Why would a powered loco have block detection resistors? That’s definitely not what caused the truck sideframe meltdown. And if the wrong resistor was used for current sense, it would simply melt down just sitting there, no short needed.
ANd what did they use, a 10 ohm resistor instead of a 10K? Because even 100 ohm is only 150ma at 15V and probably won’t melt an axle. 10 ohms gets you 1.5 amps, that could certainly heat up enough but I’d be surprised a small SMD resistor wouldn’t melt before the sideframes of a truck with metal axles and wheels. Or the axle would melt, or even the conductive paint unless it is a metal axle with the resistor soldered directly to it. That’s 22 watts into what is at best a 1/8 watt resistor. Doesn’t add up that the truck sideframe would be the part that melts in this case.
Sigh. It was not a resistor on a loco. It was an Atlas tank car. And either there was an incorrect size resistor used (apparently they come in different sizes with the same rating), or the wire cement used bridged the resistor and the thing drew too much current. Whatever the reason, the car physically got hot enough to melt. From what I am told, there was enought current draw to cause damage, but not quite enough to trip the booster breaker.
Frankly, I am tired of posting this same comment. If I can get a picture of the destroyed car I’ll post it here.
There are phootos of melted LOCO trucks from shorts. Rolling stock, coompletely different, unless you happen to have some odl car with metal trucks - but the sideframes there are usualy metal as well so nothing to melt, at least at the power a DCC system can provide.
When I refer to melted parts, it always based on the photos of LOCOMOTIVE trucks, not cars. I think I covered how the wrong resistor might melt the car truck - it wouldn;t be hard for someone inexperienced in electroonic parts to use a 10 ohm instead of 10K ohm surface mount resistoor in a detection axle, the markings can be confusing unlike the color bands on leaded resistors (and it’s easy to quick check a leaded resistor with a meter). That would draw 1.5 amps at 15V, which wouldn;t trip a booster rated for that current or higher. Wouldn;t trip a circuit breaker set for say 2 amps, either. So yeah, while it’s sitting on powered track, it would be dissipating over 22 watts of heat in a small area. Plenty to melt plastic - considering a 16V 50ma GoW bulb can do it.
We’re explicitly talking shorts here though. And even then, nothing is completely foolprooof. Inadequate wiring, especially, can cause enough of a drop to where a ‘short’ is not a short - The resistance of the inadequate wiring menas that there isn’t 0 ohms across the booter terminals, there may instead be 10 ohms. If my 5 amp booster tripped out at 1.5 amps, I’d not be too happy with it. 5 ohms or resistance - that’s still only 3 amps. Again, the 5 amp booster better NOT trip at 3 amps load. Either is enough power to generate enough heat to melt plastic. That’s why the quarter test is so important. And it’s equally important that it work WITHOUT pressing the quarter down.
Thanks everyone for the helpful comments. Your feedback was most informative. I purposely let this thread go a bit before responding to see how/where others would go.
Given that the NCE tool has a protection device, I won’t use the CB for the mainline but will for a yard. I don’t want to fry a decoder and make something blow up. While seeing smoke is great for steam, I’d prefer avoiding that on a more modern layout! This layout is my first and want to ensure it runs smoothly. Taking my time and asking questions is invalauble to just jumping without looking. I don’t have a ton of money/time, so these resources are very precious–almost as much as the level of help I get. For that and so much more, thanks!
when is a short not a short … when it’s not 0 ohms ?
i found two web pages discussing melted trucks.
The first describes a melted diesel truck and a steam locomotive wheel that melted due to wheel not being located properly. Even though they caused a circuit breaker to trip, the repeated cycling of the circuit breaker lead to heat build up that melted a diesel truck and steam loco wheel.
the second describes a wheel bridging a turnout and drawing excessive current, enough to melt plastic but not enough to trip the breaker.
i don’t understand why there was enough heat to be generated to melt plastic if these were truly metal on metal shorts (zero ohms means zero Watts, otherwise all our switches would be getting hot).
The best explanation i can come up with is that they are poor contacts. We’ve all lived with dirty track and intermitent stalls.
That’s pretty much exactly it. It’s not always a solid contact. Notice that even in cheap toggle switches you have contacts that are spring loaded so when they make contact, they do so with some force. Top quality switches and relays often use other materials than copper for the actual contact surfaces.
Add on long runs of too small a wire, adding an ohm or two of resistence. Then power feed through a couple of loose rail joiners… Again, that’s why the quarter test is a good thing. If the weight of a quarter is enough to make good enough contact to trip the breaker, odds are a model loco, which is heavier, will also make sufficient contact if it derails and shorts. Nothing’s 100% fooolproof of course, but fully passing the quarter test all over the layout is cheap insurance.
Something else worth considering - the manual reset feature of many electronic circuit breakers. If you walk away and a short occurs, it won’t keep applying power to test if the short is cleared when in the manual mode.