What is an isolation transformer?

Answer

An isolation transformer does not have a direct electrical path from the power input side to the power output side. The term is also used to define how much electrical isolation exists between the input and output windings. For example when using line-voltage input transformers to power low voltage device handled by humans, a high degree of isolation is required for safety.

Isolated transformers often use separate bobbins for the primary and secondary coil windings, but usually the windings are just wound on top of each other with insulation in between.There is usually an electrostatic shield between windings which is tied to transformer case.

Non-Isolated transformers are becoming rare. A common example is the "Variac" which is a non-isolated variable transformer. Usually the term "auto-transformer" is used to describe non-isolated transformers. They are rarely found in consumer products.

Although any transformer with a separate primary and secondary winding is an isolation transformer to some extent, The term is usually used to denote a special-purpose transformer built just for that use. It is tested and rated to withstand a very high voltage difference, called the withstand voltage, so that even if thousands of volts are applied to the primary, it will not leak through to the protected side. These transformers are used in the medical industry, to protect patients hooked up to monitoring instruments that are powered by utility mains, as well as other uses.
An 'isolation transformer' is a transformer specifically used to electrically-isolate the secondary circuit from the primary circuit. Of course, ALL mutual transformers do this, but an isolation transformer normally has a 1:1 ratio, which means its secondary voltage is the same as its primary voltage. In other words, its purpose is NOT to change the supply voltage, but to isolate the user from the supply circuit.

Isolation transformers isolate the secondary circuit from the earthed (grounded) primary circuit. As there is no earth return from the secondary circuit back to the primary supply, isolation transformer eliminate the risk of shock between the secondary circuit and earth.

An example of an isolation transformer is a bathroom shaver socket. The output is completely isolated from the earthed supply, so users cannot receive a shock between the outlet and earth.