What is the difference between a 24" F18T8 and a 24" F17T8.  The length and diameter are the same, but how are they different?  Is it something to do with the electrodes?  I once saw some failed F17T8s in a bathroom replaced with F18T8s and they would not start reliably.  If they lit they were fine, but if they didn't there was nothing you could do.  Flipping the switch on and off had no effect.  If the lights were left off for a while and switched on again, then there was a chance they would start.  With the correct lamps, they would come on dim initially and grow brighter over a couple of seconds, which is typical for rapid start.  With the F18T8s, the ends would glow orange for a  second and then the light would immediately come on at full brightness or go out and stay out.  Do you think these are programmed start ballasts?  I actually caught the maintenance guys opening one of the fixtures to take out the ballast and pointed out that the lamps were wrong!  They installed the correct lamps a few days later.  Can any of you explain the behavior with the 18 watters?

They were using the wrong lamps, if I remember, the F17T8s were made for instant start only, which is why you were seeing them not starting well

The 2' T8's are curious. A library at my college has F17T8 lamps in undercabinet fixtures in the study cubicles - they were preheat fixtures with the starters inaccessible within the fixtures. I remember one had a bad starter and I was able to light the tube by twisting it so only one pin on each end connected to the socket after it preheated.

But most of the F17's are electronic ballasted on instant-start ballasts. I had never come across an F18T8 until I bought this blacklight fixture . That fixture could potentially use F17 or F18 lamps according to the label inside - but apparently the ones you observed cannot. I'm not sure why T8 lamps exist in the same length with just a one-watt difference.

I think the 18w one was for appliances that needed a narrow 2 foot long lamp on a preheat circuit.

As for the F17T8s, I have programmed start ballasts for those and I tried some Philips 15w 2 foot T8s on those as well as some F20T12s.  All started fine.

In EU Countries, 18w 24 inch T8's are extremely common in 2x2 grid ceilings where the tubes fit nicely, usually found in 4-tube lay in trays ('troffers') taking advantage of their ability to be wired in series on the 230v mains.

17w T8's don't exist here, although I do remember once seeing a 16w T8 in a kitchen cabinet light. 

Our rule of thumb here at the supply house is, F17T8= electronic ballast instant start and
F18T8= magnetic ballast preheat circuit. Even before the advent of the F17, the 18 is rarely used except for appliance/vending use. Can't remember a single domestic or general lighting type fixture that ever used it.

I have better luck with F17T8s starting and running fine on F20 trigger ballasts, especially 1 lamp

I use some European F18W T8 lamps on Advance LC-25 preheat ballasts with Osram ST151 starters on 120V. They always start quickly and run nice and bright. They have been running for a couple of years now in kitchen under cabinet lights that get a lot of use.

I remember trying F17T8 lamps on preheat once and the starter kept glowing after the lamp struck, but not enough to close the contacts. I didn't try long term, however.

They mayhave similar power rating and similar length, but their internal construction and operating specs are quite far different:
The F18T8 was designed as a more efficient alternative of F20T12 in preheat circuits with just a series choke ballast (single at 120V, series pair on 230V) So the main specs are runninb arccurrent 0.37A, arc voltage 54V, preheat current 0.55A, no provision for reducing the ignition voltage, so the ignition voltage in the 300..400V range (so not available in other than preheat and/or special electronic ballasts). Practically it is one half of the (European) F36T8.

The F17T8 (if I remember well - if the following description really belongsvto the F17T8) is more like a half of the US F32T8, so a 0.27A/70V arc, but with coating reducing the ignition voltage, so suitable even for RS or cold start IS ballasts. In fact two of these in series may operate instead of single F32T8 on a cold start IS ballast (the type with just a single wire per lamp end; as there wont be any provision to heat the "middle"electrodes).

So the ignition voltage difference was the reason you have such problems to ignite the F18 on a F17 ballast. And once it ignites, the F18 will very likely overload the F17 ballast due to the lower arc voltage, so the ballast may overheat.

Definitely these two lamps are not electrically compatible to each other.

I have a few F18T8s, will they damage an IS ballast designed for F17/F25/F32 T8?

And will F17T8 do any harm to a F14/F15/F20 Trigger start ballast?

F17s seem to start fine on 1 lamp, 2 lamp and 3 lamp magnetic trigger start, where as the F18T8 needs to be touched to start on any of those, but works fine on IS

I have a few F18T8s, will they damage an IS ballast designed for F17/F25/F32 T8?

Yes, they may.

And will F17T8 do any harm to a F14/F15/F20 Trigger start ballast?

Depends, how the ballast will respond to the higher arc voltage of the F17T8 and how exactly it works in general. If it contains some separate starting circuit (electronic starter,...) and the starting circuit keeps activating, it may either overheat and or prematurely wear out (it would be designed with assumption of a just brief operation each start)

F17s seem to start fine on 1 lamp, 2 lamp and 3 lamp magnetic trigger start, where as the F18T8 needs to be touched to start on any of those, but works fine on IS

This is is the result of the missing starting aid in the F18. Your finger is just doing what the internal coating normally does on the F17...

I bought a f18t8 once thinking it be brighter in a f17t8 electronic fixture boy was I wrong, it was dimmer and flickered  the whole time with a greenish hue. Then I learned for preheat ballast only

I have had no trouble with F17 lamps on preheat unless they are somewhat mercury starved.

The F18T8 is an energy saving retrofit for F20T12 for preheat circuits. It can't operates on trigger start or rapid start ballasts, since it have krypton gas, which have higher starting voltage.