Tag Archives: overhead wire

More pantograph arcing

How people are finding me

I don’t know how many people who read my blog are also WordPress users who would already know this, but WordPress has this very basic feature that shows you what words people are searching for that bring them to your blog. I can’t tell who looks for what or which search engine they use or anything like that, but I’ve been getting a lot of hits lately from searches for some variant of pantograph, pantograph arcing, arcing wire, electrical arc, etc…

So this is for you, searching-for-arcing-pantographs-person (people?)  Each picture, like almost all the pictures I post can be clicked for a larger version.  Enjoy:

Starts small

Travels slightly..

Impressive, part 1

Impressive part 2, moving down the catenary

And then mostly fades

And then all together (may not load well on some computers.  Sorry):

So I’ll take this opportunity to repeat myself


You do not need that much electricity running through your body.

And for the curious, the search phrase that brings the most people here is “types of train cars“, followed by “Sandi Day“, “ATU757“, “Robertson Tunnel“, and “how fast do trains go“. I’m also starting to get people searching for my blog by name, which is pretty cool.

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.

HEAT WAVE. (and MAX hot weather operations)

Question: Why do the MAX trains go slower when it’s hot out?

I’ve been waiting months to make this post.  Not kidding – I know this question comes up every year when Portland gets a heat wave, so in February I took this picture:

Weighted catenary, February 2010

The temperature that day was somewhere in the 50°s – notice how far down on the pole the weights are hanging.

And compare the weights to this picture, same location, yesterday when it was about 88°F:

Weighted catenary system July

Edit evening of 07-08-10:

Here’s a picture from earlier today when it was about 100°F.

Weighted catenary at about 100 FYou can see that the weights are down a few inches from where they were at 88°F, and much farther down from where they were in February

I asked one of the MOW guys to explain how this works to me a while back. The pantograph on the top of the train is spring-loaded to push upward on the overhead wire, so the wires need to maintain tension. Tension in the overhead catenary wires in the high-speed areas of the alignment is kept in balance by weights like these – as warm weather makes everything expand, the weights drop further down to maintain tension in the overhead and keep it from sagging. But there comes a point where the weights hit bottom in extreme temperatures. When that happens, any additional expansion from the heat will make the catenary wires sag since the weights can’t drop any further to provide tension. So the trains run slower to avoid damage to the pantograph (and overhead wire) since the overhead wire has gone slack and can no longer provide the necessary resistance against the pantograph. Trains will drop their speeds by about 10 mph in high-speed areas when it gets to be above 90°F, and drop the speeds even more when it goes above 100°F. Plan accordingly if you will be traveling by MAX train!

Sun kink is another concern in extremely hot weather.  I have no TriMet pictures of sun kink, so here is one shamelessly borrowed from the Iowa DOT:

Sun KinkSun kink in rails

I’m no physicist so I’m not going to attempt to explain the nitty-gritty science behind it. But basically the ballast & railroad ties can keep the rail in place with normal heat expansion and contraction. However, with extreme heat, the construction of the rail can’t handle the force of expansion, causing the rails to slide laterally. There is especially high risk of this happening with big temperature swings (very hot during the day, cool at night), so during the summer it’s not uncommon for there to be slow orders for trains in areas of the alignment at risk of sun kink.

Slow OrderOld pic – I don’t remember why this flag was out, but here’s a picture of a wayside flag designating a slow order – a train must be at the speed limit posted on the yellow slow order flag when the front of the train reaches the flag, and then proceed no faster than that limit until it passes a green wayside flag.

You may also remember earlier this year when expansion joints were installed in areas of the rail on the west side and on Interstate where sun kinks are likely to develop. These joints give the rails room to expand to reduce the likelihood of the rail buckling.

104 in busTemperature in one operator’s unairconditioned bus

And my own PSA/editorializing during the heat wave – be nice to your operators! You can get off of the 100°F+ bus when you get to your destination – they’re stuck on it for their whole shift! (and even the air conditioned buses and train cabs can get the whole greenhouse effect going on too – this is not the nicest time of the year to be a transit operator! I mean, they tell you on the news not to leave your kids or pets in a car even with the windows opened because the internal temperatures can top 100°F – what about your bus passengers & operators?)