I have a building 100 feet from my house. The electrician is burying a cable to install a sub panel in the building. He is feeding it from the main panel in my house. He will be running 3 conductors and a ground from the main panel. The sub panel will have a ground wire leading back to the main panel ground. He wants to install an earth ground rod at the sub panel just outside the building. Is it right to install this extra earth ground or should the main box have the only earth ground?