T-12 bulbs with T-8 ballast?

[40cpe]

Our local water association has three 8' dual bulb fixtures in one room. One fixture went out, they replaced the T12- bulbs with no results. They called me to look at it since I'm the resident bulb changer.

I found new T-12 bulbs in the fixture and after opening it up, found it had a T-8 ballast. The ballast is original to the fixture, no evidence of it being changed. The other two fixtures are identical to this one and have T-12 bulbs and are burning. I didn't open another up, but will assume they have T-8 ballasts, too. The bulbs have the oval shaped plastic terminals on the ends with the contacts in the plastic oval.

Obviously I'm not a fluorescent expert. Will T-12 and T-8 bulbs interchange in the same fixture? Will a T-8 fixture run a T-12 bulb, and Vice Versa? I replaced the ballast with a T-12 because they have a closet full of bulbs and all is well for now. I'm just confused about how these bulbs and ballast interchange.

Thanks for any explanation before I go to bed tonight and dream about this quandary

 

[alan camby]

I think those oval ends you speak of are H.O. ends. Yes there are t8 bulbs that will fit in fixtures that were originally t12 fixtures and vise versa.
You should always use the correct bulb for the ballast installed in the fixture. Ballast will have compatible bulbs listed on it. Yes other bulbs might light and work, but it is not right. Could cause a reduction in ballast and or bulb life. I would say that it could even be a safety issue.