That all depends on the fire risk assessment (or if for purposes other than fire, the relevant availability risk assessment).

The separation to achieve 'diverse' is a function of the potential causes of any given single point of failure.

So for example if the fire, flooding, or accidental impact event that affects one cable is able to affect a cable 3 m away, that is not 'diverse' ... but if the event is highly unlikely to, or cannot, simultaneously affect both cables, that could well be'diverse'.

The guide to electrical medical locations touches on this subject. 

AMK said:

The guide to electrical medical locations touches on this subject

Agreed, although this is based on domain-specific requirements to conform to relevant HTM's. The risk assessment for other applications might lead to different decisions or approaches - more costly or less costly dependent on the requirements and considerations involved.

I agree. Referring to the original post, if diverse routes cannot be established throughout, is the optimal solution be to use cables to PH120 ? However, considering the example provided, since the cables are buried underground, the single point of failure would likely result from an impact. 

I agree. Referring to the original post, if diverse routes cannot be established throughout, is the optimal solution be to use cables to PH120 ? However, considering the example provided, since the cables are buried underground, the single point of failure would likely result from an impact.