Strip Light Problem

I had a complaint that someone was shocked changing bulbs in the strip light. I'm working in a 2-pin twist-lock environment. I checked the light and found 0 volts frame to ground with the lights off. As I added red, green, blue the voltage increased to 60 volts. However, there is no way for the person changing lights to contact ground. I then checked the voltage frame to ladder. The ladder is aluminum. The voltage was 0 with no lights up, 6 with red up, 17 with red and blue up, and 25 with RGB up. The floor is painted wood. Any ideas why I should measure a voltage in this situation? I measure with an digital meter. I've heard some people say digital meters can pick up stray voltage, so I'm planning to try my trusty analog meeter.
 
First things first- Get stuff grounded! There is no way this can be emphasized enough! It is a dangerous situation.

As far as the voltage readings go, it is a stray voltage developed through capacitance between the wiring and the frame and may include some through leakage resistance as well. If there were a direct short, you would see full line voltage.
Volt meters are a very high impedance, so stray dissipation will show up. As for the outright shock, there is more to the story but getting the equipment grounded will fix both problems.
 
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You will most likely still get a reading although it may be lower. What is at play here is the meter resistance itself. The higher the meter resistance (Ohms per volt), the more of a stray you will see. Digitals tend to have a higher meter resistance than Analogs. To kill any stray reading, you should put a 7 to 25 watt lamp in parallel with the meter, essentially, meter across the lamp. That will eliminate all visible stray voltage.

The human body tends to be between 200 ohms and 50,000 ohms depending on skin moisture. It is very sensitive to current flow. Unfortunately, in the right circumstances, a lethal flow can be as low as 10 milliamps. That's 10 thousandths (or one-hundredth) of an amp.
 
John, thanks for the technical explanation. It adds to your credibitlity. I'll try the lamp in parallel. It will be a couple of days before I can get to the analog measurement.
 
I am sorry but all fixtures should be unplugged while replacing the bulbs.
 
I concur. One of my design mentors a few years ago told me that even if all of the channels on the light board are set to "0", there is still a trace amount of current going into the dimmers. Unplugging is your best and safest solution when performing routine maintenance.
 
I concur. One of my design mentors a few years ago told me that even if all of the channels on the light board are set to "0", there is still a trace amount of current going into the dimmers. Unplugging is your best and safest solution when performing routine maintenance.

It's truth. Learned that one the hard way.


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Auto range digital meters tend to do odd things as well, especially cheaper ones. I would check the wiring throughout the fixture, if you are in an environment that still has underground outlets(like my High School) how old are your fixtures? Old fixtures have cloth or asbestos insulation, and it breaks down over time, sometimes you can get a partial short against the chassis, that will not trip the breaker, but still give you a shock.
 
Almost all "cloth" type wire is asbestos. Non-asbestos wire usually is a rubber (or silicon) covered with a cloth (usually fiberglass if high temp) jacket. True asbestos could absorb moisture and could become slightly conductive. Areas where it entered or exited a fitting or metal grommet hole were likely candidates for leakage to the frame. Towards the end of the asbestos heyday, manufacturers started wrapping the copper conductor with a thin plastic tape before applying the asbestos cover. This stuff was a little better until the plastic tape began to break apart.
Needles to say if your system is wired in asbestos, electrical leakage may be the least of your problems! Over-flexed cables would shed asbestos dust. Takes about 30 years to kill you, so the shock might get you faster, but the end result is the same.
 
Initial thoughts - were talking about DMM’s but have not mentioned true RMS DMM’’s that should correct for this misleading voltage thing possibly due to the lamp warming current of the dimmers at least in part - though a good analogue meter also has its uses in addition to a True RMS meter.

That said... stop using the light and stop testing. If someone was shocked even by way of a high resistance or low voltage shock as described, it is still a lighting fixture that should not be used due to a short which could become worse due to use or heat in resistance due to that short breaking down what is left of the insulation. Someone changing a lamp while at full voltage could kill or lead to huge liability issues after reported. You don't want to continue using this light or testing it.

That or on an un-polorized plug, if plugged in the wrong way, your hot is now the screw shell of the lamp and its possible it is contacting the reflector for the short - beyond internal wiring or cord grip breakdowns. Also in making live the screw shell to the lamp/lamp base, if someone were to touch that screw or base while screwing in a lamp they would get shocked beyond should the socket be un-insulated or broken and it were touching the reflector. They are also getting electrocuted in this way as a possible how this was reported.

The lamp warming voltage given off on a dimmer is enough to feel this "short" how ever it was caused and luckily not enough to harm someone if the trim voltage is correct. Or it could be a high resistance short in touching the frame and being a better ground than the lighting grid. Literally your fly system/lighting grid is now the neutral/ground most likely in at least part for this lighting instrument and it's not good for the wire rope or any part of the grid.
That plus the asbestos.

In metering, I note you are doing frame to a ground of some type of ground found and also to the ladder in theory you are standing on... Aluminum ladders shouldn’’t be used for lighting, wood deck/rubber feet if of a certain moisture contact can conduct a path to ground etc. Stop metering it in the air at least because should while meterring it out you slip and both grab shorted light and another pipe in the grid that would be a better path to ground... That short just traveled between your arms by way of your heart and you can die! Again stop testing this, power off and drop the fixture until replaced or safe to use.

Might have been as simple as a reversed un-polarized plug that someone touched the hot screw shell to while in lamp warmer mode, could be screw shell touching reflector or any number of things. No way to tell in the air safely and given there is a problem detected, the fixture isn’’t serviceable or safe - drop it and have it repaired or replace it. Start with that one and the rest should also get a proper look at.

Yes there is ways to test what’’s up further while still in the air, I don’’t think I would be up on an aluminum ladder trying to troubleshoot them short of breaker off and even there enough has been described to pull the fixture anyway.
Side but related story..

I now own a c.1911 Chicago Cinema Equipment 4.5" PC floodlight fixture that was once the pin-rail spot for lighting it badly at a theater I worked at. Wasn’t the original crows’s foot (Not normal Edison) porcelain sort of straight blade plug found elsewhere but instead was a Mini un-polorized twist lock plug that’s non-Nema in even the ML 1 series.
 
Plugged it in backwards at one point I suppose in moving it over some I suppose (didn’t re-wire it yet.) Went to bend a piece of ½" Sch. 40 for drape weight to bend around the end some and I got sparks on my fly rail once I touched one part of it to another. Don’t think I got shocked, instead the pipe protector for the grid to the grid now linked by way of this pipe being bent was a better path of least resistance somehow.
 
The hot was now the neutral apparently for polarization of the lamp socket and something was shorted there. All was fine as long as the hot were plugged in correctly at the plug in path of least resistance, reverse the plug and that wiring problem became a problem in electrifying basically the pipe the light was mounted on as per a higher resistance short to ground than that of the neutral. Or I could be reversed of this in concept. Protector for the pin rail this pipe assembly was not connected to the pin-rail assembly. Once you linked the two together, they would spark if plugged in backwards on a wood floor etc...
 
Old timer was fine with all of this - that was how it was done in the non-grounded and un-polarized days, just reverse the plug and move on. I did install a properly wired index light over the pin rail before I left it. This fixture went into their bone yard that I years later bought and added to the museum in making safe.

Strip lights are not nice or fun lights to work on in having worked on lots of them over the years. Lots of types out there in the un-polorized type... re-porcelaining 1928 era reflectors is really messy amongst other types and problems with getting certain lamps for them. Total service call of the fixture is necessary and probably cheaper to replace it with something more modern. That an no museums are looking for strip lights for their collections I think at this point.
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