Polyswitch Selection Guidelines GSFC Thoughts

1. ELECTRICAL SAFETY ISSUES DISCUSSION WITH SHUTTLE SMALL PAYLOADS PROJECTS (a.k.a. HITCHHIKER)

2. Polyswitch Selection GuidelinesGSFC Thoughts • Guidelines are being developed at GSFC to assist payload organizations in the selection of polyswitch devices and to demonstrate that a selected polyswitch implementation complies with (or meets the intent of) TA-92-038. • STEP 1: Generalized selection criteria to meet the mission objectives • Ensures mission success • STEP 2: Verify device selection provides the appropriate hazard controls • Battery hazards (JSC-20793) • Battery and harnessing hazards (JSC-20793 and TA-92-038) • STEP 3: Independently evaluate and test the suitability and performance of the device in the application • May not be required if sufficient margin is shown in STEP 2 • If STEP 2 CAN NOT be satisfied, revisit STEP 1 until STEP 2 is satisfied… GSFC & STP Electrical Safety Issues Clarification TIM

3. POLYSWITCH SELECTION GUIDELINE DETAILS

4. STEP 1: Generalized Selection CriteriaThe Basics • Define the circuit operating parameters • Maximum / Minimum ambient operating temperature [??? CPR safety temps???] • Tempcircuitmax and Tempcircuitmin • Normal operating current of the system • Iload • Circuit maximum operating voltage & interrupt current • Vcircuitmax and Icircuitmax • Requires knowledge of the power system capabilities • Select a device that accommodates the circuit’s maximum ambient operating temperatures and normal operating current • Ihold @ Tempmax Iload • Verify compatibility with the circuit maximums • Vdevicemax > Vcircuitmax; Idevicemax > Icircuitmax • Verify the circuit operating temperature ranges match the device’s range • Tempdevicemax > Tempcircuitmax; Tempdevicemin < Tempcircuitmin • Verify the device physically fits the application space GSFC & STP Electrical Safety Issues Clarification TIM

5. STEP 1: Generalized Selection CriteriaOther Considerations • Effect of ambient conditions on performance • Vacuum and thermal elements • Worse case thermal environment of the polyswitch in the battery box while in the Orbiter [??? CPR safety temps???] • Heat transfer SIGNIFICANTLY affects function time. • Increasing the heat transfer of the device will: • Increase device power dissipation • Increase the time-to-trip (Ttrip) • Increase the hold current (Ihold) • Decreasing the heat transfer of the device has the opposite affect • The trip, hold, and ‘tripped’ currents at the expected temperature extremes • How much slower or faster does the device work at the temperature extremes? • Temperature of the polyswitch while tripped. • Does it overheat?... What happens then? Does it open or short when it overheats? • If there is 'heat sinking‘ • Show that the polyswitch function time is not extended too far GSFC & STP Electrical Safety Issues Clarification TIM

6. STEP 1: Generalized Selection CriteriaOther Considerations (cont’d) • System Effect / Performance due to polyswitches functioning • Device Trip temperature is generally ~125 degrees C • Regardless of the ambient environment • Device voltage drop • Vdrop = Ioperate x Rmax • Inrush currents • What happens to the system with erratic operation of the device • Current / voltage spikes? • Inductive loads • When the device functions it happens quickly (on the order of miliseconds). Verify that the device voltage rating is not exceeded when switching inductive loads, where: • Device reset • Reset time • How quickly the device resets is dependent upon the power being dissipated at the time of a fault and whether the automatic reset conditions have been met. • Automatic reset conditions occur generally when: • Where, Pd= power dissipated in the device, RL= Load resistance • Resistance and leakage current in tripped state • Device does not open the circuit which allows current through the device until power is removed or reduced to the automatic reset conditions. GSFC & STP Electrical Safety Issues Clarification TIM

7. STEP 1: Generalized Selection CriteriaOther Considerations (cont’d) • Device resistance considerations • Reflow / Trip ‘jump’ affects should be considered when selecting the ‘hold’ current • Device resistance after cycling does not return to initial value for a ‘long’ time • Defines Rmax for circuit design • Lot testing of COTS polyswitches is recommended. • Resistance temperature behavior prior to tripping • Resistance increases as device warms and approaches the trip temperature • Include this feature in the power system analysis • Current faults • Low Current: ~ x2-x3 Ihold • May take a ‘long’ time to function the device • High Current:  ~ x10 Ihold • Device functions quickly • Must insure device is not damaged GSFC & STP Electrical Safety Issues Clarification TIM

8. STEP 2: Hazard Control Verification • Region ‘C’ operation of these devices is the primary concern • The device can either trip or remain in the low resistance state • If in the low resistance state then it may never function… • A quick hazard assessment can be made using the following conditions, under all thermal environments: • Dead short circuit in either battery or s/c harness • Smart short in either battery or s/c harness where current just exceeds the wire rating GSFC & STP Electrical Safety Issues Clarification TIM

9. STEP 2: Hazard Control VerificationBattery Only • Requirements • NSTS 1700.7B • 200.4a: Safe Without Services • 201.3: Functions Resulting in Catastrophic Hazards • 209.1: Hazardous Materials • 213.2: Batteries • NSTS 18798A • TA-92-038, Protection of Payload Electrical Power Circuits • Hazard considerations: • Shorting (internal / external) • Cell reversal or over-discharge • Excessive internal pressure • Overcharge • Over-temperature • Freeze / Thaw • Accumulation and ignition of hazardous gas mixture • Leakage of battery container GSFC & STP Electrical Safety Issues Clarification TIM

10. STEP 2: Hazard Control VerificationBattery Only (cont’d) • Hazard concerns: • Hazards while in the “tripped” state? Since there is a leakage current… • Can the battery overheat? • Assume fault occurs at warmest temperature determined by thermal analysis. • Can battery over discharge to cause a safety hazard? • Could a safety hazard be caused by erratic operation of the downstream hardware to inadvertent tripping of the polyswitch. • Current spike affects to the battery. • Power system glitch affect to other hazard controls? • I.e. affects on timer circuits? • Time-to-function considerations: • The polyswitch should function quickly at some current value below the short circuit capability of the battery over the s/c temperature range [??? payload safety temperature ???]. • Something < 1second [???] GSFC & STP Electrical Safety Issues Clarification TIM

11. STEP 2: Hazard Control VerificationBattery & Harnessing • Requirements: • NSTS 1700.7B • 209.1: Hazardous Materials • 213.1: Electrical Systems • 219: Flammable Atmospheres • NSTS 18798A • TA-92-038, Protection of Payload Electrical Power Circuits • NS2/81-MO82, Ignition of Flammable PLB Atmosphere • Hazard causes: • Electrical ignition • Presence of hot surface ignition source • Short circuit or load failure which cause over-current in Orbiter wiring powered from payload bus/source or damage is cause to co-located safety critical circuits resulting in removal of more than one inhibit • Electrical faults in payload while it is connected to the Space Shuttle due to short-circuit or abrasion • Improper sizing of wiring / fuses • Ignition of payload bay atmosphere during Space Shuttle entry, landing, and post landing GSFC & STP Electrical Safety Issues Clarification TIM

12. STEP 2: Hazard Control VerificationBattery & Harnessing (cont’d) • Hazard concerns: • Wiring in the battery or harness may overheat • Size wire based on the polyswitches trip current while cold. • Time-to-function considerations: • The polyswitches should function quickly at a current just exceeding the selected wire’s rating over the s/c temperature range [??? CPR safety temps???]. • Device function time should be less than the time for the wire temperature to increase from 200F to it’s rated value • {??? Something < 10 seconds ???} GSFC & STP Electrical Safety Issues Clarification TIM

13. STEP 3: Generalized Selection CriteriaIndependent Verification • Test in the ‘flight like’ configuration… • Some current margin may be sufficient to preclude testing? • Perhaps 25-30%????? • Ambient applications • W/out heat sink • Manufacturer’s data may be sufficient • W/ heat sink • Correlate or determine function time offset with data sheets over temperature • 20C, Tempcircuitmax, Tempcircuitmin • Vacuum applications • W/out heat sink • Correlate or determine function time offset with data sheets over temperature • 20C, Tempcircuitmax, Tempcircuitmin • W/ heat sink • Correlate or determine function time offset with data sheets over temperature • 20C, Tempcircuitmax, Tempcircuitmin GSFC & STP Electrical Safety Issues Clarification TIM