Wednesday, August 21, 2013

Lightning Arrester

It's time to play "Stump the Professor!"   Here's a physics-related question that has me mystified.  The page above is from the Westinghouse manual for AMM brakes from 1908.  I just don't understand how the point where the compressor circuit is connected, in connection with leaving the lights on during an electrical storm, can provide an "efficient lightning arrester."  They also don't make it clear whether they expect you to have the pole up or down during the storm.

So I'm presenting this to the collective wisdom of our readership, which I value highly.  Can anyone explain how this is supposed to work?

While we're at it, I might point out that the IRM rule book requires the operator, in case of an electrical storm, to bring the car to a stop in a safe place (i.e., not blocking Olson Road, I guess!) and pull the pole.  This is certainly the right thing to do.  If lightning were to strike the overhead line anywhere, and a car's pole were up, the pulse would be transmitted into the car with disastrous results.  If the pole is down, and you're physically underneath the wire, the probability of a strike hitting the car should be very low.


Jeron G. said...

Here's my take on it:

Keeping in mind that electricity takes the path of least resistance, it does make a bit of sense. By having the compressor be the first item tapped in after the main trolley switch, it's a quick path to a large part of the car that's grounded to the entire frame and subsequently the rail.

By having the lighting circuit tapped in as the following item, it would act as backup protection in the event of a lightning strike should there be an insufficient ground to the frame from the compressor. Each light bulb could act as an expendable fuse of sorts and since it's the lighting circuit, it's probably the circuit with the most wiring in the car therefore providing the most resistance.

Just a theory, of course.

Anonymous said...

I know the CNS&M rulebook made a similar instruction to turn on the interior lights during a lightning storm.

Perhaps early minds of the time thought it would provide a path of less resistance than the compressor and motor circuits?

Bruce Duensing said...

Sort of dovetails into placing lightning arrestors into the design of the overhead power feed, on the support poles, back in the days of yore. Light bulbs seem like a pretty iffy back up, considering the strength of the voltage spikes involved. I can recall substations being completely knocked out by a strike. Then there is the matter of the path of least resistance..maybe I am thick but I am not getting the light bulb trick.

Randall Hicks said...

No, Bruce, you are not thick, I don't understand it either. You certainly wouldn't want a high-voltage pulse being absorbed by the light bulbs in the passenger compartment; they would arc over and shatter. That wouldn't be good. On the other hand, remember that most of the time the governor will be off, so the compressor circuit is open and no current can flow. Maybe Westinghouse just thinks it's better to blow up light bulbs rather than their precious governors and compressors?

I've started looking through some of my old electric railway engineering books, without finding what I want. Cars are generally equipped with a small arc gap arrester of the usual sort, ahead of the motor choke coil, and with magnetic blowout or other means of extinguishing a DC arc. But none seems to mention leaving the light circuits on.

Here's another question: on the North Shore, for instance, would they just continue to operate through an electrical storm? The only sensible thing to do would be to stop and pull the pole, just as our rule book says, I would think.

Bruce Duensing said...

I don't know what the rule books said, but having lived a block from the CNS&M Mundelein station, inclement weather complete with lightning strikes did not bring anything on the rails to a halt. I know there was a rule change to keep the headlights on 24/7 but it had nothing to do with strikes as I have a copy of that memo. It was to provide more visibility to the public.

Anonymous said...

Let me share the results of my appropriately detailed WAG analysis (WAG of course atands for Wild A***** Guess)

I suspect this precaution originated in the wood car era. I would think a wood car struck by lightening would behave like a wooden building, lots of smoke, structrual damage, and likely a fire. Structures are protected by an array of lightning rods to divert the strike to ground. Since the lighting circuit is sort of an array of wires just under the roof, the thought was that it might divert the strike away from traveling through the wood car structure. Note that there is no discussion of turning on the floor heat circuits, which would not help.

It is also true that the lighting circuit has very little inductance, unlike the motor, air compressor, or floor heart circuits. Inductance blocks (chokes) high frequency currents, such as lightning. That is why there is a large inductor (choke) on the roof just after the air gap arrestor to try to enccourage the strike to take the air gap route.

When steel cars came along, lighening was less of a fire problem, since there are lots of low resistance paths to ground. But since no one knew the reason for turning on the lights, the rule remained.

Pete Schmidt