I am redesigning a system that currently uses two tables to record work done by a person. One table called "service" records work that is not related to a piece of equip. The other table is called "summary" for work done on equip. Tables are almost identical. I was thinking of combining these and setting the foreign key to the equip table to be null if the work does not relate to a piece of equip.

Does this violate good relational design? I have heard the term "overloading" but can't find any references.