In a message dated 8/30/2002, Don Borowski writes:
> But there is common mode supression (hot and neutral against safety ground) > offered by the transformer, assuming that the capacitive coupling between > the > primary and secondary is minimal. > > ...There is one effect of a long run of bundled conductors - the > disturbance (at > least at frequencies above several hundred kHz) becomes a floating source > at the > power outlet end. The transformer will break the common mode current loop. Hi Don: Your explanation makes sense if we focus on high frequency common mode disturbances where the inductance of the ground wire is significant enough to make a difference. In this case, having the ground wire tied to one side of the AC at the service entrance does not ensure that these two wires are at the same instantaneous potential at the AC outlet on the other end of a long wire. At low frequencies, it does not appear that the isolation transformer provides any benefit for common mode noise. Regarding my surge scenario that Bob Johnson commented on, it seems that this type of isolation transformer provides only limited protection from lightning surges. Common mode surges coming in to the service entrance will be converted to differential surges when one side is tied to local earth, and these differential surges will sail right through the transformer (subject to its frequency response transfer function). On the other hand, ground strikes that raise the local earth potential at the service entrance will not be passed through on the AC pair, but they will come right through on the ground wire. While this is different from what would happen without the isolation transformer, I'm not sure it provides any added protection. In summary, it sounds as though this type of isolation transformer is primarily intended to attenuate high frequency common mode noise. Is this correct, or have I missed something? Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. 781-721-2848 http://www.randolph-telecom.com
