Diagnose An Open Ground

Reply

  #1  
Old 03-17-15, 03:33 PM
czizzi's Avatar
Forum Topic Moderator
Thread Starter
Join Date: Jul 2007
Location: USA
Posts: 7,231
Diagnose An Open Ground

Doing some renovations at a local hotel. Updating bathrooms with new countertops, sinks, tile, the works. In the process are creating a larger countertop in the open sink area (toilet and shower different room) so that it will include a breakfast bar of sorts. Microwave/coffee maker on top and Mini-fridge under the counter. Required me the cut in two new electrical boxes for outlets. I ran the lines when the wall was open and tapped into a previously abandoned junction box. Was the old wall A/C line that is tied into the room circuit. They abandoned it because the breaker was constantly tripping in rooms when the A/C was on and someone used a hair dryer. So they ran dedicated lines just for the A/C years ago and capped off these boxes.

Have done 18 of these rooms over the past few years and never had a problem. This last room suddenly shows an open ground. I twisted the grounds together in the junction box and have a single line that runs through one outlet, continues on to end at the 2nd. Both sets of outlets are showing open ground. Short of ripping everything out, how best do I determine if it is the buildings line that has a break, or something with my install of the outlets themselves. Can't imagine that I forgot to hook up the ground on the first outlet as I specifically remember twisting them together and cutting one leg to pigtail to the outlet.

With an analog multi-meter, how best to test the lines? Obviously black and white work fine, but don't want to short out my meter by taking either to ground.
 
Sponsored Links
  #2  
Old 03-17-15, 04:15 PM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 14,587
Since an open ground will not read voltage I would suggest testing from hot to ground down the line. When you read 0 you have found the open.
 
  #3  
Old 03-17-15, 04:21 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 46,143
What type of wire is there originally..... NM, metal clad ?
 
  #4  
Old 03-17-15, 04:36 PM
czizzi's Avatar
Forum Topic Moderator
Thread Starter
Join Date: Jul 2007
Location: USA
Posts: 7,231
Can't see much of the cable itself to get a reading of the markings, but it was an older white clad 12/2 with a thinner than usual ground wire. I hooked up to it with yellow clad 12/2 romex with ground.
 
  #5  
Old 03-17-15, 04:44 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 46,143
That would have been probably cloth covered NM cable.
The ground was one or two sizes under the main conductors.
 
  #6  
Old 03-17-15, 05:02 PM
czizzi's Avatar
Forum Topic Moderator
Thread Starter
Join Date: Jul 2007
Location: USA
Posts: 7,231
No cloth covered, white vinyl covered (or whatever the normal coating is). Block wall, furring strips, paneling and drywall over the top. I removed the drywall, opened the paneling enough to run wires, notch out furring strips (and cover with nail plates) and set up the new boxes which I had to excavate into the block wall to fit the remodel boxes. None of the wires were cloth covered. I only can see in each room less than an inch of the wire covering as those wires came into the box from above. All my work was from the bottom of the box down and eventually over under the counter height.
 
  #7  
Old 03-17-15, 06:29 PM
Member
Join Date: Nov 2012
Location: USA
Posts: 3,314
I would start at the source and be sure you have a ground there,test HOT to GR. and work the circuit from there until you find the open.
Geo
 
  #8  
Old 03-17-15, 06:35 PM
czizzi's Avatar
Forum Topic Moderator
Thread Starter
Join Date: Jul 2007
Location: USA
Posts: 7,231
I have had a rather large chunk taken out of my linesman pliers from shorting hot to ground by accident in the past. Please tell me specifically what I should set the multi-meter to, and how I should test the lines. If I test hot to ground what reading should I get? And would this reading indicate that the ground is bad at the source or further down at my hook ups. Thanks for all advice guys.
 
  #9  
Old 03-17-15, 07:14 PM
pcboss's Avatar
Forum Topic Moderator
Join Date: Jul 2002
Location: Maryland
Posts: 14,587
You would set the meter to AC voltage above the level that you expect to read. 120 volts would be a normal reading.

Meters do not short like a pair of pliers.
 
  #10  
Old 03-17-15, 07:36 PM
czizzi's Avatar
Forum Topic Moderator
Thread Starter
Join Date: Jul 2007
Location: USA
Posts: 7,231
Thanks, pc, that is what I needed to hear. Will test in the morning and see what happens.
 
  #11  
Old 03-18-15, 05:55 AM
Member
Join Date: Nov 2012
Location: USA
Posts: 3,314
If you get a reading of 120 Hot to Ground at the source then move on to the next outlet until you get 0 volts Hot to Ground and the problem will be there or between there and the last outlet you texted .
Geo
 
  #12  
Old 03-18-15, 04:07 PM
czizzi's Avatar
Forum Topic Moderator
Thread Starter
Join Date: Jul 2007
Location: USA
Posts: 7,231
At the incoming source wires in the original junction box - white to black 120v - black to ground 25v - white to ground 25v

Does that make any sense?
 
  #13  
Old 03-18-15, 04:14 PM
ray2047's Avatar
Group Moderator
Join Date: Mar 2006
Location: USA
Posts: 33,050
Digital or analog meter?
.
 
  #14  
Old 03-18-15, 04:31 PM
czizzi's Avatar
Forum Topic Moderator
Thread Starter
Join Date: Jul 2007
Location: USA
Posts: 7,231
It was with an analog meter.
 
  #15  
Old 03-18-15, 05:56 PM
Member
Join Date: Nov 2012
Location: USA
Posts: 3,314
Your going to need to go back to the panel or farther back on that circuit, check in the panel and make sure the grounds are all clean and tight.
Geo
 
  #16  
Old 03-18-15, 06:34 PM
czizzi's Avatar
Forum Topic Moderator
Thread Starter
Join Date: Jul 2007
Location: USA
Posts: 7,231
Well, I'm not qualified to be messing around with a panel. I will let the owners know of the predicament. They had some vandalism a year and a half ago where someone cut the lock and disconnected the sump in the underground utility room. The room filled with water and soaked all the panels. They dried them out and tested everything. It is a waterfront hotel and the sump is the only thing that keeps the cellar from flooding. They think it was a then recently released employee, but can't prove it. They also cut all the phone lines, opened a fire hose and flooded a dozen rooms and tried to break into the laundry. New security has been installed, but this may be a lingering memento of the day a couple of years ago. Small penance compared to some of the people that go postal and leave a trail of hurt in their wake......
 
  #17  
Old 03-19-15, 06:09 AM
Member
Join Date: Nov 2012
Location: USA
Posts: 3,314
Keep us posted on the out come, that being the case I'll bet there are a lot of corroded terminals etc.
Geo
 
  #18  
Old 03-19-15, 02:51 PM
Member
Join Date: Oct 2010
Location: USA
Posts: 55
I would use a line tester-a lot of meters don't load enough to be good troubleshooters.
 
Reply

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread
Display Modes