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The twin USB sockets are not working on this model  because the cable powering them is defective. I want to replace this cable.  Its part number is ONFVF.   Unfortunately Dell Parts tell me that it is unavailable. I am posting this in the hope that someone may have the part. Of course I would be delighted to remunerate anyone who has it. 
Difficulty in finding a part for Dell Inspiron 15 5551 . Part Number ONFVF. - a 
cable