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ELECTRICAL SAFETY ISSUES DISCUSSION WITH SHUTTLE SMALL PAYLOADS PROJECTS (a.k.a. HITCHHIKER) 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-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… 10 Sep 02 GSFC & STP Electrical Safety Issues Clarification TIM 2 of 15 POLYSWITCH SELECTION GUIDELINE DETAILS STEP 1: Generalized Selection Criteria The Basics 1. 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 2. Select a device that accommodates the circuit’s maximum ambient operating temperatures and normal operating current – Ihold @ Tempmax Iload 3. Verify compatibility with the circuit maximums – Vdevicemax > Vcircuitmax; Idevicemax > Icircuitmax 4. Verify the circuit operating temperature ranges match the device’s range – Tempdevicemax > Tempcircuitmax; Tempdevicemin < Tempcircuitmin 5. Verify the device physically fits the application space 10 Sep 02 GSFC & STP Electrical Safety Issues Clarification TIM 4 of 15 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 10 Sep 02 GSFC & STP Electrical Safety Issues Clarification TIM 5 of 15 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 • 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: V L – Device reset di dt • 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. 10 Sep 02 GSFC & STP Electrical Safety Issues Clarification TIM 2 V Pd 4 RL 6 of 15 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 Ihold • May take a ‘long’ time to function the device – High Current: ~ x10 Ihold • Device functions quickly • Must insure device is not damaged 10 Sep 02 GSFC & STP Electrical Safety Issues Clarification TIM 7 of 15 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 10 Sep 02 GSFC & STP Electrical Safety Issues Clarification TIM 8 of 15 STEP 2: Hazard Control Verification Battery 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: – – – – – – – – 10 Sep 02 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 9 of 15 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 [???] 10 Sep 02 GSFC & STP Electrical Safety Issues Clarification TIM 10 of 15 STEP 2: Hazard Control Verification Battery & 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 10 Sep 02 GSFC & STP Electrical Safety Issues Clarification TIM 11 of 15 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 ???} 10 Sep 02 GSFC & STP Electrical Safety Issues Clarification TIM 12 of 15 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, 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 • 10 Sep 02 20C, Tempcircuitmax, Tempcircuitmin GSFC & STP Electrical Safety Issues Clarification TIM 13 of 15