Is there a standard that states how high the reduced speed may be, on conveyors for example, if the conveyor is not enclosed?

The C standard EN 619:2002 for continuous handling equipment and systems only states speed for cases in which people may travel on the conveyor. These are:

Continuous flow conveyors on which persons may travel:

• Floor level: Conveying speed of 0.3 m/s
• 0.3 m above floor level: Conveying speed of 0.15 m/s
• Full railings: Conveying speed of 0.5 m/s

VDI 2854 also deals with the issue of safe, reduced speed. This states that a speed is deemed to be safe if the person has time to escape from a hazardous event.