SSP-57003_Rev_L.docx

External Payload Interface Requirements Document

International Space Station Program

Revision L

September 2015

Type 4

EXPORT CONTROLLED - The technology or software is subject to the Export Administration Regulations (15 C.F.R. Parts 730-774). Export, re-export or retransfer contrary to U.S. law is prohibited. EAR99

National Aeronautics and Space Administration International Space Station Program Johnson Space Center Houston, Texas Contract No. NAS15−10000 CAGE Code 2B945

SSP 57003

Revision L

REVISION AND HISTORY page

IRN 0001 per SSCD 004089, EFF. 11−08−00 Incorporates the following PIRNs: 57003−NA−0001C, 57003−NA−0003A, 57003−NA−0004, 57003−NA−0005, 57003−NA−0006B, 57003−NA−0007B, 57003−NA−0008A, 57003−NA−0009, 57003−NA−0010A 06-21-01

Incorporates the following PIRNs: 57003−NA−0002A, 57003−NA−0012B, 57003−NA−0013C, 57003−NA−0014B, 57003−NA−0015D, 57003−NA−0016B, 57003−NA−0017C, 57003−NA−0018A, 57003−NA−0019A, 57003−NA−0023, 57003−NA−0024A, 57003−NA−0026D, 57003−NA−0027A, 57003−NA−0028 08-30-02

Incorporates the following PIRNs: 57003−NA−0035F, 57003−NA−0036, 57003−NA−0037, 57003−NA−0038B, 57003−NA−0039C, 57003−NA−0042A, 57003−NA−0043 This is the first release on the IPIC contract.

09-08-03

Incorporates the following PIRNs: 57003−NA−0032, 57003−NA−0051A, 57003−NA−0052A, 57003−NA−0054, 57003−NA−0047D, 57003−NA−0050B, 57003−NA−0056, 57003−NA−0064, 57003−NA−0031, 57003−NA−0033A, 57003−NA−0040, 57003−NA−0041, 57003−NA−0045, 57003−NA−0048, 57003−NA−0057 Incorporates PIRN 57003−NA−0046E per SSCN 003928.

08-29-06

Incorporates the following PIRNs: 57003−NA−0044A, 57003−NA−0049, 57003−NA−0050B, 57003−NA−0053A, 57003−NA−0055B, 57003−NA−0058B, 57003−NA−0062, 57003−NA−0063, 57003−NA−0065B, 57003−NA−0066 07-21-08

Incorporates the following PIRNs: 57003-NA-0067, 57003-NA-0068F, 57003-NA-0069, 57003-NA-0070, 57003-NA-0072D, 57003-NA-0073, 57003-NA-0074, 57003-NA-0078, 57003-NA-0080B, 57003-NA-0082, and 57003-NA-0083.

07-31-09

Incorporates the following PIRNs: 57003-NA-0071C, 57003-NA-0084, 57003-NA-0085A, 57003-NA-0086, 57003-NA-0087, 57003-NA-0088, 57003-NA-0089, 57003-NA-0090A, and 57003-NA-0091A.

06-24-10

Incorporates the following PIRNs: 57003-NA-0093A, 57003-NA-0094C, 57003-NA-0095A, 57003-NA-0097A, 57003-NA-0099, 57003-NA-0100, and 57003-NA-0101.

PIRN 57003-NA-0092B was incorporated into Rev. F but inadvertently omitted from the Revision/History Page.

07-18-11

Incorporates the following PIRNs: 57003-ELC-0030A, 57003-NA-0098B, 57003-NA-0102, 57003-NA-0103, 57003-NA-0104, 57003-NA-0105, 57003-NA-0106B, 57003-NA-0107A, 57003-NA-0108, 57003-NA-0109 02-03-12

Incorporates the following PIRNs: 57003-NA-0096D, 57003-NA-0110, 57003-NA-0111, 57003-NA-0112, 57003-NA-0113, 57003-NA-0114, 57003-NA-0115A, 57003-NA-0116, 57003-NA-0117 and 57003-N/A-0125 SSCN 012713 is also being incorporated at Rev J

Incorporates the following PIRNs: 57003-NA-0118, 57003-NA-0119A, 57003-NA-0120, 57003-NA-0121B, 57003-NA-0122A, 57003-NA-0123A, 57003-NA-0124, 57003-NA-0126A, (SSCN 013672 and SSCN 012443), 57003-NA-0127, 57003-NA-0128, 57003-NA-0129A, 57003-NA-0130, 57003-NA-0131A, 57003-NA-0132, 57003-NA-0133A, 57003-NA-0134A, 57003-NA-0135A, 57003-NA-0137, 57003-NA-0138A, 57003-NA-0140, 57003-NA-0141, and 57000-NA-0378 SSCN 012607 was Closed with PIRN 57003-NA-0102 at Rev H.

11-11-14

Incorporates the following SSCN 013870, New ISS Label Approval/Change Process and Form, is addressed by the updated of Appendix D; and SSCN 014480, Simplified Payload Interface Requirements, is addressed by the release and completion of Rev L.

Revision L is a total document update. The update to the document name from “Attached Payload IRD” to “External Payload IRD” was made by the RISE Team in an effort to include all types of unpressurized payloads, including Deployable Payloads, which are not externally “attached” to the ISS. No PIRNs were used for any part of this update.

03-08-16 eRU: /s/Beth Mason 3-8-16

INTERNATIONAL SPACE STATION program

External Payload Interface Requirements Document

PREFACE

This document defines sets of interface requirements for compatibility of External Payloads with the International Space Station (ISS). Payloads launching in the pressurized volume of the transportation vehicle must address a subset of requirements from SSP 57000, Pressurized Payloads Interface Requirements Document. This document is under the control of the Space Station Program Control Board (SSPCB). Any changes or revisions to this document will be delegated to the Vehicle Control Board (VCB) for approval.

DOCUMENT CONCURRENCE

TABLE OF CONTENTS

appendices

SSP 57003 September 2015 v Export Controlled – see Title Page Introduction Purpose This Payload Interface Requirements Document (IRD) is the principle source of interface design requirements for all National Aeronautics and Space Administration (NASA) developed External Payloads on the ISS. Payload developers must verify the applicable requirements in this document to ensure the safety of the ISS crew, transport vehicles, on-orbit ISS systems hardware, and neighboring payloads. This document also provides design guidance that ensures the basic operation of the payload and affects the payload’s mission success. It is the responsibility of the payload developer to design in accordance with the design guidance.

Scope This document applies to all External Payloads attached to the ISS trusses or ELC, interfacing with the MCAS, SSRMS or SPDM, and Deployable Payloads deployed from or in the vicinity of ISS and those External Payloads that launch in the Pressurized environment and exit the ISS through the Japanese Experiment Module (JEM) Airlock. This document does not contain requirements for interfaces unique to the Columbus EPF and JEM-EF. The specific launch vehicle IDD should be used for applicable payload transport requirements. Payloads that pass through the JEM Airlock will also have requirements from SSP 57000, Pressurized Payloads Interface Requirements Document applied.

US Payloads that utilize the Japanese Experiment Module – Exposed Facility (JEM-EF) and the Columbus will have additional requirements levied on them by JAXA and ESA in accordance with NASDA-ESPC-2900, JEM Payload Accommodation Handbook Vol. 3 Exposed Facility/Payload Standard Interface Control Document, and COL-RIBRE-SPE-0165, External Payloads Interface Requirements Document.

Use This document provides interface design and verification requirements on ISS External Payloads and also provides interface design guidance necessary for the successful operation of the payload. The payload developer must also provide data deliverables applicable to their payload’s design. These requirements, guidelines, and data deliverables are allocated to a payload through the payload unique ICD. The payload developer and the ISS Program must jointly agree on the interfaces and identify requirements as applicable if the interface exists, and not applicable if the interface does not exist for the payload configuration documented in the ICD. This applies to physical and environmental interfaces. For requirements where exceptions are required because the payload will not meet an applicable requirement, the payload developer must provide an exception request to the ISS Program for evaluation. Implementation of design guidance is the responsibility of the payload developer as part of the payload’s internal development and verification campaign.

All payloads must assess the general requirements in Sections 3.0 and 4.0. Depending on their on-orbit configuration, a payload will have to assess requirements from at least one of the appendices.

Each appendix has a Section 3 and Section 4, with requirements/guidelines and verification respectively, but with the appendix letter appended in front (for example, F.3.5.1 and F.4.3.5.1).

Control and Maintenance This IRD is controlled through the authority of the Vehicle Control Board (VCB). Changes to this IRD are made using the Preliminary Interface Revision Notice (PIRN) process. For the purposes of Certification of Flight Readiness (CoFR), PIRNs are applicable when the final signature is received from NASA.

SSP 57003 September 2015

1-1 Export Controlled – see Title Page Documentation Applicable and reference documents can be found in Section 5.0.

Interface Requirements This document provides requirements, guidance, and information to be used by the payload developer to successfully integrate a payload into the ISS systems.

This IRD includes several informational pointers to requirements in SSP 51700, Payload Safety Policy and Requirements for the International Space Station. These requirements are handled through the safety process. This IRD does not include a complete set of those safety requirements.

Verification of Program Furnished Equipment by the Payload Developer is not required. However, the integrated experiment assembly, using Program Furnished Equipment, does need to be verified.

Responsibilities of the payload developer are allocated as follows:

· “Shall” is intended to express a provision that is mandatory to ensure ISS systems, crew and other payloads are protected. Payload developers are expected to (1) meet these requirements found in both Section 3.0 of the main volume and in Section 3.0 of the appropriate appendices and (2) submit verification to the Program as described in Section 4.0 of the main volume and in Section 4.0 of the appropriate appendices.

· “Should” is used to denote a guideline. Guidelines are provisions intended to ensure compatibility with a provided interface or service. The Program does not collect verification or process exceptions from the payload developer for these guidelines. Payload developers are expected to take the applicable guidelines into account during design to ensure the highest probability for mission success of the payload. In addition to affecting mission success, these guidelines also are put in place to make maximum use of critical Program resources such as crew time and upmass. Failure to meet the guidelines will inherently incur risk that may or may not be tolerable to the funding organization and may have adverse impacts on critical Program resources. In some cases, the funding organization, the payload developer, and the ISS Research Integration Office may agree to elevate a “should” guideline to a “shall” requirement. This will be documented in the payload’s unique requirement set, and the payload developer will be required to submit verification to the Program for these items. Each guideline has an associated verification submittal statement that is intended to assist the payload developer with a recommended method for verification. The “Verification Submittal” associated with the guideline verification is used only in the case when a guideline is elevated to a requirement.

· Data Deliverable specifies information that is mandatory for the payload developer to provide to the ISS Program. This information is used for system integration analyses to ensure the functionality of ISS systems and the compliance with system-level requirements, the appropriate use of ISS resources, and to ensure authorizations have been obtained from selected Program offices such as the Radio Frequency Manager and ISS Payload Label Approval Team (IPLAT).

· “Must”, “will”, “may”, and so on, are associated with informational statements only and do not imply any additional verification be provided to the Program.

Structural/Mechanical and Microgravity Interface Requirements This section defines the structural/mechanical and microgravity requirements between ISS Payloads and the ISS locations that may be used for Payloads.

Fracture Control Structural design requirements are addressed in SSP 51700, Section 3.9.1, Structural Design.

Meteoroid and Orbital Debris Protection Requirement for Payloads Meteoroid and orbital debris protection are addressed in SSP 51700, Section 3.9.4.6, Meteoroid and Debris Protection.

Extravehicular Activity On-Orbit Induced Loads External components of Payload hardware exposed to Extravehicular Activity (EVA) crew shall maintain positive margins of safety for the loads defined in Table 3.1.3-1, Extravehicular Activity Induced Limit Loads.

TABLE 3.1.3-1 EXTRAVEHICULAR ACTIVITY INDUCED lIMIT LOADS (2 Pages)

4200 in·lb moment

Notes:

EVA on orbit induced loads for inadvertent kick and kickoff, pushoff loads do not apply to hardware or worksites which are assembled or maintained using robotic system.

Structural Materials Criteria and Selection Mechanical properties and selection of Payload structural materials shall be in accordance with SSP 52005, Section 5.4.1, Allowable Mechanical Properties of Structural Materials.

Note: Stress Corrosion is addressed in SSP 51700, Section 3.9.3, Stress Corrosion.

Interface Tolerance User-provided on-orbit installed payload interfacing hardware shall be designed to interface with the worst case tolerance stack up between parts and temperature differentials during installation.

Note: This requirement does not apply to the FRAM interface (i.e. the interface between the Active and Passive FRAM); it is intended for payloads that have mating pieces.

Structural/Mechanical Mechanical Stop Design

Microgravity Microgravity requirements limit the disturbing effects of Payloads on the microgravity environment. The requirements are separated into the quasi-steady category (frequencies below 0.01 Hz), the vibratory category (frequencies between 0.01 Hz and 300 Hz), and the transient category. For Payloads, the microgravity payload interface is between the payload and its attachment point.

Limit Quasi-Steady Accelerations For frequencies below 0.01 Hz, Payloads shall limit unbalanced translational average impulse to less than 10 lb·s (44.5 N·s) within any 10 to 500 second period, along any ISS coordinate system vector.

Limit Vibratory and Transient Accelerations Vibratory Requirements For frequencies between 0.01 Hz and 300 Hz, payloads shall limit produced forces to the values shown in Figure 3.1.7.2.1-1, Microgravity Acceleration Contribution Limit for a Payload and Table 3.1.7.2.1-1, Microgravity Acceleration Contribution Limit Values for a Payload are not exceeded using simultaneously the force transfer functions of Figures 3.1.7.2.1-2, One-Third Octave Narrow-Band Force Transfer Functions for Payloads, and 3.1.7.2.1-4, One-Third Octave Wide-Band Force Transfer Functions for Payloads, (Tables 3.1.7.2.1-2, One-Third Octave Narrow-Band Force Transfer Functions for Payloads, and 3.1.7.2.1-4, One-Third Octave Wide-Band Force Transfer Functions for Payloads), and the moment transfer function of Figures 3.1.7.2.1-3, One-Third Octave Moment Narrow-Band Transfer Functions For Payloads, and 3.1.7.2.1-5, One-Third Octave Moment Wide-Band Transfer Functions For Payloads, (Tables 3.1.7.2.1-3, One-Third Octave Moment Narrow-Band Transfer Functions For Payloads, and 3.1.7.2.1-5, One-Third Octave Interface Wide-Band Moment Transfer Function Values For Payloads).

FIGURE 3.1.7.2.1-1 MICROGRAVITY ACCELERATION CONTRIBUTION LIMIT

FOR A PAYLOAD

TABLE 3.1.7.2.1-1 MICROGRAVITY ACCELERATION CONTRIBUTION LIMIT VALUES FOR A PAYLOAD

Freq (Hz) micro-g

Freq (Hz) micro−g

Freq (Hz) micro−g

FIGURE 3.1.7.2.1-2 ONE-THIRD OCTAVE NARROW-BAND FORCE TRANSFER FUNCTIONS FOR PAYLOADS

TABLE 3.1.7.2.1-2 ONE-THIRD OCTAVE NARROW−BAND FORCE TRANSFER FUNCTIONS FOR PAYLOADS

FIGURE 3.1.7.2.1-3 ONE-THIRD OCTAVE MOMENT NARROW-BAND TRANSFER FUNCTIONS FOR PAYLOADS

TABLE 3.1.7.2.1-3 ONE-THIRD OCTAVE MOMENT NARROW-BAND TRANSFER FUNCTIONS FOR PAYLOADS

FIGURE 3.1.7.2.1-4 ONE-THIRD OCTAVE WIDE-BAND FORCE TRANSFER FUNCTIONS FOR PAYLOADS

TABLE 3.1.7.2.1-4 ONE-THIRD OCTAVE WIDE-BAND FORCE TRANSFER FUNCTIONS FOR PAYLOADS

FIGURE 3.1.7.2.1-5 ONE-THIRD OCTAVE MOMENT WIDE−BAND TRANSFER FUNCTIONS FOR PAYLOADS

TABLE 3.1.7.2.1-5 ONE-THIRD OCTAVE INTERFACE WIDE-BAND MOMENT TRANSFER FUNCTION VALUES FOR PAYLOADS

Transient Requirements

Angular Momentum Limits Limit Disturbance Induced ISS Attitude Rate Payloads with individual rotating parts with a mass greater than 2 kg or with rotating parts that gimbal shall limit any non-transitory disturbance (disturbance duration greater than 10 seconds) induced on the on-orbit Space Station by an individual disturbance source to an angular momentum impulse of less than the per axis values shown in Table 3.1.7.3.1-1, Maximum Angular Momentum Impulse, during any continuous nine minute period. Any non-transitory disturbance source that generates a total angular momentum impulse less than 100 ft-lb-sec (135 N-m-sec) in a 110 minute period can be ignored with respect to this requirement.

TABLE 3.1.7.3.1-1 MAXIMUM ANGULAR MOMENTUM IMPULSE

Note:

Where Hx, Hy and Hz are the absolute values of the x, y, and z components of the disturbance angular momentum relative to the Assembly Complete center of mass, XISS = -17.66 ft (-5.38 m); YISS = -1.32 ft (‑0.40 m); and ZISS = +14.80 ft (+4.51 m) in the Space Station Analysis Coordinate System, as defined in SSP 30219, Space Station Reference Coordinate Systems, Figure 4.0-1.

Limit Disturbance Induced CMG Momentum Usage Payloads with individual rotating parts with a mass greater than 2 kg or with rotating parts that gimbal shall limit any disturbance (non-transitory or transitory) induced on the on-orbit Space Station by an individual disturbance source to an angular momentum impulse that produces an estimated Control Moment Gyroscope (CMG) momentum magnitude less than 10,000 ft-lb-sec (13,558 N-m-sec) during any continuous 110 minute period when evaluated per expression in Table 3.1.7.3.2-1, CMG Momentum Usage Calculation. Any non-transitory disturbance source that generates a total angular momentum impulse less than 100 ft-lb-sec (135 N-m-sec) in a 110 minute period can be ignored with respect to this requirement.

TABLE 3.1.7.3.2-1 CMG MOMENTUM USAGE CALCULATION

Estimated CMG Momentum Usage ft·lb·sec

N·m·sec

ISS-Provided External Wireless Communication (EWC) The External Wireless system provides a Commercial Off The Shelf (COTS) solution for Payloads requiring external high data rate 802.11n wireless capability. For more details on External Wireless Communication (EWC) on ISS, please see Appendix L, Section L.3.5.2, External Wireless Communications User Requirements.

Interface with Space Station Provided External Wireless Antenna The envelope for the External Wireless Antenna is shown in Figure 3.1.8.1.1-1, External Wireless Antenna Envelope.

External Wireless Antenna Attachment Hole Pattern The External Wireless Antenna should be attached on the payload structure using the hole pattern as defined in Figure 3.1.8.1.1-1.

Note: Mount the antenna with the arrow pointed in the zenith direction.

Note: Dimensions are in inches. Attachment holes are the eight (8) outer holes in the above pictures. Only four (4) of the eight holes will be utilized for mounting of the antenna, per Section 3.1.8.1.2.

Figure 3.1.8.1.1-1 External Wireless ANTENNA ENVELOPE External Wireless Antenna Attachment Screws The External Wireless Antenna should be attached to the payload with four (4) NAS1351N ¼ inch-28 screws at the corners of the antenna in steel inserts.

Note: The mounting plate has eight (8) holes, but only four (4) of the holes will be utilized for mounting of the antenna.

Torque The External Wireless Antenna attachment screws should be torqued to 48 in-lbs maximum above running torque if lubricant is used or 110 in-lbs maximum above running torque if no lubricant is used.

Mounting Surface The External Wireless Antenna mounting surface on the payload should be alodined.

Grounding The Ground Plane for the payload antenna is the 9 inch x 9 inch aluminum plate to which the antenna is attached by 10 screws. The antenna pattern requirement for the Ground Plane is that it must have a normalized radius that is equal to or exceeds a value of 10.

Antenna characteristics are as follows:

Frequency > 5 GHz Wavelength λ = 6 cm Minimum Ground Plane radius a = 11.43 cm The normalized Ground Plane radius ε = 2·π·a/λ = 11.969 The normalized Ground Plane radius is therefore greater than 10, and the ground plane is sufficient.

External Wireless Antenna Thermal Protection The External Wireless Antenna installation should include 3 layers of non-aluminized beta cloth per STM0484-02 (Fabric, Beta, Glass, PTFE Coated, Silicone Free).

Securing of Threaded Fasteners Threaded fasteners that perform a safety-critical function shall incorporate two separate verifiable locking features. (Reference SSP 52005, Section 5.6, Fastener Requirements) Note: Fasteners that are not safety-critical, are internal to payload components, and are shown to be contained are excluded from this requirement.

Electrical Interface Requirements Power Bus Isolation

International Space Station Electrical Power System (EPS) Circuit protection Characteristics

1. The Payload connected to Interface C (defined in Figure 3.2.2-2, Interface C, including payload sites and robotic interfaces) circuit protection device shall be designed to provide trip coordination, i.e., the downstream circuit protection device disconnects a shorted circuit or an overloaded circuit from the upstream power interface, without tripping the upstream circuit protection device. The trip coordination is achieved either by a shorter trip time or lower current limitation than the upstream protection devices described in Table 3.2.2-1, Detailed Upstream Protection Characteristics, Table 3.2.2-2, PVGF to User Electrical Interface Parameters, and Figure 3.2.2-1, ITS S3/P3 and MCAS Overload Protection Characteristics.

Note: Current limiting protection devices start to limit the current when the current reaches the limiting threshold. The shaded regions in the figures show the current limit regions from the time the protection devices start to control the current within the specified range to the maximum time where the protection device trips and interrupts the current flow. Nominal current ratings are 25 amperes. The current at the S3/P3 APPI will be controlled to within the limiting level of 27.5 to 30 amperes within 1 millisecond. The current at the MCAS power interface will be controlled to within the limiting level of 13.2 to 14.4 amperes within 1 millisecond. The Remote Power Controller (RPC) will trip if the current remains in the limiting region up to the decision time of 34.5 ±3.5 milliseconds.

Overcurrent protection shall be provided at all points in the system where power is distributed to lower level (wire size not protected by upstream circuit protection device) feeder and branch lines in compliance with Table 3.2.2-3, Fuse Derating, Table 3.2.2-4, Circuit Breaker Derating, Table 3.2.2-5, Ambient Operating Temperature for Derating Relays and Switches, Table 3.2.2-6, Cycle Rate per Hour for Derating Relays and Switches, and Table 3.2.2-7, Load Application Rate for Derating Relays and Switches.

Data Deliverable: Provide the overload protection characteristics curve for the Payload using the same format as Figure 3.2.2-1 for up to 50 milliseconds.

Note: EOTP and OTCM electrical requirements are provided in 3.7.3, External Equipment Requiring SPDM Support Interface Requirements.

Table 3.2.2-1 detailed upstream protection characteristics

POWER

INTERFACE

*Trip decision time at or above limiting/trip threshold (27.5 A to 30.0 A at S3 and P3 APPI and 13.2 A to 14.4 A at MCAS power interface).

TABLE 3.2.2−2 PVGF TO USER ELECTRICAL INTERFACE PARAMETERS

(See Notes: 2 to 5)

User P/L w/15 ft PVGF harness on

SSRMS

User P/L w/30 ft PVGF harness on

SSRMS

User P/L w/15 ft PVGF harness on

SPDM

User P/L w/30 ft PVGF harness on

SPDM

Notes:

1. Protection is equivalent with SSP 30263:002, Type II or Type VI RPCM Standard ICD, depending on MSS element base location.

2. Minimum voltage includes volt drop across the PVGF harness

3. Although the min. steady state voltage case is emphasized in this table, the same voltage drops can also be applied to transient voltages as defined in SSP 30482, Volume 1.

4. The Voltage at the User Interface does not include a voltage drop for a user supplied connector.

5. To calculate the minimum voltage at the User Interface for current draws less than the maximum, interpolate between the Interface C minimum voltage of 113 Vdc at 0 amps and the minimum voltage (at maximum current) specified in this table for the appropriate configuration.

6. If a user payload is strictly tied to only one SSRMS payload power string, then the maximum current allowed could be increased to 23.3 A with a minimum voltage of 105.5 Vdc for the 15 ft harness and 104.7 Vdc for the 30 ft harness.

7. For the SPDM, only one bus/feed of power is available at a time. Current is shared between all payloads attached to the LEE, OTCMs, and EOTP PFRAM sites. The maximum combined current for all SPDM payloads is 8.2 A.

figure 3.2.2-1 its s3/P3 and MCAS overload protection characteristics

Figure 3.2.2-2 Interface C Table 3.2.2-3 Fuse Derating

Rating at 25 °C ambient. Derating of fuses allows for loss of pressure, which lowers the blow current rating and allows for a decrease of current capability with time.(1)(3)

Fuse Derating Example:

The principal stress parameter is current:

A board expected to be operating at 90 °C ambient has a calculated maximum current of 1.0 A. The additional derating required due to temperature is calculated as shown:

The total derating factor is calculated as follows:

The fuse rating is calculated as shown:

A fuse with rating equal to or greater than 5.7 A is suitable in this circuit.

Table 3.2.2-4 Circuit Breaker Derating

Circuit breaker contacts are derated by multiplying the maximum rated contact current (resistive) by the appropriate contact derating factor Note:

(1) Use series resistance to ensure that circuits do not exceed the derated level.

Circuit Breaker Derating Example:

The principal stress parameter is contact current.

A circuit breaker is to be selected to control an electrical motor rated at 17 A, full load, 24 Vdc. The circuit breaker is to be installed in an environment with an ambient temperature ranging from 10 °C to 30 °C.

The temperature derating is:

The contact current derating is:

This example, then, requires the use of a circuit breaker with a maximum thermal rating equal to or greater than 50 °C and a maximum contact rating of at least 85 A for this application.

Table 3.2.2-5 Ambient Operating Temperature for Derating Relays and Switches

Temperature Extremes Under Which the Relay/Switch May Function

(1) The factors provided pertain only to contact loads, and they are intended for derating specified loads established in the governing specifications (resistive, inductive, motor, and/or lamp loads). The users are cautioned to use the contact voltages and nominal coil voltages (currents) prescribed in the governing specifications. Utilization of reduced coil voltages and abnormal contact voltages can potentially reduce the life of the relay and compromise relay operations.

The steps for relay/switch load derating are:

Select the appropriate load (resistive, inductive, motor, or lamp) and rating from the specification.

Determine the temperature range in the application. Select the appropriate factor from Table 3.2.2-5.

Determine the cycle rate in the application. Select the appropriate factor from Table 3.2.2-6.

Determine the load application. Select the appropriate factor from Table 3.2.2-7.

Calculate the derated load by multiplying the various factors together.

Derated Load = Load × Temp Factor × Cycle Rate Factor × Application Factor.

Other examples are as follows:

Example 1:

A 1.0 A relay is operated in an environment with a temperature range of +25 °C to +70 °C. The relay is cycled at a rate of 5 cycles per hour. The load application is make, break, and carry of a resistive load.

The worst case temperature is 70 °C. From Table 3.2.2-5 select 0.85.

The cycle rate is 5 cycles/hour. From Table 3.2.2-6 select 0.9.

The load application is specified as make, break, and carry. From Table 3.2.2-7 select 0.8.

Relay derating factor is 0.85 × 0.9 × 0.8 = 0.612. The derated contact load is 0.612 × 1.0 = 0.612 A resistive load.

Example 2:

A 10 A relay is operated in an environment with a temperature range of -40 °C to +35 °C. The relay is turned on for 3 minutes every 2 hours. The load application is carry only (resistive load).

From Table 3.2.2-5 select 0.85 From Table 3.2.2-6 select 0.85 From Table 3.2.2-7 select 1.5

Table 3.2.2-6 Cycle Rate per Hour for Derating Relays and Switches

Derating Factor for Nominal Cycle Rate

Cycle Rate Per Hour

Note:

(1) The factors provided pertain only to contact loads, and they are intended for derating specified loads established in the governing specifications (resistive, inductive, motor, and/or lamp loads). The users are cautioned to use the contact voltages and nominal coil voltages (currents) prescribed in the governing specifications. Utilization of reduced coil voltages and abnormal contact voltages can potentially reduce the life of the relay and compromise relay operations.

Table 3.2.2-7 Load Application Rate for Derating Relays and Switches

(1) The factors provided pertain only to contact loads, and they are intended for derating specified loads established in the governing specifications (resistive, inductive, motor, and/or lamp loads). The users are cautioned to use the contact voltages and nominal coil voltages (currents) prescribed in the governing specifications. Utilization of reduced coil voltages and abnormal contact voltages can potentially reduce the life of the relay and compromise relay operations.

Electrical Power Consumer Constraints Wire Derating Wire derating criteria for payloads shall be per Table 3.2.3.1-1, Current Carrying Capacity of Insulated Payload Wiring (Amperes) for External Payloads, and the criteria that follows.

Table 3.2.3.1-1 Current Carrying Capacity of Insulated Payload Wiring (Amperes) for External Payloads (From SSP 51700 (TA-92-038))

Notes:

1. Wire rating information is derived from extensive testing of MB0150-048 Orbiter wiring at JSC and applies to equivalent copper wiring with any type of insulation. For convenience, information pertaining to wire with insulation ratings of 150 °C, 175 °C, and 200 °C are shown. For wire ratings other than these, refer to JSC engineering publication TM 102179, “Selection of Wires and Circuit Protection Devices for NSTS Orbiter Vehicle Payload Electric Circuits”. Wire sizes smaller than 26 gauge are not recommended for use in payloads.

2. Current Carrying Capacity of Wire – Represents the maximum sustained current in amperes which the wire can carry in a vacuum and an ambient temperature of 94 °C (200 °F) and not exceed the temperature rating of the insulation material.

3. This table does not reflect wire bundle derating, nor does NASA JSC believe bundle derating to normally be necessary. This is due to the multitude of inter-related factors involved in bundling which can either enhance or degrade the current-carrying capacity of wire. However, in unique applications where a majority of wires in a bundle are heavily loaded simultaneously, the user may utilize the wire bundle criteria:

Loss of Power SSP 51700, Section 3.1.4.1 addresses Loss of Power.

Electrical Grounding and Isolation Adherence to the following requirements satisfies the ISS single point grounding scheme.

Payload Power Isolation Users of 120 Vdc, or 28Vdc power shall be dc isolated from chassis, structure, equipment conditioned power return/reference, and signal returns by a minimum of 1 megohm.

Grounding Isolation Single Point Ground Each isolated electrical power source shall be connected to structure at no more than one point.

Electrical Power Isolation Each isolated electrical power source shall be dc isolated from chassis, structure, equipment conditioned power return/reference, and signal circuits by a minimum of 1 megohm, individually, except at the single point ground.

Signal Circuit Return Grounding Circuit conductors shall be dc isolated from chassis, structure, and equipment conditioned power return/reference, by a minimum of 1 megohm, individually, when not terminated by the signal circuit’s single point ground/reference. Balanced, differential circuits isolated from chassis, structure, and user conditioned power return/reference by a minimum of 6000 ohms complies with this requirement.

Note: Signals circuits with frequency components equal to or above four megahertz may use controlled impedance transmission and reception media such as (but not limited to):

- shielded twisted 72 ohm cable

- “twin ax” cable balanced and referenced to primary structure at a single point

- “triax” cable using the center and inner shield conductors for unbalanced transmission, referenced to primary structure at a single point with the outer shield multipoint grounded as an “overshield”.

- “coax” cable with the shield terminated 360 degrees at each end and at available intermediate point (permitted for signals with the lowest frequency component equal to or above 4 MHz).

Alternating Current Power Return

1. Each AC power supply line shall…