Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Open Ground Wiring Diagnosis

  1. Mar 13, 2005 #1
    I am rewiring my old house (35+ years old) that had non-grounded aluminum wiring (very dangerous!) with good ol' copper/grounded wiring. I have four rooms done and in each room I have put in wall outlets that go to a switch by the door. When I finished the rooms, I used my circuit tester to make sure everything was wired and working properly and there were no problems. I have now gone back and checked the switched outlets with my tester and three out of the four rooms (the newest room does not have the problem) are now showing an "Open Ground". One thing to note is that the light on the tester does not illuminate brightly. The light is dull but still lit. When I turn the switch on, the tester says there is no problem.

    I have used my meter to test continuity between the ground bar in the sub-panel and the ground wire on the outlet that the tester is showing has the open ground. I have 0 Ohms of resistance so I know there is no broken wires so now I am really confused.

    Everything I have read says this is bad as it is a potential shock hazard so I need to get it fixed. I have checked and rechecked the wiring and everything looks fine. I have also gone as far as moving the outlet from the room that does not show the open ground to the room that does show the open ground but that did not work. The problem stayed in the same room so I know it is not the outlet itself.

    Can anyone tell me why the tester is showing an open ground? Where can I start to diagnose the problem.

    Thanks in advance for your help.
  2. jcsd
  3. Mar 13, 2005 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Please provide the make and model of the tester; it's possible it's a problem with the tester, not a problem with your wiring.

    - Warren
  4. Mar 13, 2005 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    A room specific problem? Sounds more like you missed getting a good ground connection in the panel/box --- just a guess. Is the "bad" room on a separate breaker/circuit from the "good" room?
  5. Mar 13, 2005 #4
    The tester is an A.W. Sperry CA-300A. It is just a little handheld tool with three lights on it. I doubt it is a problem with the tester since one room does not have the problem. I would expect all rooms to show a problem if it was the tester. Is there anyway I can test this with a meter? Meaning, if there is an open ground, will there be any voltage on the ground wire? If so, how would you measure it?

    Each of the fours rooms are on their own 15A circuit breakers (using 14 gauge wire) fed from the same sub-panel. I verified that the ground was working properly when I installed the sub-panel.

    Thanks again for your help.
  6. Mar 13, 2005 #5


    User Avatar
    Science Advisor
    Gold Member

    I don't know anything specific about the tester but my guess is that it is lying to you. It tells you that everything is good as long as you have 120 volts to the outlet correct? It's a little gadget you plug into an energized outlet to tell you if everything is working correctly? If so, then why are you testing it with the power off? If the wall switch is off then the hot wire is interrupted. I don't think this is a valid test mode.
  7. Mar 13, 2005 #6
    I don’t think you have a problem but to make sure I’ve understood your situation:

    1 - You’ve wired the suspect outlets via a wall switch. The “hot leg” is disconnected when the switch is in the off position.

    2 – Your tester is a three light device; the illuminated lights are coded to respond to various wiring configurations such as OK, neutral-hot reversal, bad ground, etc.

    3 - When using the above tester the device reads all okay when the outlets are switched ON and a bad ground when the outlets are switched OFF, but the lamp is dim

    4 –Is what you refer to as a meter actually a digital voltmeter? If so, it may have a very high input impedance of more than 1meg ohm per volt on the AC scale.

    4 – With the above meter, you have measured 0 ohms form the ground bus in the main breaker panel to the suspect outlet.

    If all above is correct I believe you do not have a problem. The lamp in the tester that is illuminated and indicating a bad ground when the outlet is switched OFF is responding to a capacitive coupled voltage. If the bad indication disappears when you switch the outlet ON, I believe that verifies the situation.

    If your voltmeter is the high impedance type, you can probably measure the “voltage” that is illuminating the bad indicating lamp. Switch the outlet OFF and measure from the “hot leg” to ground. I think you will measure in the order of 60vac. That reading can be ignored and is what my electrician friend refers to as “phantom” voltage. If you use an old-fashioned, low input impedance voltmeter, the reading will likely be 0vac. Lacking that put a 100kohm resistor across your meter terminals to force a low input impedance reading from your digital meter.

    If what I described occurs, the only problem you have is measurement technique.
  8. Mar 13, 2005 #7
    Thanks again to everyone for all of your help.

    All of your questions/assumptions are correct.

    I am using a digital volt meter (it is a Fluke 88) to test the continuity between the ground leg at the outlet and the ground bus of the sub-panel.

    My only two concerns were:

    1 (and most important). Is this a dangerous thing for my family?
    2. Will an inspector fail the inspection if they use a similar tester?

    From all of the responses, I think the answer to number 1 is no which will help me sleep better.

    As for number 2... I have seen inspectors use similar testers (when I bought this house a year ago) so I will just have to trust that I did everything to code and there is no problem regardless of what the tester is saying.
  9. Mar 13, 2005 #8


    User Avatar
    Science Advisor
    Gold Member

    Make yourself a test light. Go to a hardware store that has a decent electrical section and get a light bulb socket with a pair of wires coming out. These are used in construction as temporary sockets in ceiling lights before painting and etc so the final fixture isn't damaged. They will be encased in rubber.

    Solder on a couple of stiff wires for probes because the wires they come with are stranded. Wrap your probes in enough tape to protect you from a shock yet allow you to probe with them. Screw in a 100 watt light bulb. Go around to all of your outlets and probe as follows:

    Start with the light switch ON.

    One probe in the narrow slot, one in the wide slot and the bulb should fully light.
    One probe in the narrow slot, one in the ground hole and the bulb should fully light.
    One proble in the wide slot, one in the ground hole and the bulb should NOT light.

    Turn the light switch off and repeat the above. The bulb should NEVER light.

    NOTE: If the outlets you are testing in this way are on a ground fault protected circuit you will trip the ground fault interrupter when you probe between the narrow slot and ground hole.
  10. Apr 10, 2005 #9

    Where is the sub-panel near the new work?

    First: when running a sub-panel you must run a 4-conductor cable, from the main panel to the sub, and the Neutral at the sub must be Isolated from ground, i.e., the neutral should be white and the grounding conductor can be bare but the isolation at the sub panel is important.

    If you ran a 3-conductor from the main panel to the sub, the neutral will double as neutral and fault path back to the breaker protecting the subpanel. This is known as objectionable current.

    At the sub panel a separate ground bar kit should be installed and the metal frame of the sub is bonded to this ground bar.
    The Neutral is left to 'float' no attachment except to the white coming into the sub from the main panel.

    When you land your wires at the sub all white wires must land on the neutral(and be isolated from the grounding conductor) and the bares go to the ground bar kit, this keeps the isolation in tack.

    If any neutrals are shared this could cause this problem. If there is any three wire circuits, i.e., Hot,hot and neutral make sure there is 240 volts between the two hots otherwise the neutral will be carrying 2 times the current it was designed for.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook