N&W AC Signaling in 1913

[NW Mailing List]

Mr. Jeff Cornelius asks: 

........................................... 
"Reading the article, I'm curious about the nature of the 'foreign 
current problem.' Is this in the track or in ground return?" 
........................................... 

Yes, Jeff, it would be interesting to know exactly what problems they were having. 

"Stray earth potentials" have always been a bugaboo for railroad signaling, and for the utilities, too. 

Perhaps a good example is a situation which occurred about 1924, when the first tests were being done with cab signals on the Sunbury & Lewistown Branch of the PRR. It was noticed that at a certain location the cab signals would pick up to Clear, when they should have been displaying some less favorable indication. After a great deal of investigation, it was determined that the problem was being caused by an improperly grounded 60 cycle AC motor in a nearby mill. The current from that motor was finding its ground through the railroad's signal equipment! Recognizing this as a serious problem that could occur almost anywhere, the railroad immediately changed the power for its cab signal system to 100 cycles (Hertz) AC. This afforded enough separation of the frequencies that stray 60 cycle commercial current would not interfere with the railroad's signal equipment. To this day, railroad signal equipment in the Northeast still operates on 100 cycle power. 

Another classic example of artificial energization of railroad signal equipment occurred in the 1990s, on Conrail's double track Philadelphia-Harrisburg Main Line (which had been the former Reading main line,) near Pottstown, Pa. The track circuits of the signal system were steady energy, non-coded DC. The utility built a 500 Kv transmission line out of its Limerick nuclear generating station, and that line paralleled the railroad for a few miles. Of course, they powered the line up in increments, until it reached full 500 Kv potential. The first hint of trouble was when crews began reporting that the cab signals on engines were picking up to favorable aspects, even this was non-cab signal territory and the cab signals on engines should have been showing Restricting all the time. Soon thereafter, a signal maintainer noticed a relay in a signal case in picked-up (energized) state, when it should have been down (unenergized.) Suspecting that there was a problem with the utility's transmission line inducing force fields into the track, the Signal Department placed an AC volt meter between the rail and a ground rod, and determined that there was indeed an induced voltage in the rails. One maintainer took his electric drill to an insulated block joint and placed one prong of the plug on one rail, and the other prong of the plug on the adjacent rail, bridging the insulated joint, and his drill spun around! The solution to the problem was to install impedance bonds and ground off the AC which was being induced into the rails by induction from the high voltage source. 

The telephone companies had a somewhat similar problem, and up until 40 years ago the AT&T Outside Plant Department had a wing called Corrosion Control. The telephone company's problem was that all its old cables (both buried and aerial) had lead jackets on them, and when buried, these lead cables became the "ground" for undesired earth potentials. Over time, so many lead molecules would leach away from the cable that pin-holes developed and water would enter the cable... which was a very serious problem because the individual conductors inside the cables were only insulated with a paper wrapping! "Electrolysis" is another name for this phenomenon. The telephone company's solution was, as I recall, to "reverse" the electrolysis by applying an AC voltage to the lead sheath of these old cables, and the let the electrons find their ground in somebody else's electrical system! Clever, indeed! The invention of plastic cable insulation was the ultimate solution for the electrolysis problem. 

And this problem has a modern chapter, too. There is currently a developing social flap about utility electrical transmission lines not being equipped with a wire to furnish a return path for the electrons to the generating stations. That is to say, the electric utilities are using the earth as the return path, and the "social consequences" of that are just now being realized. I do know that I can drive ground rods into each end my yard and connect a sensitive galvanometer between them, and read a small current. So, if the water pipe leading into my house develops a pin hole, what part did our country's electrical transmission system play in causing my problem? 

To give a specific answer to Jeff's question... I would theorize that improperly grounded electric motors used in mine operations around Elkhorn may have been the source of the "foreign current in the DC track circuits, which had become very troublesome," to which Mr. Beoddy refers. 

-- abram burnett, 
pennsylvania turnip farmer