Hi all, this is my first post. Please forgive me if I've put this in the wrong place or violated any other rules (but please tell me so I can not do it again!).

I am trying to replace an old floodlight on the back of my house. When I pulled the old light, there is no AC power to the wires. The box has black, white and bare ground. When power should be on, black and white have no AC voltage. I measured the resistance between each wire, black and white are shorted together (~1-2 ohms), but they are not shorted to ground (hundreds or more of ohms I think).

I don't know where the wires go to inside the house so I bought a Fluke Pro3000, thinking I could trace the wires back to another box and find an improperly installed wire nut or something. Unfortunately, it doesn't seem like the transmitter can properly drive the load. When I connect the transmitter to ground and either black/white, the probe doesn't even register a signal when I want right at the source. It only registers a signal when I hover over the transmitter itself.

Any suggestions for things I can try or measure to try to determine where the issue might be? I'm hoping I don't have to open every outlet and switch box to check for errors, but if I have to let me know.

Thanks so much in advance for any advice!!!

 

Is there a nearby switch for the floods ? What else is on the circuit ? Is there anything else not working?

 

Did that fixture ever work.... that you know off ?

Measuring the resistance on the wires is a critical tool in troubleshooting. However, if you've correctly written down what you found..... you have a problem. You should find the white and ground shorted. You should find no continuity between black and white - although that will depend on anything connected to the circuit.

What type of cable..... NM (romex) MC (metal clad-BX) ?
As you found out.... you can't tone into a short.
You also can't tone out metal clad wiring.

You can tone on white and ground or black and ground.


A funny story..... I work on and off in a high end condo development. The condos are on the property in all different directions so that some people's front doors face away from the road/parking lot. These units have fixtures mounted on side walls to light the sidewalk at night to get to the front doors. Some of these lights are on a different owners wall.

The people ask me to check these fixtures. They claim to have checked/replaced the bulbs. I'll ask did it ever work. I get..... I think so...... sure....... of course. I remove the fixture and check with an ohmmeter between white and ground. If it shows open.... I know the light was NEVER connected. There are 20-30 lights in this complex that were never connected. I'm talking a 15 year old complex with many original owners.

 

The floodlight is on the back of a dining room, which was built as an extension. There are 2 switch boxes in that room. One of those has 3 switches and the other has 1. If it was switched that would be the most likely place I'd think. Also in that dining room is a ceiling fixture so it could have gotten power from there. FWIW - I've replaced all the switches and the ceiling fixture a year or so ago. Its quite possible I messed something up there. Every other fixture appears to be operating as expected.

The sheathing on the wire was yellow. I'm not sure what that means.

As for toning into a short, I figured if the two wires were shorted to eachother, but not to ground, then it's not really toning into a short. But maybe the hot isn't connected somewhere, but the neutral is?

I do not know if that light ever worked. When I bought the house 2 yrs ago, the motion sensor light that was there had nearly disintegrated.

Thoughts on measurements I can make or tests I can run?

Thanks!!

 

How does an open between white and ground mean it was never connected? Should there be some high value resistance there?

 

Technically neutral and ground are the same potential. They are connected together at the panel. The white wire supplies the neutral to the circuit while the ground is just for safety.

If the two wires you clip a toner on to are shorted or are a very low resistance..... the toner will not send a tone out on it.

 

Ok - I rechecked the resistance measurements. I misstated before.

White - Black: 10M (high impedance)
White - Ground: Short
Black - Ground: This is the odd one. It flips between high impedance and short. Its like I can't get a good measurement, but I've tried several times.

I also took a plug tester and checked every plug I'd expect to be on the same circuit. They were all good, including the outdoor plugs, which are GFCI. The GFCI performed as expected.

Back to the resistances, if the hot was actually connected and really shorted to ground, wouldn't that be throwing a breaker?

PS - thanks for all the comments so far!

 

Yes.... provided that it was actually getting power and if you actually had hot there you would have smoked your meter if in ohms mode.
You said yellow jacketed wire which would be newer style NM-B (romex) cable.

Your black to ground measurement should be the same as your white to ground measurement as you said the white and ground are shorted.

 

You're right, all three nodes appear to be shorted together, though sometimes jiggling the wires changes the measurement. This could be because the stripped ends have some corrosion so maybe the meter isn't making a good connection.

In any event, if the hot wire was actually connected somewhere, I would think a breaker would be tripping.

Any ideas for debugging and finding the source of the short?

Thanks!

 

Always turn off the power before doing ohms or continuity mesurements.

When doing ohms or continuity measurements, at least one of the two places you are touching the meter probes to should not have other things connected to it. Label all wires before unhooking them.