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W&H MAIN YARDS: Head End Power

Bob MacDowell
Last Updated 5-15-94

It's connected pretty simply. There are four HEP cables, two on each side of each car. You connect them together so there's four connections on each joint, then at the end of the train, you loop the adjacent connectors back to each other.

The reason you do this is because there is a sense wire in each HEP cable. Each of the four HEP cables has six conductors: three BIG connectors (the three phases) and three small pins (one is a sense wire, the other two are ground. Maybe one of the grounds is a spare, I don't know.)o

Within each coach, all of the phases are tied together: that is, Phase-1 from all four cables is interconnected, Phase-2 is interconnected, as is Phase 3. The only wire kept unique for each of the four thru-cables, is the sense wire.

When the car is cabled as described, the result is a continuous sense wire that runs down the left side of the train, loops at the tail, then comes all the way back up to the locomotive; using the two connectors on the left side. The same arrangement exists, again, on the right side.

On each side, coaches have one socket and one HEP cable. You connect the HEP cable of each coach into the socket of the other. Locomotives have two HEP sockets. You use the HEP cable of the coach into one, and an HEP jumper cable to get between the remaining empty sockets. I don't think it matters if you crisscross the connectors on one side.

    |-------------|      |--------------|      |-------------\
 .==|             HH=====|              |=====HH              \HH==,
 `=HH  coach      |=====HH   coach      HH====HH  engine       \HH='
 ^^loopback                                ^^jumper cable       >loopback

HH = socket     === =HEP cable      You're seeing one side of the train.

Phew. And I did that in vi.  :-)
The locomotive will not energize the HEP until it sees that all the sense wires are contiguous; that is, there are no unplugged HEP connectors.

You can do tricks (and I've seen Amtrak do them): You can short-loop one side of the train, that is loop back one side at the last car, and the other side 5 cars up. The locomotive is none the wiser, although it does mean you now could have some energized but unprotected connectors in the last five cars. Of course, that also means that the last 5 cars only have two connectors serving them, so their available amperage is reduced.

""I have a good understanding of 3-phase AC but I've never been totally sure about how things are arranged on trains with the four multiple wire connectors, I don't know much about AC myself (learned a lot thanks to resident expert Dave Pierson though), and my study of Amfleet wiring predates my understanding of AC. But I'm pretty sure the wiring is as I describe above, dead simple. (Caveat: I've been wrong before.)""

"...the looped-back jumpers on the front of the locomotive and the rear of the last car,..."

Yeah, those mostly assure that there are no exposed HEP connectors which a passenger could stick a finger in, and also discourages trainmen from plugging/unplugging cars with the power on. (The sense pin is *shorter* than the power pins, of course.)


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