There are several ways a belt can break - often you can tell which one applied by inspecting it. Essentially though, if it happens to a new belt it is one of three things: too much tension, overheating, or rubbing very hard against a fixed object. Excess tension is caused by seized pulley bearings, misalignment, or incorrect installation (usually the last of these three). Overheating is caused by slipping or bending on too sharp a radius.
It is quite unlikely your belt was defective, unless you have used a washing machine belt instead of an industrial one (if you did that it would indeed wear out immediately).
You said the belt "snapped". If you mean it broke rather than cooked, the problem is probably incorrect routing of the belt, so that the automatic tensioner is not working. Usually it also involves having had trouble getting the belt on, and having to lever things around to get it fitted, and in the end it could be twanged like a bow-string. Is the cutter belt loose in the declutched position, as it should be? It declutches by slackening the belt so that it cannot drive the cutter.
While we don't have a manual for your specific model, here is a later model showing the cutter belt routing:
![[Linked Image]](https://www.outdoorking-forum.com.au/forum/uploads/usergals/2014/06/full-2772-16751-greenfield_evo_3_mk_2_cutter_belt.png)
You didn't by any chance run the belt around the outside of the guide pins did you?