Electromagnetic compatibility (EMC) is the ability of an electrical appliance to operate satisfactorily in its electromagnetic environment without adversely affecting this environment—which may also contain other equipment. High packaging densities in electronic assemblies and ever-increasing signal processing speeds often cause faults in complex electronic equipment, measurement and control systems, data processing and transmission systems and communications technology. These are attributable to electromagnetic influences.

Electromagnetic influence is the effect of electromagnetic factors on circuits, appliances, systems or living things. Properly designed electronics enclosures help to reduce electromagnetic interference, or EMI. Interference is an electromagnetic factor which may induce an undesirable influence in an electrical installation (interference voltage, current or field strength). Interference sources may be divided into:

• • Internal sources of interference – Artificial, i.e. technically induced

• • External sources of interference – Natural, e.g. lightning, electrostatic discharges

Among artificial or technically induced interference sources, a distinction must be made between the effects of electromagnetic factors created and used for business purposes (such as radio transmitters, radar etc.), and electromagnetic factors which occur within the context of operations or in the event of a failure which are not purposely generated (e.g. spark discharges on switch contacts, magnetic fields around heavy currents etc.). Interference may take the form of voltages, currents, electrical, magnetic and electromagnetic fields, which may either occur continuously, periodically, or randomly in a pulse shape.

In low-voltage networks, the following apply:

• • The most interference-intensive temporary events are caused in low-voltage networks by the switching of inductive loads, e.g. power tools, household electrical appliances, fluorescent lamps.

• • The most dangerous over-voltages (according to level, duration and power content) are caused by deactivating fuses in the event of a short-circuit (duration in the millisecond range).

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