@Trestleman1 posted:Mark, I'd still like someone to explain exactly what happens electrically during a derailment/short circuit. Why do you get high voltage knocking out a TVS sometimes and high amperage other times or do you get both problems at once. Any explanation would be helpful. Don
Hi Don:
(Disclaimer -- I respond at risk of sounding rather condescending, which is absolutely not my intent. You can also do a great deal of reading on the forum and elsewhere more information on these electrical subjects). To 'generally' answer your question:
Shorts are an issue of current flow. When a derailment occurs, you are directly connecting the 'hot' and 'return' of your AC power supply. That in turn creates a condition where there is no load / resistance to impede or regulate current flow (amps) in the system. This condition 'theoretically' demands infinite current at the ~18VAC from your power supply, which it of course cannot deliver. At the moment of shorting, your PSX current breaker, wired in-line with your AC power supply, trips near-instantly, disconnecting the circuit and preventing your supply from melting down. This entire sequence happens in less than a blink of an eye after the short condition occurs.
Voltage spikes (or 'voltage transients') are abnormal 'bursts' of volts beyond the designed supply voltage of a circuit. They do not require a short condition to occur, but will happen often (if not always) when a short condition occurs. Spikes obviously result from lightning strikes, but can occur whenever there is a electrical disturbance, (failed devices, components, shorts elsewhere, etc). Voltage spikes are not (strictly-speaking) high-current events, and thus will not blow fuses or circuit breakers unless there is a coinciding high current condition, (which is often the case). The TVS (transient-voltage-suppression) diode is connectedacross the 'hot' and 'return' of the AC power into the protected load, (as opposed to in-line, as with fuses and circuit-breakers). When a voltage spike occurs, the TVS absorbs it, preventing the load from seeing that transient voltage and blowing it up.
Now is where GRJ and Mark's responses fit in perfectly. A derailment is first a short condition, but it also generates voltage transients. The mix and magnitude of each during a derailment is as diverse as the stars in the heavens; you can never really lock down the exact details of each event. That is why, ideally, we use circuit-breakers in-line right after the AC power supply to protect against shorts and high current conditions, and TVS diodes across the loads (track and / or locomotives) to protect them against voltage spikes. GRJ (and others) have pushed this ad nauseam on the forum.