Hi @chenpan 

Thanks for reaching out to us. Sorry to reply you late. Forum has been busy recently. 

Ground line is not necessary, but lacking ground line may cause possible safety issue. The purpose of a ground wire is to give excess electrical charges a safe place to go. The solid mass of earth below our feet has a negative electrical charge, which means positive electrical charges are naturally attracted to it. A ground wire helps those positive charges get to the ground in a safe, direct and controlled way, where they can be discharged without the risk of electrical shock or fire.