I don't think there is an IEC requirement. The clearance will be stipulated by the switchgear manufacturer. It will depend on the physical space required to assemble the switchgear, and also the minimum volume of air required to ensure that arc products caused by failure of the switchgear do not reach an operator stood in front of the switchgear. This would be verified during type testing. For example you may be able to use the switchgear in a situation with a reduced ceiling height, providing you pull it forward to increase the volume of air at the rear to compensate.
Regards
Marmite
