Isolation transformer - does it offer protection

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.

BTW, the shield shunting to ground obviously requires that an outlet, it's corresponding fused circuit/panel, and the entire house are properly grounded in the first place - which a frightening number are not.

07-12-11: Jedinite24
Nope it doesn't...I had an amp plugged into an industrial 1.8 KVA Topaz Isolation transformer

While that might seem like a large isolation transformer by weight, it really isn't that large when it comes to electrical spikes. 1.8KVA=120V x 15A.

I bought six Topaz 2.4KVA Ultra-Isolators from a guy who was using three in series per channel, which is called "cascading"(?). He was doing so for common noise reduction, but the effect also works for filtering of spikes(I think). Each succeeding transformer knocks down the remaining spike until none, or very little, is passed.

Or, you can just do it right from square one and get a big daddy at your panel like Mike.

07-20-11: Dhl93449
...But some solation transformers capacitively couple very high frequency spikes from input to output without significant attenuation. So you cannot use them by themselves without additional surge protection.

I was not aware of this, thanks for the tip.