I’m kind of confused about an issue I’m having at work right now. I work for a natural gas pipeline and we cross under many railroad tracks. We use Cathodic Protection to keep our pipes from rusting. We seem to be getting a small amount of interference from some of the rails. I don’t know much about trains but I understand that they are diesel engines that power electric motors. Do these motors ground to the rail? We’re getting 500-1000mV influence on our pipe when a train passes over our pipes. Does this make sense to anybody?
Signal system and data circuits, grade crossing circuits, etc. all use the rails. But so are there other signals in lines or conduits…even fiber optic cables could be in use. You would have to check with each railroad for each location specifics.
The LION, of course, does not know for sure. But no, Diesel Electric locomotives do not use the tracks as a current conductor. Electric trains (with overhead wire or third rail) do use the tracks as a ground return.
You can look at the tracks for your self. Find a rail joint and see what kind of bonding joins the running rails. A pair of relatively light weight copper bonds between the two sides of the rail joiner (other than a insulated rail joiner of course) are part of the signal system. Two very heavy bonds on one rail or the other (but not both) with light weight bonds on the other rail indicate a rail that is being used as a ground return for electric train operations.
My guess, and of course it is only a guess, is that you are getting readings from the signal circuits. A battery puts a current across the rails perhaps in mile long blocks, the presence of a train in that block shunts the current, dropping a relay and displaying a red signal at the start to the block. Go back to your meter and check the readings. If it is the track signal, then it will be shunted before or after the train is in sight (depending on the length and locations of the blocks).
Another issue is the crossing gates. Is your crossing near a grade crossing. It could be that the signalling circuit for the crossing gates is giving you these
How many of those crossings are older gas lines that were cased? (Before 1994)
(In most cases, the casing rules for natural gas lines have been relaxed or done away with in favor of heavywall pipe. Liquid pipelines will probably always remain cased under railroad rights of way)
I’d be suspecting installation error or bad “donuts” if the casing has insufficient clearance (less than 2")…railroad industry has long noted pipeline installation problems.
I appreciate the in depth response! I was leaning more toward the “Electro-magnetic Field” that you mentioned. Because it only happens when the train is directly above the line. But I didn’t realize that the each mile or so of rail has it’s own signal where it can “know” where the train is. Pretty cool.
I will definitely be trying to contact the railroad directly. But it really helps to get some of the basic knowledge down first! Thank you!
These lines were actually installed in the forties. They are definitely all cased under the railroad’s right of way. But we monitor the resistance between the casing and the pipe and they are still functioning properly . . . (no shorting)
Again, each railroad and each location is different, but, you may be encountering cab signal territory where the impedance is measured by the shunting of a train differently than in a regular relay shunt; or speed is being measured for a grade crossing signal…so many variances and possibilities…could even be the train line data frequency from Fred to loco or controls for mid train slaves or pushers. …etc.
Where is this happening? You might have you lines under an electric powered (no diesel) line which would be grounded through the rail. In any event, the rails carry current for signal systems.
Where is this happening? You might have you lines under an electric powered (no diesel) line which would be grounded through the rail. In any event, the rails carry current for signal systems.
Railroads can be public utilities and in fact in some states are regulated by the Public Utilities Commssion or Public Utiilties Board or some such agency…it is in effect where there is a percieved monopoly for a public service such as providing transit or transportation services. Bus companies come under the same agencies…they are sort of state ICC’s.
There is a saying often attributed to one A. Lincoln that goes: "If you call a tail a leg, how many legs does a dog have?’ The answer is, of course, four. Calling the tail a leg doesn’t make it a leg.
likewise, calling a railroad a public utility doesn’t make it a public utility.
Greyhounds: are you talking philosophical or legal definition? You may be right philosophically, but then states and governments have dealt with the matter differently. Public Utilities Commssion, Public Service Commission, Utilities Service Agency, etc. all tell a differetn legal tale.
It would seem to me that if you are getting this electrical response only when a train is passing, it must be related to the train rather than to signal currents, power lines, and other fixed sources. And while the locomotives do not ground to the rails, they certainly would produce an electromagnetic field around the traction motors. I wonder if that field is strong enough to induce current flow in the pipeline.
High voltage power lines are known for inducing an easily detectable voltage in a steel fence wire running under the power line. In effect the power line and the fence form an “air core” transformer, even though the two “windings” of the virtual transformer are many feet apart. Of course, the power lines are carry alternating current. Transformers don’t work with direct current.
Up until fairly recently, diesel locomotives were generally driving their motors with DC current. But with better technology in inverters and control systems, AC current locomotives are taking over the market. So it is reasonable that transient current issues that weren’t present for many years prior are now surfacing as AC locomotives become more prevalent.
This is just speculation on may part, of course, but possibly the electrical field associated with the AC traction motor is interacting with the pipeline similarly to a fence / powerline situation. The voltages in the motor field are lower than the powerline / fence situation, but then the pipeline is only a few feet below the rails.
Apparently you are able to measure this while a train is passing, so I have to ask, does the current/voltage peak when an engine is over the pipeline – lead engine(s), and/or mid-train or rear of train engine(s), or is it uniform while the whole train (other non-powered cars) is over the pipeline?
Does the disturbance start before the train arrives and stop as soon as the end of the train has passed or does it continue for a time after the train has passed the pipeline?
Does the amount of disturbance vary based on the speed of the train passing the pipeline?
If the train stops over the pipeline, does the disturbance return to normal when a train is not present?
Answering these things might lead you to the cause… i.e.: is it a field effect from the traction motors only or possibly an induced localized static electric condition cause by the motion of the train itself.
Is the amount of electrical disturbance affected by the moisture level in the ground/ballast in the vicinity of the pipeline?
I have a couple questions for you. How deep is the line buried? What are you using for an anode? How big is the line? What kind of earth is it buried in? What is the MAOP?
I am not completely in the know on the cathodic protection, but I have worked with our guys on it. I am in processing and compression.
I’m not an electrical expert, but what are the Ground Fault Relays on the locomotives tied/ referenced to ? I presume it’s the frame and then via the trucks* and wheels to the rails = “ground”, so that the locomotive is “grounded” to the rails. Any other arrangement would run the risk of the frame of the locomotive being at a different potential (voltage) than the actual ground surface, which would create a risk of shock, etc. everytime a crew person stepped on or off the locomotive - also, sparks every time the loco coupled to a freight car (which are just about all-metal, and not insulated from the rails in any way that I know about).
(*“Truck” is railroad jargon for the complete wheel, axle, frame, spring, and suspension, etc. assemblies under each end of a locomotive or railcar.)
Even a DC-motor loco will have a magnetic field around each motor that is relatively constant with respect to the motor’s location, as the field coils are fixed in place and their voltages and currents don’t change with each rotation - only when the engineer changes the power output. But each motor’s magnetic field is moving with respect to the natural gas pipeline, and that might well be enough to induce a small current and voltage in any metal pipe.
The “proof of the pudding” would be as Semper Vaporo outlines above - use a fast recorder or oscilloscope as a train goes over the pipe. A series of either 2, 4, or 6 pulses for each locomotive would correspond to the power trucks and/ or traction motors - I’d expect that the induced voltage signal from the following cars and their trucks would be much smaller or nonexistent, though.
There were a lot of problems with signal circuits in the early days with harmonics from the three-phase AC drives. Is it possible that the railroads have only installed shielding to screen out the signaling related frequencies and their harmonics, but not other frequencies and that is causing the problems.
I also think Semper Vaporo asked some very pertinent questions and I am an EE (with a bit of NE on the side) - especially the question whether the voltage peaks only when the locomotives pass over or the whole train.
From what the original poster wrote, the problem seems to be a DC potential. It doesn’t make sense that stray fields from an AC locomotive would induce a DC voltage, what might make sense is the stray magnetic field from the “field” (stator) windings induces a voltage by its relative motion to the track.
What seems more likely to me is that the track is carrying a potential from elsewhere for some reason. When a train rolls over the track, the weight compresses the track structure and increase the current flow from the rails to ground and thus changing the potential measured on the pipe. This would be similar to the operation of a carbon pile in an old voltage regulator.