UL Standard: 600Y/347 VAC

A rating of 600/347 on a breaker just means it can ONLY be used on a Wye (4 wire) 600V distribution system, NOT on a 600V Delta (3 wire) system. 600V distribution systems exist primarily in Canada but there are pockets of industrial areas in the US still using 600V.

Ungrounded 3 wire Delta systems are still very prevalent in North America. When rating a circuit breaker for interrupting capacity, the amount of ground fault (asymmetrical) energy that can flow in a grounded neutral system is significantly less than the energy that can flow in a delta system, because potentially the phase-to-ground voltage on a delta system is significantly higher. Because IEC based systems typically don't include ungrounded delta distribution, breakers made for that market do not have to contend with the higher ground fault energy and the interrupting capacities are theoretically lower if used in an ungrounded delta system. So to try to avoid the potential confusion, UL allows them to be labeled for the Line voltage, but insists on the "slash rating" that tells the user that it is only designed to be installed on a 4 wire grounded neutral Wye system. So if you bought a breaker that says 480/277, you cannot use it on a 480V delta system. Typically they are slightly less expensive than Delta only rated breakers, but once you get above 150A frames, I don't think many mfrs have different ratings any longer, making this only applicable to smaller breakers.

Many people unfortunately do not know this an mistakenly install slash rated breakers where they should not. Inspectors are supposed to catch that. MCBs ( the new DIN rail mounted Miniature Circuit Breakers being imported to North America) are a good example that comes up a lot recently NOBODY makes an MCB that is anything other than "slash rated" and recently most mfrs have released 480/277V versions. People keep using them on control panels etc. where they have no idea in advance what kind of system they will be connected to, and end up getting in trouble when inspectors reject them in the field.