Transformer provides no attenuation for differential transients
Consider an ideal transformer with two windings, a primary and a secondary. If the number of turns in the primary is the same as the number of turns in the secondary, then the primary voltage is always equal to the secondary voltage.

Why? Consider Faraday's law of induction. It says, among other things, that the voltage \$v\$ across an inductor is equal to the rate of change of magnetic flux \$\phi\$ through it:

$$ v = {d\phi \over dt} $$

Now the two windings of the transformer are each around a common core, and that core is designed to contain most (ideally all) of the magnetic flux from the windings. Thus, the flux through either winding must be the same, thus the rate of change of flux is the same, thus the voltage is the same.

For transformers with differing number of windings the situation isn't really any different, except that the ratio of the voltages will be related by the ratio of the number of windings. This is how transformer gets its name.

When one side (let's call it the primary, though it works in either direction) is presented with a common-mode transient, the voltage across the primary doesn't change. It's as if you tried to move current through a wire by attaching both ends of it to the (+) side of a battery. You may have changed the voltage of the wire relative to some other (unspecified) reference, but since the wire is isolated from whatever that other thing is, no current flows. No current, no flux, no change in flux, no voltage, no coupling to the other side.







simulate this circuit &ndash; Schematic created using CircuitLab

As long as the transient voltage wasn't enough to blow through the insulation, and you don't have any significant capacitive coupling, or other non-ideal effects, the common-mode transient essentially never happened, as far as the secondary can tell.

On the other hand, differential mode voltages do drive a current in the primary, thus as the current increases, flux increases, and this is coupled to the other side, resulting in the same voltage appearing there.