Interference from DC and Quasi DC Fields

Generally, DC magnetic fields do not present an EMI threat to most equipment. Although the Earth's DC fields are relatively stable, there are inherent slight variations in both field direction and strength, over time. These minor temporal instabilities are usually well below the sensitivity of most electronic instruments and usually do not represent an EMI threat concern.

However, some highly specialized instruments like TEM and SEM electron microscopes and NMR or MRI imaging systems are sensitive to small variations in DC field strength or changes in direction. The movement of a mass of ferromagnetic material through the Earth's field such as occurs with a moving elevator, subway train, truck, etc. will momentarily alter or perturb both the field strength and the direction of the Earth's DC field creating a "Quasi" or "time varying DC field. This influence can be measured at relatively large distances from the source of the perturbation and changes in the DC field environment caused by external influences such as a moving subway train, can adversely impact the stability and accuracy of these sensitive instruments.

Monitor with DC field interference

Extremely high DC magnetic field levels (5 G and higher) can also cause interference with cardiac pacemakers and other devices.