EQUIPTMENT FUSES A TIMELY REMINDER

[neil t]

Hi guys just a timely reminder on fuses, as a lot of test gear comes from overseas (The USA in particular, for me anyway) the NEED to replace a fuse for a lower rating ie 500ma 250v instead of a 1amp fuse as found on the same gear run on 115v. This NEED was highlighted this week as a shorted power rail took out  a 5v switching supply in a hp3314a, after a couple of days probing desoldering and replacing more than a handful of parts from rectifier diodes, thyristor, schottky diodes, oscillator circuit, As this machine was powered up after a simple rom battery replacement and some new caps, I stood in horror for what seemed like seconds as the displayed flickered at the speed of the oscillator (33khz) and then dead.
I stood hand to head as I watched this machine turn into a $600 door stop  OMG what the F%## have I done, it turns out that I had forgotten to trim the nice nichicon cap leads, a mishap that the under board shielding did not fail to notice, Anyway the moral to the story is I should have replaced that damn fuse before POWER UP,perhaps blowing before infecting the board with to much current ,oops my bad . patient is now doing well whirring away behind me all life signs being monitored in intensive care for the next few hours. DON'T FORGET TO CHANGE THOSE BLOODY FUSES, they may just save a little bit of heart ache.

[bugi]

If the old fuse didn't trip in those seconds of horror, would a replaced fuse with half the current rating have saved things quickly enough? Considering that fuses may need quite a bit of extra current to blow within seconds...

Also, it depends on the circuit how good an effect such mains fuse adjustment has. E.g. if there are multiple power rails, and one of the lower power rails gets shorted (in a way where its electronics is self-limiting the current drain from the common power input), the total input current might not even get high enough to trip the main fuse ever.

Also, in some corner cases one may actually need to increase the fuse rating (or at least change it to a version that can handle higher short time energy), judging from warnings with some PSU units. Specifically, if inrush current is being a problem, as that peak current will typically double from the 115V to 230V change (e.g. from 40A to 80A).

Though in general, it should be a good start to change to half current rated fuse. It should be either safer that way, or at least no worse than before, and only in some rare case it might start blowing up instantly at turn on (in which case one can ask if the design was pushing the fuse too hard with inrush to begin with).

[neil t]

Hi there bugi  putting the philosophical concepts aside the main point of my little anecdote was to point out that less damage may have been done if I had paid proper attention to manufacturer
specification fuse , and that I should have erred on the side of caution., is I stated that what seemed like seconds was most likely less than a second or so, also a quick note is that these fuses specified where as slow blo and of course grateful  that it didn't do even more damage to the board being full of hard to get chips, what I didn't describe was that the 5v rail is taken from the 15v rail which sustained no damage at all and of course totally possible that the fuse made little difference if any .



best regards Neil

[wraper]

Changing the fuse would not make much difference, if any. Fuse protects from fire, not component failure.

[Electro Detective]

I check fuses on any gear that comes a knockin before going any further,

and perform start peak and operating current tests to ensure the manufacturer did their homework correctly, be it a fast blow or T fuse

or perhaps some turkey user or bad hair day repair tech wannabee put in the wrong value because 's it's what the apathetic ass "had in the parts bin at the time"   

[neil t]

I check fuses on any gear that comes a knockin before going any further,

and perform start peak and operating current tests to ensure the manufacturer did their homework correctly, be it a fast blow or T fuse

or perhaps some turkey user or bad hair day repair tech wannabee put in the wrong value because 's it's what the apathetic ass "had in the parts bin at the time" 

  Yep and this old machine had sustained some very poor quality repairs before it came here, and yes I usually replace the fuse and check voltages and caps but let this one slide through.
Any how its running sweet now (even removed a very slight shimmy in the wave form at <100mv All good