As you mention, there can be common mode to differential mode conversion at the service entrance since safety ground and neutral are tied together at that point. This will come right through the transformer to some extent. But you did mention that the transformer has a capacitor and MOV from line to neutral. These along with the leakage inductance of the transformer will provide filtering and limiting, thus transient reduction, but really no more than same capacitor and MOV and a series inductor would provide.
The transformer does not address ground bounce, but that is a whole subject in inself. If everything around the device plugged into the transformer is also bouncing the same amount, then you have a system that looks like a bunch of devices in a Faraday cage, and there is little problem, except for the currents to ground via the distributed capacitance to earth ground of the various devices. Problems usually occur when you have a number of devices referenced to different grounds at different potentials during a disturbance. Note that if you have a ground strike that raises the local earth potential at the service entrance, it will raise the potential of the AC pair as well because neutral and safety ground are tied together at the service entrance. The long and the short is that the transformer can help for only a limited problem (common mode noise). Don Borowski Schweitzer Engineering Labs Pullman, WA j...@aol.com on 08/30/2002 11:06:47 AM Please respond to j...@aol.com To: Don Borowski/SEL@SEL, john...@itesafety.com cc: emc-p...@majordomo.ieee.org Subject: Re: Curious about Tripp-Lite "isolation transformer" 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.comIn 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
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