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I’m curious as to why after replacing T12 HO bulbs they don’t come on if power was on when replacing. I come back hours later and they are finally on. Replace with power off and they work right away. Ideas? Thanks.

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    Welcome. Replace in what? Please read your post as if you aren't inside your own head and add clarity so we can help. – isherwood Dec 11 '20 at 14:35
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Depending on the ballast type the lamps need to go through a “start cycle”. Live swap is not the best thing for a ballast and although not usually dangerous it’s never a good idea to change out lamps live.

What is the start cycle, when a ballast is first fired the voltage spikes depending on the type well over 300v until the gas in the tube ionizes and starts to glow, at this point the voltage is quickly reduced to control the current this is why you need to use the correct lamp type for the ballast, wrong lamp type and the lamp life is drastically reduced or the light output is reduced. Many times the lamps will light on install on mag ballast but solid state need to go through the cycle and may have turned that lamp channel off because of the bad lamp or open socket. The next power bump the ballast goes through the start cycle sees a good lamp and it works.

With electronic ballast I pull the power to that fixture prior to relamping why most ballast I have are electronic and the channel or lamp may be shut down, if not overheated Chanel’s need to cool before they will try a restart or strike. If the lamps don’t light up I know the ballast is bad this is quite common with electronic ballast that have been trying to light a bad lamp for months.

Today I convert all my fluorescent lamps to LED and have been doing this for years. My opinion the universal led retrofit lamps are the best light color is up to you but if you want bright work area lighting go 5000k or higher light color 6500k at 1/2 the wattage of a fluorescent will appear 2x the amount of light. If in a area like a living room you want a lower color in most cases 3000k -4000k is usually preferred but you still get the same light feel fro 1/2 the wattage.

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    The biggest mistake you can make when evaluating lighting is looking at the light. Instead, look at the stuff being lit up. LEDs are notorious for that eyeball searing intensity, and that makes them seem brighter. But if you ignore the light and actually look at the workspace being lit, I find LED installations are dimly lit. What really helps is CRI, however, and going from a LOL 20 CRI ratty old green fluorescent to a 75 CRI LED helps. But going to a 90 CRI modern fluorescent helps a lot more. – Harper - Reinstate Monica Dec 11 '20 at 19:33

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