The interference of an electromagnetic pulse incident on the air-dielectric interface of a three-layer model for microstrip circuits was studied. Numerical results illustrating the power absorption by the model as a function of indicent angle and width of the pulse are presented. It is found that less than 1% of the power of the incident pulse can be absorbed by the substrate material. However, about 80% of the power of the incident pulse can be induced on the air-dielectric interface during the reflection of the pulse by the boundary. This amount of power flowing on the surface of the circuit can cause malfunctions or interference if the incident EMP has sufficiently intense wave fields'.