How Do You Fix An Outlet Whos Ground Voltage Is Incorrect?
(Post-and-tube is a very old wiring technique from the early days of residential wiring. It typically involved cotton-insulated single wires separated by ceramic insulators nailed into wall studs behind a lath/plaster wall surface.) Okay. If you have white, black and bare copper wires you’re off to a good start. The bare copper wires, by NEC code, should have a green wire nut with a copper “pigtail” attached via a screw to the box (if it’s a steel box). There should also be a pigtail from the green wire nut to the green screw on the 3-prong outlet. By code, the whites (neutral) and blacks (hot) should each have a wire nut (typically red or gray), each with a pigtail to the outlet. The white pigtail should go to the silver screw on the outlet. If your box is wired-up like this, you PROBABLY have a defective (missing, corroded, etc.) ground somewhere else. This is a dangerous condition and you should not use the outlet until this is corrected. First try tightening the wire nuts in the bo
jmg967, that would be true if the ground were actually connected to anything. It probably isn’t. Ground and neutral are supposed to be connected way back at the power entry point of your house. You only need hot and neutral to power a circuit. Normal operation of a circuit sees exactly the same CURRENT on the hot and neutral wires. Ground does not normally carry any current. Your house won’t catch fire, but the presence of a three-prong connector leaves the poor shmuch who moves in later thinking he has a grounded outlet, and he does not. ( A house inspector will check the ground integrity during an inspection and he’ll catch this, but it is generally bad form to put a 3-prong outlet in an ungrounded circuit and I am sure it’s not to code.) Anything you plug into your new ‘3-wire’ plug that uses the ground wire as a safety feature will have a floating ground. Not good. Surge and transient protection will still work, since the suppression is applied between hot and neutral. Your compute
It sounds like you have a bad ground. The resistance between the ground prong in your plug and the ground rod outside your house (or cold water pipe where it enters the ground), should be very low. Ideally it would be zero, but that never really happens. If the resistance is high, due to a broken ground conductor somewhere (not hard to believe; bare copper wires are fairly fragile) then the tiny current allowed through by your voltage meter — ideal voltage meters pass no current, but in the real world, they all allow a few mA through — decreases the voltage difference from one side to the other. If you wanted to confirm this, you could go to another outlet with a known-good ground (tested the same way), attach an extension cord to it that you know is good (test continuity of the ground conductor first, obviously), and test from the hot of the questionable outlet to the ground of the extension cord. It should show 120V. If it does, as I suspect it will, then you need to fix your ground.
This is a classic symptom of a floating ground. This is caused by by either bad or non existent connections. Connecting the ground wires to your box isn’t going to make any difference unless the box has a separate ground connection (say from metal conduit which may be what your tube are; a picture would help) but it’s a good idea for safety. This can cause a problem with computer equipment if the ground experiences transients because the electronics aren’t expecting 0V to float around. I’ve solved many computer crashing problems in commercial settings with an electrician. First place to start is in your panel. Turn your service breaker off. Check to make sure that the wire leading to the fuse/breaker controlling this circuit has an associated ground and neutral and that the ground is connected to the ground bus. If there isn’t that means you can’t really depend on the ground wire to provide a ground unless you can figure out where it terminates. Second you need to retwist all the groun
