What is a shorted turn?

A shorted winding means the insulation of the winding wires is deteriorated or burned and hence the current no longer takes the designed path. A coil or a winding on a transformer is a length of well-insulated wire wound in a tight spiral, usually around some sort of former, made of a dry insulation material such as plastic, card or wood, with a magnetic core. A coil can have anywhere from 1 turn (loop) to thousands of turns. For proper operation the windings must all be insulated from each other, that is, the conductors of the wires must not touch each other electrically. (Even though the insulated wires must of course touch each other physically.) If the insulation becomes frayed or worn, or is damaged in some way, perhaps by too much heat or voltage, the insulation on some of the wires can be damaged so that two or more of the turns end up touching each other electrically. This in effect short-circuits such turns of the winding. Because a shorted turn is a closed loop around the magnetic core, it ends up getting a massive current induced into it which causes that shorted turn to get very hot, often so much that it catches fire. This heat causes the insulation of other nearby turns to fail in the same way and this process then rapidly cascades until the whole coil of wire either catches fire or the supply of current is cut. It can be cut either by a protective breaker or fuse or, if this does not happen soon enough, sometimes a turn or turns melt under the intense heat, thus breaking the circuit. In either case, even though the current ends up being cut off, the coil itself is ruined and has to be replaced before the appliance it is serving can be used again.