It is common for MCCBs with 250a frames and above to be available as "sealed" noninterchangeable trip (NIT)and "unsealed" interchangeable trip (IT) breakers.
Yes, breaking the seal of a NIT breaker will void UL.
However, IT breaker you will not void UL unless there is an attempt to modify or repair the breaker. You can take the cover off because you are allowed to replace the trip unit and to add UL listed accessories in the field. I would like to see more technicians and electricians remove the covers of breakers where there are allowed to do so. If a breaker has interrupted some pretty significant faults the arc chute area will look like world war three. Most breakers may look almost brand new if they have never been opened or closed or simply opened and closed under normal load.
But refurbishing the breaker is another issue. How does one define refurbishing?
If there is no issue with taking the cover off there is no issue with taking it off to inspect the moving and stationary contacts. Most breakers that I've have had apart you can remove the arc chutes for a better view of the contacts. They slip right back in place. There is nothing in the instruction manual that I'm familiar with, that is Westinghouse and C-H, which prevents you from doing so. A breaker can look bright and shiny on the outside but you can't tell what the inside is like until you take the cover off.
If one attempts to refurbish the inside of the breaker cleaning can remove the lubrication that is required for the moving parts. Unless you are factory trained and authorized to do so the breaker may not function properly. And contact spring tension, latching mechanism, etc, etc. 40+ years ago Westinghouse do offer renewal parts for MCCBs which I found to be very interesting to say the lease. But things were a bit looser back then and liability was not as big an issue as it is today. I may have some old renewal parts sheets to prove it if I haven?t thrown them away.
And how about those breakers that have been submersed in food water? How many of those have been cleaned up and sold at a "good price" on the open market.
I sad to know that there are those who believe that as long as the hole in the panel is filled and the breaker is not required protect by tripping nobody will ever know the difference. One could conceivably put a dummy breaker in and as long as there are not overloads or short circuits nobody would ever know.
I would be very hesitant to purchase sealed breakers on the used market because I couldn?t tell what the breaker is actually like on the inside.
With and IT breaker that is not sealed I can simply remove the cover and inspect it. If it is clean while not looking as though it had been cleaned (refurbished), the contacts and arc chutes are in reasonable good shape and there is no sign of tampering or heating I would be inclined to accept it. With a used breaker I would feel much more comfortable if it were also tested according to NEMA AB4 a test procedure that I understand NETA uses.