How to add a string to a .txt file in all rows excluding few characters using sed or awk

I have a text file named xid.txt:

xid: SC48028 id: artf398444
xid: indv1000 id: indv24519
xid: SC32173 id: artf398402
xid: SC21033 id: artf398372
xid: 1001 id: tracker4868
xid: wiki1000 id: wiki10709
xid: proj1234 id: proj12556

I need to add a string 'PT_' before 'SC48028' , 'SC32173' … so on. The string 'SC…' can start with any combination may be 'AC…' or 'DL..'

Required output:

xid: PT_SC48028 id: artf398444
xid: indv1000 id: indv24519
xid: PT_SC32173 id: artf398402
xid: PT_SC21033 id: artf398372
xid: 1001 id: tracker4868
xid: wiki1000 id: wiki10709
xid: proj1234 id: proj12556

If you see the above output, we should not insert 'PT_' before strings which start with 'i' , 'p' , 'w' & 'numerical' . I have tried a few basic commands for my requirement using insert/append in sed.