277v on hot wire with switch off, 0v when switch on.

[EL-Mech-Tech]

I was replacing some Metal-halide fixtures with new LED Fixtures. They were hard wired, but I could not turn off breaker. They had a separate light switch, so I turned it off there. I checked for voltage on the switched Hot wire (BLK) to ground and it read 277v, turn the switch on it is 0v. The only thing I can think of, that would cause those readings, were if the hot wire is actually Neutral (WHT).

I thought it may have been reverse in a junction box when the lights were install but all is correct. The wires are in FMC and go from this room through two others above drop ceilings, then to an electrical room. I checked the breaker box and it is wired correct there.

I have some colleges that thought a three way switch would get these readings, I did not agree but I checked anyway and the switch is not a 3-way.

Am I correct in my thinking? Or is there something I am missing.

Thanks

 

[Little Bill]

Where are you taking the voltage measurements at?
Were the MH fixtures working before?
The LED replacements are not working?

 

[__dan]

If you're changing the light fixtures you would have to LOTO the power and verify it is off. Normally the LOTO is at the breaker. That is locked off. You say you cannot access the breaker and the LOTO failed at the verification stage.

Depending on where you work, the switch would not be used for the LOTO, and, it would be a terminating offense to work it with power on, or not locked out, or measuring power on the circuit you are reworking.

Electro mech tech, some places don't hire electricians and use mechanics who also know electrical for electrical jobs. I would have to ask if you are an electrician or a mechanic who got this assignment because of no electrician. That is very common I see.

There are a few reasons you could have 277 at the light with the switch off, probably most or all of them bad. If it were me I would trace and examine the wiring back to the source to see why, but if you're not familiar with looking at wiring like that it would be time to call an electrician, before someone gets shocked or working it live costs you your job. Using safety rule violation to turn over the work force is also very common.

You have to ask yourself if troubleshooting this electrical problem is within your specialty or not.

 

[EL-Mech-Tech]

Where are you taking the voltage measurements at?
Were the MH fixtures working before?
The LED replacements are not working?

I took the reading at the junction box where the fixture is wired in. I followed wiring back to the switch and to the opening where it leaves the room. Everything is the room is wired White to White, Black to Black, Green to Green.

 

[EL-Mech-Tech]

If you're changing the light fixtures you would have to LOTO the power and verify it is off. Normally the LOTO is at the breaker. That is locked off. You say you cannot access the breaker and the LOTO failed at the verification stage.

Depending on where you work, the switch would not be used for the LOTO, and, it would be a terminating offense to work it with power on, or not locked out, or measuring power on the circuit you are reworking.

Electro mech tech, some places don't hire electricians and use mechanics who also know electrical for electrical jobs. I would have to ask if you are an electrician or a mechanic who got this assignment because of no electrician. That is very common I see.

There are a few reasons you could have 277 at the light with the switch off, probably most or all of them bad. If it were me I would trace and examine the wiring back to the source to see why, but if you're not familiar with looking at wiring like that it would be time to call an electrician, before someone gets shocked or working it live costs you your job. Using safety rule violation to turn over the work force is also very common.

You have to ask yourself if troubleshooting this electrical problem is within your specialty or not.

There are lock outs for light switches, which I was using. You can lock out at a disconnecting device downstream from a breaker as long as the work to be done is down stream from the disconnecting device.

Otherwise when ever you work on a piece of equipment that is on a circuit with other equipment you would have to bring down multiple machines to work on one. That is why disconnects exist.

The problem is when I went to verify, I found voltage where there should not have been.

 

[__dan]

At the fixture verify the neutral is solid, no Volts above ground (< 250 milliVolts to ground) then resistance to ground, less than a few Ohms neutral to ground. One of the causes would be bad neutral and Voltage on the neutral, then through some load to where you are testing.

If there's never any Voltage on the neutral, then the Voltage is coming in on the hot. Again it's possible you have a shared neutral on a multiwire circuit and Voltage is coming back though some other load.

Also something could be miswired and I've seen hots from one transformer with a neutral from another. I would work back to the source tracing the wiring and possible shutting off other circuits to make the Voltage go away.

If you have a circuit tracer breaker finder put that on it first, hot to neutral with or without the switch on, or both ways. See if it traces clean to the breaker, meaning, if the circuit tracer starts to detect signal going everywhere, you would have something miswired with shared conductors from another system. If it traces clean back to the panel, dump that breaker. A lighting branch circuit would be only lighting and no other equipment. If you don't have a circuit tracer, get one. The Ideal unit is great.

Also how much Voltage. Is it the full 277 or is it 90 something Volts. It's possible to induce a floating Voltage on an open circuit run with other conductors as long as the load is disconnected. If the neutral were connected to the load, the load would pull the other side down to zero where the neutral is.

 

[Dennis Alwon]

It appears there is a break in your wire somewhere. When the load is introduced the voltage will drop to zero if the conductor is damaged. There is enough contact to read voltage but not enough to operate the light. I am guessing these circuits are outdoors.

 

[rambojoe]

Or a timer/ems/single leg photocell system... Id also triple check voltage on original lights. What type of facility and where are the lights?
"A break" is a great way of looking at lights... You will look into every possible way they are controlled. Even occupancy sensors are used incorrectly.

 

[coop3339]

Could it be a switched neutral? With the switch off it would appear as a hot. When the switch is closed it would look like 0v to ground.

 

[LarryFine]

Could it be a switched neutral? With the switch off it would appear as a hot. When the switch is closed it would look like 0v to ground.

Test the unswitched terminal against both a known neutral/ground and a known hot.

 

[guschash]

Are these lights wired for 480 . There was time when we wired all our high bay at 480.