The SmartPi measures the electrical current in the same way as a current clamp meter in an indirect way. One advantage is that the circuit does not have to be disconnected and the measurement is contactless.
The current I flowing through the conductor generates a magnetic field H which runs in a ring around the conductor (Figure 7).
The current is measured according to the transformer principle with the conductor as the primary winding and the current sensor as the secondary winding.
If a current sensor is now placed around the conductor, the magnetic field of the conductor induces a current in the current clamp that can be measured (Figure 6).
If current flows through two conductors in opposite directions, the resulting magnetic field is cancelled out because the two magnetic fields are directed in opposite directions (Figure 8).
The current sensor can't measure current.

Phase and neutral conductor must not be connected together through one current terminal.
If the consumption of an electrical appliance (e.g. television)  to be measured, the individual conductors in the supply line must be separated. The current clamp has only to be placed around the conductor in which the current flows into the device.
If the current sensor is placed around the entire supply line, the SmartPi cannot measure consumption.