Consider a 3-pole circuit breaker with the pole sections side by side so that pole 1 and pole 3 have pole 2 in the middle. This is a very typical circuit breaker design for molded case breakers from companies like Cutler-Hammer and Square-D. These typically have a voltage rating giving a single number, and suitable for that voltage line to line and relative to ground. For example a 480 volt rating would allow use in a 480Y/277 volt WYE system, as well as a 480 volt corner grounded delta system. These breakers typically have ampere ratings as high as 1200. This would be in contrast to typical miniature branch breakers that have a dual voltage rating, one for line to ground and the other for line to line, such as 120/240 or 480/277.
Here's what I want to learn more about. Presumably these breakers might not have any greater line to line voltage rating than line to ground because they may be constructed with no extra insulation levels between the poles. The insulation between poles could very well be just the same as the insulation to the outside. This would contrast with placing two or three single pole breakers side by side where you would have double the insulation between poles, and some amount of greater voltage capability between them (perhaps).
What I want to focus on in a 3-pole molded case breaker is the voltage "endurance" capability (I'm avoiding the term "rating" here so as not to be confused with formal standard ratings such as UL, or manufacturer specifications that could incur legal liability) between the FAR poles, e.g. between pole 1 and pole 3 of a three pole breaker. Is there any reason there could not be an even higher voltage "endurance" between these far poles, as long as the middle pole in between does not have a relative voltage any greater than the formal rating?
Consider a 480 volt 3 pole breaker, such as Square-D model FAL34100 or others like it. It can be used on a 480 volt delta three phase power system. But what about a single phase system where there are two poles at 180 degrees phasing, with 480 volts relative to ground on each, e.g. a 480-0-480 system, with the middle pole connected to the grounded neutral wire of this system? Between pole 1 and pole 2 there would be 480 volts. Between pole 2 and pole 3 there would be
480 volts. But between pole 1 and pole 3 there would be 960 volts! But these poles are well separated by the middle pole.If that's a little high for you, then try a lower voltage breaker such as the QDL32100 (rated for 240 volts, including 240 delta) being used on a 240-0-240 single phase system connected as described, where it would see 480 volts between pole 1 and pole 3.
In what way could a breaker like that NOT have a voltage "endurance" capability of 960 volts between poles 1 and 3 when the middle pole is connected to the grounded conductor that has no more than 480 volts relative to either phase?
Please note that I am NOT asking about formal specification or rating. I'm NOT asking of such a connection would be in compliance with any electrical code, or if such a connection would be consistent with the purpose it is safety listed for. I am asking about the physics of the design, and any aspect of electricity that would make it not possible for a breaker to generally be capable of doing this. such systems are in common use.