Let’s continue with the discussion about shielding of magnetic fields at kHz frequencies.
Consider two 50 cm long wires forming a current loop with a total current of 500A. Each wire is shielded with a 1 mm thick copper shield. At both ends, a resistor connects the shields. Only for single digit mOhm resistance values there is significant shielding. Already at 50 mOhm shielding is no longer present.
Because the shielding material is copper, this is a case of conductive shielding. By analyzing the path of the induced current, we can understand why the connection impedance needs to be so low: In case of high connection impedance, the induced current will form a loop in each shield separated (red loops). The magnetic field created by these separated loops will cancel out and there is no shielding effect. In case of low connection impedance, the green loop will form and a magnetic field opposite to the source loop will create a shielding effect.
Which loop is dominant depends on the ratio of the connection impedance to the length impedance of the shield. This leads to an important finding: Even the thickest shield made of pure gold will be useless if your bonding is crap. 😊
Magnetic Field Shielding-kHz
