Thanks all, never thought about the caulk gun being partially insulated, just looked at it and they are painted and have caulk residude here and there from many non train jobs, guess the paint was not enough of insulation as there is no way the pure metal parts could have toucher where it happened (yes I just tried the location).
I suspect the breakdown voltage of even plastic rail joiners is high enough for any voltage likely encountered on a model railroad. The heating point most likely was the contact between the caulk gun and the rail. But from the description, it was enough of a low impedence that the breaker was tripping.
I did get a junk boxc of electronic stuff when I was a kid, and one thing in there was a couple of feet of real high voltage wire - the conductor was maybe #20 in size, but the insulation, all silicon, was bigger around than #4. I forget what voltage was marked on it - either 60K or 100K. Not sure what the neighbor who gave me the stuff used it for, but morer than 40 years later, my friend used a piece of it to rebuild a small engine he was working on - needed a new plug wire.
--Randy
Modeling the Reading Railroad in the 1950's
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rrebellI don't see why it would not see the caulk gun as a direct short.
It is possible that even one comparatively massive short might produce sufficient damage to impair gap insulation; repeated 'restarts' would only enhance the prospective issue.
I wonder if any comparative analysis of different methods of making gaps, or materials used in them, has been conducted (no pun intended) with this specific issue in mind. Some techniques, particularly the idea of cutting a longer gap and then backfilling it with dielectric material like epoxy, should produce much better integrity against both sparking and carbonization than, say, small polyethylene pins in the center of a one-piece slide-on joiner.
Those cycles of off and on are the circuit breaker operating. The power is periodically re-applied to see if the short still exists, then it flips off again.
If it didn;t do that, and instead kept the power on - it would be because whatever is across the rail, plus the wiring back to the base system, was not flowing enough current. Insufficient feeders and/or too small bus wire adds reistence to the circuit, Combined with a painted metal object across the track, especially a not so heavy one, there is additional (and significant) resistence in the contact points.
That the breaker was cycling means the wiring is actually good. But it if wasn't, there could be enough resistence in the circuit so that it was flowing less than 5 amps, and the breaker would never trip. A mere 3.3 ohms, at 15 volts means the current draw is 4.55 amps. For a 5 amp system, that's not a short. It can supply 4.55 amps all day long. That's over 65 watts - and 65 watts of heat can easily melt plastic.
But even when the breaker cuts power, it also periodically restores it, sees there is still a short, then cuts it. That's putting short bursts of at least 5 amps, possibly more as short term (couple of seconds at the most) the booster can likely supply more than 5 amps. Repeating that every couple of seconds puts bursts of up to 75 watts into the short - now we're talking real heat.
Power was off and when I turned it on it went through its cycles of on and off, didn't take me long to turn it off but long enough. Lots of feeders and yes it happened near a switch. The Digitrax has the power to just turn off with a direct short and has done so but I don't see why it would not see the caulk gun as a direct short.
The other thing is, all these breakers will periodically turn the power back on. I've never had a problem, because I don't leave the power on when I'm not there, and if I left something on the layout and I turn the power on, I'll be there to see/hear it (I can hear the 'singing' you get with a DCC short, despite being old and being told I never listen). However, my plan on my new, largest layout I've ever built, is to set the breakers for manual reset.
Did the breaker trip, either immediately or after some time? How much current can your system supply? Did you try the quarter test? How far is the "scene of the crime" from your base station or booster?
This is a good argument, seldom mentioned, for dividing a layout into power districts with separate breakers. You can have the individual breakers trip at perhaps 2 amps, while the base station might continue pumping out 5 amps until that higher threshold is reached.
It takes an iron man to play with a toy iron horse.
I think this is the whole gist of the "quarter test". If you can simply lay a coin across the rails, without pressing it down, it should trip the breaker.
If it doesn't, you don't have enough feeders. If your short was near an insulated joiner the current had to be fed from the other side of the joint. Maybe it was going through a turnout or maybe there weren't enough feeders close enough to where you set the caulking gun down.
The booster just thinks it is a heavy load and keeps pumping watts out (5 amps? 8 amps?) a small cross-section piece of rail will heat up like a soldering iron, or bulb filament.
You need plenty of feeders in order to pass the "quarter test". If you have a loose wire joint in your house wiring (or too small of an extension cord) and you have a short, the breaker may not trip if the loose wire joint absorbs some of the current but will generate heat in return. Enough heat to set the house on fire.
Good Luck, Ed
Earlier this year I did a stupid (all fixed same day), I left as caulk gun over the hill on my layout and created a short when I turned on the system. It did not hurt the Digitrax stuff but hurt the trackwork causing shorts by elininating gaps from plastic insulated joiners. How can this happen when the Digitrax has a breaker? I was just going over the years train wins and fails and it got me to wonder about this.