IEC 60601-1 - Medical Electrical Equipment Safety 15.5: Mains supply transformers of ME equipment and transformers providing separation in accordance with 8.5

What does 15.5 require?

Transformers are protected against overheating if any output winding is overloaded or short-circuited: each winding is tested in turn at the most adverse primary voltage between 90 and 110 percent of rated, rated frequency and the other windings between no load and normal load, with the short or resistive load at the winding ends or the first point that can be shorted in single fault condition, protective components included where a short or overload they would not cover is unlikely (one means of operator protection plus high-integrity components); no winding opens, no hazardous situation occurs, winding temperatures stay within Table 31 by insulation class (150 to 210 degrees at 25 degrees ambient), and the transformer then passes the dielectric strength test between primary and secondary and each to the frame. The short-circuit test shorts the output winding until either thermal stability is reached or the protective device trips, directly across the winding where the 15.5.2 induced-voltage test is not applied; the overload test, applied after rectification where appropriate and omitted if the short-circuit test ended without a protective device operating, either steps the load up from normal use to the minimum current that operates the device and then, with an external device shunted, loads the winding for 30 minutes at the Table 32 multiple of an IEC 60127-1 fuse rating, at the fuse manufacturer's 30-minute clearing current for other fuses, or to thermal stability just below the operating current of other devices. Winding insulation between turns and layers, where transformer failure could be hazardous, withstands after humidity preconditioning five times rated voltage at five times rated frequency for windings up to 500 V or 60 Hz, or twice voltage and frequency above, capped so that the highest-voltage winding sees no more than the Table 6 one-means test voltage, with the core and screen connected as the neutral point or each primary side in turn, other windings open or earthed points connected to the core, raised from half over 10 s and held for one minute away from resonance, with no flashover, breakdown or deterioration (transformers above 1 kHz tested under 8.8.3 instead). Transformers that form a means of protection under 8.5 keep the end turns from shifting past the insulation between windings, give a single-turn earthed screen an insulated overlap of at least 3 mm and a width at least the primary's axial length, provide double sleeving of at least 0.3 mm wall extending 20 mm beyond toroidal windings, meet 8.8.2 between primary and secondary, and meet the 8.9.4 distances with enamelled wire counting 1 mm each for patient protection creepage, joints measured through unless heat-sealed or adhesive-filled, and moulded transformers deemed to have no creepage where bubble-free insulation of at least 1 mm at 250 V separates the windings.