Discussion related to commuter rail and rapid transit operations in the Chicago area including the South Shore Line, Metra Rail, and Chicago Transit Authority.

Moderators: JamesT4, metraRI

  by JLJ061
I noticed on Metra Electric territory the line has periodic "Air Gaps" in the catenary and was wondering what they are for and how they are supposed to work?

  by Nasadowsk
Section breaks, in all likelihood. They don't need to be long on DC systems, but are long on AC system phase gaps, with an insulated section in between to prevent arcing, and pop the breakers if there is any (since it'd be a phase to phase arc). Breaks where the phase is the same on both sides on AC systems can be short ones like DC.

The catenary is sectioned so that parts can be protected seperately, much like your house has a bunch of breakers - imagine if you only had one, and everytime you overloaded anything, it popped...

There's two ways sections are done. The most common is an insulated piece with arc horns in the wire. Both AC and DC systems use this.

The other way is to raise the wire up up up above the pan's reach, and then come back down - this is done on some drawbridges, mostly noteably Cos Cob on the New Haven line (NEC). The pan litterally comes off the wire, and 'flys' in the air as the car goes accross the bridge, then comes under the wire and the wire drops down. A lot like a third rail gap.

The New Haven's pans didn't reach as high as the PRR ones did, and ledgend has it, the first GG-1 above New York got hung up at Cos Cob. I don't know about Chicago, but out east, there are some wild variations in wire height - the PRR had 'high wire' and 'low wire' signs to indicate extremes to the crews, as there were restrictions on operating at both (most noteably, not to drop the pan in low wire territory, as there wouldn't be enough space to break and arc, and high side circuit breakers on rail equipment are a recent innovation.)

  by JLJ061
Ok, these types of air gaps aren't section insulators like on the South Shore that I'm aware of.

From what it appears these types of air gaps run for about 100 feet, where the catenary kind of "breaks up" from a solid piece to three smaller pieces, then form back to one solid piece again.

What's really strange is I have seen South Shore motormen while running on Metra Electric actually be applying power to the traction motors while running through these gaps, which I thought was a no-no!

  by MikeF
JLJ061 wrote:What's really strange is I have seen South Shore motormen while running on Metra Electric actually be applying power to the traction motors while running through these gaps, which I thought was a no-no!
The South Shore has not had motormen since the '60s; the guys you're referring to are engineers. While it's a good idea to shut off while running through section breakers, as it saves wear on the pan shoes and the wire hardware, it's not necessary. The modern MU cars draw plenty of current even when coasting so it doesn't really make a huge difference.

  by Nasadowsk
On a DC system, yeah, it's a good idea to cut power (a few systems require it, noteably Cleavland's RTA), on an AC system, it's required to cut power at a phase break, period - you will open the breakers at the sub (by design).

Metra and the South Shore may/may not have a restriction on it, it depends on the type of gaps used and the power system design. Since they're DC, it's less critical (unless the gap runs a more than a few feet for whatever reason, at which point it becomes more important). It's bad form to cause excessive wear and tear on the overhead system if can be avoid, anyway. This doesn't apply to third rail the hardware is designed to arc - catenary isn't really supposed to.

  by Tadman
I deal with industrial electrification every day, and believe it or not the friction does not cause most of the wear - it's the arc. That's why we put double collector shoes on a electric crane now, rather than single, to protect the inverters and protect the life of the conductor bar.

Incidentally, the electric cranes I looked at today were at the Pullman Factory, which is now an aluminum warehouse. They were original equipment from 1906, and lifted, among other things, the original CSS cars.

  by byte
That's interesting ... was this the factory MK/Amerail did the assembly of the 8400s in?
  by Tadman
Just found this old thread - the answer is no, the MK/Amerail plant is the one at 103rd just off the Bishop Ford Fwy. It's was one of the newer Pullman plants on the south side.

The plant I was speaking of in the previous post was the Standard plant in Hammond, on 165th. Standard, as in the second half of the name Pullman-Standard.
  by dinwitty
boy, I hope the AC is in sync. MUed cars could stretch across the gaps.

Some lines used 3 phase AC.

There's a dam near here with a generator thats tied into the main electric grid, they have to sync that generator in before flipping its big switch in. I don't want to be around if that is ever screwed up.
  by mtuandrew
dinwitty wrote:boy, I hope the AC is in sync. MUed cars could stretch across the gaps.
Isn't that the idea? Another poster here told me that FRA regulations forbid individual cars to be bussed together anymore - that is, each individual car is fed by its own pantograph or shoe. It's a good idea anyway, like you suggest, so a train of AC-powered MUs won't set off any fireworks when spanning electrical blocks of different phase, frequency, or voltage. As for the MU control signal, it shouldn't matter if it is in a different phase than the motor power supply.