I have un ungrounded metal lamp. I have no reason to believe that there is a
problem with it, but with the lamp turned off I am seeing about 2V between
the lamp frame and ground, and with it turned on about 4V between the lamp
frame and ground. All as measured with a digital multimeter. Can anyone
explain why to me?
Digital meters depending on the type are notorious for low voltage readings
that are false.
Try turning the meter on and letting the probes be in free air. Take a
reading, now wave them about..
Try an RMS meter if you have one.
Sure, the frame of the lamp is connected to the wide blade of the plug and that
connects the frame to the neutral conductor in the outlet.
Depending on the other loads on the same branch circuit that the outlet is on
there will be up to several volts dropped (I * R) along the neutral conductor
back to your main panel where neutral and ground are connected together.
Since ground conductors USUALLY do not have any current flowing through
them, the voltage drop across them will be essentially zero. Using a water pipe
as your ground reference will probably give you the same indication.
On second thought, and after reading your post again, the wide blade of your
lamp plug should not be connected to the lamp frame. Unplug the lamp and
measure the resistance of each blade to the lamp frame with the switch on and
off. In both cases the resistance to the frame should be very high. If not,
find the short, and fix it.
"False" is not the word I'd choose. DMMs have a very high input
impedance - as a result, even a few picofarads of capacitance between
the power circuit and the lamp frame could provide a high impedance
that would allow this reading to appear on the meter. 120 V / 2 V = 60,
typical DMM input is 10 meg ohms, and 600 megohms is the impedance of
about 4.5 picofarads at 60 Hz.
In my old steelmaking days we had a mimic panel for a dust collector
that was quite hard to read - it used neon lamps to indicate damper
positions, and since the lamps were connected to a couple hundred
metres of cable, it was very easy for stray capacitance to provide
enough voltage to allow the neons to light.
RMS isn't so much the problem as the high impedance. If you're looking
for hazardous leakage current, use a low-impedance tester (a Wiggy or
Square D) or a purpose-built leakage current test set.
Incorrect measurement technique. Put a load - like a
100 watt bulb - across the meter leads, then take your
measurements. A digital meter doesn't put enough load
on branch circuit wiring to avoid erroneous low voltage
The static voltage or minute power is either capacitively coupled or induced
from conductor to frame. The power should be canceled in theory, with the
zip cord conductors direction of current being 180 degrees out of phase.
Perhaps one of the conductors is longer at the beak-out point or one
conductor is closer to the frame. Either way, it would appear to involve
It is all relative. One of the conductors is usually tied to ground. If you
measure voltage relative to ground, the induced voltage, due mainly to
capacitive coupling from each conductor, is somewhere between the hot and