Re: Large Gauge Ground Wire in Parallel with Loconet Gound Wires == Groundloop?

Mark Gurries
 


On Dec 14, 2017, at 1:00 PM, modelrr@... [WiringForDCC] <WiringForDCC@...> wrote:



In Allan Gartner's Wiring For DCC website, Booster Network Wiring, RECOMMENDATION #4-6: Run a Heavy Ground Wire Between Your Boosters:, the wiring diagram indicates cutting the two ground wires in the booster Loconet wires and adding a large gauge ground connection between the command station and the each of the boosters.  (Yes he was discussing wiring Digitrax equipment which is what my club has.)

The stated reason is to prevent ground loops causing possible erratic behavior.  

On Mark Gurries' website or the only mention of ground loops is in stating not to connect the booster network ground to house earth ground.  Again, erratic behavior may result.

Larry Puckett, the DCCguy.com website also advocates the ground wire but does not mention ground loops.  He did not mention them in his recent Model Railroader article.

Question: does the large gauge booster network ground wire and the parallel Loconet ground wires constitute a ground loop that may cause problems.  Or are they just wires in parallel.  


Your mixing up two separate problems.

1) Ground Loops involving Earth Ground versus DCC system ground (Booster Ground/ Booster common)

2) The problem of Booster Track Current flowing in Loconet Ground wires.


Your question has nothing to do with Earth Ground.  So #1 is not being discussed.   Your question is #2.


Setup: Two Booster Districts meet each other at a rail joint consisting of double insulated rail joiners (gaps).   Booster X powers the track on the left side of the rail joiners and Booster Y powers the track on the right.  (This has nothing to do with existence of DCC circuit breakers).

Situation:  As a given engine moves across the double insulated rail joiners, the engine will become powered by BOTH boosters at the same time until is completes the crossing.

Operating Problem:  Any locomotive with offset wheel pickup will stall unless the booster X and booster Y have a ground connection between them.  Even if you have a locomotive with all wheel pickup, electrical pickup is never perfect and there is the potential for the locomotive to have offset wheel pickup momentarily for multiple of reasons.  In this case the locomotive will not cross cleany unless you have ground connection between the two boosters involved.

Electrical Problem.  At that moment in time where offset wheel pickup is taking place, the only way current can continue to flow is if the locomotive current is carried between the two boosters via the ground wire.  No ground wire, then you have an open circuit situation and the locomotive loses power.

LOCONET SITUATION:   The loconet cable consist of 6 26AWG or 28AWG wires.  Two of these wires are in parallel to form the “ground" connection.  The ground wires carry the following current

1) Loconet return current.   Very Low current.  Loconet communication.
2) Railsync return current.  Low current.  Booster Signal communication and low current power source for plug in throttles and some loconet devices.
3) Booster ground current.   Momentary Very HIGH Current.  The current flowing depends on the size and efficiency of the locomotive motors for a given scale.

These small gauge wires are NOT designed to support high current flow.   The voltage drop in the wire will be proportional to the length of the wire and the magnitude of the current flowing in it.   

LOCONET PROBLEM:  When you have a locomotive with an offset wheel pickup problem, the locomotive current will flow in the LOCONET Ground wires.  These Motor currents are much much higher the Loconet or railsynce current in the same ground wires.  The higher the current draw of the motor, the higher the voltage drop that will appear across the ground wire between the two boosters.  The momentarily very high voltage drop can prevent reliable communication of Loconet or Railsynce or both at the same time.  The problem can go away one the locomotive completes the power district crossing.

SOLUTION:  Since we cannot change the gauge of the wire in the Loconet cable to support the motor currents, then a parallel wire of a significantly higher gauge must be run between the boosters ground terminals.   This effectively reduces the voltage drop in the loconet ground.  Furthermore since it runs directly from booster to booster with lower resistance than found in the Loconet cable, the motor current will freely choose the lower resistance path and flow in the large wire between the two boosters removing most of that current flow from Loconet wires.  In other words, you greatly relieved the Loconet ground of the responsibility of supporting locomotive current flow.  Loconet and Railsynce communication will be more reliable now that it is being unmolested by the independent and very noisy locomotive current.


Is it a ground loop?  Technically yes.   But in this specific case the design of the loconet bus brings with it an electrical design flaw straight from the factory.   The permitting of Locomotive current to flow on the Loconet ground.    With small layouts and/or small scale layouts in practice this is not much of a problem.  The loconet cables are short and/or the locomotive currents are low.   But it become a big problem on very large layouts and/or large scale layouts where the loconet cables are long and the locomotive current can be very high.   Adding the large gauge parallel ground wire permitting a ground loop to be establish is the LESSER of the two evils.   


In the case of my club all the Loconet connections are daisy chained with a lot of intermediate connections.  So the large gauge wire is shorter with no interrupting connections.

Great.  That is the best you can do.


Best Regards,

Mark Gurries
Electrical Engineer
DCC Website & NMRA DCC Clinics: www.markgurries.com



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