When a partial discharge occurs inside a metal-clad panel, the pulse current seeks earth and in doing so produces a very short voltage step on the outer surface of the enclosure. That step, in the order of millivolts to tens of volts with a nanosecond rise time, is what is known as TEV.
Its great practical advantage is that it is measured from the outside, by resting a capacitive sensor against the panel plate, without opening it and without taking it out of service. That makes TEV inspection a quick round which can be repeated often across a whole medium-voltage switchroom.
The reading is expressed in dBmV and interpreted by comparison: between neighbouring panels, against the background level of the room and against the history of the same point. A single figure means little; what flags a problem is a panel that stands out from its peers or that rises over time.