We’ve already concluded that for low frequencies the Star or SPG ground works best and at the high frequencies, the so-called equipotential ground plane with a MPG system works best. No one grounding system is perfect. On top of that, most of the equipment these days operates at frequencies from DC to daylight, so the grounding design has to cover all frequencies including the mid-range.
There is a grounding configuration we have yet to discuss—which is not really a grounding system at all—called the Floating Point Ground (FPG), in which all electrical/electronic circuits are deliberately isolated from everything at least by conductors. Believe it or not, there are some advantages to doing this, particularly if some of the equipment circuits are sensitive to circulating ground currents or low level magnetic fields. It gets used from time to time in spacecraft and on shipboard, it is often used for power systems to maintain weapons operation in the event of a short to ground and to minimize electrogalvanic corrosion.
Implementation of a FPG is very difficult and at the circuit level requires extensive use of optical couplers, fiber optics, and isolation transformers. At the box level, it requires plastic boxes, insulated spacers, isolated panels, etc. A disadvantage of the FPG is that all isolated conductors become one plate of a capacitor with respect to ground and in a high static field environment, floating objects (whether insulators or conductors) can accumulate charge. When charge levels become high enough corona/streamer discharges can occur and/or arc over to nearby conductors that are at a different charge potential or that are grounded.
For a system that has a large range of operating frequencies, all three forms of ground systems are used. An example of such a combination might be a platform monitoring system where data from various analog sensors such as pressure, temperature, strain, vibration, etc is being multiplexed to a digitizer, which then sends its output to a microwave telemetry transmitter. The construction of some sensors requires them to be grounded. Other sensors are floated. The digitizer may have both SPG and MPG internally and the microwave telemetry transmitter probably only uses a MPG system.
The choice of grounding system is determined by frequency principally because of the inductance of the grounding conductor. This is directly related to conductor length. Many EMC guidelines suggest that the length should be held to less than λ/20. This is based on the assumption that the ground system will be a disaster at λ/2. More likely it will be a disaster long before the length reaches λ/4 because this simple guideline does not consider the capacitance of the device in conjunction with the inductance of the ground conductor.
The first parallel resonance occurs at F = 1/ (2π (L * C)0.5) with F(Hz), L(H) and C(F). When resonance occurs, the impedance becomes infinite. Not really of course, but it does get very high and the ground doesn’t work at that frequency. Murphy’s Law says that is the frequency at which the widget generates the most RF noise. To minimize the problem and provide a better rule of thumb for those concrete thinkers, plan on the SPG being usable to around λ/50. In addition, a MPG is absolutely required after λ/10 and a combination of the two (Hybrid) is used when the frequencies fall in the middle. There are always exceptions, but that’s EMC!
– Ron Brewer