Bachmann Locos and track-powered cars - shorting length?

[jviss]

I'm planning a conversion to DCC and I have hit an issue.  I want a programming track that's a spur: a switch form the main line, and and double-gapped spur that's wired to the Prog A and B terminals so I can drive a loco onto it for service mode programming.  Despite the vendor's manual showing just this, their tech support people advise strongly against it, saying "[y]ou can easily damage the program circuit if you have the main track set to Rail A and Rail B and the siding set for the program circuit and the loco creates a bridge between the two circuits." 

So, I'm thinking of a design that would obviate this danger, and I need to know over how long a track distance a Bachmann loco or track-powered car will short the rails - which I've called the "shorting length" in the title of the post.

Clearly a wheel bridging a gap will short the rail, but if this could be mitigated in some way, what's the next distance?  On my BBH 4-6-0's, the leading truck picks up power, as do the drivers, so it would be from the leading wheel to the last driver - I don't have one handy to measure now, but you get the idea.  For a passenger coach with track powered lighting, and both trucks fitted with pickups it would be first axel to last, right?

Any ideas?

Thanks,

jv

[aspoz]

Seems logical, but don't forget that some locos have pickups in their tenders as well as in the loco body, and the Annie might be one although mine got converted so long ago I can't remember.  

Usual way I've done this in the past is to have a section of track ahead of the programming area which is switched dead when I select programming but is live when I've selected the main line.  That way I have to get the loco in the right area and I can't get a short (unless I mess up in the wiring which touch wood hasn't happened yet).  Mind you, I've only done that in N; all my large scale stuff is battery!

[Hunt]

As the Digitrax techs advised, best to use a track physically not part of the layout for programming track. Use the programming track to assign decoder address. Then place the locomotive on the main track. You can do most all other CV programming on the main using Ops mode programming, which directs programming to a specific decoder address.

To have a powered drive on drive off spur on the layout also serve as the programming track --
Simple way - Use a Double Pole Double Throw (DPDT) toggle switch. The wiring is covered in several places in the Digitrax Tech Support Depot.

Use second toggle to cut power to a length of track before programming track if you cannot trust yourself with only the one DPDT. 

[Hunt]

The Bachmann Dynamis Pro (Dynamis  +  Pro Box)  is designed to automatically handle a drive on - drive off layout spur (without a manual DPDT switch), which also serves as the programming track.

[Kevin Strong]

I'd figure 3' long for the length of the longest loco or passenger car. Probably a K-36 would be the longest narrow gauge loco that's been commercially produced. I think Hunt's suggestion of a DPDT-controlled spur that lets you select between main and program makes the most sense. You can use a center-off DPDT, and completely kill the power to the spur if need be.

Later,

K

[jviss]

Thanks all.

I'm thinking a DPDT switch plus a "guard track" between the main and programming spur, that is longer than my maximum shorting length, and is unpowered when the switch is in the programming position. 

I should probably check whether the switch is break before make, or perhaps use one with a center-off position, just to be safe.

[Hunt]

Only want one toggle switch for all - a dead track between layout track and programming track, then use a quality break-before-make 4PDT center off switch. Be sure toggle switch rating is compatible with the current and voltage available to track.