What people call an isolation transformer usually has a 1:1 ratio
(although technically ALL transformers provide isolation
).
Dr,
To some degree, all transformers provide some good isolation.
That brings back memories from 30 years ago.
In a hospital ICU, or in a research setting, things get much more specific.
There is a capacitive coupling (Electrostatic coupling) between the transformer coil windings.
High impulse signals (transients from elevator motors, lightning, etc) can couple from primary into secondary circuits, and eventually end up coupling through power supplies for devices connected intravenously into a patient. These signals can have a high frequency characteristic (1 Mega Hz for example) which acts differently than what we experience at 60 Hz.
In a patient having 1 K Ohm blood path resistance, a 10 micro Amp signal would be pushed by 10 milli Volts. Some patients (such as those in ICU) cannot tolerate even a few milli Volts extra signal directed into their hearts.
This is where the real "Isolation Transformer" comes into the system, as the first line of defense against transient signals.
To reduce the electrostatic coupling between the primary and secondary windings, there is a designed-in capacitive shield which was grounded to the primary system. The secondary system carried its own ground as a perfectly 'derived' power system.
Transient signals are generated by high frequency radio, elevator motors shutting down, lightning hitting the building, etc. These transient signals can couple through an electronic device power supply and directly onto the devices circuit board, and possibly end up inside the patient. I have seen and resolved examples of transients of all these types. We had radio signals passing through our research instruments. We had elevator motors shutdown (generating an inductive pulse in the power system for the building) and shutting down a research lab. We had lightning entering a printer signal cable, and finding its way into everything in a lab (back door effect).
In designing electronic instruments for research on human patients, I had to incorporate ferrite beads on ALL wires/conductors from the outside of the device cabinet. I had to design-in instrumentation amp circuits that had common-mode rejection ratio into the 100 dB range. We put the instruments into a Faraday shielded room, and then everything was quiet enough to gather data. We gathered data via skin conduction on patients. We had three AM radio stations that would force their signals into out instrumentation, unless we went to extremes to keep out the transient signals.
The point is,
that a patient connected intra-venously to equipment can be affected by transient signals.
That is where the true "isolation transformer" comes into play.
The NEC specs 5 milli Amps as the trigger level for GFCI devices.
In my duties, I measured at 10 micro Amps level for hot-ground leakage.
These standards were set by the JACH for hospitals,
and equipment was specifically designed to measure at these levels.
The real "Isolation Transformer" fits into these scheme of preventing transients
from entering the patient device electrical system.
That brings back memories from 30 years ago.
That is getting OFF the Subject, however.
So, back to 208 / 380 Volts.
