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I have a broken fluorescent tube (housing is intact, but it flickers instead of lighting up properly) that I have to change (bottommost type in the image).

I can turn it about 20 degrees, but it does not come off easily. Is there a rule (of thumb) on how many degrees one has to rotate such lamps? In other words: do I have to rotate further or apply more downwards force to remove the lamp from the brackets? I don't want to break the glass...

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What type of bulb does it use? – Comintern Jan 10 at 16:32
    
Wiki to the rescue. See added image. – Jasper Jan 10 at 16:50
    
alfeema's answer is correct then - those are almost always 90 degrees. The resistance you're feeling is from the socket contacts pressing against the two connectors on the end of the bulb. They should pretty much just drop out after you turn them to the right angle. – Comintern Jan 10 at 16:55
    
In cheap lamps sometimes they get stuck behind the contact and will never turn without breaking the pin off or the fixture itself. Spread the housing until you can sneak one end out. – Mazura Jan 10 at 19:52
up vote 5 down vote accepted

The bottom two are bi-pin fluorescent tubes. There are two types of bi-pin socket you will encounter on straight tube T5, T8 and T12 fluorescent lights.

This type requires 90 degree rotation.

Removal can be a real pain the first time you do it as the pins can weld to the brass contacts and you will end up breaking the socket if you get rough with them. I use silicone spray with a straw to shoot some into the socket when I encounter one that's semi-spotwelded itself in. It's non-conductive and prevents future issues. Rotate the tube 90 degrees so they align with the slot and pull out. Also, if the tube is too long to allow it to drop out of the fixture opening, there often will be a slot in one end to drop the pins into so the other end will clear the opening.

Align the pins vertical, insert the tube until it stops and rotate until you feel both pins click in place, it is possible to have only one set engage in the indent in the brass contacts that are visible.

enter image description here

This type pushes straight into the socket and removal is straightforward.

enter image description here

The top two tubes in that picture are compact fluorescent and pull straight out of the socket.

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Re silicone spray: can it lead to poor contact on the new lamp? – Jasper Jan 10 at 20:59
1  
In 20 years of changing fluorescent tubes, the answer has been no. If you had any experience with electrical contact cleaner (R-11) as was used for old TV tuners and electrical switches, some of it came with silicone lubricant. The wiping action as you install the tubes will ensure metal contact. I got tired of replacing broken sockets. – Fiasco Labs Jan 10 at 21:11

According to this video, 90 degrees:

https://www.youtube.com/watch?v=oh3a_p7C104

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Two hands on each end did the trick. – Jasper Jan 10 at 17:29

When installing a bi-pin fluorescent bulb into an overhead fixture, you slide it up into the two end sockets evenly with the pins in a 0-180 degrees (vertical) orientation, and then turn it so that the pins are in a 90-270 degree (horizontal) orientation.

If you are close enough to the socket you will be able to see the brass pins as the bulb is rotated. Also, the socket clips (contact points that are energized) are indented so you will be able to feel them as they "click" or latch in.

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For me, 0 degrees is "3 o'clock". Perhaps 12/6 and 3/9 o'clock is more generally understood. – Jasper Jan 10 at 18:50
    
90. Mathematical convention: starting at 3 o'clock and counting counterclockwise – Jasper Jan 10 at 18:58
    
Compasses use 0 degrees as "North", which is also up. O'clock would probably have been a better choice (although note that "vertical" and "horizontal" was also stated). – Daniel Griscom Jan 10 at 23:33

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