It has been a long time since I have read up on how an alternator works, but IIRC, by exciting the alternator (when you put the fuse in) you are starting the charging process.
HOWEVER, the early TNs will exhibit a 'charge lamp lit' at low rpm at start up until the engine is rev'ed up enough. IIRC there is a technical bulletin to replace a resistor or diode that fixes this (presumably by exciting the alternator at a lower rpm). I have never looked this up, so give mine a little bump in rpm until the lamp goes out (I have a first year machine).