"Equipment" is plural. If you want to use the word in the singular, you have to say "piece of equipment". "A piece of equipment is supposed to be used as a reserve." Just out of curiosity, what's the context? I might be able to give you a more natural sentence.

No. It is not clear what you are referring to but my guess is that you are trying to say that if the usual equipment is unavailable, spare or additional equipment is available. The word "equipment" is not used with an indefinite article - it is either "the"or simply "equipment".