Isolating transformers for medical location

Medical isolating transformers are the main element of sets for supplying IT insulated electric network in medical rooms. The technical requirements for these transformers specified in EN 61558-2-15 standard are very strict, in order to ensure a reliable and safe operation. 

These transformers can be made in 1-phase (ET1MED type) and 3-phase (ET3MED type) versions. In accordance with above-mentioned standard the range of allowed power is from from 0.5 to 10kVA. Moreover the primary (supply) voltage is limited to 1000V, and the maximal secondary voltage is limited to 250V. Rated frequency of medical separation transformers should not exceed 500Hz.

In order to meet earth leakage current requirements, reinforced insulation is applied in the construction of these transformers. In accordance with EN 61558-2-15, the leakage current between the transformer secondary winding and earth, measured in no-load state, should not exceed 0.5mA. The leakage current between the screen and earth, measured in no-load state, should not exceed 3.5mA.

Other requirements relate to no-load secondary voltage, no-load current and short-circuit voltage. ΔW difference between UWJ no-load secondary voltage and UWOB secondary voltage under load, expressed as the percentage of secondary voltage under load, should not exceed 5%:

No-load current of medical separation transformers should be limited to maximum 3% of rated primary current when suppled with rated voltage, and the short-circuit voltage should not exceed 3% of rated supply voltage.

Connection diagrams of medical separation transformers of ET3MED and ET1MED types.

In emergency states the medical transformers must operate with overload. An overload level of 1.6xIn for 1 hour or 2xIn for 30minutes is allowed. The transformer operating temperature is supervised by a relay cooperating with temperature sensors. Medical transformers of ET1MED and ET3MED types are equipped with PTC and PT1000 temperature sensors, which are installed in windings and allow for efficient control of transformer operation.
An important parameter specified in EN 61558-2-15 standard is a inrush current. The referred standard requires that transformer construction itself (without application of additional elements in installation) should limit the value of inrush current to 12-times peak value of the rated primary current.

Example of inrush current for ELHAND transformer of type ET1MED-6,3kVA, under load .