Grounding Transformers in Control Circuits

this used for AC only. There is another board that has a transformer on it that take the 120v AC to 12v DC. we have isolation transformer on other equipment and they are grounded. our electrical engineer can not explain why they do and why they don't. the transformer is use to step down the high voltage (208/240/480 3 phase coming into the machine) into a control voltage.

If that other board is in anyways tied to the problem board, it could cause problems. The problem board has no ground so DC voltages will build up on the secondary side. This is not a problem if it is all AC, however it will cause problems if tied to a DC circuit. Either a ground, or a capacitor isolation link to the DC circuit should fix the problem.

do you use an EMC filter after the transformer? that could help

no, i don't. it is a standard step down transformer(208/240/480) down to 120 for controls in 3 phase machine.

Then you should check the noise on the voltages during operation

Sounds like a classic ground loop problem. By changing the leads, or adding a transformer you are altering (or more importantly) breaking the ground loop path. You now have a magnetically coupled circuit (no galvanic connection) from the power source to the load. Ground currents cannot flow in this case and the problem goes away. You can search the internet for ground loops (and find much more information than I could put here). I personally fix this type of problem by using transformers all the time. As a matter of fact, I had to add one in my entertainment set on the S-video line to eliminate the 60 Hz hum from the amplifier, due to ground loops.