A horizontal ladder type cable-tray runs parallel to a pedestrian walkway, which is about ten feet way. The cable tray carries cables from an outside 750kVA transformer to indoor 208v switchgear. The AF IE value on the load side of the transformer secondary (208v) CB is 90 cal/cm^2, which implies a 21 ft Flash Protection Boundary for the cable-tray.

Does this imply that the walkway should be closed?

How would you handle this?

The arc flash boundary only applies when someone is "interacting" with the equipment. However, there is always a risk of cable failure.

Hazzard vs Risk....

Is there a risk of it failing and what is the probability of it?

Is there a risk of the transformer in your alley/yard failing and exploding? Do you avoid using that area of your yard?

If there is no exposed energized parts, and you are not interacting with the cables, then there should not be an AF Hazard risk. I would not be concerned about the remote chance or risk of a cable failure. This is not a known predictable failure.