1.0 SCOPE

 

 

2.0 APPLICABLE DOCUMENTS

 

 

3.0 DEFINITIONS

 

 

4.0 REQUIREMENTS

 

 

4.1 Design philosophy

4.2 Environmental requirements

4.3 Electrical requirements

4.4 High voltage design and construction

4.5 Pyrotechnic design and construction

4.6 EMI design and construction

4.7 Grounding

4.8 Wire and cable

4.9 Conductors

4.10 Lay of wire

4.11 Harness diameter

4.12 Harness length

4.13 Harness bend radius

4.14 Breakouts

4.15 Shielding

4.16 Shield terminations

4.17 Electrical connectors

4.18 Connector protective covers

4.19 Connector sealing grommets

4.20 Conductor terminations

4.21 Terminal size

4.22 Insulation sleeving

4.23 Terminal junctions

4.24 Wire splices

4.25 Connector potting

4.26 Connector location

4.27 Harness lacing or tying

4.28 Harness clamping

 

 

5.0 PARTS AND MATERIAL

 

 

5.1 General

5.2 Dissimilar metals

5.3 Wire and cable

5.4 Shield braid

5.5 Solder sleeves

5.6 Electrical connectors

5.7 Connector protective covers

5.8 Solder

5.9 Flux

5.10 Sleeving

5.11 Terminal junctions

5.12 Wire Splices

5.13 Connector Potting compound

5.14 Lacing tape

5.15 Convolute tubing and accessories

 

 

6.0 PROCEDURES

 

 

6.1 General

6.2 Lay of wire

6.3 Shield termination

6.4 Crimp connections

6.5 Solder connections

6.6 Connector potting

 

 

7.0 QUALITY ASSURANCE

 

 

7.1 Compliance

7.2 Visual inspection

7.3 Functional test

7.4 Continuity

7.5 Insulation resistance

7.6 Contact retention

 

 

8.0 CONTAMINATION CONTROL

 

 

9.0 RECOMMENDATIONS FOR AN IMPROVED ELECTRICAL HARNESS

 

 

9.1 Connectors

9.2 General Design Practices

9.3 EMI/EMC Related Practices

9.4 Fabrication Recommendations

9.5 General Recommendations

 

 

 ac             alternating current

 C              Celsius

 CMA            Circular Mill Area

 dc             direct current

 DSC            Differential Scanning Calorimetry

 EED            Electro-explosive Device

 EMI            Electromagnetic Interference

 ESD            Electrostatic Discharge

 ft             feet

 GSFC           Goddard Space Flight Center

 HSC            High Strength Copper Alloy

 Hz             Hertz

 lb             pound

 MSFC           Marshall Space Flight Center

 NHB            NASA Handbook

 OD             Outside Diameter

 PVT            Physical Verification Test

 QCM            Quartz Crystal Microbalance

 RF             Radio Frequency

 rms            root mean square

 TFE            Polytetrafluoroethylene

 Vac            Volts alternating current

 Vdc            Volts direct current

 Zero-G         Zero Gravity

1.0 SCOPE

1.1 The purpose of this standard can be described as two-fold. Initially, it establishes the minimum requirements for the design of space vehicle electrical harnesses. Additionally, in Section 9, this document provides a number of recommendations, based upon decades of harness development experience, for the design, manufacturing, handling, and integration of flight electrical harnesses.

1.2 Classification- Electrical harnesses shall be of the following types, styles, and configurations.

1.2.1 Types- Harnesses shall be of four basic types depending on the intended application as follows (see 1.3):

Type I Enclosed in fluorocarbon elastomer convolute tubing.

Type II Enclosed in a glass or other approved braid.

Type III Open bundle (not enclosed, general use).

Type IV Enclosed in a overall EMI/EMC shield.
 
 

Style A Continuous lace

Style B Spot tied

Style C Tie wraps

Style D Combination of any two of the above styles
 
 

Configuration R - Random lay

Configuration T - Twisted lay
 
 

1.3.1 Type I and II - These harnesses may be used in all areas where abrasion protection is required.

1.3.2 Type III - This type of harness may be used as a "general use" harness in all areas, where it shall not be subjected to abrasion.

Type IV - This type of harness may be used in all areas where overall harness EMI/EMC shield is required.

2.0 APPLICABLE DOCUMENTS

2.1 The following documents form a part of this standard to the extent specified herein.

Specifications

Federal

QQ-B-575        Braid, Wire Copper, Tin-Coated Tubular

Mil-C-17        Cables, Radio-Frequency, Coaxial, Dual Coaxial,

                Twin Conductor, and Twin Lead



Mil-C-39012     Connector, Coaxial, Radio-Frequency, General Specification for



Mil-I-22129     Insulation Tubing, Electrical, Poly-tetrafluorethylene Resin,

                nonrigid



Mil-I-23053     Insulation Sleeving, Electrical, Heat Shrinkable, General

                Specification for



Mil-T-43435     Tape, Impregnated, Lacing, and Tying



Mil-C-27500     Cable, Electrical Shielded and Unshielded, AEROSPACE



MIL-W-22759     Wire, Electric, Fluoropolymer-Insulated Copper or Copper Alloy

GSFC-302-S-016  Crimping of Electrical Connections



GSFC-S-311-P-4  Connectors (and Contacts), Electrical Rectangular, for

                Spacecraft use, General Specification for



GSFC-S-700-42   Connectors, Electrical, Rectangular (Power and Coaxial Contacts)

                for Space Flight Use, General Specification for



GSFC-S-313-023  A Pressure Applied Urethane Resin Insulation for Potting

                Connectors



GSFC-S-313-025  RF Shielding of Connectors With an Insulation and an

                Electrical Conductive Polymer

MSFC-SPEC-101   Flammability Requirements and Test Procedure for

                Materials in Gaseous Oxygen Environment



MSFC-SPEC-222   Resin Compounds, Electrical and Environmental Insulation,

                Epoxy, Specification for



40M38277        Connectors, Electrical, Circular, Miniature, High Density,

                Environmental Resisting, Specification for



40M38298        Connectors, Electrical, Special, Miniature Circular, Environment

                Resisting 200 C, Specification for



40M39569        Connectors, Electrical, Miniature Circular, Environmental Resisting,

                200 C, Specification for



40M39580        Connector, Electrical, Zero-G, Specification for



40M39589        Junctions and Junction Devises, Electrical Distribution and

                Bussing, Specification for

Military

MIL-STD-202     Test Methods for Electronic and Electrical Component Parts



MIL-STD-889     Dissimilar Metals



MIL-STD-975F    NASA Standard Electrical, Electronic, and Electromechanical

                (EEE) Parts List



MIL-STD-21122   Clamp, Loop, Cushioned, Wedge, POLYTETRAFLUOROETHYLENE, TYPE V

                CLASS A and B

NASA

NHB 5300.4(3A-2)        Requirements for Soldering Electrical Connections



NHB 5300.4(1D-2)        Safety, Reliability, Maintainability, and Quality Provisions

                        for Space Shuttle Program



NHB 5300.4(3G)          Requirements for Interconnecting Cables, Harnesses, and Wiring



NHB 5300.4(3H)          Requirements for Crimping and Wire Wrap



NHB 8060.1              Flammability, Odor, and Offgassing Requirements and Test

                        Procedures for Materials in Environments Which Support

                        Combustion

Reference Publication 1124      Outgassing Data for Selecting Spacecraft Materials



PPL-20                          GSFC Preferred Parts List

For the purpose of this standard, the definitions listed in this section shall apply.

3.1 Harness- One or more insulated wires or cables with or without helical twist; with or without common covering, jacket, or braid; with or without breakouts; assembled with two or more electrical termination devices and so arranged that as a unit, can be assembled and handled as one assembly.

3.2 Wire- A single metallic conductor of solid, stranded, or tinsel construction, designed to carry current in an electrical circuit. It may be bare or insulated, but does not have a metallic covering, sheath, or shield.

3.3 Electrical cable- Two or more insulated conductors, solid or stranded, contained in a common covering (sheath, shield, or jacket); two or more insulated conductors twisted or molded together without common covering; or one or more insulated conductors with a metallic covering shield or outer conductor (insulated or uninsulated).

3.4 Coaxial cable- Cable used for transmission of radio frequencies (RF), assembled or not, with two RF (coaxial) termination devices.

3.5 Shield- A metallic cover over an insulated conductor or conductors.

3.6 Length of lay- The axial length of one complete turn of the helix.

4.0 REQUIREMENTS

Designs shall conform to the requirements specified herein.

4.1 Design philosophy- Only parts and materials with the ability to function satisfactorily in the environments to be encountered, and which conform to such governmental specifications as are specifically applicable under the contract, shall be used. Only fabrication methods and techniques adequate to assure high quality electrical harnesses shall be used.

4.2 Environmental requirements- Electrical harnesses shall be designed to withstand the maximum specified operational environments.

4.3 Electrical requirements- The harness assemblies shall be designed to provide optimum power transfer from the transmitter or power supply bus to the receiver or load. The harness assemblies shall be designed to meet total performance requirements, to minimize susceptibility to electromagnetic interference from external sources, and to minimize electromagnetic interference problems.

4.3.1 Electrical wire current carrying capacity- Wires shall be of such cross section as to provide ample and safe current carrying capacity. The maximum design current in any wire shall be limited so that "wire total temperature" will never exceed the rated wire temperature. "Wire total temperature" is defined as maximum ambient temperature plus temperature rise.

4.3.2 Current carrying capacity (exposed space environment)- Table 1 shows the current carrying characteristics of wire at 28 ± 4 Vdc in an exposed space environment. Tests were conducted at a maximum ambient temperature of 75 C.

NOTES:

1. Column two indicates the maximum design current for any individual wire in a bundle of the size wire specified in column one.

2. Column one is the recommended minimum wire size for the design current specified in column two.

3. The current as specified in column two will generate a temperature rise of approximately 50 C above ambient temperature in a vacuum environment. Precaution should be taken so that the total temperature of the wire (ambient plus rise) does not exceed the continuous operating temperature rating of the wire.

4.4 High voltage design and construction- Circuits carrying potentials in excess of 200Vac, rms, or 300Vdc through critical pressure environments should be terminated in single contact high voltage connectors. If the design requires that high voltage circuits be terminated in multicontact connectors, contacts should be selected which are the most distant from ground potentials. Shielded wire should not be used in high voltage circuits unless required by special designs. High voltage connectors must be kept free of any contamination which would decrease the voltage flashover characteristics.

4.5 Pyrotechnic design and construction- The pyro bus shall be capable of supplying 5 amps minimum to the electro-explosive device (EED) over the expected temperature and voltage range of the spacecraft.

4.6 EMI design and construction- Harness assemblies shall be so designed and constructed as to minimize induced interference resulting from electromagnetic coupling of signals from wire to wire within the harness assembly. Program requirements will state relative importance of isolation in an interference control specification.

4.6.1 Isolation of Signals- Separate connectors can be used to isolate signals in radio frequency, audio frequency, and power circuits; ac signals can be separated from dc signals; high from medium from low level signals; pulse from continuous wave signals. Where a combination of signals circuits are terminated in a single connector, like grouping of signal wires can be brought out and laced separately in the harness bundle.

4.6.2 Wire and cable types- for specific applications, the following types of wire or cable should be considered to minimize EMI:

(a) Single conductor shielded- For sensitive signals and to minimize crosstalk between wires.

(b) Twisted pair- For audio frequency circuits grounded at a single point and for balanced signal and power circuits.

(c) Single or double shielded coaxial cables or triax cables- For high frequency pulses over long cable runs.
 
 

4.6.4 External shielding of harness assemblies- Copper braid is most effective for electrostatic shielding of harnesses carrying higher frequency circuits. Steel braid or steel tapes should be used in providing electromagnetic shielding for low frequency circuits, specifically, the dc and ac power circuits. The copper and steel braids and tapes may be used in combination. Shielding should be of the type that provides maximum coverage, with 85 percent as a minimum cover limit.

4.6.5 Isolation of harnesses- Harness assemblies carrying high level impulse signal, low level signal, and power circuits should be physically separated from each other by the maximum distance available under the design and space requirements. As a design objective, a minimum spacing of two inches should separate harnesses carrying circuits of varying signal levels. Where practicable, only those harnesses or harness segments carrying circuits of the same signal level group should be clamped under the same cable clamp. Only one end of the external shielding applied to the harness assembly should be grounded.

4.7 Grounding- The chassis of all electronic modules, major assemblies and instruments shall be electrically bonded to the structure. The resistance between any two assemblies is nominally less than 2.5 milliohm, and may be established by use of ground straps or wire between the assembly and structure.

4.8 Wire and cable- Wire and cable, as specified in 5.3, shall be used. The size of individual wires shall be a minimum of 24. Exceptions include power harnesses where the size of individual wires shall be a minimum of 22, and thermistor wiring where wire sizes smaller that 24 may be considered for use.

4.9 Conductors

4.9.1 Wire and cable- Wire and cable conductors shall be high conductive stranded copper or copper alloy. Wire sizes 22 and larger shall be constructed of copper. Wire size 24 and smaller shall be high strength copper alloy (HSC), for adequate termination strength and flex life. HSC wire should be checked for magnetic cleanliness.

4.9.2 Coaxial- Coaxial conductors shall be high conductive copper, copper alloy, or copper weld.

4.10 Lay of wire

4.10.1 Configuration R- Random lay has no pattern. The wires run at random, weaving in and out. Random lay is used when normal flexibility is required.

4.10.2 Configuration T- Twisted lay shall be employed where maximum flexibility is required.

4.11 Harness diameter- The diameter of the main body of a harness assembly shall be kept to a minimum, consistent with the requirements for routing, installation, and ease of handling. As a design objective, the maximum harness diameter should be limited to one inch.

4.11.1 Estimating harness diameter- This method of estimating harness size is based upon determining the units required for the number of wires of each diameter used, adding the number of units, then converting the total units to harness diameter. Using the nomographs included after this page, use these steps to estimate harness diameter:

(a) Determine nominal diameter of insulated wires or the average diameter of multiple shielded or twisted conductors.

(b) One of the nomographs converts wire diameter or average outside diameter to units for 10 wires; conversion to more or less than 10 is linear.

(c) The other nomograph converts total units to harness diameter.
 
 

4.12 Harness length- Harness length shall provide enough additional wire length for reworking the connection at least one time. Conductors connecting contacts within the same connector shall extend one to two inches from the rear of the connector.

4.13 Harness bend radius

4.13.1 Optimum bend radius- The optimum bend radius for harness Type I through Type IV should approach 10 times the outside diameter (OD) of the harness.

4.13.2 Minimum bend radius

4.13.2.1 Coaxial cable- The minimum bend radius for coaxial radio frequency cable or harness assemblies containing such cable shall not be less than six times the OD of the cable or harness.

4.13.2.2 Wire size larger than 10- The minimum bend radius for harness assemblies containing wire larger than size 10 shall not be less than six times the OD of the harness.

4.13.2.3 Wire size 10 or smaller- The minimum bend radius for harness assemblies containing only wire size 10 or smaller shall not be less than 3 times the OD of the harness.

4.14 Breakouts- The location of breakouts shall be specified on the design documentation.

4.14.1 Bend radius at breakout- Breakouts shall meet the bend radius requirements as specified in 4.13.

4.14.2 Support- Breakouts shall be secured immediately prior to their emergence from the bundle. Breakouts shall be long enough to provide proper support at installation. Breakouts shall be secured by connector backshells or clamps as close to the connector as practical.

4.15 Shielding- Shield braid constructed of copper, as specified in 5.4, or knitted wire mesh shall be used for custom shielding applications.

4.16 Shield terminations- Shields are normally terminated at only one end of the harness; however, an overall shield encompassing individual shields is usually terminated at both ends. Shields may be terminated with solder sleeves, or hand soldered.

4.16.1 Shields terminating in connector contacts- Shields that terminate in connector contacts using solder sleeves, or hand soldered with jumper wires, will use a 22awg jumper wire. If the connector contact size does not permit use of the recommended wire size, the jumper wire size shall be selected to accommodate the connector contact.

4.16.2 Shields terminating at connector shell- Shields should terminate to the connector shell through special connector adapters. Continuity to the connector shell is made by either screwing the adapter to the back end of the connector, or soldering the braid to the backshell.

4.17 Electrical connectors

4.17.1 General purpose connectors- Connectors not required to be mated or demated in space shall be as specified in 5.6.1.

4.17.2 Zero-G connectors- Connectors requiring mating and demating by an astronaut in space shall be as specified in 5.6.2.

4.17.3 Coaxial connectors- Coaxial connectors specified in 5.6.3 shall be used for terminating coaxial cables.

4.17.4 Special purpose connectors- Connectors to be used for Pyrotechnics shall be as specified in 5.6.4.

4.18 Connector protective covers

4.18.1 General purpose connector covers- All demated connectors shall have the mating surfaces protected by covers (see 5.7.1) during storage, handling, and installation of harnesses.

4.18.2 Zero-G connector covers- Zero-G connector covers shall be as specified in 5.7.2.

4.19 Connector sealing grommets- Wire sealing grommets on connectors and terminal junctions shall accommodate only one wire per grommet hole to ensure proper moisture sealing. If the wire diameter is smaller than the grommet hole was designed for, the wire OD shall be built-up by installing heat reactive tubing as required. Caution, the OD of the wire and shrink tubing must not exceed the capacity of the extraction tool.

4.20 Conductor terminations- The preferred method of terminating conductors is with a crimp termination. However, where necessary, solder terminations are acceptable. Conductor size shall not exceed the size of the terminal, contact, solder cup, crimp barrel, etc. Furthermore, conductors shall not be modified (reduced in size) to permit the use of undersized terminations.

4.21 Contact size- Contact size is determined by the connector body (shell) and shall not be modified in any way to accept oversized conductors.

4.22 Insulation sleeving- Unless insulation is integral with the termination, all terminations except those potted shall be covered with insulation as specified in 5.10.

4.23 Terminal junctions- Terminal junction devices specified in 5.11. may be utilized as electrical distribution and busing junctions.

4.24 Wire splices- Use of electrical wire splices for initial design and harness modifications shall be held to an absolute minimum. Splices shall not be located within three inches of a connector, and shall be staggered along the length of the harness. Splices shall not be located in the harness where undue stresses may be placed upon the wires terminated in the splice or adjoining wires upon installation of the harness assembly.

4.25 Connector potting- Connectors not having environmental sealing grommets shall be potted as specified in 6.6. Connectors with sealing grommets shall not be potted.

4.26 Connector location- Where practicable, connectors shall be so located as to permit visual inspection for proper mating and lock after installation.

4.27 Harness lacing or tying- Harnesses shall be secured with lacing tape or tie wraps.

4.28 Harness clamping- Harness movement shall be controlled by means of cable clamps or tie wraps and bases.

5.0 PARTS AND MATERIAL

5.1 General- Only the parts and materials specified herein shall be used.

5.2 Dissimilar metals- Dissimilar metals shall be defined and classified in accordance with MIL-STD-889. The joints of dissimilar metals in contact with each other shall be protected against electrolytic corrosion.

5.3 Wire and cable

5.3.1 Wire (unshielded)- Wire shall be in accordance with Specification MIL-W-22759, and GSFC Publication PPL-20.

5.3.2 Cable (shielded wire)- Cable shall be in accordance with Specifications MIL-C-27500 and MIL-STD-975F.

5.3.3 Coaxial cable- Coaxial cable shall be in accordance with the specification and RG numbers shown in GSFC PPL-20, except where special design considerations necessitate more stringent environmental or performance requirements. Where other cable is used, as a minimum, it shall comply with the environmental and performance requirements of specification MIL-C-17.

5.4 Shield braid- Copper shield braid for custom applications shall be in accordance with Specification QQ-B-575.

5.5 Solder sleeves- Solder sleeves shall be in accordance with Drawing MSFC-DWG-50M02239.

5.6 Electrical connectors

5.6.1 General purpose electrical connectors- General purpose connectors shall be in accordance with Specifications MSFC 40M39569, 40M38277, and GSFC S-311-P-4.

5.6.2 Zero-G electrical connectors- Zero-G electrical connectors shall be in accordance with Specification 40M39580.

5.6.3 Coaxial connectors- Coaxial connectors shall be in accordance with Specification MIL-C-39012, except where special design considerations necessitate more stringent environmental or performance requirements. When other connectors are used, as a minimum, they shall comply with the environmental and performance requirements of Specification MIL-C-39012.

5.6.4 Special purpose connectors- Connectors to be used in pyrotechnic circuits mating to the EED will be in accordance with Specification MSFC 40M38298.

5.7 Connector protective covers

5.7.1 General purpose connector covers- General purpose circular connector protective covers shall be in accordance with Specification 40M39569 and 40M38277. GSFC S-311-P-4 connectors shall use an ESD compatible, non-carbon filled protective cover.
Zero-G connector protective covers- Zero-G connector protective covers shall be in accordance with Specification 40M39580.

5.8 Solder- Solder shall be as specified in NHB 5300.4 (3A).

5.9 Flux- Flux shall be as specified in NHB 5300.4(3A-2).

5.10 Sleeving

5.10.1 General purpose insulation sleeving- Insulation sleeving shall be in accordance with Specification MIL-I-22129 or NASA Reference Publication 1124 rev 2.

5.10.2 Heat reactive insulation sleeving- Heat reactive insulation sleeving shall be in accordance with Specifications MIL-I-23053/ and meet the contamination requirements of the project.

5.11 Terminal junctions- Terminal junctions shall be in accordance with Specification 40M39589.

5.12 Wire Splices- Wires shall be spliced with single
junction devices, or a male/female connector contact pair held together with shrink tubing.

5.13 Connector Potting compound- Potting compound shall be in accordance with Materials Processing Document S-313-023.

5.14 Lacing tape- Lacing tape shall be in accordance with Specification MIL-T-43435, and GSFC Reference Publication 1124.

5.15 Convolute tubing and accessories- Convolute tubing and termination hardware shall be in accordance with MSFC Specification 40M51284.

6.0 PROCEDURES

6.1 General- Procedures specified herein are only to be used for design, and are not intended to be used as detailed manufacturing procedures.

6.2 Lay of wire

6.2.1 Twisted lay- When twisting is specified, it shall begin as close to the termination as practical without causing undue stress on the connector. All breakouts shall be twisted at least one and a half turns over its length, or it shall not be twisted.

6.2.2 Length of lay- The length of lay shall be 8 to 16 times the OD of the harness.
6.3 Shield termination- Shields shall be terminated by one of the following classes:

Class 1 - Solder, pig tail, nominal shield coverage

Class 2 - Solder, pig tail, maximum shield coverage

6.3.1 Class 1 - Shield termination- The outer insulation and shield shall be removed approximately two inches from the end of the wire. The outer jacket shall have an opening made by cutting the jacket radially and sliding the jacket towards the end of the wire. An opening of approximately 1/4 to 5/16 inch shall be made, staggering the openings on adjacent wires from 1 inch to 4 inches from the ends of their shields. The shield shall be terminated by soldering a wire to the exposed braid. The braid opening and shield termination shall than be cleaned and insulated by used of heat reactive tubing.

6.3.2 Class 2 - Shield termination- Shall be terminated as in Class 1, except the shield will extend to within 1/4 inch of the connector back. The shield termination will be from 1 to 1 1/2 inches from the end of the shield. Shield termination wire length shall be kept to a minimum, with the absolute maximum length of 2 inches.

6.3.3 Shield Jumper Wire Termination- Jumper wires may be carried through a connector contact individually or gathered in stub splices and connected to a contact or to the connector backshell hardware.

6.3.3.1 Stub splice common jumper wire termination- Jumper wires of individual cable shields of a harness shall be terminated in stub splices. Not more than five wires, including jumper wires, shall be terminated in one stub splice.

6.3.4 Floating shields- The shields shall be floated by folding back over the conductor insulation and covered by heat shrink tubing.

6.4 Crimp connections- Crimping shall comply with the requirements of NHB 5300.4(3H).

6.5 Solder connections- Soldering shall be in accordance with NHB 5300.4(3A-2).

6.6 Connector potting- Connector potting shall be in accordance with Document GSFC S-313-023.

7.0 QUALITY ASSURANCE

7.1 Compliance- The supplier is responsible for the performance of all inspection requirements as specified herein. GSFC reserves the right to monitor fabrication and inspection of harness assemblies to insure compliance with the requirements of this standard. Inspection records of the tests shall be kept complete and available to GSFC.

7.2 Visual inspection- Harnesses shall be visually inspected to insure compliance with the following requirements:

(a) Applicable harness design documentation

(b) Materials

(c) Design and construction

(d) Identification of component

(e) Workmanship

7.3 Functional test- The following test shall be conducted on each harness assembly manufactured to this specification prior to submission for GSFC inspection and acceptance. In conducting functional test on harness assemblies, hand probes shall not be used directly in harness connectors. Adapter cables or breakout boxes shall be used. On potted connectors the functional test shall be performed prior to connector potting.

7.4 Continuity- Harness assemblies shall be tested for point to point electrical continuity in accordance with the applicable wiring diagrams or wire lists. Measurements shall be taken with a milliohm meter and recorded on either the wiring diagrams or wire lists. Values, within a connector bundle of the same conductor size and wire length, shall be consistent within 10%.

7.5 Insulation resistance- The insulation resistance between each conductor and every other conductor, each conductor and shields, and between each conductor and connector shell shall be greater than 100 megohms at an applied voltage of 500Vdc for a maximum of one minute.

7.6 Contact retention- Every contact of every connector shall be push tested to verify that the contact is fully seated and locked into the connector. The contacts should be pushed to a minimum value of 6.5 lbs but not to exceed 7.0 lbs for the 22 and 20 awg contacts. Larger contacts should be tested to the value specified in table 9.1 of NHB 5300.4(3G).

8.0 CONTAMINATION CONTROL

8.1 Fabrication- All braid will be ultrasonically cleaned with an approved solvent before being incorporated into the harness. All wire and cable will be wiped with a clean lint free cloth and an approved solvent before being incorporated into the harness.

8.2 Certification- After harness fabrication is complete and certified, the complete harness will be cleaned with an approved solvent, inspected with both black and white light and vacuum baked at a temperature of 20 C above maximum environmental test temperature. The bakeout will continue until a chamber pressure of 1X10^-6 Torr is reached and the QCM requirements have been fulfilled.

9.0 RECOMMENDATIONS FOR AN IMPROVED ELECTRICAL HARNESS

There are a variety of harness design and fabrication practices that are deemed extremely appropriate for flight harnesses, but lie outside of what are considered minimum requirements (as defined in sections 1 through 8). This section serves to encapsulate these practices, which have been learned through the experiences of many individuals over many years. It is expected that this section, more so than any other in this standard, will be a "living" section.

9.1 Connectors

9.1.1 When selecting connectors for bulkhead use and where access and visibility is limited, connectors that can be mated and locked without additional hardware are preferred.

9.1.2 When "D" type connectors have been selected for use between major module assemblies and applications such as; turn-on connectors, arming connectors, etc., screw retaining hardware should be replaced, torqued, and staked at the time of the final mating. This should help prevent hardware loss and major schedule impacts while disassembling the spacecraft to retrieve hardware.

9.1.3 When selecting "D" type connectors, shell size 4 is the least preferred due to the long profile and tendency to roll during demating.

9.2 General Design Practices

9.2.1 Harness securing style A (continuous lace), as noted in section 1.2.2, is not an often practiced method due to the slippage that can occur in the lacing.

9.2.2 When a single connector is to be used for more than one type of signal, or for both power and signals, separate the signal types as far apart from each other and the power as possible.

9.2.3 Try to influence the subsystem engineer during package design and connector layout so that the used pins are equally distributed across the connector. If necessary, use unwired contacts to balance the contact density. This is to help prevent the tendency for the connector to roll during demating, bending the male contacts at the wired end of the connector.

9.2.4 Female connectors should be used in any application that could have a significant voltage on any contact when the connector is demated.

9.2.5 For calculating harness weight, a one inch diameter unshielded cable will weigh approximately one lb/ft. Furthermore, a two inch diameter cable will weigh approximately 4.25 lbs/ft, and a three inch diameter cable will weigh approximately 10.5 lbs/ft.

9.3 EMI/EMC Related Practices

9.3.1 Although it is normally stated that shields are terminated at the source end only, it is easier at time of fabrication to terminate both ends and then disconnect one end if noise problems dictate. However, this is an electrical systems issue, and harness designers need to adhere to the overall electrical system design guidelines, if any, as provided by the appropriate system engineers.

9.3.2 Source end only shield termination is normally used to reduce low frequency noise interference. Terminating both ends of a shield is normally used to reduce higher frequency (RF) interference.

9.3.3 If Ferrite beads are used on the harness for RF noise reduction, they should be potted or staked to prevent damage to the harness insulation.

9.3.4 Penetrations into an RF tight Spacecraft should be made by means of filter pin connectors or braided cable with RF backshells.

9.4 Fabrication Recommendations

9.4.1 Do not load up the unused cavities of the larger sizes of the high density connectors with contacts. The insertion/extraction force of a connector contact pair can vary between 2.5 and 12 oz. The mating/demating force for a pair of 104 pin connectors could be as high as 75 lbs. Packages must be designed to handle these forces or contacts screened and selected to fall in the low end of the insertion/extraction force curve.

9.4.2 If a contact will not fully seat into the connector try using the contact extraction tool without a contact to size the retaining clip.

9.4.3 When trying to remove a contact with a shielded wire or coax attached and the amount of wire is too short to allow the tool to be "backed" on, cut a small V in the tip of the plastic tool. This will allow the tool to be directly inserted on the contact.

9.4.4 After torquing, connector hardware should be staked with an approved epoxy. The staking will be an indication that the hardware was torqued and that it has not been disturbed.

9.4.5 When potting non-environmental type connectors, mating connectors must be used to maintain pin alignment.

9.4.6 When potting non-environmental type connectors, the potting material must be thick enough to prevent it from flowing into the connector pin cavity.

9.4.7 When crimping a wire whose gauge is smaller than the connector contact is rated for, find the CMA for the contact and the wire to be used. Subtract the wire CMA from the contact CMA, find a wire or wires from Table 4 that matches the difference and use this combination of wires as a "slug" to crimp with the wire to be used. Once the "slug" and wire have been crimped, the slug and wire must be covered with shrink tubing. Test samples should be pulled and inspected before doing the actual flight operation.

9.4.8 For applications where disconnects are useful, such as thermistors and heaters, a mating pair of connector contacts, crimped and insulated with Kynar shrink tubing, may be used. Pull tests should be performed on equivalent samples to verify the wire breaks before the pins separate.

*Note: Selecting wire with the largest number of strands provides the greatest flexibility.

9.4.9 Crimp schedules should be generated for every wire/contact combination. All crimp tools being used for that program should be verified against the crimp schedule.

9.4.10 When using High Strength Copper Alloy wire, crimp schedules must be generated since the wire is of a much harder material.

9.4.11 To generate a crimp schedule, make 5 samples of a wire/contact combination at the number specified on the contact positioner. Additionally, make 5 samples each at one number above and one number below the number specified on the contact positioner. Pull each sample, keeping a record of each pull value and whether the wire broke or pulled out. Plot each of the crimp settings and select the one that is most consistent and where the wires broke outside of the crimp area, or pulled out of the contact.

9.4.12 Teflon wire must be screened before use. As a minimum a Physical Verification Test (PVT) and a Differential Scanning Calorimetry (DSC) test must be performed. The DSC, although not normally performed by the manufacturer, will indicate whether the insulation material was fully sinitered during manufacturing. This test measures the melting point of the insulation. If the wire has not been fully sinitered, the insulation will show two melting points rather than one. Oversinitering, curing the insulation at too high of a temperature, will show only one melting point but at a temperature higher than a properly sinitered wire. Undersinitering and oversinitering has been shown to be a factor in insulation cracking. DSC will only show if a wire has been undersinitered. In order to show oversinitering, additional testing such as wrap back and spark testing are necessary.

9.4.13 When terminating Gore coax using the window technique, the center conductor and insulator are so limp they must be supported with shrink tubing. A technique successfully used is to cut and remove the outer jacket and shield approximately 1/2 inch from the end of the wire. Push back the center insulator or heat the end of the insulation with a heat gun which will cause the insulation to shrink allowing the contact to be crimped on the exposed wire. Cover the center insulator with Kynar shrink tubing. This will provide support preventing the center conductor and insulator from collapsing. Cut the outer jacket between 1/2 and one inch from the end of the jacket/braid, slide the outer jacket over the shrink tubing opening a window of approximately 1/4 inch. Terminate the shield, keeping the shield wire to the shortest length possible, and insulate the shield termination with shrink tubing.

9.4.14 After cleaning the shield terminations of Gore coax cables that have a wrapped inner insulation, make sure that the alcohol has dried before performing insulation resistance tests. If the alcohol has not dried, leakage will be indicated by the test.

9.4.15 Use of tie wraps with plastic locking features is not recommended due to slippage that can be caused by thermal relaxation. Tie wraps with metal locking features have proven to be more reliable.

9.5 General Recommendations

9.5.1 Connector savers should be installed on every package connector, and not removed until the package has been integrated and ready for final mate to the flight harness.

9.5.2 Never probe directly on a flight connector. Use a connector saver or a breakout box.

9.5.3 A mate/demate log should be maintained once the harness has been delivered to I&T. Connector inspections should be instituted once the connector mate/demate count reaches 15, and reinspected every 10 mate/demates thereafter. These inspections should be performed by Quality Assurance personnel.

9.5.4 When demating "D" connectors, a demating tool or other mechanical aid should be used to insure the connectors remain parallel during demating.

9.5.5 Circular connector backshells should be torqued to the values shown in Table 5.

*Note: The information in the table below is based upon data in MIL-C-85049, Paragraph 4.6.12, Table 3, as well as applicable M85049 slash (/) sheets covering self-locking coupling nuts. The values in the table apply to lubricated threads tightened properly without excessive pressure to squeeze them out of round

(Self-locking torque values are 80% of non-locking.)