third rail question

A railfan was telling me last night that to make a closed circuit between the substation and a train the current goes from the third rail to the train and back to the substation via the wheels to the regular rails to large cables. This seems incredible to me. It never occurred to me how the circuit was closed, but this seems hazardous. If it is true, how much of the track is energized? How much current is in the wheels and track? I don't think the guy was pulling my leg, but he may be incorrect. I apologize for being a little dense on this, but there are probably others out there that would to know how this works. Thanks.

Thats basically how it works..although someone with a little more knowledge of third rail ops can probably provide you with a more detailed explanation. If you look at the wheels of the MU's (especially at night), you'll notice small sparks at times from the return current.

You must touch both the 3rd rail and tracks to complete the circuit. Being direct current, the tracks are always the ground....

Shocking, isn't it? If its a third rail equipped train, such as found on the LI, MNCRR and the NYCTA, it's 600-750 volts DC and several hundred amps. It it uses catenary then it can be 11,500 - 12,500 volts AC (or 24 kV depending where you are in the country) around 60 + amps, depending on what the train is doing. What's amazing is they also run other things through the track as well, such as the "code" or the signal system to the engine simutaneously through the running rail. Just remember to give it a wide berth (especially that AC stuff) as less than 1/2 an amp can permanently put you in the clear.

Glad to hear you get a charge out it...

From an equipment standpoint, I can tell you that virtually all metal parts of the cars are bonded electrically to "high [voltage] ground"...to wit...the running rails.
There are many ground straps connected to carbody. Ultimately, there is a connection to a "truck to carbody" ground strap.
The electricity is transferred to the axle and wheelset via "axle ground brushes" that ride on the turning axle. Then from wheels to running rail via metal on metal contact.

All of your high voltage circuits end up with a cable bolted right to a metal piece of the car. (Traction Motors, M/A or inverters, heat)

The fact that the metal parts of the car are used as ground for the high voltage circuitry is exactly why the battery voltage circuits have to "float". (Originate and end at the battery set. If a battery voltage wire touches car body metal, we call it a "GTL" -Grounded TrainLine...no good...if the continuity is high enough, the GTL has to be tracked down and removed.)

Think of how much easier wiring the cars would have been if the manufacturer could just attach all the negatives from each component in the battery circuitry to the car body, just like is done on your automobile.
But that path is already taken.

Ahhh, the return current is the same as the third rail current. And yes, can be a few thousand amps on a long train.

Creepy thing - watch trains at Mineola. Notice that the tracks click and clunk a minute or so before an electric comes. But not a diesel. Spooky, huh?

I think it's caused by magnetic fields induced as a train draws current, and anything loose nearby just getting shifted as a result.

While we're on the subject, what's the normal accelerating / cruising 9at, say 80mph) currents of the various cars, plus the 'idle' (i.e. HEP load) current when they're standing?

To tie in with what Krispy said about signals also in the running rails:

Many of you headenders probably wonder what the black boxes between the running rails, with a heavy copper buss bar connecting them, are.
You'll see these scattered around, with a slew of them around interlockings.
They are Impedence Bonds....they let the DC return current through, but keep all of the AC signals from passing through to the next section of track. There will always be insulated joints somewhere between the boxes. That's why the heavy buss bar is necessary.

Nas,

I've always noticed that too about the clunking sounds, but never made the same assumption you have... Good thinking!

Joe

What I love is how at some places, there's 1/2 dozen 4/0 lines welded onto the third rail for current. The most we ever get to play with at work is 2 #2 lines (elevators just don't use much power, since they're counterweighted)

Phil,

The buss line cable and traction motor circuit cable on the M-1 is 535 MCM.
The M-7s have gone to redundant, smaller cabling.
I'm sure you've seen there are 2 cables from each contact shoe hanger.

Tom

Since it's sorta being discussed, is the 3rd rail power jumpered between the "married" sets of MU's? Or is it each unit on it's own?

yes on each married pair the shoes are all alive even if one shoe only touches rail.
the cars have a 600 volt bus jumper between A and B car.

How was the 3rd rail power jumpered on the old Ping-Pongs and 72's? Did they consist of married sets? Thanks...

This is a bit off topic. If all of the shoes in a car are live when one third rail shoe touches the third rail, how did passengers avoid injury or electrocution when boarding an electric MU car on a low platform? Were people just more careful then or were injuries underreported?

P.S. In Ziel and Fosters "Steel Rails to the Sunrise" there is a picture on page 120 of Jamaica with low platforms and third rail in place.

Phil,

In re T/M currents:
I'm trying to remember. I haven't seen a graph from the Test Train in years. If I get a chance to nab one, I will.
But, the initial T/M circuit (series) in an M-1 or M-3 is about 3 ohms.
E=IR nets about 250 amps per car for starters with rail voltage of 750.
The progress points are 288 amps for P-1 rate and (I forget) let's say for now 360-380 amps for P-2 rate.
This makes sense when you look at a power test on an M-1 or M-3.
An ideal power test is 2 and 1 ( 2 points of power with handle in P-1, and an additional point of power when handle moved to P-2.)
An initial T/M circuit makeup draws around 250 amps (first point of power), but if my Master Controller handle is in P-1, I'm not stopping the cam until I'm over 288, so another point of power is allowed.
(Dynamic brake is when you really crank some current.
But since you asked about 3rd rail draw, it does not apply.)

We had discussed in the old forums that the M-3 originally accelerated faster than the M-1s, and had to be calmed down so the 3s were not pushing the 1s.
The M-3 originally power tested 2 and 2 or 3 and 1. Both were acceptable.
Now, they power test the same as M-1s.

I'm trying to get data on the M-7.

I don't recall ever putting an Amprobe on an M/A, so I don't know what the draw is. The fuse is rated 250 amps, so it's somewhat less than that.

If it's winter, you also can't overlook the overhead heat and floor heat circuits. First stage overhead heat measures 60 ohms per each end of each car;
Second stage is 30 ohms per each end of car; floor heat is 70 ohms per side of car.
All of these could be on at the same time.
Do your E=IRs and you come up with some decent amperages there, too.
PTR 1 through 4 (third rail voltage potential sensing relays) and cab heat draw (1 amp) are negligable, so that's about it.