EMP stands for electromagnetic pulse which is a field of intense energy (CRS Report for Congress). Because EMP works by line of sight, when an explosive device creates an EMP shockwave, anything that can 'see' the explosion is susceptible. EMP is basically an intense blanket of lightning. A blanket of lightning that will typically fry any piece of modern circuitry or damage anything that is wired (atomic archive). A big scare for United States security is if one of these devices were to explode at a high altitude, giving the name HEMP device (High-Altitude Electromagnetic Pulse). If a large enough weapon were to explode 500km above earth's crust, centered on the US, most of the 48 state's coveted electronic devices and infrastructures would be destroyed.