ABSTRACT
The modern approach to achieve electromagnetic compatibility is based on a risk management approach. Risks are identified early in the design and are addressed as soon as possible. After successful full-compliance testing conformance is assumed and risks should be negligible. Nevertheless, incidentally unexpected electromagnetic interference may occur and then troubleshooting is required. A structured approach will help to find the root cause in an efficient manner, i.e., the effort required increases per step:
Have changes been made in the installation or system?
This question typically can be answered without visiting the site of installation. An assessment can be made if the reported interference may be related to the change.
Have changes been made in the environment of the installation or system?
This question typically can be answered without visiting the site of installation. Examples of changes in the environment are the placement of a power transformer in the basement, or mobile antennas on the roof of a nearby building. An assessment can be made if the reported interference may be related to the change.
Has software been updated? What is the version number?
Software contains bugs. Sometimes they introduce intermittent problems which mimic electromagnetic interference problems. When the software version is known, it can be checked for known issues, either in this specific release, or if they have been solved in a newer release.
Perform visual inspection.
Are there any loose connectors?
232Are any of the (ground) connections oxidized or damaged?
This not only applies to grounding straps, but also to filter connections to enclosures.
Have any pigtails been used?
Are cables properly separated?
Cables connected to sensitive equipment should not be routed next to emitting power cables, like variable frequency drive cabling.
Perform functional variation.
Changing the mode of operation of the mal-performing equipment may give indications toward the disturbance cause. Examples are as follows:
When an external magnetic field interferes with an electron microscope, the amount of image distortion will depend on the acceleration voltage.
The (audible) output noise of an audio amplifier with a shorted input may remain constant when the volume is increased or it may alter.
Pulse width-modulated power sources are notorious for their noise production. Changing the output power might change the nature of the observed interference.
Verify the input voltages of the mal-performing equipment. Is the interference frequency the mains fundamental or a harmonic? Note that in harsh environments reliable voltage measurements are difficult and skipping to step 7 may be preferred.
Perform common mode current measurements.
Start with measuring common mode currents over cable bundles or grounding structures (see Figure 8.6). When the threshold of 5 µA is exceeded, try to determine which cable(s) in the bundle are the culprit.
Perform near-field measurements.
When common mode currents indicate problems with filters, electromagnetic field leakage between the filter and housing may be confirmed with near-field probes (see Figure 8.9). Also when connections between different parts of an enclosure are marked as suspect during visual inspection, near-field probes will be able to indicate if indeed field is emitted from them.
In case problems occur intermittently or only during lightning strikes, inject current into the system and repeat steps 5 and 6 (synchronized with the injection).
