High Power Microwaves (HPM) can disturb and/or destroy electronic equipment. This report provides a systematic approach describing the disturbance and destruction processes. We show outgoing from measurement results on real rather well protected electronic equipment, how the electromagnetic coupling to the electronic equipment can be described by the here proposed quantity: receiving transfer effectiveness. The smaller the receiving transfer effectiveness is, the better protected the most critical electronic component is. The receiving transfer effectiveness consists of the product of four factors. The two first factors, the impedance mismatch factor and the radiation efficiency are the fundamental parts in the protection. It is striking to see how the radiation efficiency can be close to 0 dB four a few discrete frequencies compared to its typical value of minus a few tens of dB. The product of the last two factors, the partial directivity, depends on from which direction the test object is irradiated, together with the orientation and type of polarisation in use. By help of the knowledge gained, we show in a practical example, how the uncertainty in a Radiated Susceptibility test can be lowered by interchanging testing in the directional domain by testing in frequency domain.