Isolation transformer - does it offer protection

what good does an iso do, then?

.

Most of what is sold as an isolation transformer like these come in a box that contains more than just the transformer so you get RF filtering along with surge and spike protection. The transformer itself does some things that might benefit your system including the ability to break a ground loop and offer balanced power. It will also filter out some noise.

.

Once found some retail site that claimed 8:1 spike reduction. Even had a picture of an oscilloscope for evidence. Never seen that confirmed by a manufacturer and doubt they would even test for something that was not it's intent. Seems logical that they just aren't fast enough.

FYI, the discontinued PS audio ultimate outlet used a balanced (balun) isolation transformer plus filtering that works well to reduce noise-- it also claims to stop spikes, but I have no way to test. Keces also makes a high end iso sold through ebay-- looks well made.

07-12-11: Jedinite24
Nope it doesn't and I can't I can't tell you why

07-12-11: Herman
It may attenuate a spike but won't eliminate it.

An ideal transformer will pass any changes in the primary to the secondary so any spikes, surges, sags, or distortions in the incoming signal theoretically get passed through.

I think shielded isolation transformers can effectively attenuate (filter) voltage transients (voltage spikes) through two mechanisms, if their rating is sufficient to handle the level of spike;
1) The Faraday Shield shunts energy to ground(& common mode noise)
2) Lenz's Law (see animation at link) which loosely translates to "The Induced current is such as to OPPOSE the CHANGE in applied field."

Electricity and controls for HVAC/R By Stephen L. Herman, Bennie L. Sparkman

The reason a transformer can greatly reduce any voltage spikes before they reach the secondary is because of rise time of current through an inductor. The current in an inductor rises at an exponential rate, Figure 18-3. As the current rises in value, the expanding magnetic field cuts through the conductors of the coil and induces a voltage that is opposed to the applied voltage. The amount of induced voltage is proportional to the rate of change of current. This simply means that the faster the current attempts to increase, the greater the opposition to that increase will be. Spike voltages and currents are generally very short in duration, which means they increase in value very rapidly, Figure 18-4. This rapid change of value causes the opposition to the change to increase just as rapidly. By the time the spike has been transferred to the secondary winding of the transformer, it has been eliminated or greatly reduced, Figure 18-5.