120v transformer 24v bond question

I was troubleshooting a ac unit and noticed a transformer was fed 120 on the primary(black white) and was bonded to the frame by the green wire. On the secondary side 24v I noticed the black was bonded to the frame and white went back to the transformer. All ac power, typically yes dc you can use the frame for the common but for ac I've never seen this. Is this a common practice?