Will a Polyfuse interfere with the DCS signal

Matt, I wouldn't think so, from what I've read a polyfuse or polyswitch is made of crystalline components that turn from a (for lack of better terms) solid conductive state to a liquid  non-conductive state as the temperature rises.  When it cools, it turns back into a conductive state.

I'm using them in my BPRC systems (2.4Ghz) that I used to replace all my PS2 systems in my 7 steam engines.  They came with the systems when I bought them and are located between the motor and receiver and battery and receiver.

Dont polyfuses have a slow reaction time?

right around 300 ms to 1 second depending but they will trip faster if yo put more current through them. They aren't the fastest protection but Im thinking they should work for what I want to use them for. They are affordable and self resetting. Another option is mini blade fuses and holders. More money and I am planning on putting these things in like 70 locos over the course of time

 Does anybody know if they will mess with a DCS signal?

Yeah no...No yeah...Lol

DCS seems to hate any resistance in a circuit. Polyfuses once tripped can take years to reset themselves back to their original low resistance design...Here's a Wiki statement about them...

" The device may not return to its original resistance value; it will most likely stabilize at a significantly higher resistance (up to 4 times initial value). It could take hours, days, weeks or even years for the device to return to a resistance value similar to its original value, if at all.^[1]" 

The PTC shouldn't have any effect on the DCS signal.

When a PTC is subjected to very high overloads, they trip pretty quickly.  If they're subjected to mild overloads, they can have an extended trip time.

I use the PTC in passenger cars between the two pickups to protect the wiring, and I also use them when I add a tether between the locomotive and tender to share power on small steamers.  Since I've actually had the experience of cooking the wiring in a passenger car and my added tether, I think it's well worth it.

But initial resistance is around .002 to .004 ohms so the change is resistance is still negligible

I have PTC's in a number of MTH locomotives, no effect that I've been able to detect.

Sweet, I'm going to send an order off to DigiKey and get a bunch. They're only .28 each. I was thinking a 2 amp hold current with around a 4 amp trip current

That should be plenty unless you're really working those locomotives.

You know, for as cheap as they are, the manufacturers should really start including them as standard equipment on all new locomotives.

Oh wait, that would be too easy, and cut down on the fried electronics business.

They could toss in the TVS protection as well Elliot, but that would cost them another 25 cents or so.

If Lionel did that it may very well push the MSRP of a diesel into the $1000 range

By all means John! And those prices are retail, imagine what they are wholesale bulk.

I guess the arguments against, beside what I said earlier, would be: space inside the engine and labor to install. Chinese labor costs are skyrocketing.

Everybody knows there is no room inside the and O gauge loco to fit something the size of a penny

You mean it's not a TARDIS? Bigger on the inside than it is on the outside. Oh, aren't we going to have fun? And when I say we, I mean you!

I think im coming down with the hebrew basement flu, cough cough

Matt, on a more serious note, I reread your original post, and even though I am not very familiar with the TIU, depending on the power supplies used, your potential max current in a bridging event could reach the combined max output of both supplies. I use some crazy 20 amp transformers. That's a potential 40 amps in some locations.

OUCH!!! I can smell the smoke now.

Even though I don't use DCS, my layout still has power districts, most of which end at a switch. When I get done wiring, I plan to go back and review which districts are on which of the six supplies. The goal being to minimize the number of transitions between supplies, by grouping as many contiguous districts on a single supply as possible.

Im running 10 amp bricks so the potential short between districts is 20 amps. Ive already nuked the sound board in one Lionel loco because of a derail on a switch between tow districts. You're the one who clued me in to what happened. It never occurred to me how that would be an issue