REG NASA-LLIS-0658-2000 Lessons Learned Electrical Grounding Practices for Aerospace Hardware.pdf

1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-03a71 Center Point of Contact: MSFCa71 Submitted by: Wil HarkinsSubject: Electrical Grounding Practices for Aerospace Hardware Practice: Practice: Electrical grounding procedures must adhere to a proven set of requirements

2、 and design approaches to produce safe and trouble-free electrical and electronic circuits. Proper grounding is fundamental for reliable electronic circuits.Programs that Certify Usage: Programs That Certified Usage: Saturn I, IB and V launch vehicles, Space Shuttle Solid Rocket Booster, Internation

3、al Space Station, MSFC-developed payloads and experiments.Center to Contact for Information: MSFCImplementation Method: This Lessons Learned is based on Reliability Practice No. PD-ED-1214; from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.Benefits:Ground

4、ing procedures used in the design and assembly of electrical and electronic systems will protect personnel and circuits from hazardous currents and damaging fault conditions. Benefits are prevention of potential damage to delicate space flight systems, subsystems and components, and protection of de

5、velopment, operations, and maintenance personnel.Implementation Method:Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-System Grounding Requirements and Design Approaches:The design of electrical and electronic systems should comply with the followin

6、g as a minimum: (1) a ground reference plane should be established that will hold the grounds for all systems, subsystems, equipment metallic components, surfaces, and electrical and electronic parts at the potential of the base structure; (2) within equipment, power should have dedicated returns; (

7、3) except for a single-point reference, all electrical signal and power grounds should be electrically isolated from each other, and each separately derived electrical system should be electrically connected to structure at only one point; and (4) a dedicated power return should be used except where

8、 necessary to support system requirements.The grounding within electrical or electronic enclosures is at the discretion of the circuit designer. The following design approaches should be considered for the design of these systems: (1) within equipment, conditioned electrical power should be DC-isola

9、ted from chassis and structure except at a single point; (2) within equipment, the single-point reference should be routed external to the equipment for termination to ground, or routed directly to the chassis for termination; (3) the control power bus return should be independent of the primary ele

10、ctrical power return and should be referenced to the return system at a single point; (4) secondary and tertiary electrical power should be single-point grounded and should be returned to that single reference ground point; (5) when all single-point grounds are not terminated to chassis or structure

11、 secondary and tertiary electrical power should be dc isolated by a minimum of 1 megohm; (6) power conversion performed to supply conditioned power to several devices or functions should reestablish a single-point ground reference for the serviced equipment or functions; (7) equipment should not de

12、pend on other equipment for reference or grounding, either signal or power, unless it is also dependent upon the other equipment for its power; (8) signal circuits with frequencies below 2 MHz, with interfaces external to equipment, should be balanced and isolated from chassis; (9) all returns and r

13、eferences should be brought out of equipment on individual connector pins; (10) shield connections should be made to connector shells or to connector pins that are, or will be, grounded when mated; (11) single-ended circuits with the lowest frequency component equal to or above 2 MHz should be coupl

14、ed by coaxial cable with the shield terminated 360 degrees at each end; and (12) external to an equipment, single-ended electrical signals should be prohibited for signal frequencies below 2 MHz except where electrical isolation is maintained.refer to D descriptionD Schematic Examples:An example of

15、grounding implementation concepts is shown on Figure 1. This figure reflects the stated grounding requirements and design considerations and shows two feeder, cabling and load configurations. At frequencies below 2 MHz (Figures 1 and 2), the emphasis is on circuitry requiring internal grounding with

16、 interfaces to external equipment. For frequencies equal to and above 2 MHz, the emphasis is on external connections between equipment and the proper grounding of shielding to prevent electromagnetic coupling.Single-Point/Multiple-Point Grounding:Although the establishment of a ground reference plan

17、e Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-requires a single-point ground, the actual practice of complying with this requirement in a system design is controversial. Modern electronic systems seldom have only one ground plane and, to reduce p

18、otential interference, as many ground planes as possible are sometimes used. From Figure 2, a grouping of ground planes connected by the shortest route back to a system ground point where they form an overall system potential reference, could be called a single-point ground system. However, problems

19、 with this scheme arise when interconnecting shielded cabling is used having significant lengths with respect to the wavelength of signal frequencies and parasitic capacitance exists between equipment housings or between subsystems and the grounds of other subsystems. It can be argued that a “multip

20、le-point“ ground system, which bonds each subsystem or equipment as directly as possible to a low impedance equipotential ground plane, can minimize these electromagnetic interference problems. An example of such a system is shown in Figure 3 where each subsystem is connected directly to a common gr

21、ound plane, ideally a flat, equipotential plate.In practice, the selection of a grounding scheme is dependent on the highest significant operating frequency of low-level circuits relative to the physical separation of the equipment. As shown in Figure 4, single-point grounding works best at low freq

22、uencies and small dimensions and multiple-point grounding works best at high frequencies and large dimensions. For transitional situations, one or the other may perform better as shown in Figure 4. For this crossover region, hybrid grounds perform best when portions of the low-frequency systems use

23、single-point grounds and the high-frequency portions use multiple-point grounds.Shock Prevention:Proper grounding protects personnel from accidental contact with metallic elements that may have hazardous voltage potentials due to system faults or accidental contact between energized elements and equ

24、ipment chassis, frame or cabinet structure. Case voltage rise is limited to reduce currents to levels that do not produce adverse reactions and possible secondary effects. Typically, case voltage rise is limited to prevent hazardous currents. Table 1 summarizes the alternating and direct currents an

25、d their shock effects.Table 1. Summary Effects of Electrical Shock Alternating Current (60Hz) Direct Current (DC) Reation(mA) 0.5-1 1-3 3-21 21-40 40-100 100 (mA) 0-4 4-15 15-80 80-160 160-300 300 Perception Surprise Reflex action Muscular inhibition Respiratory block Usually fatal Bonding:The integ

26、rity of interconnected conductive elements is ensured by electrical bonding, a process in which Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-components or modules are electrically connected to provide a low-impedance conductor. Bonding practices s

27、hould comply with MIL-STD-5087B or with SSP 30245. Bonding procedures require the use of specified clamps, standard parts, bolt and screw attachments, washers and materials to ensure consistent bonding of equipment under various temperatures and corrosion environments. The use of jumper cables is di

28、scouraged except across movable vibration or thermal isolation joints.Surface preparation for bonded joints should begin by removing all anodic film, grease, paint, lacquer, or other high-resistance properties from the faying surfaces. A typical bonding hardware configuration is shown in Figure 5. T

29、he use of scrapers, abrasives or chemical cleaning methods to provide a clean, smooth bonding surface is dependent on the type of joint (i.e., metal-to-metal, metal-to-nonmetal or nonmetal-to-nonmetal). Example bonding impedances for selected bonding classes are shown in Table 2.refer to D descripti

30、onD refer to D descriptionD Table 2. Example Bonding Impedances and Bonding Class Bonding Class ImpedanceProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-A (Antenna installation) H (Shock hazard) R (RF potentials) S (Static charge) DC resistance 1 ohm

31、 (conductive structure) 1000 ohms (composites) 1000 ohms (conductive subassemblies) 1 ohm (pipe and hose)refer to D descriptionD Cabling/Connector Grounding:Cabling extending outside grounded enclosures is vulnerable to radiated emissions if cable lengths are a significant portion of the wavelength

32、of the systems highest operating frequencies. Adequate shielding and grounding are required to ensure proper system operation. Figure 6 shows typical grounding practices for shielded cabling and connectors. Shield terminations at connectors are gripped by the connector back shell to provide a low im

33、pedance 360 degree connection. Soldered connections are not recommended due to the difficulty in repair and wiring changes, and the use of foil in some cable shielding. Where cabling enters enclosures, the box connector or partition penetration in Figure 6 may be used. For cabling where the overall

34、shield ends in a terminal strip, the termination may look like the configuration shown in Figure 7.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D descriptionDrefer to D descriptionDrefer to D descriptionDTechnical Rationale:Through many y

35、ears of designing and fabricating electrical circuits and electronic devices for launch vehicles, experiments and payloads, the Marshall Space Flight Center has developed procedures and techniques for designing reliable and safe aerospace electronic systems. Design criteria were built upon a solid f

36、oundation of industry and government practices. Military standards were used at the outset, and procedures unique to the space program were added as refinements. Practical experience reflected in the standard procedures and techniques resulted in reliable circuits that presented minimum hazard to pe

37、rsonnel and equipment.Related Practices:1. Reliability Preferred Practice No. PD-ED-1202, “High Voltage Power Supply Design and Manufacturing Practices,“ Lewis Research Center.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-2. Reliability Preferred P

38、ractice No. PD-ED-1210, “Assessment and Control of Electrical Charges,“ Goddard Space Flight Center.3. Reliability Preferred Practice No. PD-ED-1206, “Power Line Filters,“ Goddard Space Flight Center.References:1. SSP 30240 Revision A, “Space Station Grounding Requirements,“ September 1991.2. MSFC-S

39、PEC-521B, “Electromagnetic Compatibility Requirements on Payload Equipment and Systems,“ August 15, 1990.3. MIL-STD-461C, “Electromagnetic Emission and Susceptibility,“ August 1986.4. MIL-STD-462, “Electromagnetic Interference Characteristics,“ July 1967.5. MIL-STD-463A, “Definitions and System of U

40、nits, Electromagnetic Compatibility Technology Interference,“ June 1966.6. MIL-STD-5087B, “Bonding, Electrical and Lightning Protection for Aerospace Systems,“ Amendment 3, December 24, 1984.7. Denny, Hugh W., “Grounding for the Control of EMI,“ 1983.8. White, Donald R.J., “Electromagnetic Interfere

41、nce and Compatibility,“ Vol.3, “A Handbook on EMI Control Methods and Techniques,“ 1973.9. SSP 30242 “Space Station Cable/Wire Design and Control Requirements for Electromagnetic Compatibility, “September 1991.10. SSP 30245 “Space Station Electrical Bonding Requirements,“ September 1991.Impact of No

42、n-Practice: Impact of Non-Practice: The impacts of failure to adhere to proven and acceptable grounding practices for the design and fabrication of electrical and electronic parts are: (1) potential damage to delicate space flight systems, subsystems, components and experiments; (2) creation of spar

43、ks or overheated components or connections, creating a fire hazard or a thermal imbalance that cannot be counteracted by environmental control systems; (3) danger to ground or flight crew personnel from electrical shock; or (4) damage to vehicle, payloads and ground systems due to atmospheric lightn

44、ing. Ultimately, lack of proper grounding could cause death due to electrical shock or mission failure due to excessive heating, shorts, or fire.Related Practices: N/AAdditional Info: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Approval Info: a71 Approval Date: 2000-03-03a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-