q9 said:
Just as an excercise, I'd like to know how to do what I asked to do...I doubt very much that I will in fact implement it on the bike, it was just a simple thought for an excercise...something to keep me amused on a rainy weekend...
If you insists.
The analysis is very easy and straight forward. You consider a blackbox with +12V as your input and "light" as your output. Then you examine every element inside the blackbox to see what effect it has on the output if it fails. After that, you either do a design to make sure that failure mode cannot happen, or reduces its effect to acceptable consequence or put in backup/duplicate circuit or whatever.
As an example in your case, the elements are:
a. the cable from +12V to fuse holder
-open circuited
-shorted to other cable
-shorted to bodywork
b. fuse holder itself
-open circuited
-bad contact
-short circuited
c. fuse element itself
-open
-bad contact inside element
d. the cable from fuse holder to lamp switch
-open circuited
-shorted to other cable
-shorted to bodywork
e. the lamp switch itself
-open circuited
-intermittent contacts
-contacts stuck together
f. the cable from switch to lamp
-open circuited
-shorted to other cable
-shorted to bodywork
g. the lamp bulb holder
-open circuited
-shorted
-intermittent, bad contact
h. the lamp bulb itself
-filament open
-filament open and short across internal connection, high current
I'll let you work out the consequence and the possible remedy.
In fact, it is not difficult to see that the only solution to all the above is a completely duplicated setup(including the lamp) with separate cable run.