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| Scorched rail 28/04/2013 at 18:11 #44132 | |
John
884 posts |
Just been reading an incident in the control log about a persistent short-circuit caused by a mountain bike thrown on the track. Prior to the discovery of the bike, a recharge was attempted resulting in scorching to the rail-head. It was noted on the log that P-Way would be required to inspect the rail(s). I was wondering, what are the implications of this type of damage? Log in to reply |
| Scorched rail 28/04/2013 at 18:19 #44133 | |
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Stephen Fulcher
2026 posts |
If memory serves me correctly, the rail would normally be replaced for a scorched top. I am not sure totally what the implications of this are, possibly some form of weak spot?
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| Scorched rail 28/04/2013 at 18:19 #44135 | |
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Rickurd
128 posts |
It could have weakened the structure of the steel I suppose, but I'm no expert
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| Scorched rail 28/04/2013 at 19:20 #44138 | |
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easilyconfused
14 posts |
Before I discovered IT as a job I was a mechanical test engineer for Rolls Royce Aero Engines. Localised heating of steel and many other alloys can cause brittle areas that are prone to brittle stress fracture. One of our standard tests was passing a very high current through test articles and then subjecting them to fatigue and stress tests to measure the properties of the materials. The results from these tests were interesting from an engineering perspective - a brittle surface zone on a test subject can reduce the fatigue life by more than 50% in the right conditions. Fatigue might sound the last issue for a railway line but if you watch as trains pass the rails flex under each axle and then rebound. Do that millions of times and the rails fail. Regards John Log in to reply The following user said thank you: John |
| Scorched rail 28/04/2013 at 22:27 #44144 | |
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vontrapp
210 posts |
When I did my third-rail isolation training, 1200v dc, one man let go of the isolation-pole and it caused it to bounce between the live-rail and the running-rail. There were sparks everywhere. Is it true that the amps double when an insecure isolation occurs? And it didn't blow the circuit-breakers, either. In this case of a scorched-rail, why was the breaker reset? I would have thought that the line should have been examined. Or, is it a common occurance that the breakers activate?
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| Scorched rail 29/04/2013 at 05:24 #44146 | |
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pilotman
189 posts |
One of the problems with 3rd rail traction at a relatively low voltage is fault detection. The normal motor starting current is so high that it is difficult to separate a train starting from a stand from a fault. Also the sytem is not grounded, so no reliable way to detect an earth fault either.
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| Scorched rail 29/04/2013 at 09:23 #44147 | |
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Haraubrad
103 posts |
As I understood it before I retired 11 years ago. They would reset the circuit breakers 3 times before investigating why they had tripped, 3rd rail only. Overhead was investigated before a reset. Aubrey Log in to reply |
| Scorched rail 29/04/2013 at 09:35 #44148 | |
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pilotman
189 posts |
On the National Transmission network there is what is called "Auto-reclose" in place. Circuit breakers are reclosed automatically after a fault, in case it is transient, e.g. a lightning strike. If they come out again they are locked out and the fault has to be investigated. As I mentioned the problem with 3rd rail is that in many cases the breakers stay in. Many faults (especially to ground) are therefore burnt out by the enormous current. Log in to reply |
| Scorched rail 10/05/2013 at 11:10 #44518 | |
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button_pusher
56 posts |
" said:When I did my third-rail isolation training, 1200v dc, one man let go of the isolation-pole and it caused it to bounce between the live-rail and the running-rail. There were sparks everywhere. Is it true that the amps double when an insecure isolation occurs? And it didn't blow the circuit-breakers, either. In this case of a scorched-rail, why was the breaker reset? I would have thought that the line should have been examined. Or, is it a common occurance that the breakers activate?With any electrical circuit, as soon as you complete the circuit i.e. close a switch you get whats known as an "inrush current" which can be 20-40 times the steady state current however only last for around 10 milliseconds It's more prevalent on circuits that have some kind of "drive" on them such as motors/capacitors/transformers when an initial current surge is required to get the thing moving/up to operating "temperature" so to speak. Log in to reply |