AC/DC overhead lines mixing

[dm1]

There are numerous street running lines with 1200V electrification, so the safety case isn't as critical.

The big issue with 25kV is that the voltage is high enough that flashover can occur even without a conductive object (the proverbial umbrella) touching the wire itself, at a not insignificant distance. Therefore clearances need to be much larger to reflect this. With DC electrification at a much lower voltage the distance at which flashover occurs is significantly smaller.

I imagine the safety case will assume that you won't deliberately try and touch the wire - as with DC third rail. People naturally assume that the wire can only hurt you if you touch it. With 1200V DC this is more or less the case. With 15 or 25kV AC it definitely isn't.

 

[biko]

That's a clever set up. Have they ever done something similar at the Belgian border? Certainly a lot simpler than some of the border station arrangements elsewhere I remember from youthful travels with OHLE voltage switching capability through platforms to allow traction changes.

Yes, pretty sure the Belgian EMUs that run to Rosendaal and Maastricht go at half voltage on the Dutch side.

Yes indeed. There is a section without catenary just over the border on the Dutch side on both lines. The Belgian 3000V capable trains run at half power on 1500V, although only older units can do it.
Modern ones don’t like low voltages.

For the switchover between Dutch 1500V DC and German 15000V AC, the voltage can change on the catenary of most tracks in Venlo and Bad Bentheim. The disadvantage is that all trains need to stop while the voltage is changed even if they are multi voltage units.

 

[Bald Rick]

There are numerous street running lines with 1200V electrification, so the safety case isn't as critical.

The big issue with 25kV is that the voltage is high enough that flashover can occur even without a conductive object (the proverbial umbrella) touching the wire itself, at a not insignificant distance. Therefore clearances need to be much larger to reflect this. With DC electrification at a much lower voltage the distance at which flashover occurs is significantly smaller.

I imagine the safety case will assume that you won't deliberately try and touch the wire - as with DC third rail. People naturally assume that the wire can only hurt you if you touch it. With 1200V DC this is more or less the case. With 15 or 25kV AC it definitely isn't.

Noting this - and that I’m not an electrical engineer -but the ‘significantly smaller’ distance for 1200DC is about 50-100mm less than 25kV AC. That’s just a slightly larger umbrella.

 

[dm1]

Noting this - and that I’m not an electrical engineer -but the ‘significantly smaller’ distance for 1200DC is about 50-100mm less than 25kV AC. That’s just a slightly larger umbrella.

Assuming air can withstand up to 1kV/mm the difference is between ≈1mm for DC and 25mm for (15kV RMS ≈ 26kV peak) AC.

And that's the absolute minimum - in reality the standards require at least double that if not more, so the difference is then larger. It might not seem like much, but it's not an insignificant difference.

 

[AM9]

Assuming air can withstand up to 1kV/mm the difference is between ≈1mm for DC and 25mm for (15kV RMS ≈ 26kV peak) AC.

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And that's the absolute minimum - in reality the standards require at least double that if not more, so the difference is then larger. It might not seem like much, but it's not an insignificant difference.

Yes, I've noticed the assumption by so many here that OLE voltage can jump much further than is actually the case. The actual distance in dry air (which is almost normal for the UK) is approximately 1mm/kV, so for a 25kV ac conductor, the distance is the maximum allowed rms voltage on the line x the sqare root of 2, therefore:

29,000 x 1.414 ~ 41,000V, so the actual distance over which an arc would be initiated over is less than 50mm.​

The clearance for safety reasons encompasses many other issues, mostly mechanical that have a far bigger impact, e.g. catenary/conductor wire movement, surface tracking on insulators (mainly pollution induced), extreme water paths (rain is rarely a contiguous water stream, and clean water is not a good conductor anyway), occasional wildlife intrusion etc.. Under the same circumstances*, the arcing distance for DC conductors with non-inductive loads* is in the order of 2mm so the actual structural clearances for low-medium voltage DC as @Bald Rick says, will only be about 50-100mm less.

*As there aren't any high speed mainline OLE installations of 1500VDC in the UK, it is difficult to make a direct comparison with 25kV on OLE clearances in the UK. Apart from the only heavy rail DC OLE being for relatively low speeds, the currents on that installation are about 3 times that on a typical 25kV line, so the conductors are heavier and subject to less movement in their registration. The arcing distance of DC conductors is much more variable where inductive loads are concerned (as in heavy traction motors), and a danger there is unlike ac (where the potential passes through zero 100 times per second allowing an arc to extinguish), resulting in an arc being sustained and even able to extend in length due to inductive currents continuing to flow.

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[Philip Phlopp]

Noting this - and that I’m not an electrical engineer -but the ‘significantly smaller’ distance for 1200DC is about 50-100mm less than 25kV AC. That’s just a slightly larger umbrella.

And it's the increasing prevalence of larger golf umbrellas which is worrying ORR, RSSB and ourselves.

 

[Bald Rick]

And it's the increasing prevalence of larger golf umbrellas which is worrying ORR, RSSB and ourselves.

Perhaps the answer to clearance issues is to ban golf. No loss, some would say.

 

[Meerkat]

Perhaps the answer to clearance issues is to ban golf. No loss, some would say.

I have never played golf but need a big umbrella - I’m 6’3” and if the umbrella ain’t large I get wet legs!

 

[Philip Phlopp]

Perhaps the answer to clearance issues is to ban golf. No loss, some would say.

It would make as much sense as the decision to bugger about with the clearance requirements.

It was always a slightly odd issue anyway, most golf umbrellas I see are lightning safe (is it an EU rule perhaps) and not only do they present no risk of arcing/shock, but they could safely be used to poke the OLE should one wish.

 

[Bald Rick]

but they could safely be used to poke the OLE should one wish.

But please, children, don’t thy this at home!!

 

[Philip Phlopp]

But please, children, don’t thy this at home!!

They're too short to reach anyway...