Types of Medical Isolation Transformer (MIT)


Although the BS EN 60601 standard specifies what a Medical Isolation Transformer is and what tests and criteria it must meet, there are several different medical applications that require Isolation and each requires a different design of Transformer.


Isolation Transformers provide Galvanic Isolation between two (or more) windings called the Primary and Secondary. Galvanic Isolation provides Secondary protection against problems like spikes and noise on the primary. It also prevents certain problems like Earth Leakage on the secondary from propagating to the Primary.

Advance ultra-Isolation Transformer (AIT) and Medical Isolation Transformer (MIT) series type test results

Earth Isolation is NOT implied by the title, In the UK and certain other countries the mains supply is earth referenced and it is possible to remove this reference by using an isolation transformer without an earth reference on the secondary. Also possible is to connect a 'clean earth' to the secondary to allow correct operation of noise filters on the load.

More Advice on Earthing Power Conditioners

The BS EN 60601 standard acknowledges these different approaches to a Medical Isolation Transformer and defines them as two 'TYPES'

Type 'F' transformers are earth free on the secondary, and when used in Isolated Power Systems (IPS) must be sufficiently physically insulated from earth to have a secondary leakage of less than 5mA. This effectively means a transformer much larger than conventional designs to prevent any stray capacitance to earth metalwork on the secondary. Isolated Power Systems are typically used in Patient Care areas where applied parts are commonly used. To create a true IPS system the earth free transformer must be permanently fault tested using an insulation monitor; any failure to earth must be instantly alarmed both audibly and visually in the Patient Care Area. In this way additional levels of protection can be afforded to vulnerable patients to whom even small amounts of Earth Leakage could be fatal.

Type 'B' Medical Isolation Transformers are used to provide a 'Mains' like supply to medical equipment. In the UK this means that the Secondary is EARTHED to form a quasi-neutral. In this way devices which carry a CE mark which predates BS EN 60601 can be helped to pass the new standard for things like earth leakage and conducted noise emissions and immunity. These transformers are typically used on PCs and other devices used in Patient Care areas where their natural Earth Leakage might possibly cause nuisance tripping of the room supply and reduce the burden of new wiring to meet the new standard. Additionally isolation of devices with high leakage prevents other medical systems on the same circuit which may have applied parts from becoming the preferred route to ground.

While Advance does not claim that fitting an MIT unit will make any older equipment meet the new standard it will help with lessening of earth leakage and reduction of nuisance trips.

The different CLASS of transformers recognised by the standard further complicates the situation. In simple terms CLASS I transformers are conventional units in a metal box which is safety earthed. Class II transformers are fitted with a fully enclosed non-conductive case (normally plastic). Because of the nature of Class II transformers the Secondary is ALWAYS earth free or 'floating'. While this doesn't preclude the Class II transformer for applications where reduction of earth leakage is a priority its lack of a Secondary Earth reference may cause certain groups of devices to interact in ways they don't when a mains connection allows them to filter noise using the mains neutral. For this reason Advance recommends that Class II transformers are fitted to only ONE load.

Advance Class II transformers are made in a resin sealed plastic enclosure, which makes them suitable to be used even in Wet Rooms like operating theatres and lavage areas. While clearly the plug and socket cannot be made waterproof it is even possible to connect this type of transformer with proper glanding and terminal protection so that it could be periodically immersed with no fault.

In summary whatever your Medical Isolation Transformer problem Advance has the solution but don't be surprised if you are asked about your application so that we can ensure You get the right solution.

Medical Isolation Transformer (class II 500VA)

Medical Isolation Transformer class II