Fast acting fuse in each engine?

Derailments do not generally cause a short in an engine, they usually cause a voltage spike. A fuse in an engine would only protect the components in the loco if their were an internal catastrophic short, which would occur due to component failure, and not a derailment. Voltage spikes are controlled with a TVS installed in the engine, or at the power supply. A fuse in an engine would be unnecessary as it would probably never blow, as internal shorts are very rare at the most.  A TVS would be necessary, as voltage spikes are more common to occur.

Larry

Thanks, Larry. Value of TVS protection not in doubt! Would like to know if others have experience or knowledge re: whether a fast-acting fuse inside a loco would in some circumstances reduce or prevent damage and repair expense for a loco's electronics?

Most folks here do as TrainLarry stated above, install a TVS in the engine, as well as at other locations around the layout. The recommended one here on the forum is a 1.5KE36CA, here is one from Digi-Key. They have a good selection of electronic parts, reasonable prices and good shipping rates suing USPS first class mail.

Not saying it hasn't been discussed, but I don't recall ever reading a thread here on the forum about fuses being installed in engines? I do try to keep up with the recommended practices by the many knowledgeable electronics folks around here and follow their suggestions. 

I agree with Larry, the TVS is good protection, I use them but I haven't considered a fuse.

I have an unusual one, I just put a 1.3A PTC in series with the motor leads of my K-Line Interurban that I have a Cruise Commander Lite installed in.  For reasons I don't fully understand, the unit smoked the FET drivers last modular club show.  I don't know if it stalled on the switch or the motor had a fault of some kind.  The motors seem fine, so I decided to protect the board in case of a similar incident. 

When I use a tether to connect the tender rollers to the locomotive for better power over switches, I put a 3A PTC in series with the tether to protect the wiring in the case of a derailment.

A 10-amp fuse in a loco would be worthless.  Much too much capacity.  Check the amperage drawn under the max load you'll be using, and use the smallest fuse with greater capacity.  Fuses do not blow quickly on slight (say 150% overloads.

I agree with TrainLarry and the Gunrunner. 

A 6 amp fuse might even be on the high side but a 10 amp fuse would almost let you do soldering.

MTH puts a 7 amp breaker in their Z-1000.

Lee Fritz

As  gunrunnerjohn said, a PTC [PTC Resettable Fuses - AKA PolySwitch, Polyfuse, PolyTron, MultiFuse] is ideal for engines running on modular layouts or when interior fuses are needed.  PTC protection is available in ranges of 1.5A to 4A.

See more information on this Circuit Protection page.

I'll agree with the general trend.

A TVS is far more likely to be the protection needed.

Point on that, There is no such thing as too many, but you do get into the realm of reduced improvement. One per engine and one at each power drop would seem to be the max needed.

But a PTC isn't a bad backup in case of a derail to eliminate smoked wires, this also affects lighted cars.

Correct Russell, and I put them in passenger cars when I'm putting in LED lighting.  It's optional, but a very useful addition.  I use low current ones for LED use, since I'm only drawing a total of 20-30 ma for the lighting.

Thanks, Susan and GRJohn. Just did some website looking. Mouser has a 5.5A hold, 10A trip, "max 16v DC" PTC that seems as close as I can find to enough current for a high-draw loco (2 motors, smoke, etc) with the lowest trip amperage I have found so far.  What values do you use in a loco, GRJohn? Will less "hold" amperage work and not trip in sustained running with a heavy but not excessive train as load, can either of you say?

Actually, if you're putting them in the locomotive, you'll want a much lower value.  The trip current is only after a significant period of time, it takes more of an overload to trip them quickly.  Truthfully, I'd be looking at a 3-4A trip for large locomotives, I've never seen one that would pull a lot more than that.  I have a .7A hold, 1.3A trip in my Interurban set, and it's never tripped, and I don't expect it to.  I use the RUEF300 for locomotives, and I think that's overkill to be truthful.  At the time I didn't have a lower value.

Thanks, GRJ. The Mouser RUEF300 has a 3 amp hold current, 6 amp trip current. The constant loads for a large diesel, say, would include 2 can motors under load, lights, a smoke unit operating, and the electronic board and sound system loads.  Do all those fit under a steady hold current of 3 amps?  As I understand these PTC devices, the internal resistance would remain low at the "hold" current indefinitely presuming ordinary adequate cooling, but there would be some change as sustained current remained above the hold current value, meaning increased heat and resistance until tripping.  So one would want a hold value at least as large as any forseeable constant total current draw.  Can someone (including a factory person reading this) offer measured results from actual locomotive current draws on a meter during use under load?  I can't (yet) but will post such when I am able to test an actual 2-motor PS2 or similar engine in loaded use. Meantime information to guide an initial resettable breaker purchase appreciated, if experience is out there. 

I'd start with the one that has no more than 4 amps trip current first in the biggest thing you have.  Once you're convinced yourself that will work, you might want to go even lower.

If you have a locomotive that requires more than 3-4 amps continuous, something is seriously wrong.  I have only put them in a couple of places so far, but I'm thinking that might be another protection that I start putting in. 

I measured the current of a dual-motored diesel with 45 cars, on level track with smoke running, it was drawing about 2.5 amps.  It would increase slightly on grades, but not that much.  The PTC that trips at 3 amps, for instance, will pass that for some amount of time before tripping.

This is like fuses, one that's way too high for the application is no better than none.  If you put one in that's too low for the application, you'll know pretty quickly and no harm done.

For what it's worth, on my 6 track circuits, I use 5-amp breakers on 5 of the circuits & 7 on the 6th, which regularly handles 3 trains and would occasionally blow if one of the trains was passenger cars.

Sounds about right, unless you have a pretty heavy load, you'd be surprised how little power the locomotive actually requires.  The passenger car issue is why I'm so hot on LED lighting.

Thanks, guys. Based on your detailed responses, have just ordered 10 of Mouser's item 650-RKEF250, 2.5A hold, 5A trip, 60V max units.  Will try them out.

Please refresh my recollection.  I understand what blow means, but what does hold mean?

The "hold" current is the continuous rated current.  The PTC will not open the circuit at that level of current.  The Trip current is a bit more nebulous, but between the hold and trip, the PTC may open given a long enough sustained current.

On my bench tests, I find that at the "trip" current it takes quite a while for a typical PTC to open the circuit, some lasted more than a minute.

I quit using PTC's in locomotives years ago. Once they trip they aren't ever the same again as it can take weeks for the resistance of those things to return to spec's...if ever. On a DCS layout the last thing you need is resistance in the wiring esp. onboard the locomotive.

PTC's in lighted cars is the only place for them.

From Wiki;

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 resistance value similar to its original value, if at all.^[2] Since a PPTC device has an inherently higher resistance than a metallic fuse or circuit breaker at ambient temperature, it may be difficult or impossible to use in circuits that cannot tolerate significant reductions in operating voltage, forcing the engineer to choose the latter in a design.

Hard to believe that even at 4x the original resistance, it's pretty hard for me to believe that this would be a significant issue.

Let's take the Littlefuse 60R250XMR as an example.

From the Littlefuse 60R250XMR Datasheet.

R1max = Maximum resistance of device at 20°C measured one hour after tripping or reflow soldering of 260°C for 20 sec.

So we add in the .130 resistance maximum for a maximum 2.5 amp load and we get a .325V drop maximum at full current.  I'm not seeing that this is of major concern, and you'd typically have much less current and voltage drop, not to mention this is the maximum resistance.

I don't always believe everything that's typed into Wikipedia.

FWIW, I just installed some analog meters (volt & amp) on my two main tracks. Granted they are the cheapies from ebay, but seem to be fairly accurate. With both Lionel and MTH modern diesels, I have barely been getting above 1 amp with one train of 8-10 cars running and smoke on. Adding a lighted caboose (non LED) adds just a tad more amperage, barely noticeable. Track is currently all flat, no grades. All command control using 1-PH-180 per track. So far I don't have any passenger cars either, so can't try that yet.

Well, the passenger cars and the caboose are a moot point for this discussion, as the PTC is only passing the current for the locomotive.  Your results are similar to mine, I just don't see the high currents that are being discussed.  I'm ordering some lower value ones to try in my locomotives and see how things work out.