Overcurrent protection devices (OCPDs) need protection, too. Not from overcurrent, but from physical damage [240.30(A)]. You can provide that protection in one of two ways:
1. Put them in something: an enclosure, cabinet, cutout box, or equipment assembly.
2. Mount them on something: open-type switchboards, panelboards, or control boards.
The second option is available only if what you’re mounting the OCPD on is:
1. In a room or free from dampness and ignitable material, and
2. Accessible only to qualified personnel.
The operating handle of an OCPD can be accessible without opening a door or cover [240.30(B)]. In the typical industrial setting, this accessibility is usually a plus because you have the (additional) protection of the enclosure in the event of a fault when manually operating a breaker. In other settings, for example an administrative office, it’s probably safer to keep access behind a locked cover so that unqualified personnel don’t reach in and manually operate OCPDs.
If you’re using an enclosure to protect your OCPDs, mount it in a vertical position if practicable [240.33]. If you do end up having to mount the enclosure horizontally, that most likely means you’ll be operating the breakers vertically. In that case, ensure the “up” position of the breaker is the “on” position.