Why no flashover with steam under the wires?

[CosherB]

Having seen many main line steam locos on 25KV electrified lines, I wonder why there are no flash-overs? Not from the safety valves as that's pure H2O, but from the exhaust. That comprises a lot of partially-combusted carbon particles, including heavy smoke particles, partially burned ashes, and small coals.

Why do flash-overs not occur?

 

[starrymarkb]

It has happened, a Kettle managed to short out Liverpool Street station.

[youtube]vQTinmPjS_o[/youtube]

 

[CosherB]

Thanks for that Starry. So it does happen, but is, apparently, no big deal?

 

[ChrisCooper]

Of cource when the first 25kV electrification was taking place, steam was still pretty common. I think it was one reason for the use of 6.25kV in areas of limited clearance. Another concern was the fact that the soot might damage the overhead or the insulators, perhaps even causing shorts across the insulators. I don't think the problems were anything line what was predicted though.

 

[scotsman]

Flash-overs generally don't occur because exhaust gases are mainly composed of carbon, not in the form of graphite (which is conductive). As for the steam, well, water is a poor conductor and for it to cause a flash-over would mean that there was a very high saturation of water particles around the OLE

 

[LexyBoy]

Contrary to popular belief, 25 kV is not capable of jumping huge distances. The electric field strength at which the insulative properties of air breaks down is around 3 kV/mm, i.e. 25 kV won’t spark unless the air gap is ~8 mm or less (it’s slightly easier for moist air, but not drastically). Adding conductive particles to the air (e.g. water droplets) will reduce the dielectric breakdown threshold, but it needs to be reduced by ~100 fold in order to be able to spark over to ‘ground’ (the loco in this case), which would be very unusual – probably only possible in very confined situations such as in the video.

Not that I’d put my hands anywhere near OHLE of course

 

[starrymarkb]

Thanks for that Starry. So it does happen, but is, apparently, no big deal?

Not for the kettle, but apparently it did trip out everything else!

 

[John Webb]

Contrary to popular belief, 25 kV is not capable of jumping huge distances. The electric field strength at which the insulative properties of air breaks down is around 3 kV/mm, i.e. 25 kV won’t spark unless the air gap is ~8 mm or less (it’s slightly easier for moist air, but not drastically). Adding conductive particles to the air (e.g. water droplets) will reduce the dielectric breakdown threshold, but it needs to be reduced by ~100 fold in order to be able to spark over to ‘ground’ (the loco in this case), which would be very unusual – probably only possible in very confined situations such as in the video.

Not that I’d put my hands anywhere near OHLE of course

The air gap and electric field stength quoted above are measured using spherical electrodes probably at least a couple of cm in diameter. If electrodes are not spherical but pointed or otherwise irregular, the electric field strength in their vicinity can be much higher leading to breakdown of the air over a much larger gap - hence the possibility of flashover to parts of the human body from distances greater than the 8mm quoted above. So I'd definately keep my hands well away fron the OHLE!

 

[matchmaker]

Before steam was withdrawn, certain classes (Princess Coronations being one) were barred from travelling under 25kv OHL south of Crewe. These locos carried a diagonal yellow stripe on the cabsite.

 

[LexyBoy]

The air gap and electric field stength quoted above are measured using spherical electrodes probably at least a couple of cm in diameter. If electrodes are not spherical but pointed or otherwise irregular, the electric field strength in their vicinity can be much higher leading to breakdown of the air over a much larger gap - hence the possibility of flashover to parts of the human body from distances greater than the 8mm quoted above. So I'd definately keep my hands well away fron the OHLE!

This is also true - and is why lightning conductors have a spike at the top. Even so, there is a very large safety margin with the spacing of OHLE and anything conductive (rightly so!).