“Service yard” means any yard area utilized for storage of materials or vehicles accessory to, or used in conjunction with, the principal use of the lot or building, or used for garbage or trash containers, or for the location of mechanical equipment accessory to the principal building or use. All commercial and public service yards shall be fenced so as not to be visible from the street, and such fences shall be a minimum of six (6) feet above grade, or as otherwise approved by H.A.R.C.