Thanks ET, for the explanation. Purely for interest - I can't figure out if the "jump" distance is proportionate to the voltage. If it is, would the values be different with the autotransformer setup, and an effective 50kV, than for a simple 25kV catenary?
3 metres seems a very sensible thing to tell young children - and grown ups for that matter. My own boyhood experiments on conductivity of various materials in various states, conducted with an electric cattle fence and lengths of grass and straw of varying wetness, led to a short sharp shock rather than anything serious. I'm sure the curious mind is present in modern-day children still, at least some of them, and the message must be to take absolutely no chances with the electrics on the railway.
"Grown-ups" have been known to do daft things, like driving across a level crossing in a lorry with an aerial on top, or walking under power lines with a long carbon-fibre fishing rod. Blindingly obvious after the event, of course.
Yes the AT system is 50kV between the ATF (auto transformer feeder wire) and the catenary both of which are 25kV to earth, therefore the electrical clearances and safety distances are the same.
It is extremely unlikely for a power arc at 25kV to "jump" 3 metres but that would need to be initiated by almost direct contact and the atmospheric conditions would have lend themselves to ionisation, and finally the protection schemes (circuit breaker trips) would clear the fault before the arc got established over that distance.
The rule of thumb electrical clearance in air for 25kV is 600mm, however there are reduced clearances down to 150mm in the Thameslink tunnels in the Kings Cross area.
The railways and the
ORR» take a lot of care to keep people away from live
OLE▸ , before any new scheme is energised a screening and protection walkout is done
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