Kevin Ricks wrote:
E Z Peaces wrote:
Kevin Ricks wrote:
E Z Peaces wrote:
Kevin Ricks wrote:
If the load is switched off, all 3 may still glow but may be
somewhat dim.
Phantom voltage is likely to come from capacitance between the
conductors.
A man used a Fluke 1507 to check voltage at the end of his new 50'
extension cord. He found 120V between hot and neutral, and each
read 40V from ground.
He thought the cord was defective. Then he remembered he'd used an
adapter, so his ground was open at the outlet.
Hot and neutral would have had the same capacitance to the ground
conductor, so without the meter in the circuit, the ground conductor
would have been 60V from each.
The meter has an input resistance of 3.0Mohms, so the ground
conductor must have had 6.6Mohms capacitive reactance to each other
conductor and a total of 3.3Mohms capacitive reactance. (Actually,
it would be lower because the meter would have a little capacitance.)
That would amount to about 8 picofarads per foot between each two
conductors in the cord. I suppose Romex would have roughly that much.
Using that cord as a model, I would expect an outlet on a Romex
cable with the neutral disconnected at the breaker box and no
appliances connected to provide one of the bulbs in a tester with
120VAC and the other two with 60 volts through approximately 3
million ohms.
I don't know if 60V through 3Mohms will light a bulb enough to see,
but the manufacturer of the tester could avoid the possibility by
shunting each neon bulb with a 100k resistor, or maybe even a
million ohms.
Phantom voltages can fool you when using a voltmeter, however in this
case, (Open neutral circuit with no other loads), you have 1 neon
directly connected between hot and ground -glows brightly.
The open neutral would make the other 2 neons be in series between
hot and ground. So they would in theory glow 1/2 bright. Now that I
think of it this is more likely the case for the OP. All 3 bulbs may
glow in both case (load or no load) but for different reasons.
Kevin
Uh-oh! My calculations would apply if I were using one bulb to check
voltages sequentially, but a tester uses three bulbs at once. Doh!
Here's the link Smitty posted.
http://www.professionalequipment.com...productid=7918
I wonder how it's designed to show only one light with an open neutral.
Don't know for sure it could have other components maybe enough series
resistance to keep the other 2 bulbs off in an open neutral condition.
In my experience the other 2 neons will glow dimly and may not even be
noticed in bright lighting conditions. I once thought I had a bad tester
until found the fault and then thought about it. Now I disregard the dim
lit neons. My tester is 10 or 15 years old though.
Keep in mind though that these testers ware designed to test the
condition of the outlet they are IN, not the rest of the wiring.
There is a difference if the neutral wire is disconnected right at the
outlet or nearest pigtail connection and an open neutral on down the
line with other loads interfering. I think even the more sophisticated
testers would have a problem in that situation.
FYI - This page shows some diagrams of testers. This one works
differently than the commercially available units. The 3 lights stay on
for correct wiring and one or more of the lights go off with a fault
condition.
http://www.giangrandi.ch/electronics/neon/simple.html
Kevin
Zeners! They could also be used to prevent bulbs from glowing in case
of an open connection, as in Smitty's link.
I wonder if the circuit in the link you posted has advantages over a
resistor-zener-zener-bulb in series between each pair of terminals.