Arc across blown fuse (or open contacts of a Circuit Breaker):
240 VAC OCPDs are likely not going to have enough of a gap, and Arc will not be extinguished, when opened under load on a >240 VAC Circuit.
Throw open a 240 VAC EXO Switch (Disconnect), and a 600 VAC EXO Switch - both being the same duty rating (General or Heavy).
Notice the amount of travel for the handle, plus the intensity of the "THUNK!!!"
which results from opening the Switch.
Also notice the amount of travel for the "Knife-Blades" inside the Switch.
Opening the 240 VAC Switch under load, when connected to a 480 VAC Circuit, will likely produce an Arc between the contact points, which will continue to run for a long time - until something stops it (like the Fire Department, etc.).
The Arc containment and extinguishment considerations are the primary reasoning behind having Equipment ratings for 240 VAC and less, or upto 600 VAC.
You could use 600 VAC rated Equipment on a 240 VAC System, and have no undesirable results.
The same cannot be said of the opposite - using 240 VAC EQuipment on a 480 VAC or 600 VAC System will hide the disasterous effects, until the time comes to unmask its effects!... either a Fault or Overload scenario.
Now, there are "Dual Listings" on things, which may apply to the quoted text below:
I remember seeing a 250 volt panel full of 250 volt breakers running a 480 volt log home cutting line that had been in service for years.
Have seen Panelboards with devices which appeared to be "Too Small To Be >240 VAC Rated", but in actuality were the correct rating.
Square D's type "EHB" frames is a good example.
Have also seen the misapplication mentioned by Poorboy - and that is really scarry!
Yes, it has been working this way for years, and it is difficult to explain what the disasterous results will be ... someday, which makes it even more frustrating to have someone justify their "Who Cares?" attitude!
Been there too, Poorboy!
I have a feeling this kind of thing occurs from time to time, and am wondering what the greatest dangers of it are. Would a dead short be more likely to cause parts to fly? Is it mostly to handle the fault situations that the construction is different?
You are correct in feeling the reasons behind why there is 240 VAC rated Equipment and 600 VAC rated Equipment!
edited a stupid grammatical blunder!
hope there's not any more!
[This message has been edited by Scott35 (edited 07-17-2005).]