[Boatanchors] BOATANCHOR FUSING

[Brian Clarke]

Hi John,

I didn't see any responses to your Q.

Here's my take.

If you have an inductor-input filter, then it doesn't need to have much overload built 
into the size of the fuse. And as most transformers are very high efficiency, it 
doesn't matter whether you fuse the primary or the secondary - load safety wise.
However, if we follow CISPR or NEMA rules, then we need to allow for the 
possibility of the primary shorting to the case - in which case, there should be a 
fuse in the phase leg of the primary, particularly if you put the device into a metal 
case.

If you have a capacitor-input filter, a different set of criteria occur. You now have 
a very high charging current at turn-on that is many times the running current. If 
you fuse to handle the start-up, you have no protection for normal operation. If 
it's a tiny little peripheral device [10 W or so], I'ld use say a 0.5 A fuse. But if 
you are approaching 500 W input, I'ld put in a slow-start circuit; then the fuse 
can be chosen for normal operation. 

73 de Brian, VK2GCE.
  John asked:

  Something I have been thinking about is the possibility of fusing the power 
  tranny secondary, either one leg of the secondary or the center tap.  This 
  would be in addition to any present or added fuse in the primary.  Does anyone 
  have any feelings about this pro or con?