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

2 of Sep 02GSFC & STP Electrical Safety Issues Clarification TIM Polyswitch Selection Guidelines GSFC 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 –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 and TA ) –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…

POLYSWITCH SELECTION GUIDELINE DETAILS

4 of Sep 02GSFC & STP Electrical Safety Issues Clarification TIM STEP 1: Generalized Selection Criteria The Basics 1.Define the circuit operating parameters –Maximum / Minimum ambient operating temperature [??? CPR safety temps???] Temp circuitmax and Temp circuitmin –Normal operating current of the system I load –Circuit maximum operating voltage & interrupt current V circuitmax and I circuitmax Requires knowledge of the power system capabilities 2.Select a device that accommodates the circuit’s maximum ambient operating temperatures and normal operating current –I Temp max  I load 3.Verify compatibility with the circuit maximums –V devicemax > V circuitmax ; I devicemax > I circuitmax 4.Verify the circuit operating temperature ranges match the device’s range –Temp devicemax > Temp circuitmax ; Temp devicemin < Temp circuitmin 5.Verify the device physically fits the application space

5 of Sep 02GSFC & STP Electrical Safety Issues Clarification TIM STEP 1: Generalized Selection Criteria Other 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

6 of Sep 02GSFC & STP Electrical Safety Issues Clarification TIM STEP 1: Generalized Selection Criteria Other 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 V drop = I operate x R max –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.

7 of Sep 02GSFC & STP Electrical Safety Issues Clarification TIM STEP 1: Generalized Selection Criteria Other 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 I hold May take a ‘long’ time to function the device –High Current:  ~ x10 I hold Device functions quickly Must insure device is not damaged

8 of Sep 02GSFC & STP Electrical Safety Issues Clarification TIM 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

9 of Sep 02GSFC & STP Electrical Safety Issues Clarification TIM STEP 2: Hazard Control Verification Battery Only Requirements –NSTS B 200.4a: Safe Without Services 201.3: Functions Resulting in Catastrophic Hazards 209.1: Hazardous Materials 213.2: Batteries –NSTS 18798A TA , 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

10 of Sep 02GSFC & STP Electrical Safety Issues Clarification TIM STEP 2: Hazard Control Verification Battery 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 [???]

11 of Sep 02GSFC & STP Electrical Safety Issues Clarification TIM STEP 2: Hazard Control Verification Battery & Harnessing Requirements: –NSTS B 209.1: Hazardous Materials 213.1: Electrical Systems 219: Flammable Atmospheres –NSTS 18798A TA , 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

12 of Sep 02GSFC & STP Electrical Safety Issues Clarification TIM STEP 2: Hazard Control Verification Battery & 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 ???}

13 of Sep 02GSFC & STP Electrical Safety Issues Clarification TIM STEP 3: Generalized Selection Criteria Independent 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, Temp circuitmax, Temp circuitmin –Vacuum applications W/out heat sink –Correlate or determine function time offset with data sheets over temperature 20C, Temp circuitmax, Temp circuitmin W/ heat sink – Correlate or determine function time offset with data sheets over temperature 20C, Temp circuitmax, Temp circuitmin