Short answer
The accepted red line for 25 kV (27.5 kV) OCS on conventional railways: when the distance between a worker, or anything the worker carries, and a live part is less than 2000 mm, the work must be treated as work on a de-energised OCS — which answers the frequent question of "one metre or two?", the answer being 2 m as a starting point (the standing interpretation under Rail General Transport document 2017 No. 25). On the equipment side the static clearances are not less than 300 mm from a live part to a fixed earthed structure, and not less than 350 mm to rolling stock or loaded goods.
The two sets of figures serve different purposes
300 mm and 350 mm are the minimum insulation clearances for OCS design and structure gauge checking, and they govern equipment to equipment distances; 2000 mm is the personal work safety distance, governing person to live part, and it is the lower limit for work without de-energisation rather than a safety margin — any work needing a shorter distance must first be isolated, tested for voltage and earthed, and all three steps are mandatory. Related red lines: anything above 2 m counts as work at height, so a safety harness is compulsory; insulating tools must have passed their test and be within the valid period. High-speed railways follow their own safety working rules.
Related reading
Crossover and turnout maintenance standard | Dropper spacing requirements
Reference: Siwei OCS Encyclopedia — OCS safety distances (CC BY-SA 4.0)