Fault currents and circuit breakers? Lionelsoni?

[Dogdoc1]

I have read several of the post by lionelsoni and understand the danger of a fault current between 2 outputs on the SAME transformer such as a KW if a roller pickup stalls between two power blocks.  My question is this: If I place a circuit breaker (say a 6 amp) between the output and the track for both a and b outputs on the same transformer should this make it reasonably safe( as long as wire size is appropriate for 6 amps or whatever circuit breaker is chosen? I think this would protect the transformer from the potential high fault currents since there is no internal breaker between those outputs? Also, do I under correctly that between two separate transformers, the fault current would hit the internal breakers of the the two transformers and be safe without additional inline breakers? Lionelsoni?

thanks very much . Great info on this forum!

dogdoc

 

[lionelsoni]

Dogdoc, your reasoning is correct.  But here is another consideration:

Deliberately running between differently powered blocks protected by individual circuit breakers may be safe in the sense that an overcurrent will be stopped before any fire danger can arise.  But, when the circuit breaker interrupts the fault current, the very abrupt current change may result in an overvoltage spike from the stray inductance of the transformers.  Traditional toy-train electronics may tolerate this, but modern semiconductor electronics are often less forgiving.

My advice is to protect all circuits against overcurrent (using circuit breakers) and overvoltage (using transient-voltage suppressors), while also assigning blocks to transformer circuits in a way that never requires running between differently-powered blocks.  For example, power the center rail of each block through an SPDT-CO switch that selects which of two transformer outputs (or neither) is to power the block.

[Dogdoc1]

That's what I will do but prior to reading your old post I never worried about it. Glad to learn something so thank you very much. Also, I use an ammeter to evaluate current draw and compare that to the transformers circuit breaker rating to determine how much load I can put on the transformer. I use a clamp type on the common wire( but can us output wire). So with switch lights and old steamer running with cars I pull a little over 5 amps at 16 or so volts. Well within 10 amp breaker on kw. What size inline  output breakers would you choose. 10 amp?

[Dogdoc1]

Thank you for your reply. Would you use in line circuit breakers equal to the transformers amperage? I am using kw which I think is 10 amp. Again thank you Dogdoc

[lionelsoni]

The circuit-breaker's rating should be no more than the wire can safely carry.  For 16 AWG, that's 10 amperes; for 18 AWG, 8 amperes; for 20 AWG, 5 amperes.  If you want to use 10-ampere breakers (and 16 AWG or heavier wire), that's fine.