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GROUNDING SYSTEM DESIGN FOR ISOLATED LOCATIONS AND PLANT SYSTEMS

GROUNDING SYSTEM DESIGN FOR ISOLATED LOCATIONS AND PLANT SYSTEMS . Marcus O. Durham THEWAY Corp / U of Tulsa Robert A. Durham Central & Southwest Srvcs, WTU. GROUNDING Introduction. Common feature not always effective

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GROUNDING SYSTEM DESIGN FOR ISOLATED LOCATIONS AND PLANT SYSTEMS

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  1. GROUNDING SYSTEM DESIGN FOR ISOLATED LOCATIONS AND PLANT SYSTEMS Marcus O. Durham THEWAY Corp / U of Tulsa Robert A. Durham Central & Southwest Srvcs, WTU

  2. GROUNDINGIntroduction • Common feature not always effective • Technology well defined application is art

  3. PRESENT TECHNOLOGYIntroduction • NEC 35 pp requirements -- not design • Green RP industrial -- not electronics • Emerald RP power quality sensitive equip • Lightning shield & shunt discharges - not devices • Previous requirements & design procedures

  4. PRESENT TECHNOLOGYIntroduction • NEC 35 pp requirements -- not design • Green RP industrial -- not electronics • Emerald RP power quality sensitive equip • Lightning shield & shunt discharges - not devices • Previous requirements & design procedures

  5. PROJECT10 Case Studies of Ground Problems • Situation present problem • Environment influences effectiveness of system • Analysis investigates alternatives & unique • Summary brief response to situation little things mean a lot -- pay attention to detail

  6. REMOTE SENSOR ERRATICCase 1 Situation very low readings prior to lightning Environment petrochemical plant, Alabama Gulf Coast pipeline station 1/2 mile soil = sand , > 50,000 ohm-cm isoceraunic 80 TSDY , intense combination = one of most difficult

  7. REMOTE SENSOR ERRATICCase 1 Situation very low readings prior to lightning Environment petrochemical plant, Alabama Gulf Coast pipeline station 1/2 mile soil = sand , > 50,000 ohm-cm isoceraunic 80 TSDY , intense combination = one of most difficult

  8. REMOTE SENSOR ERRATICCause • V diff common (-) to remote gnd signal current fluctuation • Cloud crossing creates potential varies w / location under storm • Strike = saturate ground system elevated potential to outside

  9. REMOTE SENSOR ERRATICEffect • Inside = equipotential • Egress = difference • Trigger protectors short time • Dump excessive transient I on lines • Time - delay on response • Isolate remote ground , metallic bonding won’t

  10. EE REMOTE SENSOR ERRATICNo Such Thing As Common Ground AWG 2/0 bond Lo R = 0.0003 / ft Hi Z to lighting L = 0.5 uH / ft f = 1 Mhz XL = 3 ohm / ft (4 order, 10000X) I surge > 3000 A V drop = 9000 v / ft

  11. UNUSED POINTS INDUCE ERRORSCase 2 Situation unused analog damaged during storm Environment pipeline station S . Texas cap rock soil = rock 15 , 000 to 100 , 000 ohm-cm isoceraunic 38 TSDY

  12. UNUSED POINTS INDUCE ERRORSCause • Extra input cards • Multi - conductor cable • Surges induced on every wire • Analog in more susceptible to spurious • Digital less prone but surges coupled • Never let float

  13. UNUSED POINTS INDUCE ERRORSConnections Load on unused Join intermediate terminals Open field ends Care to isolate shield from other ground

  14. Digital Digital Analog Analog In Out In Out _ _ _ _ + + + + GND 250 W LOAD UNUSED POINTS INDUCE ERRORSTermination

  15. FAILURE DURING STORMCase 3 Situation sensors commonly failed w/o direct hit Environment power generation, SW Oklahoma pipeline gas yard 200 yds soil = river bottom, 3000 ohm-cm, excellent isoceraunic 55 TSDY

  16. FAILURE DURING STORMWhat Is It? • Remote xmitters in yard • Existing unshielded cable - entry lightning energy • Effective grounds in plant & yard • No strike lose DCS input • Close strike to towers, tanks severe differential voltage lose transmitter also

  17. MOV FAILURE DURING STORMFix • Isolate unshielded cable • Common mode DCS • Differential mode xmitter • In - line both • MOV 36 V, 160 J • Woops, commercials ground protector

  18. DATA FAILURE BETWEEN BLDGSCase 4 Situation perimeter comm lines , terminals 5 X Environment petrochemical plant, Alabama Gulf Coast pipeline station 1/2 mile soil = sand , > 50,000 ohm-cm isoceraunic 80 TSDY , intense combination = one of most difficult

  19. DATA FAILURE BETWEEN BLDGSTap Protection • Different ground potential • Isolate signal & protection • Prefer fiber • Twisted - pair, shield 1 end • 2 Com, 1 Dif MOV

  20. DATA FAILURE BETWEEN BLDGSAC & Remote Protection • Gas tube • MOV or Transzorb • Inductor comm type frequency / baud length V & I

  21. xmitter + + _ _ s xmitter + + _ _ s Isolated Signal Ground PROTECTIONLocal , AC & Remote

  22. PROTECTION BLOWS FUSESCase 5 Situation protectors operate , fuses blow Environment petrochemical plant, Alabama Gulf Coast pipeline station 1/2 mile soil = sand , > 50,000 ohm-cm isoceraunic 80 TSDY , intense combination = one of most difficult

  23. PROTECTION BLOWS FUSESFollow Through Current • Gas tubes long • Zener short • MOV 15 sec • Shorts transmitter • 90 fuses per storm

  24. PROTECTION BLOWS FUSESNon - Incendive Resistor • Small minimize loop compliance • Large limit I • Power continuous • 4 - 20 ma, 24 VDC ~ 600 ohm • In positive lead

  25. +24V fuse limiting protector indicator wire X P -24V wire xmitter (gnd) PROTECTION BLOWS FUSESResistance Total = Sensor + Wire + Add = 250 + 17 /kft + 330 > 580

  26. PROTECTION BLOWS FUSES Loop Compliance Resistor = 24V / 50 ma > 480 ohm I loop = ( 600 / Total R) * 20 ma < 20 P = (0.020) 2 * 330 = 0.13 W

  27. I S D I S PROTECTION BLOWS FUSESSemiconductor Failure • Generally shorts • Positive lead blow fuse • Negative lead ground loop

  28. CURRENT IN ELECTRODESCase 6 Situation I = 14 A in both ground beds Environment petrochemical plant, Alabama Gulf Coast pipeline station 1/2 mile soil = sand , > 50,000 ohm-cm isoceraunic 80 TSDY , intense combination = one of most

  29. CURRENT IN ELECTRODESNon - Isolation • Marshalling panel pos, neg, gnd • Chassis short to shield • Each small circulating I • 1400 digital inputs large I

  30. CURRENT IN ELECTRODESShields • Shorted together • Do not crop to jacket • Insulate individual ends • Do not touch metal • Terminate on shield strip • Single point ground

  31. Supply PLC Xducer 24+ 24+ + _ 24- 24- s 24+ + _ 24- s Source 120V Load H H N N G G CURRENT IN ELECTRODESSingle Point

  32. FAN MOTOR ON TOP OF BOILER Case 7 Situation Fails when lightning hits surrounding Environment power generation, W Texas soil = shallow top soil on rock, ~ 100,000 ohm-cm isoceraunic 43TSDY

  33. FAN MOTOR ON TOP OF BOILER Equipment • 4160 V motor • 70 ft above earth • 5 kv unshielded triplex • In tray & conduit • Acts as antenna • 2 grounding conductors

  34. FAN MOTOR ON TOP OF BOILER Gnd Conductor Z ( AWG 4 ) L = 0.279 uHy / ft L = 56 uHy / 100 ft L = 0.5 uHy / ft typical XL = 220 ohm @ 1 MHz XL >> R

  35. FAN MOTOR ON TOP OF BOILER Fix • Lightning arrestors on terminals • MOV • Bond all metal • Shield cable

  36. STRUCTURE ATTRACTS LIGHTNINGCase 8 Situation Damages xmitters & boards Frequent hits Environment research facility NE Oklahoma soil = loam, 6500 ohm-cm isoceraunic 55 TSDY

  37. STRUCTURE ATTRACTS LIGHTNINGDissipate Energy • Not tallest structure • More conductive than others • Bond around base pins • Bond framework • 1 / 0 AWG • Non - corrosive terminals • Gound guy wires

  38. STRUCTURE ATTRACTS LIGHTNINGGround Network • Rat - race ring • If > 50 ft, criss - cross • Bond to existing grid • Radials 1 / 0 AWG • 5 ft rods, 20 ft spacing

  39. HIGH RESISTIVITY SOILCase 9 Situation Single motor from distribution system Environment pump SE New Mexico soil = 6500 ohm-cm on rock at 4 ft isoceraunic 47 TSDY

  40. HIGH RESISTIVITY SOIL5 / 8 “ x 8 ‘ Rod • Dwight’s formula R = [ p / 191.5 b] [ ln ( 48 b / a ) - 1 ] R = 177 ohm , 1 rod R = 76 ohm, 3 rods > 25 ohm >> 5 ohm

  41. ACME CONCRETE HIGH RESISTIVITY SOILConcrete • Retain moisture • Alkalinity free ions • Resistivity 3000 ohm - cm • 1 rod 10.6 ohm • 3 rod 4.6 ohm

  42. SHOCK ON GROUNDED PANELCase 10 Situation Grounded panel permitted shock Environment pumping panel, NE Oklahoma soil = loam, 6500 ohm-cm isoceraunic 55 TSDY

  43. SHOCK ON GROUNDED PANELSystem • Power panel outdoors • Overhead distribution, 277 / 480 Y gnd • 5 / 8 “ x 8 ‘ rod • Occasional shocks • Inspected returned to service

  44. SHOCK ON GROUNDED PANELProblem • Secondary pinch ~ 50 ohm • Circulating I = 1 amp • Two ground paths • Gnd rod ~ 25 ohm • Body ~ 40,000 to 1,000 ohm

  45. 277/480 v 120 v SHOCK ON GROUNDED PANEL

  46. SHOCK ON GROUNDED PANELBody Current 0.6 ma 40 K 24.4 ma 1 K 341 ma 14 A Install lo Z ground

  47. SHOCK ON GROUNDED PANELGFCI • Personnel 6 ma • Equipment 30 ma • Not required for this

  48. CURRENT IMPACT MAPhysiology 1 sensation 8 painful shock 15 can’t let go 20 possible damage 50 ventricular fibrillation 100 probable death

  49. REVIEW Grounding systems Various environments Diverse problems Common solutions Little things mean a lot

  50. SUMMARY 1. Environment - thunderstorms vs hits 2. Earth resistivity - multiple in concrete 3. Equipotential network 4. Single point connection power neutral signal common shield

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