One of my kw transformers needed a new breaker as the original one was engaging at 6 amps. I replaced it with the often recommended 10 amp 12 volt automotive auto reset breaker. To test I have a Ohmite 1 ohm 300 watt resister that hooked to the track power circuit on transformer post a and u on the kw transformer. With a direct short (a hooked to u), it engaged almost immediately . With the resistor across the post a and u producing about 14 amps (at 14 volts with 1 ohm resister) the new breaker would not engage even after a minute or so. Defective new breaker? Or normal at 40 percent overload?.i know Lionel allowed up to 45 seconds at 20 amps with the original breaker to open. I would think a modern breaker would pop at 11 or 12 amps quickly but I am not an electrical engineer. Any comments appreciated.




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In my opinion, 10 amps is too high for a KW.  10 amps is right for a 275-watt ZW.  I would go to a 7-amp.  FYI, LIonel literature from that era said to apply a 75% factor in determining maximum continuous power demand.

Breakers, like fuses, do not pop immediately upon a slight overamperage. If you muck around on the internet, you'll find charts by fuse mfrs showing how soon they open

An alternative is an electronic circuit breaker as used in the Lionel PH-180, schematics for which are available on the forum, but as of now, finished units are not available.

Those circuit breakers are thermally operated, and they will take a significant overload for quite a while before tripping.  I use a 2A thermal breaker on my test bench, and it regularly sees three-four amps for seconds without tripping.

So guys is 14 amps on a 10 amp breaker a low  over current? Would you not expect it to trip in a minute or so? It does trip immediately with a direct short. They are inexpensive so I may buy another and try it maybe lower amperage as you suggest but most use 10 for kw and 15 for zw.

thanks for the fast replies! I am deficient in electrical knowledge!

Here's a typical thermal breaker trip curve.

Note that at 135% of it's rating, it will take an hour to trip!  A minute or two at 140% seems pretty plausible, it can take up to 40 seconds with a 200% overload!

Potter & Blumfield W57-XB7A4A10-10 Thermal Circuit Breaker


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Based on that graph ,I would say my 10 amp breaker is fine and all those breakers really protect against is a dead direct short. I could never get it trip at 14 amps but I only did it for a couple of minutes as I did not want to damage my transformer. Nothing got real hot on the transformer at that amperage. In realty a 7 or 7.5 amp would likely run indefinitely at 10 amps or one **** of a long time thus really being closer to a 10 amp rating( if you want something to trip a little above 10 amps as a rated10 amp breaker will not.)

Based on that graph, I'd seriously recommend no more than a 7A breaker for a KW.  You're realistically are only going to get around 140 watts output from the KW.  Remember, PW transformers are rated on input power, don't expect more than around 70-75% of that on the outputs.

gunrunnerjohn posted:

Based on that graph, I'd seriously recommend no more than a 7A breaker for a KW.  You're realistically are only going to get around 140 watts output from the KW.  Remember, PW transformers are rated on input power, don't expect more than around 70-75% of that on the outputs.

Can you explain what is meant by input power? 


That's the max power that's drawn from the 110V line.  However, based on transformer efficiency, you can't expect to get that much coming out.  Lionel's guidelines for PW were 75%, Personally I consider that's a bit optimistic, but let's use that figure.  That translates to 140W output, around 7 amps at full throttle.    I can tell you that running a KW with a continuous 7 amps out for a period of time will result in it getting pretty warm.  Not dangerously warm, but you'll certainly know it's pushing some current.

Remember, that breaker is only there to protect the transformer, if you want to really protect the load, you need to do more than just a thermal breaker on the transformer.

PSX-AC may be a bit pricey, but if your running command locomotives, it's cheaper than a repair. 

I just installed one on my track power channel and it detected a short where my ZW-L alone didn't.  The short was due to a price sticker I missed on the bottom of the piece of Fastrack that was contacting center and outer rail tabs. The PSX tripped instantly.  I'm not installing them on my accessory power channels.


PSX-AC...u only have to buy them once. Around 200$ for the four outputs of a ZW-anything. However if you’re only running command control, Powerhouse 180s are good with their electronic breakers.


Well, I absolutely agree with the recommendations to ideally use a 7 or so amp breaker in the kw. However , the 10 amp automotive type is what is typically been sold for the kw over the years. The one I have is one I acquired 10 or 15 years ago from a parts vender and is still sold by venders for the kw(15 for the zw). I installed the 10 amp yesterday before this thread. This evening after work I hooked on the layout and shorted the track with a car(no delicate electronics to fry). I measured a 20 amp fault current with ammeter (adjusted with voltage)and the breaker opened in a very short period less than 15 seconds. This is well within the original specification of 15 to 45 seconds noted on the chart for the Lionel 5 f test set for the kw with 20 amp current flow. So I probably will not burn up my transformer with a short but I think I will use resettable magnetic circuit breakers of 7 or so amp between transformer and track Along with TVs. Also some of the less than 10 amp auto reset breakers are kinda hard to find.

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