NASA-STD-4003: “Electrical Bonding for NASA Launch Vehicles, Spacecraft, Payloads, and Flight Equipment”
NASA-STD-4003 is unusual for its focus on bonding–which is more of a concern for aerospace projects than for most others. If you look in Henry Ott’s classic EMC Engineering or Michel Mardiguian’s useful Controlling Radiated Emissions by Design, you won’t see bonding mentioned much. But for aerospace, given its severe electromagnetic environments (lightning, plasma charging, EMP, etc.), bonding is critical. Following the guidance of NASA-STD-4003 gets you a long way to meeting the requirements of MIL-STD-464. 4003 can be freely downloaded here.The most recent revision is Rev A with changes, dated January 2016.
4003 separates bonds into different categories depending on their purpose. There are five categories, as seen in the main summary table below.
This table illustrates how broadly applicable this document is: although NASA spacecraft rarely use chassis for power return, the Class C bond category covers exactly that case–useful for aircraft and other vehicles types, beyond just space missions.
One challenge with NASA-STD-4003 is that some of the numbers it has in the summary table have been imposed as hard-and-fast requirements without an understanding of where those numbers come from and how they can be tailored to particular programs.
One example is “2.5 mOhms” for RF and lightning. Lightning and RF currents both have high frequency components. We’re trying to establish a low impedance path for these currents, but 2.5 mOhms is a DC resistance value. The reason for this is simply that it is MUCH easier to measure the DC resistance of an installed bond with a milliohm meter than it is to measure the high frequency impedance of a joint. IN GENERAL, if you can establish a 2.5 mOhm bond upon installation, you have likely also created a low impedance joint, since you have to have excellent metal-to-metal contact to get 2.5 mOhm at DC. But that doesn’t mean that if you have an 8 mOhm joint that your hardware will fail; it means you need to put more thought into the specific mission parameters are driving the need for bonding at that joint.
TIP:
Bonding measurements per NASA-STD-4003 are taken per joint, not on a point-to-point basis. Example: I have a cable harness going from one metal box to another with an overbraid shield installed, connected to MIL-STD-38999 connector backshells on both ends. Let’s say the bonding requirement for this particular shield is deemed to be the 2.5 mOhm Class R category. In this case, we do not need the measurement from box to box to be less than or equal to 2.5 mOhm. Here are the measurement requirements in that case:
Box A to connector backshell A < 2.5 mOhm
Connector backshell A to shield < 2.5 mOhm
Shield to connector backshell B < 2.5 mOhm
Connector backshell B to Box B < 2.5 mOhm
OR: Box A to Box B < 10 mOhm
TIP:
The origin of the 2.5 mOhm requirement goes back at least 70 years, and derives from the need to keep the voltage developed across any single joint under 500 V in the event of a 200 kA lightning strike. Nothing in its origin relates to controlling the impedance of structure for RF systems. Even the 500 V number is somewhat arbitrary and likely stems from the need to prevent sparking between structural elements surrounding fuel tanks. The 2.5 mOhm number has been adopted because it is a conservative value that is easy to measure and, if met, generally ensures proper system functioning. However, for new systems, and particularly any systems using novel materials such as composite airframes, new analysis should be done to determine what value of Class R bonding is needed for that particular project. This is a case where detailed simulations can give good answers early in the program and help drive reasonable requirements development.
One excellent way I’ve seen to capture bonding information is in a diagram such as the example below. In it you see places where different metallic elements join together. At each joint there’s a circle specifying the purpose of that particular joint. So from one box to structure a joint is necessary to carry fault current. Between two different kinds of structural panels, Class L bonds are needed, and to ensure proper functioning of an RF system Class R bonds are needed. In one area where a joint is needed both for lightning protection and for RF performance, a hybrid joint is specified. (In this case, the lowest bond value requirement is imposed.) This kind of diagram gives an at-a-glance image for the people setting structural bonding requirements, as well as a quick reference for people monitoring installations. It also helps ensure that if bonding requirements change later in the program, a relaxation of the Class L bond requirement (for example), won’t be blindly applied to a joint that has both a Class L and Class R function.
