" ominal means the normal condition in which an equipment functions, in case of voltage, it is the voltage under normal conditions."

"Rated voltage means the highest voltage it could have."

I think the above is correct, as explained here :

http://answers.yahoo.com/question/index?qid=20071022211737AAV05Y0
"

__Rated voltage__ could also be the maximum voltage that a type of wire, plug, socket or circuit breaker is designed for. In that case, the rated voltage may be considerably higher than the nominal voltage.

__Nominal voltage__ is the standard value that is used when referring to a voltage level. If the nominal voltage is 220 volts, the actual voltage might be 5 or 10% higher or lower."
I understand your problem when it comes to understand the difference between "rated" and "nominal", as they are sometimes used as synonyms.

In fact, it all depends on the margins used to design a given piece of equipment.

A simple way to look at it is to compare a family car and a sports car, having maximum (rated) speeds of repectively 140km/h and 250 km/h :

If you consider that both cars have to be used on a highway at 120 km/h (nominal speed), the 'rated speed' of the family car will be very close to its 'nominal speed', while there is a big difference between the 'rated speed' and the 'nominal speed' of the sportscar.

So in French, "rated voltage" is sometimes used for "voltage nominal", but it is also translated as "voltage

__maximum admis__"

I hope this helps ....