Current normally flows in two directions through a conductor: On the one hand it follows - as designed - the path of the conductor and on the other hand through the surrounding insulation of the conductor towards the outside. The latter current flow must be held as small as possible by ever better and higher-quality insulation.

In the tan delta measurement, the current flow is measured in both directions. The flow directions are indicated by two arrows that are connected with each other in a pointer diagram (see above). One indicates the effective direction (following the conductor path), the other indicates the flow out through the insulation. This results in a current arrow that contains both components, a so-called apparent current.

The angle produced from the two current arrows of the effective and apparent current is the delta angle. The tangent of this angle is the measurand that indicates the quality of the insulation. During measurement, five measurement points are recorded and evaluated. The measurement is started at a fifth of the rated voltage which represents the maximum value of the electrical voltage in normal operation. The measurement voltage is increased in fifths until the full rated voltage is reached. The evaluation of the measurement results is based on three aspects:

1. The level of the first measured value (at a fifth of the rated voltage)
2. The rise in value between the initial value and the average value (0.6 x rated voltage)
3. The rise in value between the individual measurement points

Various regulations and standards specify how the measurement results should be evaluated. These results can be used to decide whether measures for insulation optimisation are required and what they should look like.

It is sensible to continuously monitor such measurement results over years or during every standstill to evaluate the ageing of the insulation. This enables timely replacement in good time or a general overhaul at planned standstill times.
Surprising and often extremely expensive unplanned downtimes of systems are minimised.