When using a standard 120V receptacle tester on all receptacles fed from a 600V/120V transformer all three lights appear. There is no indication of what this means. When tested with a multimeter all readings are proper. Receptacle tester is working properly when pluged into plugs from other sources.


HELP

Sounds like the transformer may be a WYE or grounded Delta to create a neutral return for 120 volt, and that the neutral is not grounded to a grounding electrode, which can be building steel..

Can you provide more detail on the wiring of the transformer, make and model, and perhaps some pictures?