I don't think I have ever seen a grounded secondary on a low voltage step down transformer (120v to less than 50v for example) when a 3 conductor connection with chassis ground is used. Are these types of transformers always double insulated or is grounding the secondary just considered not necessary if the primary is below a certain voltage and a 3 wire grounded connection is used?
I have never seen such a thing before, either. The secondary winding and the circuit after that are isolated and do not require grounding/earthing to be "safe". Indeed most bench power supplies have isolated outputs, and a separate "PE" green-wire safety ground terminal so that the user can connect one side of the power supply to PE where that is indicated.
I suggest you take a ohm-meter and connect it between circuit ground and the PE prong of various appliances, e.g. oscilloscope, audio amplifier, function generator, analyzers, computer, printer, ...
Indeed most bench power supplies have isolated outputs, and a separate "PE" green-wire safety ground terminal so that the user can connect one side of the power supply to PE where that is indicated.
And if you look at them you will find the required insulation to do that. Bench supplies are effectively mixed class I/II appliances (yes, you can do that). EN/VDE says about that "Geräte, welche teilweise nach Schutzklasse II, jedoch auch teilweise nach Schutzklasse I gebaut sind, werden als Schutzklasse I eingestuft." ("Appliances, which are partly construced as appliance class II, partly as class I, are rated as class I appliances"). Which is why you'll find the class I symbol on your bench supply. Also relevant here is the EN 61010-1, which describes the safety mechanisms. As most bench supplies, as mentioned, are partly class I you can also use a shield winding between primary and floating secondary to avoid double/reinforced insulation. Most supplies will have that winding anyway to cut down on coupling capacitance.
btw I don't think OP's question was about a bench supply