ESD is one of four categories of potentially damaging transients that are dangerous to electronic circuitry. The lowest voltage category is caused by broad-band noise than spans a very wide frequency spectrum. The next higher level is the Pulse Category that ranges from low frequencies up to microwave frequencies. The third level are Surge transients that typically occur at the lower frequencies. ESD is one of the most damaging types of transients requiring a quick response time to protect against the high frequency content while being able to cope with a high level of voltage.