Transcript PolySwitch Guidelines as Presented to JSC FSRP 09-20

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…
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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
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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
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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.
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V
Pd
4  RL
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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
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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
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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:
–
–
–
–
–
–
–
–
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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
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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 [???]
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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
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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 ???}
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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
•
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20C, Tempcircuitmax, Tempcircuitmin
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