Does the NEC state that a logbook must be kept at all electrical subtations and a record of any work done inside the substation must be written in such a logbook?

N0. The NEC does not have a say in the operation of Utility Power Stations. Those type of operations are governed by different rules.

However, utility owned business offices or office buildings on utility company property are regulated by the NEC. Not the power generation sites.

The code that is used to cover utility installations is the National Electrical Safety Code.


National Electrical Safety Code Committee (http://grouper.ieee.org/groups/nesc/)

And yes, everything that occurs in a generation and substation needs to be logged:

National electrical safety code - United States. National Bureau of Standards - Google Books (http://books.google.com/books?id=w0sKAAAAIAAJ&pg=PA294&lpg=PA294&dq=national+electrical+safety+code+logbook&source=bl&ots=xF-7WQlvup&sig=XCEC5B77sLUHuvGyOXc5Or-syYo&hl=en&ei=cVm6TrqXEKfA2gWi2rHLBw&sa=X&oi=book_result&ct=result&resnum=3&ved=0CF0Q6AEwAg#v=onepage&q&f=false)

Does the NEC state that a logbook must be kept at all electrical subtations and a record of any work done inside the substation must be written in such a logbook?This is not exactly an every day question. What is your reasoning behind asking this?