Is a 250V and 660W lamp compatible with both: 

* 120VAC at 60Hz, and
* 220VAC at 50Hz?

And why? Also, does the frequency of the mains have any bearing on the lamp? 

In North America I was very surprised to notice a lamp certified for 250V but plugged into an 120VAC mains. My naive intuition is that the lamp should be certified for 120V. And similarly, the same 250V lamp would probably be compatible with a 220VAC mains (as in Europe), but I would have expected that a lamp in Europe should be certified for 220V. 

But I'm most likely wrong here, so:  

**Why is a 250V lamp compatible with both 120VAC and 220VAC mains?**

PS To avoid confusion, by "lamp" I mean a [light fixture](http://en.wikipedia.org/wiki/Light_fixture) and NOT a [light bulb](http://en.wikipedia.org/wiki/Light_bulb).