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.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
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.
greg - Philadelphia & Reading / Reading
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!
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.
rrinkerWhy 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.
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.
gregc rrinker Or the truck sideframes - you cna find any number of pictures of melted plastic sideframes because the one truck was shorted across the rails and the metal part got hot enough to melt the plastic sideframes. I asked for more information when someone mentioned this is a post. They said the wrong size resistors were used across the axles for occupancy detection. During continuos use, heat built up in the trucks, melting them.
rrinker
Or the truck sideframes - you cna find any number of pictures of melted plastic sideframes because the one truck was shorted across the rails and the metal part got hot enough to melt the plastic sideframes.
I asked for more information when someone mentioned this is a post. They said the wrong size resistors were used across the axles for occupancy detection. During continuos use, heat built up in the trucks, melting them.
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.
richhotrain rrinker No, a derailment/running a switch short won;t hurt the decoder - or at least shouldn't............ Unless it is a Proto 2000 S1/S3 with the orange wire grounded on the brush. Rich
rrinker No, a derailment/running a switch short won;t hurt the decoder - or at least shouldn't............
No, a derailment/running a switch short won;t hurt the decoder - or at least shouldn't............
Unless it is a Proto 2000 S1/S3 with the orange wire grounded on the brush.
Rich
That falls under the "motor output shorted to power input" category.
Here is a short description of the pluses and minuses of using the tail light bulbs.
It is by Dick Bronson of rr-cirkits. It mentions melting truck components, as well as a risk of fire if the hot bulb contacts a flammable material. I think he is much more knowledgeable on this subject then those of us who think the bulb idea is not the best alternative.
However, for those who insist that the bulb is a good idea, he also suggests a circuit that can be used to eliminate any bulb issues.
See: http://www.rr-cirkits.com/Notebook/short.html
anecdotal comments leads to confused understanding. some modelers seem willing to buy components for protection they don't need.
BigDaddyHe claimed that when he talked to NCE, they said the Power Cab was designed to deal with occasional shorts, but in the long run that could be harmful to the unit.
when it is supplying 5A at 15V, it is the locomitives that are disspating 75W of power as kinetic energy and heat. Nothing wrong with this.
if a throttle is designed to supply 5A, it should not be damaged if there is a short and its power supply is limited to 5A. However, the throttle must now disspate 75W of heat and this may cause damage over a long period of time.
when a bulb is used, the heat is dissipated in the bulb, not the throttle.
BigDaddy The SB5 and Powercab have built in over current sensing for basic self-protection that will continuously try to reset until destroyed if left uncorrected. They do not have manual breakers that will trip or fuses that will blow
The PowerCab uses a sense resistor to measure current. It appears that it shuts itself down when it measures an excessive amount of current. But unlike a circuit breaker, this doesn't happen immediately. This can degrade throttle components due heat when this happens repeatedly (such as when distracted)d.
rrinkerOr the truck sideframes - you cna find any number of pictures of melted plastic sideframes because the one truck was shorted across the rails and the metal part got hot enough to melt the plastic sideframes.
BigDaddy I remember a post by Brendan in his epic layout thread.
I remember a post by Brendan in his epic layout thread.
Alton Junction
rrinker It's the post right above Greg's
Thanks Randy, I blew right past that one.
I remember a post by Brendan in his epic layout thread. He claimed that when he talked to NCE, they said the Power Cab was designed to deal with occasional shorts, but in the long run that could be harmful to the unit.
NCE website says the same thing. The SB5 and Powercab have built in over current sensing for basic self-protection that will continuously try to reset until destroyed if left uncorrected. They do not have manual breakers that will trip or fuses that will blow.
Why would it be uncorrected? Well suppose you are running on a loop and run upstairs to get an adult beverage, then the phone rings or there is an emergency Honey-do. Meanwhile downstairs there is a derailment.
Henry
COB Potomac & Northern
Shenandoah Valley
BigDaddy Greg i would like to see Randy's last comment in context. Could you please post a link to that thread?
Greg i would like to see Randy's last comment in context. Could you please post a link to that thread?
It's the post right above Greg's
I've given up on explaining Kirchoff's Laws in every post
No, a derailment/running a switch short won;t hurt the decoder - or at least shouldn't, there are plenty that seem to act up after something like that happens - probably the brains getting scrambled by the voltage spikes since a short like that is seldom instant in terms of the microcontroller's speed. The sort of short that fries decoders will fry the decoder the instant power is applied - motor output shorted, motor output connected to a rail input, etc.
What CAN get fried are internal connections - say the link between the front truck and the rear truck if the loco derails and is crossways ont he track, so the short is flowing through the connection linking say the two left-side pickups. Or the truck sideframes - you cna find any number of pictures of melted plastic sideframes because the one truck was shorted across the rails and the metal part got hot enough to melt the plastic sideframes. Usually this is from a 5 amp or more system with no lower current limiting protection - 5 amps at 15 volts is 75 watts and 75 watts is a good bit of heat. Even the 2.1 amp 1157 bulbs - that's over 30 watts and can still get quite warm. Under an amp with the smaller bulbs and now you're down under 15 watts - getting to Christmas light territory and those you cna usually touch even when on for extended periods of time (if anyoen remembers the old incandescent Christmas lights, with the big bulbs, not the little twinkly light kind). Of course, there's a tradeoff - too low a current light bulb and it will light up and suck power just when running a slightly higher current loco. Not usually a problem with most modern locos, but an old BB loco or multiple locos in MU might need a different rated bulb to allow legitimate operation while still limiting shorts.
rrinker Whatever current flows through the bulb, flows through the part of the loco causing the short.
i wish Randy had explained this better.
the most likely cause of a short is when it derails, bridges between powered points and main rails, runs threw an unthrown turnout, ... This is effectively a metal short across the track, reducing the track voltage and voltage to the decoder to zero which cannot result in damage to the decoder.
it's been suggested that simply limiting the current can cause damage to the decoder. i don't see how a properly installed decoder can cause a short unless the output wires of the decoder (function or motor) somehow becoming disconnected and touch something metallic.
rrinker there is ZERO need for any additional circuit breaker besides the one built in to the PowerCab.
i use a bulb because it is difficult for me to find a track fault when the powerCab takes several seconds to cycle after detecting a short.
gregc The PowerCab can only supply 1.35A, a bulb that draws less than 1.35A is needed. Any bulb drawing more than 1.35A won't work. An 1156 auto bulb draws over 2A and won't work. i finally found a #211-2 T-3 festoon bulb which draws 0.97 amps. I soldered wires directly to the bulb. An alternative may be a B-6 bulb that draws 0.94 A and looks like it fits in an auto socket. while the PowerCab will shutdown if there is short, it is difficult to track down the short because it takes several seconds for the powercab to cycle each time a short occurs. the bulb also starts to glow if there is a larger than expected amount of current being drawn. I have some older locomotives and see this happen occasionally which means they need some maintanance. BigDaddy I believe the only cheaper alterantive is an automotive light bulb and my understanding of previous threads in the forum is that it may not prevent damage to a loco or decoder. unless the decoder is wired improperly and is the cause of the short, it cannot be damaged. It doesn't make to sense to use a 1A circuit breaker to protect a decoder when the throttle is intended to supply enough current for more than one locomotive. a short across the track can damage the throttle and cannot damage a decoder (the track voltage is zero). As long as the current is limited to what the throttle is designed for there can be no damage. multiple circuit breakers or bulbs are often used to help identify which section (power district) of a layout has a short.
The PowerCab can only supply 1.35A, a bulb that draws less than 1.35A is needed. Any bulb drawing more than 1.35A won't work. An 1156 auto bulb draws over 2A and won't work.
i finally found a #211-2 T-3 festoon bulb which draws 0.97 amps. I soldered wires directly to the bulb. An alternative may be a B-6 bulb that draws 0.94 A and looks like it fits in an auto socket.
while the PowerCab will shutdown if there is short, it is difficult to track down the short because it takes several seconds for the powercab to cycle each time a short occurs.
the bulb also starts to glow if there is a larger than expected amount of current being drawn. I have some older locomotives and see this happen occasionally which means they need some maintanance.
BigDaddy I believe the only cheaper alterantive is an automotive light bulb and my understanding of previous threads in the forum is that it may not prevent damage to a loco or decoder.
unless the decoder is wired improperly and is the cause of the short, it cannot be damaged. It doesn't make to sense to use a 1A circuit breaker to protect a decoder when the throttle is intended to supply enough current for more than one locomotive.
a short across the track can damage the throttle and cannot damage a decoder (the track voltage is zero). As long as the current is limited to what the throttle is designed for there can be no damage.
multiple circuit breakers or bulbs are often used to help identify which section (power district) of a layout has a short.
That's what the CP6 is, 6 of the smaller bulbs that will work witht he PowerCab, with sockets and screw terminals for attaching the wires.
For the OP, it doesn't get any cheaper than that. But bulbs may not protect the loco from a meltdown. Whatever current flows through the bulb, flows through the part of the loco causing the short. The PowerCab will see less than its maximum capacity and won;t shut down, nor will it be damaged.
The even cheaper alternative: If you just run trains by yourself, with no other people running trains at the same time with a second cab, there is ZERO need for any additional circuit breaker besides the one built in to the PowerCab. DCC circuit breakers serve 2 basic purposes: limit the short circuit current in any given section of the railroad, CRITICAL when using 8 and 10 amp boosters with HO and smaller scales, and also preventing a short in one are from shutting downt he whole layout. This makes sense on a larger layout with multiple people running trains, one person derails, the whole thing doesn't come to a sudden halt. But when you are the only one running trains, one at a time, it's not critical.
I just remembered this discussion if you still want a CB.
http://cs.trains.com/mrr/f/744/p/255477/2856260.aspx
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
Our road with DCC on our HOn3 modular layout has been a learning experience. We used NCE because most of the group disliked Digitrax for various reasons. However, the module wiring doesn't really change for any specific DCC system.
Because live frogs are essential in HOn3, shorts from running a turnout set against you happen during operations. Having the entire layout shut down every time that happened was miserable. So we mandated that a minimum each module set must have it's own circuit breaker.
The next discovery was that all circuit breakers are not created equal. Shorts were still being passed through to the Power Pro and shutting down the whole layout because the module circuit breaker wasn't tripping fast enough. Went back and replaced the slow-acting circuit breakers with faster ones.
The next step in our journey was that shorts were still occasionally getting passed through to the Power Pro. Because we embraced Free-mo like concepts, module sets have to be reversible end-for-end. It turns out that many DCC circuit breakers only break one leg of the track power, and not both. If we had a combination of trains spanning the module track boundary, and the two adjacent modules had circuit breakers where one broke one rail connection, and the breaker on the other module broke the other rail connection, the short could be passed through the train spanning the module gap and bypass both breakers. That one took a few tries to figure out. So now we use breakers that break both legs of the circuit.
For the OP, if you are going to add a circuit breaker in front of the Power Cab, it must act faster than the internal breaker in the Power Cab to do any good. If you are going to use mulitple breakers, and they are single leg breakers, you want to wire them all to break the same rail's power.
just our experiences and lessons
Fred W
Do you have segregated districts? A CB on each will trip if there Is a short in one district and allow he rest of the layout to keep running.
I separate the mainline from 2 yards so 3 CB's.
I have NCE EB1 breakers but it seems many on this forum prefer the PBX.
Gary
richg1998 I have used the Power Cab on my home layout for many years and never found a need for a circuit breaker. The Cab protects itself if that is an issue. Rich
I have used the Power Cab on my home layout for many years and never found a need for a circuit breaker. The Cab protects itself if that is an issue.
An NCE command station/booster will provide the primary circuit breaker support. Any separate circuit breakers are used to protect separate power districts. If you don't have any separate power districts on your layout, you won't need separate circuit breaker support.
As far as curcuit breaker wiring is concerned, your bus wires, not your feeders, should provide the input and the output to the circuit breaker. Feeders from your track then connect into the ouput wires from the circuit breaker.
BigDaddyI believe the only cheaper alterantive is an automotive light bulb and my understanding of previous threads in the forum is that it may not prevent damage to a loco or decoder.
Another vote for the PSX-1 circuit breaker. I have two of them because I have two power districts on my layout. While I can appreciate being on a budget, I question saving 10$ per unit in the event something happens and the fix is a lot more than 10$. Plus the trip current of the NCE unit is 1 amp. The PSX-1 has a higher trip current.
Bear "It's all about having fun."
I believe the only cheaper alterantive is an automotive light bulb and my understanding of previous threads in the forum is that it may not prevent damage to a loco or decoder.
My alternative isn't cheaper, it is a PSX. The Power Cab is a low power system, as opposed to a system with boosters. The PSX has to be configured for a lower amp and according to Mr DCC a normally closed push button reset is required. http://mrdccu.com/curriculum/nce/101.html
You can also wire an led into the PSX so you have a visible indicator that the cirucuit has been tripped. There is a small LED on the PSX but as that is mounted under the layout, one doesn't always think to look at it.
They all get wired the same way DCC system to circuit breaker to track. Adding an auto reverser to a circuit breaker gets tricky because the two devices operate at different speeds. You can ask if you get to that point.
What's a suitable circuit breaker to use w/ the NCE starter cab sytsem? Someone suggested that the NCE 524227 (CP6) Circuit Protector would work. Any cheaper alternatives? I prefer having something reliable to warn of a short. How to wire that breaker? I thought to run feeders from the track to the breaker and another pair of wires from there to the NCE terminal.
Yes, I've read quite a bit about DCC and already installed decoders in the locos.
Thanks,
Lee