Pantographs and the Overhead Wire

All about the pantographs, baby

PantographThis is a pantograph.

The pantograph, frequently abbreviated to “pan”, is the spring-loaded arm that’s part of how the train draws power from the overhead wires, aka catenary.

pantograph at Sunset TCBy design, the pantographs press upward on the catenary. Where there isn’t a lot of clearance between the catenary and the train, the pantograph folds nearly flat on itself, as seen here at Sunset Transit Center.

The bow collector (presumably named that because it’s shaped like a bow) at the top of the pantograph arm is topped by a carbon shoe, which is the part that directly contacts the wire (which, not coincidentally, is called the contact wire). The carbon shoe is gradually worn down by the overhead wire and eventually needs to be replaced. To wear it down evenly, the overhead wires zigzag back and forth instead of going in a straight line which would only wear down one part of the carbon shoe and potentially break the pantograph.

This is not a TriMet video, though I’d love to set up a camera on top of a MAX train to get something similar – it shows both how the spring-loaded pan rises and falls depending on how much distance there is to the wire above as well as the back-and-forth zigzagging of the wires so that the carbon shoe wears evenly.

The Overhead Wire, aka Catenary

The overhead wire ranges in voltage from 675-925 volts, averaging around 750 volts, direct current.  In other words..


I used to have a link here to a Philadelphia news article where a 15 year old climbed a cat pole to touch the overhead wire but the link expired. He survived but was severely burned. A simple Googling shows a lot of fatalities that happened when people climbed on top of trains or up cat poles to touch the wires or the pantograph. I shouldn’t have to say “don’t touch something high-voltage” because it should be obvious that that’s a really bad idea, but I’m all about spreading the safety message even when it means stating the obvious.

Low-speed and high-speed areas

CBDCBD near PGE Park

In low-speed areas, such as downtown Portland or in the yards, a single-wire trolley system is used. Throughout the downtown alignment, you’ll see how the contact wire is a single wire strung through other supporting wires.

East of Beaverton Transit Center

In high-speed areas, the overhead wires look like this. The upper wire is called the messenger wire, which supports the lower contact wire.

On some of the catenary poles, you’ll see tension weights hanging. I’ve already gone over how the weights work in another post – but to summarize, they rise and fall as the temperature changes to keep tension in the overhead wire.

Section Isolators

Section isolator, CBD

Section isolator, near Beaverton Transit Center

Throughout the alignment are section isolators (also known as section insulators) in the overhead wire. These unpowered breaks in the line allow for power to be turned off in one area without needing to shut down the entire system. To prevent arcing and other problems, a MAX operator going under an isolator won’t draw power until both pantographs are clear of the isolator.

I’ve seen brighter sparks than this, but they’re extremely hard to photograph.

arcBetter example of arcing

Those pictures are examples of arcing downtown where the Yellow-Green alignment crosses the Blue & Red alignment. If you’re on a train going through where the lines cross over and all the lights in the train go out briefly, this is why. Arcing is not a desirable phenomenon, and excessive arcing can sometimes be indicative of a very serious problem.

It seems that pictures of arcing between pantographs and overhead wires is a popular topic that people search for, so I’ve added some additional photos of arcing.  And then I added some more!

Willow Creek, looking westWillow Creek, C and P signs (click for larger to see the P sign)

As a visual reminder of where the section isolators are in high-speed areas where a train is likely to be in a propulsion mode greater than those used in low speed areas, there are C and P signs along the alignment associated with the isolators. When the front of the train reaches the C sign which will be located prior to the isolator, the operator must coast, and therefore not draw power from the catenary until reaching the P sign, at which point the operator can resume a propulsion mode drawing power because both pantographs have passed beyond the isolator.

Willow Creek IsolatorWillow Creek C sign and section isolator

In most sections of the alignment, unless you know the isolator is there, you most likely won’t be able to feel the train going into coast and back, but offhand I can think of two sections where it’s fairly obvious to passengers – westbound out of Willow Creek (where this picture was taken) and eastbound out of Sunset. At Willow Creek, the isolator is so close to the platform that the train will not pick up much speed before the operator must coast, so westbound departures from Willow Creek often feel very slow.

Sunset eastboundEastbound from Sunset Transit Center

At Sunset, the isolator is on an upward hill – note the C sign at the base of the hill – so gravity is working against the train coasting uphill and you can feel a slight jerk and drop in speed as the train goes into coast. If the operator doesn’t pick up enough speed leaving Sunset before coasting up the hill, the ascent will feel unusually slow.

And…. I think that about covers the basics. Really the biggest (only?) takeaway message from this for TriMet passengers is don’t ever touch the overhead wires since knowing the rest of this stuff isn’t a prerequisite for riding the trains but staying away from the catenary wires is just a good idea! But as always, I think the intricacies of the system and all of the things that go into making it work are fascinating, even if most people don’t care/don’t need to care about them.

8 responses to “Pantographs and the Overhead Wire

  1. Another extremely informative post!

  2. Great, simple description. Also, kudos for not using the oft-used mispelling, cantenary.

    Josh Collins
    TriMet Operations

    • Thanks – and I try to proofread before I post, but I do mess up sometimes… We’ll not discuss the number of times I’ve gotten break/brake mixed up, especially when talking about brake faults and therefore both spellings of the word are in my head thinking of what happens when a brake breaks.

  3. Pingback: Portland Transit Blogging 2010

  4. Hi
    My friends and me are working on a term project on analyzing the reliability of a pantograph..
    We have a set of components but we couldn’t find the components and their functions anywhere.
    Could you help out on these:
    upper arm, frame,lower arm, pull rope, ADD valve, insulator, silent block (springs),main air hose, rocker,precision pressure regulator.
    Its for the old diamond shaped pantograph.
    Thanks a lot.

    • I’m sorry, I don’t know much about those types of pantographs. I think you could safely assume that the upper/lower arms and a few of those other parts are what they sound like, but this isn’t something I really know anything about. Good luck on your project though!

  5. BETWEEN Pantograph and overhead wire is there any friction ,if there is frictions why does the surface area of wire(conductor) not changes with it’s use in number of time

    • The copper in the overhead wire is harder than the carbon shoe of the pantograph, so it’s the shoe of the pantograph that will wear down first. However it is possible for the integrity of the wires to degrade so regular inspections are done to spot and address problem areas.

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