $1.29, no big loss if wrong. Took the old
switch off, turned the power back on and checked wire to wire just for
grins, still 80 V. Uh oh.
Put the new switch in. Turned power on, checked the socket, 120 V.
Put bulb in, (CFL), lights up fine.
Well, I have a working porch light again, but I'm left with the 80 V
mystery. I don't know any way to get 80 V on a normal residential
power setup. The meter was a digital Radio Shack multimeter. If I'd
had time I'd have checked again with the Simpson analog, digitals
sometimes give funny readings, but I've never seen 80 V. What am I
If you measured across the two wires that were connected to the
switch, you're measuring the potential from hot *through the bulb* to
neutral. If you want to sleep better, pull the switch off again and
measure from hot to ground, you should then read 120V or thereabouts.
If the bulb is a CFL there may be some odd effects causing an
incorrect voltage reading the way you measured it.
I started with the assumption the switch was bad, did the checking
mostly out of curiosity (and of course, when trying to figure out
which breaker controls the circuit and the light is NOT working, it's
handy to have a way to know).
And yes I've had trouble with digital meters and phantom results on no
load circuits, that's why I asked (and why I have a Simpson on the
shelf, but unwinding all those probe wires is a pain, not as bad as
winding them up to get them back in the case though).
But on 120 V circuits normally that meter reads okay or floats a bit.
I left out a couple of things. The fixture had no lamp in it, so that
circuit should be open. The kitchen lights are on the same circuit,
they went out when I got the right breaker (if I'd known that ahead of
time I wouldn't need a meter, technically).
The 80V terminal to terminal on the switch was with the switch off AND
no lamp in the fixture. With switch on, it went to 0 as I'd expect.
It was consistent with several measurements and was the same as wire
to wire with the switch removed (and still no lamp in the fixture).
That surprised me. And then when I went outside and put the same
meter on the fixture and got 120 V I was suprised again.
After work tonight I'll put the analog on it and see.
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