I remember getting an Advance F20T12 trigger start ballast from a Home Depot store in the USA that also required a circuit interrupting lampholder.

Interesting. I think I remember some years back seeing a one lamp F20T12 ballast (could have been for one F40T12) at a True Value Hardware store and it mentioned "circuit interrupting lamp holder) and I always wondered what the heck that was for and why, because the only circuit interrupting lamp holds I've ever known were the stationary sockets on a slimline fixture. 

If anyone on the forum knows why some one lamp trigger start / rapid start ballasts specify one circuit interrupting lampholder, I would love to know

Cheers  😀

@Kyle1500
That is an excellent question, I have wondered the same thing. Theoretically the only ballast that should need those is instant start, any ballast that heats cathodes before striking shouldn't need them. And they are hard to find. Doesn't make sense. I have only seen it on smaller wattages of around 20w or less, but I haven't exactly been looking super hard so I could be wrong.

If anyone on the forum knows why some one lamp trigger start / rapid start ballasts specify one circuit interrupting lampholder, I would love to know

Cheers  😀

I think main idea for interrupting lampholders is electrical safety, to de-energize the circuit until the lamp is completely inserted, so no one will get shocked from the free tube end. Completely unheard of in ex-USSR countries, and probably in Europe/Asia too. Not necessary for actual ballast/tube operation.

@RRK
But shouldn't that only apply to instant-start ballasts? A partially inserted tube on an energized TS/RS ballast shouldn't be able to strike and kill you like it would on a IS ballast because both cathodes aren't warm. Unless maybe the OCV is high enough to strike on only one heated cathode, but that sounds unlikely.

Are you going to bet with your life if tube fill will really strike or not?

@RRK
But shouldn't that only apply to instant-start ballasts? A partially inserted tube on an energized TS/RS ballast shouldn't be able to strike and kill you like it would on a IS ballast because both cathodes aren't warm. Unless maybe the OCV is high enough to strike on only one heated cathode, but that sounds unlikely.

The "not striking" is just not reliable enough to base a human safety on it. It normally does not strike, but some mains spike or disruption (e.g. someone switches some load ON or OFF,...), or even static electrical field outside of the lamp and it strikes with cold cathodes anyway.
For lamp life it is enough such cold starts won't happen too frequently. And that is the criteria how the systems specifications are designed. Not that it should NEVER cold start.
So if you need to ensure people won't get zapped via the discharge in the lamp, the system needs to have some interlock mechanism in it, ensuring the voltage is applied only after the lamp is fully inserted.
For instance the sockets commonly used in Europe for bipin fluorescents allow the lamp to be inserted only when the slit is vertical, but then the contacts to the lamp are not connected. You need to twist the lamp in them to connect the power, but then there are no exposed prongs anymore.
Or electronic ballasts need to sense the presence of at least the cold side filament before they start.
And generally the short pins makes the risk of touching them when manipulating the lamp in live fixture somewhat lower.

With the long pins the risk of someone touching them when manipulating the lamp is way too high, plus the required starting voltage way too high to be really dangerous, the sockets require explicite cutout contacts/switches in the sockets...

I remember getting an Advance F20T12 trigger start ballast from a Home Depot store in the USA that also required a circuit interrupting lampholder.

Home Depot, no never got one there...but Hugh M Woods(a store now long since out of business), yes I bought a couple of those ballasts(don't remember brand). I never used them lampholders - just going with a standard one instead(I don't remember the stores even selling the interrupting type)
Also I've had various 1xF40 fixtures that had those...my tendency was to get rid of the circuit interrupting lampholder because they suck!

@RRK @Medved
Ahh I see, that does make sense I suppose.

Though at least in the US I really don't know if it is worth the effort, since our type A and B plug prongs can easily be touched and screw-base sockets can easily be probed with fingers. But I suppose it can't hurt to add extra niche protection where possible.