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| Re: Amtrak train destroys minivan carrying 6 people (1400230) | |||
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Re: Amtrak train destroys minivan carrying 6 people |
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Posted by SelkirkTMO on Mon Jun 27 21:43:11 2016, in response to Re: Amtrak train destroys minivan carrying 6 people, posted by Henry R32 #3730 on Mon Jun 27 20:59:13 2016. Actually, the apparatus at the grade crossing is the least expensive part of protecting it. There's a lot more involved in being able to determine when a train is approaching, hope this copypasta helps to explain why traffic light or crossbucks doesn't really cost that much more over the other:There are three basic types of crossing circuits. 1) The first is called a DC Stick Circuit or "Easer-Weaser." It involves three separate track circuits separated by insulated joints. Let's say a westbound train enters the east approach track. This shunts the east track causing the east track relay to drop, which in turn de-energizes the crossing relay (XR). De-energizing the XR in turn activates the bell, de-energizes the crossing gate relay (XGR) causing the gates to drop and de-energizes the crossing repeater relay (XRP). The latter feeds energy to a flasher (EOR) relay, which cycles the lights by swapping polarity through the lighting circuit. Now the train reaches the island, which is always a separate circuit in its own right regardless of what kind of crossing control system is used. Whenever a train is in the island, the XR drops and crossing must remain activated until the island clears. OK, so in this case the island track relay drops. Also, what's called a stick relay has picked at this time, in this case the west stick relay. So even though the train has entered the west approach track and the west track relay is down, the stick relay circuits "wraps" around the track relay contacts. As far as the rest of the crossing circuit is concerned, there's no train any more and the crossing recovers once the island is clear. We call them Easer-Weaser for East Stick/West Stick, which has the mnenomic of ESR/WSR. 2) The second is called a motion detector. This is essentially a complicated on-off switch with a power supply for the XR or a motion detector relay whose contacts feed energy to the XR. Motion detectors do their work on the track via AC current and monitor what's called "phase angle." (That's the relationship between inductive reactance and resistance. For an AC crossing circuit the relationship is more a parallel resistor/inductor circuit. Thus the phase angle is measured using current vectors rather than resistance/impedance vectors as in series.) A perfect phase angle in a wholly inductive reactance circuit is 90 degrees. But we don't live in a perfect world, and railroad tracks certainly aren't perfect. Dirt, mud, rust, poor ballast, salt, even wet ties create lots of ways for current to go elsewhere. Most phase angles I see are in the 65- to 79-degree range. Anyway, the motion detector looks for a change in the phase angle, usually an increase of 2 percent, or a drop in the impedance distance (RX) of 0.2 volts. Doesn't take much to get that. As a train enters an AC crossing approach, the phase angle increases and the RX drops. As soon as the MD senses that change, it removes power from the relay and the crossing activates. When the phase angle begins to increase, the crossing recovers. But it will never recover while a train is occupying the island. The island has its own high frequency AC circuit. It doesn't matter how fast or slow a train is moving in a motion detector crossing circuit, it's on and sometimes on for a long time when it's a slow train. However. ... 3) . .. it does matter with predictors. A predictor is a smarter kind of motion detector. Rather than activate the crossing immediately, the predictor uses the decreasing RX value to determine the train's speed. It then performs a time-versus-distance calculation and activates the crossing at a pre-set time before the train reaches the crossing. It's the same time for every train regardless of speed because we program the approach length and warning time into the predictor's CPU. One caveat to the last sentence: If a train speeds up or slows down in the crossing approach, the predictor gets a little baffled. Also, if a train stops in the crossing approach, the crossing recovers after pre-set time. The predictor then swtiches itself over to become a motion detector. As soon as it senses movement, a change in phase or RX, it activates immediately. When the train clears the island, the predictor senses the rapid increase in RX and energizes the relay. I won't go into pre-empts, interties, wraps, uni-directional, bi-directional, remotes, and auxililary crossing circuits. It's a lot of Inside Baseball. |
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