It’s often said, by myself and many other EMC instructors, that good bonding is key for EMC success. And that’s true! But it leaves out some nuance: how good does “good” need to be?
It turns out that depends on what you need the bond to do for you. We usually talk in terms of preventing radio frequency interference (RFI), but there are several reasons you may need to pay close attention to your bonding scheme.
NASA-STD-4003A is a great document that packs a lot of practical information and guidance into a relatively short page count—less than 50, in the most recent version I have. It’s also publicly available for free! It defines bonding as: “The process of providing good electrical connection across faying surface mechanical interfaces to minimize electrical potential differences between equipment and individual parts of structure.” It then goes on to define five classes of bond:
- Class C, when structure will be carrying return currents, as in traditional internal combustion engine vehicles
- Class H, for hazard control when there might be a short to case or other fault that results in currents flowing on structure
- Class R, bonding to prevent RFI or enable RF systems to operate properly
- Class L, to conduct currents resulting from a direct or nearby lightning strike
- Class S, to control static buildup on surfaces and prevent ESD
For each of these classes, there are different aspects to consider. What impedance should you shoot for, over what frequency range? What DC resistance value makes sense for verification measurements? How much current will the bond potentially have to carry? If the only purpose of a bond is to prevent static charge concentration, you can have a surprisingly high impedance bond and still be effective. However, to protect against RFI, you need a very low resistance/impedance—but maybe not much current-carrying capacity, etc.
If you’re designing a new platform from scratch, it may be helpful to create a bonding diagram like the one in Figure 1. This shows, for each expected metal bond, what class of bond is needed—or if multiple classes are needed at a particular joint, such as bonding the case of RF equipment. That joint may need to handle both Class R and Class H requirements. In this case, meeting the lower impedance of Class R will also satisfy Class H, and making sure it can handle hazard currents will be more than adequate to carry RF currents.
There aren’t as many standards relating to bonding as there are to, say, ESD or radiated emissions. The expectation is that you’ll create a system with bonding that supports meeting those other requirements. But if you’re looking for some guidance on how to do that, you could do a lot worse than look up NASA-STD-4003A.
