The maximum unsupported span of a cable depends to a degree on its cross-sectional area, although annex D of the OSG gives the single figure of 3m,
This is very conservative and can safely be exceeded by a significant margin for higher cross-section cables up to about 20m for 6mm2. However, it is critical that the cable is not over-tensioned if the copper cores are the only strength. The tension rises to infinity if one attempts to pull the cable straight, and the exact tension can be solved by assuming it forms a coshine (hyperbolic cosine) curve, or to with about 20% by pretending it is a simple  triangle of forces with all the weight of the cable in the centre. (https://www.youtube.com/watch?app=desktop&v=OBoPpTExyBI )

In practice to be safely self supporting over more than about 10m often requires levels of central droop of about 75cm to 1m and that can give problems with clearance for vehicles beneath etc, and then a steel tension wire is preferred anyway to allow a more level run.

Even so be careful what you attach the end anchors on to -  it can be  embarrassing if it pulls out part of the top course of bricks, or removes a soffit board from the gable ends or something ;-)

Mike.

(the breaking strain of soft copper wire is about 200N/mm2 (20kg force per square mm )but outdoors stay well below  20% of that after allowing for a 2mm layer of ice all-round and a 100mph wind....)

Is 6m heigh enough on a farm.  There was a recent story in the news of a tipper lorry hitting an over head power line on a farm.  The driver was electricuted when the tipper with the hydraulic arm of the tipper lorry extended came into contact with an 11kV overhead power line.

Indeed - cable routes along hedge lines and similar where nothing will drive under are preferred to going across the yard, but of course that is not always possible.
However, it is not the most dangerous thing on most farms by a long way - that is seems is currently cows.  https://www.hse.gov.uk/agriculture/resources/fatal.htm  farms are 21times more fatalities per person than the UK industry average apparently.

It used to be worse, hence the old public safety films to scare us from playing about there
https://www.youtube.com/watch?v=_we-3Uqu5sw   This is the short version.
Junior school me recalls this and this other one about railway safety (The Finishing Line) as being indelible memories.
https://www.youtube.com/watch?v=SXGqwCbeFD8

Mike.

It's no longer a working farm, all the outbuildings have been turned in to units for small businesses. Also 705.522 says minimum 6m is acceptable. It may be possible to reduce the length of the span.