I had a vertically mounted delta loop on 80 meters many years ago. The base of the triangle was about 8 feet above ground. I installed forty 60-foot radials centered under the horizontal section. I would short out the loop and feed it on 160 meters with an L network at ground level where the coax came in...worked very well. I don't remember having any connection between the radial center and the electric mains entrance ground and never had any problems.
That sounds right. Since mine is horizontal I believe I will center my radials below the feedpoint, as suggested by WB6BYU, and lay them out on the ground underneath the entire plane of the loop. Will take data on it before and after and let you know how it does. Tnx for the suggestions.