10 1/2 Easy Electrical Tips in September CTT

[sir james I]

While this is good information to have, in all my years of running 3 rail trains I have never had a problem running trains from one power output to another. As long as they are in phase of course.

[servoguy]

I, also, have a BS in math and an MSEE with 50 years of experience.  Bob Nelson and I are in perfect agreement about the damage fault currents can do.  I suggest everyone follow his advice and avoid smoking your layouts.  

I have heard of passenger cars with two rollers burning the wiring because one roller was powered by one transformer output and the other roller was powered by another transformer output.  

I have owned Lionel trains since 1950 and have many years of experience with them.

[lion88roar]

You do this by isolating sections of track into blocks, then using switches you chose which power source is supply power to each block.

[nhrail]

Bob,

I follow your advice religiously. I am on the other side of young with no academic electrical knowledge. Some argue that your advice is somewhat theoretical and, therefore, not applicable in the "real world." I favor your approach. The practical aspect is, I think, undeniable and I have not seen an answer to the following basic question. Please excuse me you have previously offered such advice. I have searched and have not been able to find it.

Is there a "convenient" way to safely move a locomotive, on track, from one power source to another as desired? Of course "convenient" is a relative term. I suppose I'm looking for a solution to a recurring issue that the unknowing can apply :-)

Best,

John Mucci

[lionelsoni]

A low-enough voltage difference, a high enough track or wiring resistance, and a short enough duration all can make the likelihood of damage arbitrarily low.  Many modern "transformers" will not have the problem at all.  You will have to give the specifics of the setup for anyone to explain a particular result.  As I pointed out, these details were not given in the article; so I made up some plausible ones.  The danger is that someone will take the "tip" to be unconditionally safe and try it on his own layout, with bad results.

Lionel once effectively admitted that the circuit-breaker arrangement that their postwar transformers shared was flawed, in the fine print of the service manual for the KW:  http://pictures.olsenstoy.com/cd/transfmr/pskw2.pdf

I have an interesting exhibit.  It is the actual transformer out of a type-Z that I bought for parts.  That transformer lacks a whistle control but is otherwise very similar electrically to the ZW shown in the article.  There is a section of the exposed secondary winding that is badly burned.  It is in the middle of the winding, with undamaged turns beyond it at either end.  I think it is pretty clear that the fault current that must have burned it had to flow through two of the rollers stationed at either end of the burned turns.  Otherwise the same current would have flowed throughout the secondary winding, and it would all be burned.  So at some time there was a short circuit between two of the output terminals; and a current flowed, long enough and heavy enough, to severely damage the transformer; but the circuit breaker did not trip.  I don't know how to post the pictures here; but I will happily e-mail them to anyone who would be willing to do that for me.

(It's not a "reverse" current, but a fault current.)

(My job does not require me to be licensed, but I hold BSEE and MSEE degrees and have 47 years of experience.)

[BigAl 956]

I have heard this before. While I understand the logic behind this fear I can tell you that this never happens. Most model railroad hobbyists at one time or another park trains on sidings and across different blocks where there is a difference in voltage and there is no damage whatsoever. Lionel enthusiasts have been doing this for centuries now and there is just no credible evidence of this practice being a problem. Recently on another forum we had a few operators trial parking various cars and engines across different blocks set at different voltage levels. In no case was any damage or overheating reported. The current flow was negligible. Maybe there is an EE out there who can explain why reverse current flow does not occurs .

[lionelsoni]

Stan Trzoniec's article includes the suggestion of using one ZW output for level track and another for a grade up a hill on the same loop.  Some of you could have predicted that I would object to that.  The problem is that every time the train makes the transition from one block to the other, its pickups create a short circuit across a part of the ZW's secondary winding, unprotected by the circuit breaker.

I have long advised against this practice for level track, where there is at least a chance of setting the two blocks to the same voltage.  But in this case, the voltages are deliberately set different.  I don't know what his settings, track length, and feeder arrangements are; but here is an example of how things can go wrong:

Suppose that the voltage difference is 5 volts and that the total resistance from the ZW's A terminal to its D terminal, through the train's pickup, is 100 milliohms.  This could be about 25 feet of center rail or 30 feet of 14 AWG feeder, or a combination.  Now 14 AWG would ordinarily be safe for wiring a ZW, but not in this case.  The fault current when the pickup crosses the gap is 5 volts divided by 100 milliohms, or 50 amperes.

The ZW may well be able to supply this, since the low voltage of 5 volts means that the power drawn is only 250 watts, not a great overload for the primary winding.  But the power dissipated in any 14 AWG feeders will be about 10 times what the wire can stand:  Wire is rated according to the current that will not heat it to the point of melting the insulation.  The current here is 3 1/3 times the wire's rating; so the temperature rise is over 10 times what the insulation can tolerate, assuming of course that the layout was wired with 14 AWG.

All of this is guesswork, since we don't know the particulars of the actual wiring.  But the numbers I used are plausible, and the situation could be dangerous if the train ever stalled across the gap.

The other tip that my "1/2" refers to is number 7, wiring a signal to a turnout.  This is a good idea; but it should be noted that it does not work for Lionel O27 turnouts, which also have 3 terminals, albeit in a different order.  The problem is that it depends on the the internal contacts that shut off the solenoids of 022 and similar Lionel turnouts.  (Some brands of O27 turnouts also have these contacts and can be used in this way).