An isolation transformer is a transformer used to transfer electrical power from a source of alternating current (AC) power to some equipment or device while isolating the powered device from the power source, usually for safety. Isolation transformers provide galvanic isolation and are used to protect against electric shock, to suppress electrical noise in sensitive devices, or to transfer power between two circuits which must not be connected together.
Suitably designed isolation transformers block interference caused by ground loops. Isolation transformers with electrostatic shields are used for power supplies for sensitive equipment such as computers or laboratory instruments.
Strictly speaking any true transformer, whether used to transfer signals or power, is isolating, as the primary and secondary are not connected by conductors but only by induction.
However, only transformers whose primary purpose is to isolate circuits (opposed to the more common transformer function of voltage conversion), are routinely described as isolation transformers.
Given this function, a transformer sold for isolation is often built with special insulation between primary and secondary, and is tested, specified, and marked to withstand a high voltage between windings, typically in the 1000 to 4000 volt range.
Sometimes the term is exceptionally used to clarify that some transformer, although not primarily intended for isolation, is a true transformer rather than an autotransformer (whose primary and secondary are not isolated from each other). Even step-down power transformers required, amongst other things, to protect low-voltage equipment from mains voltage by isolating the secondary and primary such as are used in older "wall warts", are not usually described specifically as "isolation transformers".
Hospital grade isolation transformers are also used for the power supply of devices not at ground potential. Electrical isolation is considered to be particularly important on medical equipment, and special standards apply. Often the system must additionally be designed so that fault conditions do not interrupt power, but generate a warning. These are typically found in operating surgery rooms.