Technical Corner

DCC CIRCUIT BREAKER WIRING ERROR SHORTS BOOSTER

I recently read of the Narrow Minded Narrow Gaugers having a problem with their FREEMO-like modular layout.  Mike Condor, one of the members, explained the difficulty on the NCE-DCC Yahoo list, and solicited help. 

Part of what Mike reported is that “Each set of modules has a circuit breaker, and are gapped from the adjacent modules.  CB's are a mix of PSX-1's and NCE EB-1's, all at stock (standard) settings... .”

Mike further said “We had a number of times when a short in a module shut down the whole system. The CB did not shut down that particular module.  All cleared fairly quickly, but it was annoying.”

Well, sure it would be annoying to build in circuit breakers in each set of modules only to have a short in a single module “shut down the whole system.“   To add electronic insult to injury, the circuit breaker in the errant module did not trip.  The layout was not supposed to work that way!  The same symptom/problem could come up on a permanent home layout; it is not unique to sectional or modular layouts.

I learned, in the considerable number of years I have been designing, building, and fixing things that everything will work just the way it was built.  Not the way it was intended to work but the way it was actually built.  Symptoms may vary, operation may, at times, appear to be what was intended.  The symptoms may get a bit weird during fault conditions.  The Narrow Minded Narrow Gaugers ran trains just fine, until a short occurred and then the weird symptoms began.  I installed PSX breakers on the Denver HO Club layout, and I installed EB1 breakers on the Estes Valley Division layouts.  I had a feeling that I knew what happened, though not exactly how or why.

There are many terminals and wires that can be connected to either of these DCC circuit breakers, with the PSX obviously having the most features and potential connections.  Reduced to basics, the PSX and EB1 DCC circuit breakers have two terminals for track power input and terminals for two track power output.  During overload or short conditions, these DCC circuit breakers open only one of the two DCC track power wires, the input and output wires for the other rail are connected straight through via traces on the printed circuit board. 

The symptoms of wiring errors vary depending upon whether the layout is wired “Common Rail” or “No Common Rail.”  To begin to understand the problem, look at the diagram titled COMMON RAIL WIRING.

The diagram shows three modules, or three power districts on a home layout.  Across the top is the DCC track power bus from the booster.  A simplified representation of the certainly more complex trackwork is shown across the bottom.  This diagram could represent an around the room layout that is divided into three power districts.  It could represent just three power districts in a far larger layout.  Note that rail B is designated the “Common Rail.”   Each of the three districts includes its own DCC circuit breaker, drawn in a simplified format.  This represents either a PSX or EB1 circuit breaker.  Terminals #1 & #2 are the DCC track power input, and terminals #3 & #4 are the outputs.  Note that the circuit breaker interrupts only one side of the DCC track power, the side that passes through from terminal 1 to terminal 3.  Terminals #2 & #4 are tied together inside the circuit breaker.  For COMMON RAIL WIRING, terminal #4 must be the one that connects to the common rail.  The three modules or power districts are labeled “N,” “N+1,” and “N+2.” Note that the circuit breaker in N+1 is wired incorrectly.  The error could have been in any one of the three (or more) power districts, I put it in the middle for purpose of illustration.

The A track power bus connects to the A rails in all three modules, and the B track power bus connects to the B rails.  All seems to be in order.  However, the wiring error in the N+1 circuit breaker connects track power A to the A rails through Terminals #2 & #4 which are tied together inside the circuit breaker.  Now, suppose a misaligned turnout or other problem causes a rail-to-rail short in the N+1 power district.  Track power bus A is hard wired to the N+1 A rails, and the short ties the N+1 A & B rails together, and the B rails in both of the other two districts are hard wired to the B track power bus through terminals #2 & #4 or their respective circuit breakers.  The short circuit is directly across the track power bus A & B wires, the booster is shorted, shutting down the whole system. Also note that none of the three circuit breakers in the three power districts will trip.  Just as Mike said, and annoying too!   Well, that is easy to fix, just eliminate the COMMON RAIL WIRING, and all will be OK regardless of the wiring of breaker N+1.  Not necessarily so, as will be seen in the diagram titled NO COMMON RAIL WIRING.

Note that this wiring configuration does not have a common rail, but that the same circuit breaker wiring error is in module N+1.  Now, suppose the short again occurs in module N+1.  The current flows from the A track power bus wire, through the internal connection in the circuit breaker, to the A rail, through the short to the B rail, and back through the circuit breaker to the B track power bus.  The circuit breaker detects the over current condition caused by the short and trips, removing power from district N+1. 

Mox Nix – All is well, no harm done!  Well, not quite, what about the visiting fireman with his full length lighted San Juan passenger train?  Or, if the layout is standard gauge, how about the fellow with the lighted California Zephyr? Or perhaps just a lighted caboose (way car?) on a freight train or some fine old brass trucks with side frames that will conduct power from axle to axle?  Any of those things can and will intermittently bridge from module to module and constructively create a common rail connection that comes and goes as the rolling stock moves.  If (really big if) there is nothing to bridge the gap in the B rail and tie one of the other districts to district N+1 then there is no symptom pointing toward a problem, everything appears to be fine.

The situation can go downhill from here.  The initial short may cause the circuit breaker in N+1 to open but then the short carries booster current through one of the above described rolling stock connections and incorrectly wired circuit breaker, right back to the booster, bypassing the circuit breakers altogether.  If the initial short is ‘good enough’ to cause the circuit breaker to repeatedly trip as it retries but the temporary rolling stock bridge from module to module is not a low enough resistance to cause the booster to shut down then the rolling stock quite likely will be damaged if the fault is not corrected quickly.  The probability of damage increases if the motive power or rolling stock is light weight, like the trailing truck on an RGS motor #5 shown in the photo named COOKED MY GOOSE.  In this case, the short circuit current was flowing between the two wheels at the top of the photograph. 

If one thinks heaver equipment is immune to this sort of damage, look at the photo named DAMAGED DIESEL.  The conditions were the same as those that damaged the Goose.  The wheels at the top of the photograph were bridging a gap and the short circuit current path was from rail to wheel, from wheel to axle, from axle to bearing, from bearing to side frame, and then from side frame to bearing, from bearing to axle, from axle to wheel, and from wheel to rail.  All these little resistances add up, and the current through the resistances consumes power and the power makes things hot and hot plastic things melt.

Boosters may require substantial overload to cause them to shut down.  Derailed cars or locomotives or stuff run up against a turnout not aligned correctly may not cause a low enough resistance short to cause the booster to shut down.  Derailed cars or locomotives or stuff run up against a turnout not aligned correctly are not the equivalent of a quarter pressed against the rails.

OK – No common rail, and follow the manufacturer’s instructions to the letter when installing the DCC circuit breakers. 

Sigh, Murphy was such an optimist.  Near the start of this column, I said “I had a feeling that I knew what happened, though not exactly how or why.”  “Been there, done that” as some wag once said; the instructions for the NCE EB1 are incorrect!  The output terminals are incorrectly identified; specifically the instructions have them reversed.  Look at the attached scan titled EB1 CORRECTED.  This excerpt from page two of the BCE EB1 instruction sheet shows the corrected connection, with some verbiage describing the error.  If using an NCE EB1, it should be wired according to the corrected diagram.

What is a person to do?

Do not connect a booster directly to a layout, even a small layout.  Include at least one DCC circuit breaker with its current trip point set as low as possible, consistent with layout operational requirements.  If there are multiple power districts, set each breaker trip point as low as possible.

Install DCC circuit breaker(s) with remote reset switches so that the breaker will not repeatedly retry.  A momentary short will trip the breaker and require manual intervention to restore power.  This will be a nuisance at first, but eventually the causes for the breaker tripping will all be found and then operation will be more enjoyable with little or no frustration.

Never leave an operating layout unattended with power on, even for a short trip to the coffee pot or telephone.  Bad things happen quickly, and a short distraction can turn into half an afternoon.  Consider how hot a 25 watt soldering iron can get, and guess how hot a five ampere booster might get something when its 70 or so watts goes somewhere you never intended.

Have fun, enjoy the hobby.

Rex G. Beistle

Submitted for publication by the Rocky Mountain Region of the National Model Railroad Association.

This original work is donated to the NMRA without expecting compensation of any sort.

If this is published by any other NMRA body, just give me credit.