Bourns, Inc. Hans-Wolfgang Oertel Vice President Technology Dehn Incor

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Bourns, Inc. Hans-Wolfgang Oertel Vice President Technology Dehn Incor

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MSPReliability White Paper
Bourns, Inc. Hans-Wolfgang Oertel Vice President Technology Dehn Incorporated Richard Chadwick Vice President Engineering
OVERVIEW Bourns MSPProtector unit utilized many tele-communications applications where precise protection needed Broad Band Services (xDSL) Plain Telephone Services (POTS) Central Offices (CO), Network Interface Devices (NIDs) Optical Network Units (ONUs), well Cable Comm Cable Access Units, where protects wire input power connections. This protector consists four functional elements: three element discharge tube (GDT), matched metal oxide varistors (MOVs) fast transient protectors "Fail Safe" with switch grade short circuit contacts thermal overload protection safety. These four elements make Multi Stage Protector assembly (MSPTM). proper operation these elements concert result total protection lines across which they installed. RELIABILITY MTBF MOVs polycrystalline ceramic. They composed primarily zinc oxide with small additions bismuth, cobalt, manganese other metal oxides. individual zinc oxide crystalline grains highly conductive. intergranular boundaries, which composed added oxides, highly resistive. sintering process during production produces "micro-varistor" where ever zinc oxide grains direct contact. This micro-varistor essentially acts bi-directional zener diode with protection level approximately volts. cylindrical shape. zener voltage series mean number individual micro-varistors which span thickness cylinder, thus equal thickness cylinder divided diameter zinc oxide grains, times volt zener voltage. failure occurs fusing individual zinc oxide grains. This fusing function number severity surges encountered, excessive leakage currents caused high temperature and/or sustained swells line voltage. fusing zinc oxide grains reduces number grain boundaries, thus reduces zener voltage MOV. When zener voltage drops within applied circuit voltage, grain growth rate increases exponentially solid impedance zinc oxide channel created across MOV. resistance channel proportional bulk state resistivity zinc oxide times channel diameter. This resistance generates sufficient heat activate MSPfailsafe shorting mechanism. failure mode MOVs MSPgas tube protector assembly thus impedance which current driven dead short. equipment will remain protected resistance short, will non-functional grounding signal. failsafe shorting mechanism prevents fire hazard. estimate MTBF thus requires that evaluate surge capability long term temperature accelerated overvoltage immunity MSPgas tube protector assembly.
RELIABILITY MTBF: SURGE CAPABILITY Requirements surge capability telephone surge protectors United States contained Telcordia GR-CORE-1361. Telcordia GR-CORE-1361 surge requirements based Bell Labs estimates surge life required ensure field survival rate after years station protector telecommunications application (customer line) high lightning (60-100 thunderstorm days year) environment found Southeastern United States. basic surge life requirements this specification have remained unchanged years. Post-mortem analysis failed protectors reported BellCore 1997 showed that field failures invariably failure parallel back-up gap, contact corrosion, insect infestation. (Note that since MSPgas tube protector assembly uses place back-up gap, this failure mode eliminated.) units analyzed, tube found good condition. Since tubes tested were designed meet Bellcore GR-CORE-1361, lack failed tubes BellCore analysis considered field verification adequacy surge life requirements. Bourns station protectors have been deployed approximately million telephone lines United States, based this field experience, Bourns full agreement with these conclusions. surge capability MSPgas tube protector assembly easily exceeds requirements GR-CORE-1361. This specification requires that Breakdown voltage stay within range 265-600 Impulse Breakdown V/µs remain under 1000 duration test. simulation years induced lightning, GR-CORE-1361 requires surges 10/1000 shown following table, MSPgas tube protector assembly withstand more than 5000 surges 10/1000 some times Telcordia requirement.
MSPgas tube protector assembly after 5000 operations 10/1000 Breakdown kV/s Insulation Impulse Breakdown V/µs
Resistance >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 Summary 374, 419, 547, 576,
designates Ground protector unit designates Ring Ground protector unit simulation years conducted lightning currents, GR-CORE-1361 requires surges 2000 10/250 shown table below, MSPgas tube protector assembly withstand more than surges 2000 10/250 some times Telcordia requirement. MSPgas tube protector assembly after operations 10/250 Insulation
Resistance >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 >100 Summary Breakdown kV/s 374, 432, Impulse Breakdown V/µs 567, 602,
previous data demonstrates capability MSPgas tube protector assembly exceed surge capability required service life years Southeastern United States. seen world lightning activity below, Southeastern United States comparable lightning activity lightning spots rest world. This represents four years accumulated data from Optical Transient Detector Satellite launched NASA 1995.
Based above, Bourns extremely confident that MSPgas tube protector assembly will exceed survival rate after years highest lightning areas most world. RELIABILITY MTBF: TEMPERATURE ACCELERATED OVERVOLTA IMMUNITY stated previously, failure from zinc oxide grain growth function number severity surges encountered, excessive leakage currents caused high temperature and/or sustained swells line voltage. Given robust surge capability MSPMulti Stage Protector assembly following explores long-term temperature accelerated overvoltage immunity. basic life maximum steady state temperature zinc oxide shown below. line graph represents number years sustain maximum continuous operating voltage (MCOV) without zener voltage changing more than ±10%. This relationship, which originally derived when MOVs were first commercially developed 1970s, been proven multi-year accelerated testing.
Bourns reports that they have manufactured based surge protectors protection power service telecommunications facilities since 1979. Further, they report that they have employed MCOV nominal line neutral power systems, MCOV nominal line neutral power systems. Bourns surge protectors used majority telecommunications installations United States. current failure rate heavy duty units designed meet Telcordia surge requirements, based actual repairs preformed repair parts supplied, approximately 0.1% year. Throughout year field history, Bourns reports that they have noted increased incidence failure with age. MOVs MSPprotector assembly have zener voltage ±10%, MCOV Vrms. MCOV thus fully three times typical Cable Plant line voltage Given derating, Bourns field history with considerably less derating, Bourns confident that MOVs MSPgas tube protector assembly will demonstrate MTBF greater than years worst temperature environments found application. Based foregoing analysis, Bourns' professional opinion that MOVs assembly will have greater than survival after years most severe surge temperature environments world.
DESCRIPTION OPERATION SWITCH GRADE FAILSAFE technologies MSPMulti Stage Protector assembly Bourns switch grade failsafe. switch grade failsafe provides more reliable operation than more traditional solder compression burn through type failsafes. purpose failsafe create dead short ground should protector overheat sustained conduction. Original failsafes were solder compression design. this design, solder pellet placed spring-loaded sandwich. When protector generates sufficient heat melt solder, pellet compresses spring action closes failsafe shorting contacts. With this type design, thermal expansion/contraction during device cooling day/night temperature extremes remove spring pressure cause additional arcing. least expensive type failsafe design insulation burn through type. With insulation burn through type design, heat from protector must burn through insulator separating spring loaded ground clip from protector line terminals. Reliability limited ability completely burn insulator material without creating fire hazard. offshoot insulation burn through design additionally uses insulator back-up gap. This requires that insulator material approximately 0.003 inch thick, porous allow arcing slightly higher voltage than protector assembly. This type design prone failure from moisture adsorption/absorption contamination 0.003 inch insulation media. Bourns switch grade failsafe uses tin/lead alloy electrical grade spring loaded contacts separated 0.010 inch minimum (0.015-0.020 inch nominal). Since there media between contacts which become contaminated absorb moisture reliability greatly improved typical failures caused contamination excessive thermal accumulation will occur. When failsafe activated, contacts close creating short circuit less than There insulating media between contacts which create fire hazard, there solder pellet that cause additional arcing thermal expansion/contraction.
failsafe mechanism consists spring loaded tin/lead plated electrical grade contacts. outer clip electrically connected tube center lead. inner conductor tube electrode. spacing between contacts greater than 0.010 inch (typically 0.015-0.020 inch). There insulating media between contacts, which could absorb moisture become contaminated.
metal soldered clip. inside directly rests pellet, placing spring loaded sandwich between tube electrode ground clip. solder used eutectic alloy silver, copper antimony tin. specially formulated long-term creep resistance high shear tensile strength. Since eutectic, single temperature point divides solid liquid states with intermediate plastic state. eutectic melting point Heat generated either tube will melt eutectic alloy causing tin/lead plated spring loaded contacts snap closed.
With failsafe activated, tin/lead plated electrical grade contacts held closed spring tension. This creates dead short from each input ground less than milliohms. There neither insulation burn through media that degrade contact resistance, there compressed solder pellet which create thermal cycling problems.
SUMMARY MSPassembly will have greater than survival after years most severe surge temperature environments most World. MSPassembly, switch grade failsafe MOVs place commonly used back-up gaps will eliminate early field failures caused thermal overload conditions contamination.

Bourns, Inc. Hans-Wolfgang Oertel Vice President Technology Dehn Incor

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