Editor’s note: This question was asked in response to Interference Technology’s recent webinar by Keith Armstrong. To view the webinar, click here.
Question: What is the purpose of a bonding terminal on a 19 inch metal chassis if unwanted signals don’t flow to ground?
Answer: Bonding terminals are provided on racks, frames and cabinets so that wires or braid straps can be fitted, like those visible in the photograph on slide 31 of my presentation, to provide small, local, low-impedance return paths for the stray CM currents that have coupled into the external metalwork. The CM currents ‘prefer’ to flow in these local paths to return to the electronic components that created them, instead of spreading themselves around more widely by flowing in larger loops with higher impedances, and this reduces their radiated emissions.
Most cabinet manufacturers assume that “single-point-bonding” is required, so if left to themselves they provide bonding terminals that are too few and in the wrong places. But this has been a poor EMC design practice since before 1980.
Each electronic unit or cable is a source of CM currents into the cabinet they are contained within, so each needs at least one local strap to return CM currents. The principles of RF bonding I outlined above apply to this situation too, so we can see that – for example – using two cabinet straps each 150mm long, spaced apart by 150 millimeters, we can’t expect to reduce the emissions of CM currents at wavelengths that are shorter than 1.5 meters (i.e. have frequencies above about 200MHz). At the frequency where the straps are either a quarter or a half of a wavelength long (either 500MHz or 1GHz, in this example) we can expect them to resonate and actually increase CM emissions.
Analyzing the frequency limitations of such cabinet-RF-bonding straps tells us that we had better design our electronic units and their cables to have low CM emissions above certain frequencies (200MHz in the above two-strap example), or else provide improved RF bonding between the units and their cabinets, e.g. by using conductive gaskets. Another way in which we deal with EMC design issues early in a project where design changes cost least and cause lest delay.
(Remember not to confuse RF bonding points on a cabinet with the bonding point (or points) that are provided for safety earthing/grounding. It may be possible to combine these two functions for some of them, with careful design.)
-Keith Armstrong
