I don't know, I've always wondered, but if I had to guess, I might think that it was something to do with the 12v power supply and that the transformer is earthed. If it wasn't and a fault occurred within the transformer maybe T1 or T2 would become live, as would the thermostat connected to it. Obviously no risk if the thermostat wasn't connected to them.........Or maybe it's there to provide protection in case someone inadvertently connects 230V to T1 or T2
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