The specs are just a mess, and I cannot ethically recommend exceeding the manufacturer's rating even when I have little faith in what (if anything) might underlay that rating.
The data sheet linked by PB is dated 2006. But since then, the C7-X appears to be made by Hammond - it's not admitted that I've seen, but the construction, the (sort of unique) shipping boxes, and the labeling all point to that direction. The old data sheet gives a hipot rating - that the voltage that it will take for 60 seconds without shorting, and is not exactly a precision test of the long term safe voltage(!). There was an old rule of thumb connecting them, but I can't find it on the web today; I suspect that it is so unreliable that it is no longer quoted. The old data sheet does not provide a working voltage rating.
The DC rating from the manufacturer is a test of whether you can sue or not, if it fails. What does that mean for safety? I have no idea.
We have a long experience of no problems at up to 500vDC with the C7-X, but most of it is with the old design, originally Triad and more recently made in their Mexican factory. Those were traditional chokes, "perfect layer" wound and impregnated with varnish of some sort. The modern ones are bobbin wound, most likely without paper between each layer, and it is unclear how thoroughly they are impregnated - bobbins are more resistant to the varnish. I have no reason to expect the same high-voltage performance from the two. I have no reason to believe - or to doubt - that the Hammond-made ones pass the 1500v hipot quoted, since I have not seen any specs on the current version.
In short, you must either rely on the few specs offered, or take your chances. Ain't that a bummer?!?!
