can a principal caesura occur in the first foot of a line? i'm looking particularly at I. l 52 (lines 50 and 52 included for context)
οὐρῆας μὲν πρῶτον ἐπῴχετο καὶ κύνας ἀργούς,
αὐτὰρ ἔπειτ᾽ αὐτοῖσι βέλος ἐχεπευκὲς ἐφιεὶς
βάλλ᾽: αἰεὶ δὲ πυραὶ νεκύων καίοντο θαμειαί.
i want to place it following the first syllable, since βάλλ' is enjambment and completes the previous phrase, but i've seen it scanned with a strong caesura in the fourth foot. is there a rule against locating the principal caesura earlier in the line?