Hi Duncan:
Robert Johnson has provided an excellent answer.
I'd like to amplify on his comments.
The fuse rating is selected not for maximum normal
current, but for minimum fault current.
So, you need to examine your circuit to see what
components are likely to fault, and you need to
determine the current under that fault.
There is no need to protect a 6-amp rated filter
or similar device to 6 amps. The 6-amp rating is
a normal-condition continuous rating. The filter
will withstand much higher (2x or more) currents
for short durations.
The trip currents as a function of time:
TIME (max) UL/CSA IEC 127
---------- ------ -------
2-10 sec* -- 275%
2 min 200% --
30 min -- 210%
1 hr 135% 150%
4 hr 110% --
*depends on fuse type and rating.
Best regards,
Rich
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