The effects from undesired electromagnetic radiation, EMI, on ungrounded or unshielded conductors is commonly underestimated. An ESD event occurring outside an ESDS protective work area can still pose a risk to unshielded and ungrounded conductors within the critical work or ESDS area. Recall number M249358, October 1991, states that a discharge from an operator to the timer of a radiation therapy system caused the timer s display to blank just as treatment began. This leads to a smaller indeterminate range of 1.2 Volts for most TTL and some CMOS logic circuits and places the logic inputs from these circuit traces or cable connections susceptible to induced EMI voltages exceeding this range. I lists some logic devices, and their potential susceptibility to EM energies.
Noise margin is a quantitative measure of a device s noise immunity.
Some types of common lab stools and office chairs can radiate a series of impulsive fields from metal legs due to internal ESD when a person rises from the chair. That EMI can couple into a system through cables or open chassis and induce a noise voltage greater than the logic inputs indeterminate range and cause a single event upset. EMI effects to microprocessors or other circuit logic latch-ups in process equipment can manifest itself in a number of ways, such as random hangs, robotic malfunction, or software errors, all resulting in downtime and reduced throughput. For ungrounded conductors coupled within a capacitive circuit, this EM wave can induce a static charge, building until a discharge, breakdown, recombination, or neutralization occurs. The gap width, d , is defined by Paschen s Law, but may vary in each discharge condition.