It can be seen from Figure 9 that the disturbing current reaches more than 10% of the amplitude of the slow, and more than 15% of the amplitude of the fast, external current. These high values result from the short rise times of the digital signals. Similar high values are to be expected from any electronic switching process such as phase-angle control dimmers. The switching of the dimmer can be modelled by the onset of the trapezoidal signal. The remaining part of the 50 Hz signal only gives a negligible contribution.
So far, we have looked at the short circuit loop as a drain of EMI. In this case, the electrical properties have to be optimised to minimise the disturbing current i2(t). The property of the induced current i2(t) to generate a magnetic field that weakens the external field can also be used to shield enclosed sensitive electrical or electronic systems. In this case the electrical parameters of the short circuit loop have to be chosen to optimise the counter field generating current i2(t) and to minimise the net magnetic flux through the loop. Practical examples of this application are the shield of any shielded cable, cable trays, unused cores of cables etc. The net magnetic flux across the area of our model short circuit loop can be calculated to be: