We have been working in this restaurant repairing so much and last night we came across a box that was arching out from the 1/2" emt to the box. We killed the power to the box and this still was happening. We have seen other cases in this space that appear to be the same by evidence around the lock nut having lots of arching marks on it. There is also kitchen tables that are showing voltage. Our guess is that something has gone wrong in a refer that is causing the neutral to leak or something else from another space causing this. This building is 100 years old and 5 stories so this does not help. The City is digging a huge tunnel in the area too so if something was coming from the power company because something got damage and voltage is leaking? My real question what is the best diagnostic tool to show the true voltage that is being leaked? Does anyone know how to isolate 4" EMT from the building to the space without having to cut a piece off? Any ideas or suggestions would be great!! Thanks!!
EMT arching from box to emt bushing, prep table showing 120v, and more weird things!!
- Thread starter nicknorth
- Start date
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kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
To find out if current is coming from outside your building, open service disconnecting means. If current is still flowing, use a lot of caution here, but lift the grounded service conductor and or the grounding electrode conductor, use caution because when you open that current path if it is not part of a parallel path you will be exposed to full voltage of whatever is supplying that current. Of course in doing so you could cause problems for a neighbor - say they lost their service grounded conductor and are getting it via water pipe through your service - you possibly just opened what neutral they did have.
How can I test for this voltage? Normally we use our multimeter and find a ground, but if we are lacking a proper ground or open these conductors we might lose it? I could see running a temp GEC from the service to our testing, but would this work?
What if this is coming from the space? They don't have any large motors, the HVAC is from the building and I believe they only have heat strips that are not on anyways, but I will need to double check this. I was thinking about taking one appliance out at a time until we find something.
What if this is coming from the space? They don't have any large motors, the HVAC is from the building and I believe they only have heat strips that are not on anyways, but I will need to double check this. I was thinking about taking one appliance out at a time until we find something.
Here's some more info about the building:
It has 3 120/208 - 2,000 amp services in one mechanical room and maybe more, but I have to get with the building to find out. This space has a 40 space main lug GE 120/208 3 phase panel from the service with no ground conductor and 400 amp fuses. The mechanical room is underground and then there is 6 stories above it that are mostly IT companies. The switch gear is 50+ years old.
It has 3 120/208 - 2,000 amp services in one mechanical room and maybe more, but I have to get with the building to find out. This space has a 40 space main lug GE 120/208 3 phase panel from the service with no ground conductor and 400 amp fuses. The mechanical room is underground and then there is 6 stories above it that are mostly IT companies. The switch gear is 50+ years old.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Only thing you will test is voltage to other objects and voltage drop across sections of the current path, actual voltage applied to that path will only be measurable if you actually open that path and measure across the open portion of the path.
Measuring to the grounded conductor at the service could mislead you especially if it is not coming from your space as the service ground is likely at about same potential as what you are trying to trace down. Test to a remote probe in the earth outside or to a grounding wire on a utility pole - but be advised if that pole is part of the path to your service and the problem is on that pole it could still mislead you, so remote earth probe away from existing grounding electrodes or other bonded objects is probably best reference to true ground.
If you figure out the current is coming from within your facility then the grounded conductor at the service is likely OK for any other tests.
- Occupation
- Licensed Electrician
I agree with kwired, shut down the main or at least kill the panel to your unit and see if the problem is still there.
On this job we are notice that the conduits seem to be worn out and not making a full connection anymore so the ground path seems to be broken. I've never really seen this before, but I'm sure it happens a lot more then people notice. The only good way I see to solve this without having to install new pipe is to install a ground and bond all the boxes. What do you guys think?
We are going to do some more testing and meet with the building people today so hopefully we can start shutting down what is needed and find out about anything else electrical in this space and the building.
We are going to do some more testing and meet with the building people today so hopefully we can start shutting down what is needed and find out about anything else electrical in this space and the building.
- Location
- Illinois
- Occupation
- retired electrician
Bonding around the conduit joints will just mask the problem, not fix it. The real problem is not the arcing and sparking, it is the source of the current that is causing the arcing and sparking. You need to find and correct the real problem.
meternerd
Senior Member
- Location
- Athol, ID
- Occupation
- retired water & electric utility electrician, meter/relay tech
You would not normally see an EGC from the utility service transformers. The GEC should be bonded to the neutral (grounded conductor) directly or with a wire jumper or bus bar only at the service disconnect panel. Lift the load side neutral at the service panel and make sure the load neutral is not still grounded. If it is, you need to find out where the remote bonds are and remove them. Even on new installs, I (POCO meter guy) have found sub panels with the green bonding screw installed, bonding the neutral to ground. Only found because I always checked the neutral bus with our neutral isolated and the MBJ lifted to make sure there were no additional bonding jumpers downstream of the service disconnect. I got a lot of flack from line crews for going beyond the meter, but I found a LOT that were remotely bonded in sub panels. Missed by the inspector. Not good...parallel paths, objectionable current, etc.. In a properly installed and bonded system, the ground (EGC) and or conduit should not normally carry any load current. If you have conduit arcing, you have bonding problems. Could be deadly.
MannyBurgos
Senior Member
- Location
- Waukegan, IL
Also, make sure you don't have a bootlegged ground somewhere in the system. Bootlegged grounds are illegal ways of tricking the receptacle tester to acknowledging that the receptacles are properly grounded. This will cause EGC to become energized and depending on current, can lead to burn outs.
How do you see these bootlegged grounds or I mean what methods do they use to connect them?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
If one were to jump from the neutral terminal of a 120 volt receptacle to the EGC terminal to bootleg an EGC, but to be honest I would be even more concerned with arcing at raceway fittings because of a bootlegged neutral then a bootlegged ground. Say there was no neutral available and someone is depending on the raceway to carry their neutral load.
Both situations have dangers but the bootlegged neutral would have more potential for arcing issues at fittings.
MannyBurgos
Senior Member
- Location
- Waukegan, IL
I usually depend on my current tracer for commercial applications. What i do is isolate the grounded conductor(s) from the egc and closed circuit and use my current tracer to check for continuity between grounded conductor and EGC. You can also pull all light fixtures and devices out and check to see if the grounded conductor is terminated on the EGC after the main disconnect.
macmikeman
Senior Member
- Location
- Planet macmikeman
This is a grand opportunity to show how well afci breakers work- put one in and try it out........
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
You have to know what circuit is involved - and unless it is one of those GE's with no GF feature, it should trip anyway because of GF current if current is returning via the raceways instead of the proper neutral conductor.
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