•Diode arc-back fault occurs whenever a diode is short-circuited inside one rectifier bridge.•Diode arc-back fault generally gives a short circuit curent dynamic peak which is higher than a three-phase short circuit current peak.•Diode arc-back fault can cause mechanical damage to multi-winding transformers used for adjustable speed drive applications.•Diode arc-back fault phenomenon is treated in IEEE Std. 551, while it is instead completely disregarded by IEC Std. 60,909–0. In polypropylene petrochemical plants power adjustable speed drives (ASDs) are normally used to drive extruder or pump machines required by this type of industrial process. Each one of these ASDs is fed by a dedicated converter transformer which supplies a diode-front-end rectifier in the input converter part of the ASD. In case one diode of the input rectifier is faulted and becomes short-circuited, the converter transformer can be subjected to a dynamic short circuit stress being higher than that due to a fault occurring just at transformer terminals: this phenomenon is called diode arc-back fault in technical literature. Simulations are carried out by means of EMTP-ATP software in order to show the effect of diode arc-back fault on the dynamic peak short circuit currents to which transformer windings are exposed