In a message dated 12/31/2007 7:34:16 P.M. Eastern Standard Time, graytrainpix_@_hotmail.com writes: Second – most signal systems are “fail safe”, in that a bulb that fails to properly light up causes a train to receive a more restrictive indication. However, under the DLW system, there is one case where a failure might cause a less restrictive indication to be displayed. I.e., when a clear medium is intended at an interlocking for a diverging move, the train should see a lit-up red in the top head over a lit-up green in the bottom head. However, if the red on the top head were to fail but the green on the bottom head were to light up, the train would see a dark upper head and a lit green lower head, which indicates “clear”, as if a straight move through the interlocking were intended. A train could then arguably enter a diverging route at too high a speed. I would imagine that the DLW made some provisions in its signal wiring to prevent this, e.g. putting the top red light and bottom green light in series so that if one didn’t work, the other wouldn’t either (of cours! e, two dark heads would be taken as a STOP). Just wondered if anyone knew how this was handled. Jim I believe Gordon Davids covered this with his recent post, (part) copy as follows: The DL&W and EL were the only railroads I have ever seen that included non-illuminated arms in legitimate aspects. The yellow top arm, when illuminated over a green bottom arm, downgraded a signal indication from "Clear" to "Approach Medium." Also, the red top arm in an interlocking home signal downgraded a "Clear," single bottom arm Green, to "Clear Medium," Red over Green. If the illuminated top arm was obscured by a bird nest or anything else, the signal could display a false clear indication. When I (as a Track Department guy) expressed my displeasure over that to our signal supervisor, he told me that they used light-out relays to protect against that. But of course a relay doesn't protect against a physical obstruction obscuring the lamp. I always considered that a violation of the fail-safe principle. While not being an expert in signal issues, I believe that "light out relays" cause the signal to be dark if any light in either arm fail. Hopefully Gordon will correct me if this is not true. SMT **************************************See AOL's top rated recipes (http://food.aol.com/top-rated-recipes?NCID=aoltop00030000000004) The Erie Lackawanna Mailing List http://EL-List.railfan.net/ To Unsubscribe: http://Lists.Railfan.net/erielackunsub.html ------------------------------
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