It is important that the instrument transformers are Open Loop transformers. This means that a sinus signal on the primary side also needs to exist on the secondary side. The instrument transformers also need to offer a broadband frequency transmission in order to capture harmonics or interharmonics up to the 50th harmonic.

The terminals of the primary winding are labeled "K" and "L" or "P1" and "P2", the terminals of the secondary winding are labeled "k" and "l" or "S1" and "S2". The polarity must be applied so that the "current flow direction" is from K to L.

Example of 1-phase connection

Example of 1-phase measuring with a Rogowski coil or a current clamp