Extracting Data for Data Warehouse

Hello,

We have little confusion to taking approach for Extracting data from source to target. 

Our approach is:

1) Drop the RI Constraints 

2) Truncate the Tables

3) ReCreate the  RI Constraints

4)  Extract data from source to target

Now confusion is during this process before or during recreating RI constraints if we loose DB connection or something outage happen then we loose to ability to Recreate RI Constraints.

What your thoughts and advice?