Hi all, most luminaires I open here have a 450V rated capacitor. However I see many Brittish sellers offering capacitors marked "250VAC". Is this common place? Are they really safe across a 230V supply?

To clarify I am asking about direct connection of the capacitor across the line, not as part of a ballast that uses the capicitor to raise the lamp voltage.

There are two factor at play: Maximum voltage where the mechanism designed to prevent capacitor faults turning into dangerous conditions are effective and breakdown limit.
The first is influenced by the way how the capacitor is connected, so whether e.g. the current remains limited by the circuit even when capacitor turning into a short circuit or not. So it is possible for a capacitor to operate up to 450VAC as a motor run where the motor winding is limiting the current in case of a short circuit (assume the motor has a thermal overload protection), but only up to 275VAC when it is directly connected to mains (as a PFC,...), where a short circuit would mean kA short circuit current exploding the thing even before the breaker could respond.

The breakdown limit is mainly driven by a mechanism called Time Dependent Dielectric Breakdown, where exposure of a solid dielectric to high fields gradually draws in and traps charged particles, which then later on contribute to the formation of a conductive path. So as the name suggest, it makes the breakdown voltage dependent on the time how long the thing is exposed to it, higher voltages yielding shorter time till breakdown.
In practice this mechanism dictates how long would be the lifetime of the capacitor when exposed to a given voltage. So the maximum usable voltage depends on how long run time the capacitor is supposed to last.
Now generally motor applications are operated for way shorter times before needing major service or replacement just because they are mechanical devices (so things like bearings, sealing rings, belts just won't last untouched), so adding the capacitor on that list is not that big deal, so a lifetime of around few-k hours is good enough. On the other hand commercial lighting gear is expected to outlast quite a few lamps, so the lifetime requirement there is in the 100khour ballpark. So the same capacitor may be good at 450VAC motor run application for 4khour required lifetime, but only up to 275VAC as a lighting capacitor where it is supposed to serve 100khours. Or may be used up to 550VAC as a motor start (so being disconnected once the motor spools up), with 100 hour total operation lifetime making it good for decades of machine service life.
Also the anticipated operating temperature of the given application contributes significantly to the lifetime.
Liquid dielectrics are way better at TDDB, as there the affected spots either rise up or drop down, leaving fresh oil in the working zone, but then usually the solid support materials like electrode separation paper in capacitors become the weakest point.

With modern quality components it is usually the TDDB, what dictates all the ratings, as usually the protection features could be designed without much size and cost penalty. Making the TDDB going to higher voltages and time at once means just thicker dielectric and that means the component becomes bigger and more expensive.

So if you look into a more detailed datasheet, you will find not just one voltage rating, but a whole table with the rating varying even factor of 2 (so a range from 250VAC till 550VAC is not uncommon) depending on the reliability requirements. But often just one of the ratings is actually printed on the case, mainly with smaller size units. Larger ones often have the complete rating table on it (usually 3..5 voltage/service life and temperature range levels).

And on top of that come various safety standards, mandating its rating to be directly on the component label.

John, are you doing this for hobby of for professional installation?

For a hobby, it is OK, use and forget.
For professional installation, I'd look a bit more carefully, but most likely OK too.

For a series capacitor in a leading magnetic ballast, 450VAC rating is needed.

Modern dry film capacitors rarely fail with dead short and a firework, instead they are prone to capacitance degradation due to micro-arcing. Even good quality German made ones used in the expensive Lival fixtures.

Personally I always use PFC capacitors. It's true that for non-commercial use they make little or no difference to what you pay for the electricity, but it can certainly make a difference in how much current the lamp/luminaire uses. It can also make it more efficient; I have a Canadian Philips Goldeye 18W SOX bulkhead, it did not come with a capacitor and on its own runs at a very poor power factor of 0.24. Adding a PFC raised the PF to over 0.9 and also considerably reduced the operating temperature of the ballast. Why the manufacturers did not include one is a mystery to me, but no way would I run this fixture without one again.

All the fixtures I've seen here in the UK that do have PFC capacitors come with 250V rated ones and they all work fine. Some of the hybrid circuits with series capacitors do indeed use 450V or even 650V rated ones, but they tend to be the exception here.

Thanks Rommie, so it is common practice in the UK. I did suspect that might be the case.

RRK yes this is just for hobby use. Understandable needing the 450V unit if it were a series circuit but this is just for power factor correction direct across the line.

Medved thank you for the detailed explanation! I've only seen one PFC capacitor fail which was on a little magnetically ballasted PL-S inspection lamp. The lamp happened to be plugged into the outlet adjacent to an inverter welder. The welder was an older unit (no PFC on the input side, just single phase bridge rectifier) and apparently must have been imposing a fair amount of harmonics across the supply. Not sure if you're familiar with these lamps but the ballast is contained inside the 3 pin plug itself. The leads of the capacitor are connected directly to the pins of the plug, so there's minimal impedance. After some time it must have become too much and stinky capacitor smoke started pouring out of the ballast

Solution of course was to remove the capacitor as it really doesn't make much difference on a single 11W lamp! In this instance though I have a 400W lamp on a long cable run so the PFC capacitor will make a worthwhile reduction in voltage drop hence why I would like to fit one.