fuses question

My understanding has always been that if the fuse is subject to episodes of overloading that cause the metal to heat up without blowing and over a long period of time, then it will not last as long.

The reason is that heat causes small expansion and contraction of the metal wire which, in turn, can cause minute cracks in the metal wire and make the wire more susceptible to blowing at lesser and lesser loads.

Blowing the 20A fuse sounds to me like they saw a substantial load.  I suspect that three of them going all in a short timeframe sounds like something happening on the layout.  I'd be looking for the offending locomotive or rolling stock first.

most common I would think is your having a lot of derailments or one of your engines have developed a short. I know  it can be easy to think your locomotive is on the track but only has to be off by a little bit and you have a short. like John said that quite a high current fuse t blow for no reason! 

Walt, do you have a stray wire or piece of track that might have made a short during construction? Are these separate loops? Has the same train been on all 3?

I'll apologize up front for the lengthy reply but feel like comments from me are appropriate at this point.

thank you all for offering your thoughts.  for 2 of the fuses I know when they blew but don't know why.  I am having an issue with the 2nd AIU in line (posted about this on the DCS forum back then) - it was acting erratically and for the most part would not activate an accessory nor fire a switch.  I blew the 1st fuse somehow when I tried to activate a switch, which at the time was NOT hooked up to anything except power feeds and the AIU.  I blew the 2nd when I changed the TIU port from the bad one to a known good one.  Tried the switch command again and that one blew.  Now I was down 2 of the 4 ports.  I decided to just give up on the 2nd AIU, change the layout design, and wait until I tear down the layout to take things to an authorized repair person.

The 3rd fuse is a real mystery.  I only right now have one PS3 engine out for testing.  It's been running fine for 2 weeks while I continue to build.  I ran it one evening to check clearances and it ran fine.  Did a normal DCS shutdown and went to bed.  Next day I tried to apply power to the trackage that it was on but there was no output from the TIU.  I checked the input port and it was good but nothing coming out.  THis was on a variable port so I thought maybe that somehow it lost the 'FIXED' setting, so I reset it and tried again. Again nothing.  So unless a mysterious person broke into my house overnight and shorted it out, this one makes zero sense.

I had no choice at this point but to tear out the TIU and check fuses.  That's when I found the 3 dead ones.  The 4th one, which I was also using that port to test with that same engine, was OK.

I really can't explain how a switch not connected to any trackage other that a power feed from the TIU, which was being activated thru the AIU, would cause the fuses to blow.  I had the switch using track power at the time not an external source.  BTW: I tried 2 different switches just to make sure it wasn't something with the switch.

I front end my TIU with fast blow fuses.  10A as I recall.  The trannie that I use for track power has extremely fast breakers to boot.  So fast that they will pop before the 10A fuse blows.

so my only thought was age.

I hate electronics!

As always, thanks - walt