TonyK
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The artist formerly known as Four Track, Now!
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« Reply #1830 on: July 07, 2016, 10:43:35 » |
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Surely the roof of a lorry or bus is less than 3m from the wires at a level crossing?
It doesn't provide the same tempting target as a small finger stuck on the end of of a body full of water and pointed incautiously close. Think why lightning hits the conductor rather than the roof. |
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Now, please!
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ray951
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« Reply #1831 on: July 07, 2016, 12:19:50 » |
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Although Didcot - Oxford isn't due to be electrified until 2019 I noticed today that they have started installing what looks like an electrical sub-station/feed-in at Radley.
This is the first noticeable work on electrification on this route for several months, although still loads of piling to complete.
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chrisr_75
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« Reply #1833 on: July 07, 2016, 13:45:36 » |
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3m is standard HSE▸ guidance for 11kv and 33kv electrical transmission cables, so that would also make sense for 25kv rail traction supply. The 'live zone', which is where you are basically guaranteed a shock, is somewhat less. I can try to dig out my notes from my 'electrical awareness' training if anyone is particularly interested.
There is a degree of safety margin in these values as the likelihood of a flashover can be influenced by many factors such as atmospheric humidity, the presence of smoke and so on, which can make air.p more conductive.
As I recall, the 440kv transmission lines (on the biggest metal pylons) have a live zone of several metres, which is why the cables are so high off the ground.
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Bmblbzzz
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« Reply #1834 on: July 07, 2016, 16:19:00 » |
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Have to add that to the list with the Wales and the Olympic Swimming Pool. |
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Waiting at Pilning for the midnight sleeper to Prague.
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TonyK
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Posts: 6594
The artist formerly known as Four Track, Now!
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« Reply #1835 on: July 07, 2016, 17:08:44 » |
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...3m (or half the height of a giraffe to use a non-SI unit favoured by NR» )...
What is that expressed in terms of double-decker buses or Nelson's Column? My first flying instructor said he had gone metric with his units a couple of years before I started. I was taught to level out for landing at "the height of a medium-sized white van". Previous students began the landing flare at "the height of a baby dinosaur". You have to move with the times. |
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Now, please!
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Electric train
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« Reply #1836 on: July 07, 2016, 22:31:20 » |
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The original BR▸ standard for a safe distance was 9 feet or 2.75 meters, if I recall correctly BS EN 50122 states 3 meters. Often the challenge to achieve even the 2.75 meters is the train pantograph horns in stations which has a low wire height and there are a couple of stations on the network where the ORR» has given NR a derogation after a risk assessment has been done on each location. 3 meters is a much simpler distance to use than 2.75 and school children will not understand 9 feet. The "jumping" distance for 25kV is less than 6 inches 150 mm. The normal min clearance for 25kV is 2 feet 600mm although there are some areas of special reduced electrical clearance of 175 mm. |
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Starship just experienced what we call a rapid unscheduled disassembly, or a RUD, during ascent,”
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LiskeardRich
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« Reply #1837 on: July 07, 2016, 22:33:22 » |
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3 meters is a much simpler distance to use than 2.75 and school children will not understand 9 feet.
If you start telling my daughter about 9 feet, she would start by walking heel to toe for 9 steps. She only has little feet. |
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All posts are my own personal believes, opinions and understandings!
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Trowres
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« Reply #1838 on: July 07, 2016, 23:10:05 » |
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In the early days of 25kV electrification in UK▸ , it was reported that a test was conducted by deliberately sagging the overhead wire above the chimney of a steam loco until a flashover occurred. The gap had to be reduced to about one inch, if I recall correctly. This fits with other sources of information. e.g:(from USA):- https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9870Of course allowance would have to be made for transient overvoltage and movements - but the schools document does seem to suggest that the electricity can arc over 3m. If it did, I don't think I'd want to go anywhere near a platform on an electrified route.  |
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ellendune
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« Reply #1839 on: July 08, 2016, 07:41:26 » |
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Of course if you have metal objects in proximity you can get induced currents in them without touching the cable. I assume permanent metal objects such as bridges are earthed to avoid this but what if a person is carrying something metal such as a metal walking stick, umbrella or golf club - who close would that need to be to be a problem?
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TonyK
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Posts: 6594
The artist formerly known as Four Track, Now!
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« Reply #1840 on: July 08, 2016, 07:55:25 » |
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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.
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Now, please!
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patch38
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« Reply #1841 on: July 08, 2016, 09:32:38 » |
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"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.
Probably blindingly obvious during the event... |
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Bmblbzzz
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« Reply #1842 on: July 08, 2016, 10:33:40 » |
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3 meters is a much simpler distance to use than 2.75 and school children will not understand 9 feet.
If you start telling my daughter about 9 feet, she would start by walking heel to toe for 9 steps. She only has little feet. I'm pretty sure schools use both feet and metres nowadays. |
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Waiting at Pilning for the midnight sleeper to Prague.
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Tim
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« Reply #1843 on: July 08, 2016, 11:48:23 » |
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3 meters is a much simpler distance to use than 2.75 and school children will not understand 9 feet.
If you start telling my daughter about 9 feet, she would start by walking heel to toe for 9 steps. She only has little feet. I'm pretty sure schools use both feet and metres nowadays. Indeed they start off using "natural" units of measurements like handspans, feet and strides. I don't think my kids would know what an inch is yet though, although they talk about miles. |
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ChrisB
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« Reply #1844 on: July 08, 2016, 14:09:37 » |
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Network Rail confirm in their Oxford area e-update that IEP▸ testing between Reading & Didcot commences 16 July
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