1

I have a wall in my garage which has (on the same circuit with a breaker) 4 outlets and an outdoor light with a switch. For the sake of this question and attempted schematic below, I will use the following notations:

  • pipe character | to denote 14 gauge three strand wire source lines into a junction / outlet box

  • carrot character ^ to denote a line leaving the box / switch (14 gauge 4 strand wire)

  • plus character + to denote a wire splice / junction box

  • (s) to denote a switch

  • PS denotes the input power from the box with circuit breakers

  • and single letters labeling wall outlets

                      PS
                      |
                   ---+---
                ^  |     |
               (S)-B     |          
    D -----C-------------A  
    

So what I hope is clear from this diagram is that the line that feeds the wall in question from my garage electrical box is spliced in the attic and feeds two lines - (1) the B line which consists of an outlet (B) a switch, and a light; and (2) the A line which has three outlets, (A & C & D) chained together in series. For clarity on this question, all tests have been verified with and without an outlet - and the GFCI was removed for diagnostic clarity.

The point of this question is that outlets C & D report an open ground on using a an outlet tester. A & B are fine, and this has been verified with direct continuity testing to the circuit panel box.

  • The first thing I did was to thoroughly wire the grounds in the outlet boxes of C & D.
  • When this didn't work, I tested for continuity between grounds C & D --- Grounds were continuous
  • So then I tested for continuity between grounds C and A --- NO GROUND CONTINUITY
  • Tested for continuity between white wires in C and A --- White wires were continuous as expected as the outlet receives power
  • Tested for continuity between black wires in C and A --- Black wires were continuous as expected as the outlet receives power
  • Similarly - Tested for continuity between grounds C and the Circuit breaker box (PS) --- NO GROUND CONTINUITY
  • Tested continuity between White and Ground wires in A - Continuous as expected.
  • Traced the location of my wire using a cheap(ish) circuit tracer and know roughly where my lines are behind the finished insulated wall.
  • Tried connecting the circuit tracer in various pair combinations between the white, black, and ground wires of A and C

So I see a few possibilities to explain the broken ground:

  • there is a physical wire break somewhere, anywhere between A and C
  • there may be a outlet box behind the drywall somewhere, which I've been unable to find with a level or magnets, and like outlets C & D did not get a properly wired ground connection

There's a few things confusing me however:

  1. When I apply the energized probe of the circuit tracer direct to the ground wire in A the entire wall gets energized and the probe is overwhelmed by signal. I don't expect drywall or paint to be electrical conductors so I'm truly shocked that a couple of AA batteries are energizing the wall with a field that the induction wand is picking up on so strongly (several feet away). Anyone have any thoughts or explanations?
  • When I try this on the ground wire at outlet C, I can't reproduce the overwhelming positive signal. Instead the signal during testing is only ever so slightly higher than signal detected when using two insulated wires (black and white). Even in this case I have to have the probe right up against the wall. To me this makes sense as I think the two electric fields being sent down each strand of the wires connected to the tester (not continuous) are detectable with the induction wand (probe) and if one isn't insulated that signal will be a bit stronger with less insulation based field attenuation.
  • Unfortunately this cheap electric field induction tester isn't precise enough to find a broken wire. I bought it hoping there would be a significant signal drop off, but this wasn't the case. Anyone know of a trick to make these scanners a little more sensitive (I've tried what I think is referred to as a remote ground, but with no noticeable difference)?
  1. A friend has suggested that a drywall screw may have broken the ground wire and has remained in direct contact effectively bridging the ground (energized during testing) with the drywall, dust, paint, and whatnot on the wall. Anyone ever seen this before? Any good way of finding the offending screw invisible under paint and/or mud?

  2. When I test continuity between White and Ground in A it's continuous - as expected, but for some reason this isn't enough to make the ground in C when the outlet is wired up and you'd expect continuity given that the neutral whites are continuous between A & C. Would the broken ground explain this? Or does this hint at more fundamental problems in outlet box C?

Thanks for the help!

2
  • 1
    Re: 3. Ground is connected to neutral (white) only at the electrical panel, and neutral is not connected to the ground wire at C, so ground not continuous to neutral at C is as expected if ground wire is broken between A and C.
    – Armand
    Sep 11 at 4:05
  • 1
    What kind of cable? Any conduit? What is the wall made of - drywall or maybe plaster? Endoscopes with 15-foot cables are less than $50 online these days - maybe you could run one out of one the electrical boxes to see what's inside the wall?
    – Armand
    Sep 11 at 4:10

1 Answer 1

2

As commented by Armand, ground and neutral are connected in the breaker panel. So it is expected to have continuity between ground and neutral in most places, and the lack of that continuity is exactly the same as the open ground problem that your 3-light tester detected.

It sounds like you truly have an open ground.

I believe what you are calling "14 gauge three strand wire" is what is usually referred to as NM 14/2. NM = non-metallic cable (a.k.a., Romex, which is a brand name), 14 = 14 AWG, 2 = 2 insulated conductors (black hot, white neutral in typical usage) and the bare ground is not counted in the number. Similarly, "14 gauge four strand wire" is NM 14/3 = black/red/white.

In fact, most of the time the ground gets ignored because as long as it isn't broken, it is extremely simple - connect it to every metal box, connect it to every device, all grounds together (unlike hots which are very picky and neutrals which have to be separated by device), etc. But in your case it truly doe matter.

The most common problems are rodents and nails/screws. I doubt it is a rodent because only the ground is broken, but anything is possible. A nail or screw through a cable happens quite a bit, despite rules about protecting cables from damage. A hidden junction box is also a possibility.

I can't explain the strange reading of the circuit tracer. The only thing that comes to mind is if the broken ground wire is in contact with metal mesh holding plaster - but that would be plaster, not drywall.

At this point, I would suggest cutting a hole roughly half-way between A and C to check the cable. Hopefully if you put the tracer on ground at C then if you pick a random spot in the cable you will be able to find if it is good at that location or not. The end result will likely be replacing a section of cable - possibly the entire run from A to C.

There are two other methods which would make this safe but probably not code compliant:

  • Retrofit ground. If you have an old ungrounded receptacle, you can legitimately run a separate ground wire (i.e., not part of a cable or conduit) to another grounded location. However, that is meant for retrofit and I am pretty sure it would not apply here.

  • GFCI in lieu of ground. You indicated you have GFCI, temporarily removed. If you never had grounding, GFCI could be installed to provide largely equivalent protection without running a ground wire. (Some labeling would be required, but that's easy). However, once you have grounding I am pretty sure you can't rely on GFCI as a solution.

0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.