U-Shaped Fluorescent Tubes

[Little Bill]

What is the strip that is around the u-shaped fluorescent tubes? It kind of looks like a reflective material and is definitely conductive. I had a customer saying he was getting shocked when trying to change one of those tubes. I asked him if he was touching the metal brace at the socket end of the tube. He said no, just the strip around the tube.
I checked the tube and measured 30V from the strip to ground, and the same thing from the metal brace to ground. This is a 2-tube light, and I think it is 22.5". I removed 1 of the tubes and measured 100V to ground. I didn't have a wiggy meter and didn't try to load it to see if it was phantom voltage. I did see a blister on the guy's hand that he said was from the shock he received.
I connected the EGC to the light properly because it was just bent around the screw but the screw wasn't tightened at all. I removed the ballast and looked at it but didn't see anything wrong with it. I then checked the tombstones and wiring. I reinserted the tombstones to make sure they were secure because 1 was loose. I put everything back and the voltage on the strip went away but there was about 20V on the metal bracket.
I told him I would check more later if he wanted me to as I was there doing another job and didn't have time to do anything further today.
Any thoughts on this, especially what that strip is and what it's for?

 

[jim dungar]

During start up this strip helps the arc make the U-turn. Fluorescent lamps, rapid start type, use their capacitance to ground as a starting aid (this is why they have a label saying they need to be mounted within inches of ground).

 

[sd4524]

"During start up this strip helps the arc make the U-turn." - That sounds like something you tell a first year apprentice just to mess with him.
I never noticed this or knew that. Thanks

 

[stevebea]

"During start up this strip helps the arc make the U-turn." - That sounds like something you tell a first year apprentice just to mess with him.
I never noticed this or knew that. Thanks

Sometimes fact is stranger than fiction !:blink:

 

[Little Bill]

I don't think anyone has answered why this strip would have voltage on it on the outside.
Anyone.....?

 

[kwired]

Probably capacitive coupling. Is it in contact with a grounded surface someplace to make the strip become grounded? It most likely is supposed to be.

 

[stevebea]

Probably capacitive coupling. Is it in contact with a grounded surface someplace to make the strip become grounded? It most likely is supposed to be.

Actually the strip Bill is refering too does not make contact with the fixture at all BUT it needs to be within 1/2" of the grounded fixture housing to work correctly.

 

[kwired]

Actually the strip Bill is refering too does not make contact with the fixture at all BUT it needs to be within 1/2" of the grounded fixture housing to work correctly.

You sure about that? A T12 lamp is 1-1/4 inches diameter. Centerline of this lamp is already 3/4 inches from the fixture if the lamp is in direct contact with the fixture. A T8 may be able to make it within 1/2 inch easier.

I figured the metallic strip on the lamp probably made contact with the metal clip that holds the lamp in the fixture and was grounded through that.

Been a while since I seen one of these. I have always avoided using them unless necessary. 2x2 troffers with those cost more than 2x4 troffer, that was when T12's were the ones being sold, not really sure what is available in T8 variety.

 

[stevebea]

I figured the metallic strip on the lamp probably made contact with the metal clip that holds the lamp in the fixture and was grounded through that.

You may be right on that...... in fact it makes alot of sense.:ashamed1:

 

[hurk27]

You sure about that? A T12 lamp is 1-1/4 inches diameter.

A T-12 is 12/8th's or 1.5" a T-8/8ths or is 1" the "T" is the number of 8ths of an inch of the diameter of the lamp.

Just nit-picking:lol:

I know these lamps get darken ends faster if the lamp is not close to a grounded housing or cover, as the starting of the lamp is longer, as for the voltage on the strip I agree with the others as being from capacitive coupling, I think its just there to help with the capacitive coupling to ground as U lamps are mounted on clips putting them farther from the housing of the fixture.
My WAG

 

[kwired]

A T-12 is 12/8th's or 1.5" a T-8/8ths or is 1" the "T" is the number of 8ths of an inch of the diameter of the lamp.

Just nit-picking:lol:

I know these lamps get darken ends faster if the lamp is not close to a grounded housing or cover, as the starting of the lamp is longer, as for the voltage on the strip I agree with the others as being from capacitive coupling, I think its just there to help with the capacitive coupling to ground as U lamps are mounted on clips putting them farther from the housing of the fixture.
My WAG

You are right, 12/8ths is 1.5" somehow my calculator (in my head) malfunctioned

 

[Little Bill]

A T-12 is 12/8th's or 1.5" a T-8/8ths or is 1" the "T" is the number of 8ths of an inch of the diameter of the lamp.

Just nit-picking:lol:

I know these lamps get darken ends faster if the lamp is not close to a grounded housing or cover, as the starting of the lamp is longer, as for the voltage on the strip I agree with the others as being from capacitive coupling, I think its just there to help with the capacitive coupling to ground as U lamps are mounted on clips putting them farther from the housing of the fixture.
My WAG

After I properly grounded the housing and put the socket/tombstone in correctly the voltage on the strip went away or nearly. However I still had around 18V from the housing to the metal bracket at the pin end of the tube. I also went to the panel just to make sure everything was tight there. The neutral was tight and the circuit wire to the breaker was tight but one of the phase/leg lugs was loose, but i don't think that had anything to do with the voltage on the tube.

 

[mivey]

...A T8 may be able to make it within 1/2 inch easier...

But a T8 only needs 3/4 " to the grounding plane.

They generally are:
T5: 1/4"
T8: 3/4"
Other <=0.5A: 1/2"
Other >0.5A: 1"

The U-lamps have a reputation for being hard to start so I guess they added the stripe to help.

 

[hurk27]

But a T8 only needs 3/4 " to the grounding plane.

They generally are:
T5: 1/4"
T8: 3/4"
Other <=0.5A: 1/2"
Other >0.5A: 1"

The U-lamps have a reputation for being hard to start so I guess they added the stripe to help.

WOW T-5 only 1/4" I have seen a few two lamp strip lights with more then this, I think most of the lock in tombbstones for them are longer then .5625" to the center of the lamp holder? maybe not:? sure looked like it. AS for T-12 to T-8 or T-5 conversion, wonder if this could cause a problem, never seen any as of yet?
Good info, not seen those figures yet Thanks

 

[mivey]

WOW T-5 only 1/4" I have seen a few two lamp strip lights with more then this, I think most of the lock in tombbstones for them are longer then .5625" to the center of the lamp holder? maybe not:? sure looked like it. AS for T-12 to T-8 or T-5 conversion, wonder if this could cause a problem, never seen any as of yet?
Good info, not seen those figures yet Thanks

Those distances come straight from page 17, Table 3 of ANSI_ANSLG C78.81?2010 (a free download from www.nema.org).

For single-base lamps like the u-shaped, square and circular lamps, see page 15 Table 4 of ANSI_IEC C78.901-2005 (also a free download) and you will see 1/2" specified but you will find 3/4" allowances for some lamps with wide ground planes.

Some conversion issues are discussed in this document by Halco:
www.halcolighting.com/pdf/WP/T8-IS-Retrofit.pdf

The internal tombstone shunts and re-wiring of the ends seem to be the issue with T12 to T8, of course. I have done these with no problems.

I'm not sure what would happen with going from T12 to T5 and the starting aid issue as I have not done a conversion but just replaced the whole fixture. It looks like the retrofit kits have a reflector that goes along with a pin adaptor.

 

[hurk27]

Those distances come straight from page 17, Table 3 of ANSI_ANSLG C78.81—2010 (a free download from www.nema.org).

For single-base lamps like the u-shaped, square and circular lamps, see page 15 Table 4 of ANSI_IEC C78.901-2005 (also a free download) and you will see 1/2" specified but you will find 3/4" allowances for some lamps with wide ground planes.

Some conversion issues are discussed in this document by Halco:
www.halcolighting.com/pdf/WP/T8-IS-Retrofit.pdf

Thanks

The internal tombstone shunts and re-wiring of the ends seem to be the issue with T12 to T8, of course. I have done these with no problems.

Most T-12 wont have the shunts and the most of the T-8 parallel wired ballast will only have one wire for each lamp end with a jumper between two lamps at one end, I find these to be the easiest to retrofit as all you do is connect both wire from each tombstone together and connect it to the corresponding ballast wire.

I'm not sure what would happen with going from T12 to T5 and the starting aid issue as I have not done a conversion but just replaced the whole fixture. It looks like the retrofit kits have a reflector that goes along with a pin adapter.

Now the T-12 to T-5 retrofit is a whole different animal, as the tombstones and ballast are different, and the lamps are shorter, I have found some kits that like you said come complete with every thing already assembled that just fits into the T-12 socket, but I do not like this approach as you have to feed the power supply (120 volts) to the existing tombstones, which in many cases are already old and brittle, just don't like that approach, or the fact the only one I have found are the 28 watt lamp ones, but I did find some T-5 tombstones that can be screwed to the end plates just a little in from the existing ones, and the ballast mounts inside of the fixture just like the original one did, and allows you to use the 54 watt lamps and ballast, not really a UL listed way of doing them but it works better then the kit way.

 

[Besoeker]

"During start up this strip helps the arc make the U-turn." - That sounds like something you tell a first year apprentice just to mess with him.

I wondered about that.
We have one that that's shaped like a pretzel and it has no metallic strip.