Electrical Wiring Question
I posted this on the Knots forum but understand there are several electricians on Breaktime that might know the answer to my question:
I am restoring an old house and have completely rewired it. I have one circuit that does not seem quite right (all others — over 40 circuits — work properly and have the correct voltages.) This circuit is a three-way switch circuit controlling a light. The light is in line prior to the two three way switches. I am quite certain I have wired the circuits properly. As with all my circuits, I check the voltages. On this circuit, I measure 120 volts when the switches are “on”. However, when the switches are “off”, I measure about 30 volts (there is no fixture installed yet, I am measuring across the two wires that will hook to the fixture in the future.) My system has two panel (one where the main service enters and another in the center of the house as a sub panel.) The circuit in question is on the sub panel. I was originally concerned that I was experiencing a bias voltage because of grounding (the sub panel is about 70′ from the main panel) so I installed a grounding rod circuit to the sub panel also (of course the main panel is also grounded with a grounding rod.) The sub panel grounding did not help. I then turned off all breakers in both panels (except the three-way switch circuit breaker) and checked the voltages again. I still measure about 30 volts in the off position.
My question: does anyone know why I am getting this 30 volt reading on the three way circuit?
Thanks,
Harold
Replies
Let me guess.
Getting out the Karnak Hat and looking at the envelope that has been sealed in a mason jar for the last 10 years.
Being able to determine the asnwer to the question without seeing the question, the answer is.
A DIGITAL VOLTMETER.
Opening the envelope I read the quesiton; "What device when used to testing electrical circuits will give strange readings when connected open circuits and is used to confuse the unsupsecting"?
A DIGITAL VOLTMETER.
The digital voltmeter is very sensitive. What you are reading induced voltage that is couple from the other wires.
Install a light bulb. The smallest that you have. Then measure it and it will be zero.
A older style meter with a mechanical movement would not have registered. There is not enough energy that is coupled from the other wiring to even move the most sensitive meter.
Bill and DanH: Thanks for the response. You are correct, I am using a very sensitive digital volt meter. I will take your suggestion and put a load on the circuit and retest it.
Harold
Good call on the digital, high impedance, meter. Another reason I like a Wiggy for general troubleshooting. Not to say various meters don't have a place but I like to make my initial estimation of the problem with a Wiggy. What's your favorite test instrument?A lot of folks equate a digital meter with being more accurate without understanding it can be accurate without the information, or voltage, being meaningful. A technician needs to know the limitations of his tools.Pretty common to get this sort of reading on a three-way circuit when the light is off because the energized traveler, not connected to the load, runs parallel and around the traveller that is deenergized but is connected to the load.
"A technician needs to know the limitations of his tools."
Found this out when I replaced a Greenlee Inductive Voltage Detector with one made by Fluke. Greenlee is rated down to 50VAC, Fluke 90VAC.
THe Fluke is much easier to diagnose with as it does not give false readings off the neutral or parallel conductors. Very easy to discern what's hot from what's not.
Welcome, but this question belongs in the energy, heating.insulation folder.
Yep, you're reading a "phantom" voltage due to capacitive pickup on an unconnected wire. Your meter is sufficiently high impedance that it doesn't "load" the circuit.
If you want to be 100% sure, pigtail a socket & bulb in place of the fixture and remeasure. The voltage will be zero.
Harold,
If I am not mistaken, the code prohibits a separate ground rod on a sub-panel. The ground must come from the main panel. I think a separate ground on the sub panel could create a potential difference which would result in a shock hazard.
John: Thanks for the response. Actually I think that code says the ground and neutral can not be tied together at the sub panel. I have kept the ground and neutrals seperate, I just grounded the ground. Harold
I don't *think* that there is any prohibition against another ground rod on the sub-panel. In fact the for detached building it is required.But the IMPORTANT thing is that you maintain the ground wire that goes from the sub-panel ground bus back to the main panel.The purpose of the ground inside the house is to make sure that everything metalic is at the same potential. That is why the water pipe (if metalic) and gas pipe is bonded (grounded) to the ground. In normal operation there is no current flowing through the ground wire and no voltage drop.The purpose of the earth ground is to disapate large surge currents from outside the house (lighting). It is also to reference the house to earth ground. So that you won't get shocked when you use an outside water bib.