Potential transformers are used to measure and monitor the voltage of high voltage lines by tranforming the line voltage to some easily measurable value, often 100V (nominally - see below). They are designed for relatively low power but high accuracy and high reliability. The iron core is much larger and the wire resistance lower than usually necessary for that power. This allows to draw 10 time the rated power and more, as long as the windings do not overheat (they are not designed for good heat exchange). The insulation is designed with an extremely high safety margin, as the device must not fail due to voltage transients, even of several times the rated voltage. This makes them virtually indestructible, except thermally.