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.
Dinky 24VAC 1-phase control transformers are typically fed 120VAC -- regular way.
On their (typically unfused) secondary side one wire is normally 'anchored' to ground whenever a floating neutral must be shunned. (It's not at all unusual for these circuits to 'float' -- not being grounded at all. Think sprinkler controls, door bells, etc.)
The other lead will then become the 24VAC 'hot.'
The pigtail implied in Section B can be landed upon any chassis that is also bonded back to the panel. The amount of current is tiny -- and at a very low voltage, too.
The gadget (Nordic Electronics Fast Stat) is plainly designed to get around old work limitations -- particularly those homes that started life with mere two wire control connections from the thermostat to the furnace.
So one drops in what is best described as a digital multi-plexer and de-multi-plexer pairing. As is typical in digital systems, a floating neutral/ground basis is undesirable, hence the grounding pigtail.