I would recommend that you ground your transformer. You may also be required to by your local electrical code. Now considering the interference question, your 75 VAC is actually grounded to an extent through the circuit distributed capacitance(all AC circuits are). But grounding by itself does not necessarily reduce interference. If you are using shielding against electrostatic interference(capacitive coupling), solid grounding should help as it will makes your interference signal solidly referenced to ground as is the shield thus providing better protection. If your interference is inductive(magnetic coupling), grounding will not help. Bill Mostia =========================================== William(Bill) L. Mostia, Jr. PE Independent I &E Consultant WLM Engineering Co. P.O. Box 1129 Kemah, TX 77565 [email protected] 281-334-3169 These opinions are my own and are offered on the basis of Caveat Emptor.