I am looking at a wiring schematic for a control system for a residential HVAC system. There are two different designs, the second stating it is for upgrading from single to two stage air conditioning. This second design shows what I'll presume to be a Class 2 transformer (among other devices), with one line of the load side noted to 'Connect Common to electrical ground', and a standard line drawn with ground symbol at end. Where can this electrical ground occur? Thanks.