how to fix a ground fault created when AC kicks on

how to fix a ground fault created when AC kicks on We have 2 AC units when either one kicks on we have a spike/ground fault outside the house where the units sit. Everything has been turned off and each circuit tested to determine what is causing the ground fault outside of the house in the area of the units. It has been determined by electricians that this is the problem. What could be happening with the units that is causing this voltage surge. The units were installed by different companies, at different times and have different manufacturers. If only 1 unit comes on there is still a voltage surge but it is about half of when both units are on. So that seems as if the problem exists in both units and is magnified when both units come on The problem is there is a charge in the ground outside the house where the AC units are located. When either unit kicks on we get higher voltage right outside the house. Electricians have narrowed the problem to the outside units but the HVAC person has not been able to find what in the unit or the connection that is causing this voltage. It is enough voltage that our gas line cannot be connected to the furnace. We don't know what to look for in the units to eliminate this problem. If some neutral line is touching something it shouldn't or if something might be cracked allowing voltage to leach out into the ground. I need some direction to help the HVAC team to research and look at to correct the voltage into the ground outside the units.