2x4 Lay-In Fluorescent Fixtures

[eddie_kerri]

A long time ago, I can remember changing 4 foot T12 lamps out in a shoe store while the circuit was energized (stupid). The 2x4 fixtures were butted end to end and I remember how hard it was to remove one old lamp to install a new lamp. The process seemed to go as follows: I would rotate the lamp, pull one end of the lamp out of the tombstone while the other end was still in the tombstone. The cathodes on one end of the lamp would drag across the metal housing until I got the lamp low enough to remove the other end out of the tombstone. My question is: why didn't the circuit trip when one end of the lamp was touching the metal housing of the fixture and the other end still in the tombstone? My understanding is the output voltage of a ballast is very high and would seemingly trip the circuit very quickly based on this scenario. I don't know why I was thinking about this today at work, but it popped into my head so I thought I would present this question to the forum.

 

[cadpoint]

The tub is full of gas and doesn't make a continual contact, as it was in direct contact with any tumb stone.

Yes in fact one end is in contact but the other element is not till you brush across the housing and run current on the EGC.

Remember that it's higher voltage but very lower current.

I hope you have your PPE on, gloves and eye protection!

 

[electrofelon]

Ok I'll be honest: never thought twice about changing a floroscent hot and probably done it 11,256 times, and no I wasnt wearing PPE either.

 

[mjf]

Ok I'll be honest: never thought twice about changing a floroscent hot and probably done it 11,256 times, and no I wasnt wearing PPE either.

11,256.....:lol:

 

[cadpoint]

I've only always worked with eye protectoion, so it's really second nature!

Besides it's over head work, drilled into me by to many job site rules, OSHA rules,...
I will stop and make comments to anyone in the field and working O.H.,

"What, you don't want to see your Sweety tonight" ???

 

[steve66]

Yes, the ballast needs to fire the lamp to get it to conduct, and then there is enough resistance in the lamp to keep it from shorting from one socket, through the lamp, to the other end of the fixture.

Now if you get one pin in the socket, and the other pin on the same end against the metal frame, you will proably make sparks fly. I did that at my house once, and now I take to time to shut off fixtures first.

 

[kwired]

At same time the output of the transformer (ballast) is separately derived (not connected) directly to the incoming lines. Remember those were magnetic ballasts and you had a transformer inside the ballast. Short those output leads directly together and you short circuit that transformer or a portion of it. The output windings have a high enough impedance they will not be able to supply much current, that doesn't mean you can't damage them if they remain shorted for a long enough period of time though.

This transformer was not a simple two wire to two wire transformer though. You would have a higher voltage from one end of the lamp to the other, but a fairly low voltage from pin to pin via another winding which the purpose was for preheating the lamp cathodes so they will conduct well enough to strike the arc between cathodes. Electronic ballasts generally do not preheat cathodes, I think most of them can develop a high enough pulse to start the arc without the need to preheat.

 

[kbsparky]

I have blown fluorescent tubes when changing them out hot. Shorting one of the prongs to ground can burn out the cathode instantly, rendering the bulb useless.

I suppose that one could conceivably blow a ballast doing this, especially the newer electronic ones.

I still change out tubes hot, using extra care to avoid this scenario. After years of experience, I can do it with relative ease.