Grounding Design of Kano Hydro Power Station(1x8MW+1x2MW) in Kano State of Nigeria

AUTHOR PANKAJ SACHDEVA,JOINT GROUP PRESIDENT,SKIPPER ELECTRICAL(I) LIMITED. M.L.SACHDEVA,FORMER CE,CEA AND SR. CONSULTANT,SKIPPER ELECTRICAL(I) LIMITED S.L.NARASIMHAN,FORMER EE,K.P.C,BANGALORE AND DIRECTOR(ELECTRICAL),BHEC

PVT. LTD

PRESENTED BY SOURAV GHOSH,ENGINEER AT SKIPPER ELECTRICAL(I) LIMITED

ABOUT M/S SKIPPER.

PRESENCE IN 50 COUNTRIES,MOSTLY EXPORT ORIENTD

MANUFACTURING PLANT : IN INDIA -3 NOS. BHIWADI IN NIGERIA – 2NOS LOGAS IN DUBAI - 1No

RANGE OF PRODUCT PRESNTLY UPTO 220KV SYSTEM & UPGRADING TO 400KV. POWER TRANSFORMER- UPTO 65MVA,220KV S/S CT,PT,ISOLATORS,CBs C&R PANELS, CAPACITOR,LT PANELS ETC.

PACKAGED S/S AND MOBILE SUBSTATION

PLANNING, DESIGN, ERECTION, TESTING & COMMISSIONING OF AIS and GIS

SCADA AND COMMUNICATION

NEW TRANSFORMER PLANT - UPTO 800KV TR.( Under Building) Railway Electrification

SYNOPSIS

THIS PAPER DEALS WITH GROUND MAT DESIGN OF POWER HOUSE AND SWITCHYARD WITH DIFFERENT SOIL RESISTIVITY VALUES AND DIFFERENT CALCULATED TOUCH AND STEP POTENTIALS WHICH IN SOME CASES EXCEED THE LIMIT VALUES

SUGGESTIONS ON REVIEWING OF PROVISIONS OF THE GROUND MAT DESIGN OF SMALL HYDRO POWER HOUSES.

INTRODUCTION

GENERATING STATIONS AND SWITCHYARDS HAVE TO BE EFFECTIVELY EARTHED FROM SAFETY CONSIDERATION COVERED UNDER LEGAL PROVISIONS OF ELECTRICITY REGULATIONS, ACTS, COMMISSIONS, ETC.

GUIDANCE PROVIDED UNDER SAFETY CODES AND STANDARDS.

EACH COUNTRY HAS HIS OWN GOVERNING REGULATIONS AND STANDARDS / CODES FOR GROUNDING OF ELECTRICAL INSTALLATIONS.

DESIGN CONCEPT

THE DESIGN OF GROUND MAT IS TAKEN UP CONSIDERING THE FOLLOWING:

PLOT LAY OUT OF S/S / LAYOUT OF A POWER HOUSE(L X B ) SOIL RESISTIVITY (OHM-M) SURFACE RESISTIVITY -RIVER STONE/CRUSHED

STONE/WASHED GRANITE/ CONCRETE ( OHM-M) MAXIMUM PERMISSIBLE GROUND RESISTANCE OF SUBSTATION

/POWER HOUSE (OHM) SHORT CIRCUIT CURRENT (KA) AND ITS DURATION (SEC) DURATION OF SHOCK (SEC) DEPTH OF BURIAL OF GROUND MAT MEMBER (MM) MATERIAL OF GROUND MAT ( STEEL/COPPER) STEP POTENTIAL ( VOLTS) TOUCH POTENTIAL ( VOLTS)

MEASUREMENT OF SOIL RESISTIVITYA TYPICAL EARTH TESTER HAS FOUR (4)

TERMINALS.C1, P1, C2, P2 AND FOUR (4) ELECTRODES SUPPLIED WITH THE INSTRUMENT ARE DRIVEN IN THE GROUND AT SPECIFIED EQUAL DISTANCES THRU CABLE AND CONNECTED TO THE INSTRUMENT IN THE ORDER OF C1, P1 AND P2, C2.

SPECIFIC RESISTIVITY =2 Π.A. R

WENNER METHOD FOR CALCULATING SOIL RESISTIVITY

ρ=2ΠaR

Notes(i)DEPTH OF BURIED ELECTRODES ASSUMED NEGLIGIBLE AS COMPARED TO SPACING BETWEEN ELECTRODES.ii)MORE THE DISTANCE BETWEEN ELECTRODES MORE THE DEPTH AT WHICH SOIL RESISTIVITY IS MEASURED.iii) IF SOIL OF HIGH RESISTIVITY IS AT TOP SURFACE AND SOIL RESISTIVITY AT DEEP LEVEL IS TO BE MEASURED, TWO OR MORE RESONANCE MODEL OF SOIL RESISTIVITY MEASUREMENT IS ADOPTED.

STANDARDS AND CODES

IS: 3043(4th Reprint June 2007 including Amendment No 1/ Jan 2007 and Amendment 2 / Jan 2010): Code of Practice for Earthing Electrical Installations.

CBI&P Publication No 223: Design of Earthing Mat of High Voltage Substations

IEEE 80-2000: Guide for Safety in AC Substation Grounding

IEEE 665-1995: Guide for Generating Station Grounding.

BS: 7430 -1998: Code of Practice for Earthing

Estimation OF EARTHHING MATERIAL PRELIMINARY SOIL RESISTIVITY ( INITIAL ESTIMATION)

AS THE SWITCHYARD PLANNED TO BE ERECTED FIRST, THE SOIL RESISTIVITY WAS MEASURED OVER A LEVEL PORTION OF LAND EARMARKED FOR POWER HOUSE.

THE SOIL RESISTIVITY MEASSUREMENT MEASUREMENT= 80 OHM-M EARTHING MATERIAL ( COPPER ) ESTIMATED PROCUREMENT ACTION INITIATED

FINAL SOIL RESISTIVITY (FINAL MEASUREMENT & ESTIMATION) THE EXCAVATION OF POWER HOUSE COMPLETED AND SWITCHYARD APPROX

LEVELLED SOIL RESISTIVITY LEVELS MEASURED AS UNDER

MAIN POWER HOUSE ( 8MW RBU) - 670 OHM-M CANAL BAY UNIT ( 2MW) - 120 OHM-M SWITCHGEAR ROOM & SERVICE BAY - 120 OHM-M OUT DOOR SWITCHYARD (ODY) - 120 OHM-M

POWER HOUSE

THE SITE OF POWER HOUSE IS EXCAVATED FIRST TO THE DESIRED GRADES AND SITE IS LEVELED.

THE GROUND MAT MEMBERS ARE LAID ON THE SURFACE OF CLEANED ROCK IN CASE OF ROCKY SURFACE OR BORROWS MADE AT SPECIFIED DEPTH (650MM) IN FISSURED ROCK OR HARD SOIL.

THE GROUND MEMBERS AND RISERS (WHERE REQUIRED) ARE JOINTED AT CROSSING POINTS BY WELDING/ BRAZING/ EXOTHERMIC MOLD.

THE CONCRETE FLOOR IS LAID DOWN. THE END MEMBERS (X AND Y AXIS) OF GROUND MAT EXTENDED BEYOND CONCRETE LAYER ARE LEFT FOR EXTENDING TO NEXT FLOOR ELEVATION FOR LAYING

ANOTHER RING OF MAT AT THAT FLOOR FOR MAKING CONNECTIONS TO EQUIPMENT AND SO ON.

DESIGN STEPS AREA OF CONDUCTOR,

(AS PER IS: 3043 CL. NO 12.2.2.1)

SPACING FACTOR FOR MESH VOLTAGE = Km

CHECK FOR ADEQUACY OF (L) FOR E STEP AND E MESH

ESTEP (TOLERABLE) = ( RK +6CS X PS)… (AS PER EQ -30 OF IEEE-80)

DESIGN STEPSIK = CURRENT RMS FLOWING THRU BODY

WEIGHT 70KG= 0.157 / √ TS

RK IS THE RESISTANCE OF BODY IN OHMS = 1000 OHMS (As per Eq -33 of IEEE-80) (As per Eq -80 of IEEE-80) CALCULATION OF GRID RESISTANCE (As per IS

3043 Cl No 9.2.3 )

RESISTANCE OF VARIOUS ELECTRODES (IACSIT Intern. Journal of Engg of Techn Vol4, No3, June 2012

S,No Details of Earthing System

Resistance Symbols

1 Single Earth Electrode =Soil Res()l=Length of strip(cm)

d= Dia of rod (cm)2 Multiple Earth Electrode Ri=Resis of one rod in

isol=Soil Res()

n= Nos of rodsS=Dist between rods

(m) = Factor as per

Table3 Strip Conductor Earthing =Soil Res()

l=Length of strip(cm)W= depth of burial of

electrode (cm)t=width (cm)

4 Earthing Plate =Soil Res()A= Area of both side

of Plate(m2)

Factor for Parallel ElectrodesNo of Electrodes Factor

2 1.00

3 1.65

4 2.15

5 2.54

6 2.87

7 3.15

8 3.39

9 3.61

10 3.81

GROUND MAT IN POWER HOUSE

THE GROUNDING SYSTEM IN A POWER HOUSE COMPRISES OF:

GROUNDING SYSTEM FOR POWER HOUSE (PH)

GROUNDING SYSTEM FOR SWITCHYARD (SY)

DESIGN ASPECT OF GROUND MAT LIMIT THE OVERALL POTENTIAL RISE. PROTECT PERSONNEL & DEVICES FROM OVER

VOLTAGES. LOW IMPEDANCE PATH TO FAULT CURRENT FOR

RELIABLE & PROMPT OPERATION OF PROTECTIVE DEVICES DURING GROUND FAULT.

STEP POTENTIAL WITHIN THE PERMISSIBLE LIMIT. TOUCH POTENTIAL WITHIN THE PERMISSBLE LIMIT.

SECURITY AND SAFETY FENCE AROUND SWITCHYARD.

MANDATORY CONNECTING ALL THE METALLIC PARTS OR STATIC PARTS OF ELECTRICAL EQUIPMENT MINIMUM AT TWO POINTS.

EARTHING OF LIGHTNING PROTECTIVE DEVICES.

DESIGN ASPECT OF GROUND MAT SOIL AND SURFACE RESISTIVITY SIZE( DEPENDING ON AREA) AND TYPE OF EARTH SYSTEM DEPTH OF BURIAL OF ELECTRODE MATERIAL OF GROUND MAT MOISTURE CONTENT OF THE SOIL. SHORT CIRCUIT RATING AND DURATION OF SHORT CIRCUIT DURATION OF SHOCK PERMISSIBLE GROUNDING RESISTANCE PERMISSIBLE TOUCH AND STEP POTENTIALS

DATA AND DESIGN THE FAULT LEVEL AT 33KV LEVEL (STEP UP VOLTAGE)

HAS BEEN CONSIDERED AS 31.5KA

THE DESIGN OF EARTH MAT 60% OF 31.5 KA HAS BEEN CONSIDERED IN ACCORDANCE WITH BS-7354-CLAUSE NO 7.3.2

THE MATERIAL OF EARTH MAT HAS BEEN CONSIDERED AS “COPPER FLAT” AND COPPER CLAD STEEL RODS

BASED ON THE AVAILABLE INPUTS,THE FOLLOWING PARAMETERS HAVE BEEN CONSIDERED FOR PRELIMINARY DESIGN IN ACCORDANCE WITH IEEE-80

INPUT DATA (Tiga Power House) Sl. no Symbol Descriptions Values PH Values ODY Units

1 Isc Fault current (60% of 31.5kA) 18900 18900 Amps

2 ρSoil resistivity (average of P1 & P2 is 75.45Ω & 81.02Ω) 80 80 Ω-m

3 ρs Surface layer resistivity 3000 3000 Ω-m

4 DSpacing between parallel conductor 4 1.6 Metre

5 LPH Length of PH and ODY 36 41.6 Metre

6 BPH Breadth of PH and ODY 28 22.4 Metre

SUMMARY OF THE RESULT Tolerable Calculat

edPower House Step voltage

(ESTEP)3938.839 V > 653.24 V

Touch voltage(ETOUCH)

1151.23 V EMESH calculated >

801.17 V

Switch Yard Step voltage (ESTEP)

3938.83 V > 2204.41 V

Touch voltage (ETOUCH)

1151.23 V EMESH calculated >

685.02V

Grid resistance 0.117Ω

DESIGN AND ITS RESULTS OF GROUND MAT OF POWER HOUSE AFTER EXCAVATION COMPLETED

INPUT DATA

Sl. no

Symbol

Descriptions

Values PH 8MW & TRL

Values PH-2MW

Values ODY

Values SB & SG room

Units

1 Isc Fault current (60% of 31.5kA) 18900 18900 18900 18900 Amps2 Ρ Soil resistivity 670.5 112 112 112 Ω-m3 ρs Surface layer resistivity 5000 5000 5000 5000 Ω-m

4 DEquivalent Diameter of the grid conductor 0.020 0.020 0.020 0.020 Metre

5 DSpacing between parallel

conductor1.5

3

4

4 Metre

6L

Length of PH, ODY & Service bay including switch gear room

16.7

12

53.5

28 Metre

7B

Breadth of PH, ODY & Service bay including switch gear room

23.4

23.4

38.75

23.4 Metre

SUMMARY OF THE RESULTLocation Particulars Tolerable Calculate

dLimits

Power House 8MW Step voltage (ESTEP ) 6483.32 V < 12590.27 V

Unsafe

Touch voltage (ETOUCH ) 1787.35V < EMESH calculated

1952.42 V Unsafe

Grid resistance RG 0.364 Ω Safe Power House 2MW Step voltage (ESTEP ) 6416.71V > 492.07 V Safe

Touch voltage (ETOUCH ) 1770.70V > EMESH calculated

396.784 V Safe

Grid resistance RG 0.061Ω SafeSwitch yard Step voltage (ESTEP ) 6416.71V > 715.70 V Safe

Touch voltage (ETOUCH ) 1770.70V > EMESH calculated

579.43 V Safe

Grid resistance RG 0.061 Ω SafeService bay & Switch gear room

Step voltage (ESTEP ) 6416.71 V > 774.68 V Safe

Touch voltage (ETOUCH ) 1770.70V > EMESH calculated

540.62 V Safe

Grid resistance RG 0.061 Ω Safe

TO LIMIT THE STEP AND TOUCH POTENTIAL WITH IN POWER HOUSE

IN CASE OF HIGH RESISTIVITY SOIL, IT HAS BEEN REVEALED THAT IT IS DIFFICULT TO MEET THE SAFETY CRITERIA OF TOUCH AND STEP POTENTIALS.

IT HAS BEEN FOUND THAT LARGE LAND AREA IS REQUIRED FOR MEETING THE REQUIREMENT.

FOR CONSIDERING THE COST EFFECTIVE SOLUTION, IT HAS BEEN NECESSARY TO RE-EXAMINE THE REQUIREMENT

IEEE-665,GUIDE FOR GENERATING STATION GROUNDINGCL NO 5.1: GROUNDING PRINCIPLE STIPULATES THAT

TOUCH AND STEP POTENTIAL SHALL MEET THE PERMISSIBLE LIMITS

CL NO 5.2.7 STIPULATES THAT “THE MAXIMUM ALLOWABLE TOUCH AND STEP VOLTAGES ARE THE CRITERIA THAT SHOULD BE MET TO ENSURE A SAFE DESIGN. IF THE TOUCH AND STEP VOLTAGES OF THE GRID DESIGN ARE BELOW THE MAXIMUM VALUES, THEN THE DESIGN IS CONSIDERED ADEQUATE.”

IEEE-665,GUIDE FOR GENERATING STATION GROUNDING CL NO 5.2.2 STATES THAT GENERATING STATIONS DIFFER FROM

SUBSTATIONS IN THAT THE PERSONNEL TO BE PROTECTED ARE GENERALLY WORKING INDOORS. BECAUSE THEY ARE NOT IN DIRECT CONTACT WITH THE EARTH OR WITH A LAYER OF CRUSHED ROCK COVERING THE EARTH, THEY ARE NOT EXPOSED TO MANY OF THE STEP AND TOUCH VOLTAGE CONDITIONS THAT PERSONNEL IN SUBSTATIONS ARE EXPOSED.

THE CEA NOTIFICATION NO-211 DATED 20TH AUG 2010 ALSO STATES THAT TOUCH AND STEP POTENTIAL SHALL BE MAINTAINED WITHIN ACCEPTABLE LIMITS.

DELIBERATION

ACCORDINGLY, TAKING COGNIZANCE OF PRACTICES IN VOGUE IN VARIOUS HYDRO POWER STATIONS, THE CRITERIA FOR STEP AND TOUCH POTENTIAL HAS NOT BEEN IGNORED AND ADOPTED AS CRITERIA FOR SAFE DESIGN BUT THIS NEEDS TO BE DELIBERATED IN THE CONFERENCE.

SURFACE LAYER RESISTIVITY IN THE PRELIMINARY DESIGN CALCULATIONS, THE SURFACE

LAYER RESISTIVITY WAS CONSIDERED AS 5000 Ω- M.

THE VALUES INDICATED FOR DRY AND WET CONDITIONS FOR THE VARIOUS SURFACE MATERIALS ARE WIDELY VARYING VIZ: FOR CONCRETE UNDER DRY CONDITION IT IS 1X106 TO 1X109OHM-M AND 21 TO 100 OHM-M FOR WET CONDITIONS.

IT IS ALSO INDICATES THAT 1X109 IS USED FOR OVEN DRIED CONCRETE.

THE VALUE OF 10,000 Ω M WAS CONSIDERED AS CONCRETE MAY NOT BE FULLY DRIED AT MIV FLOOR LEVEL.

FINAL DESIGN OF GROUND MAT OF POWER HOUSE MAIN PURPOSE OF THIS REWORK HAS BEEN TO UNDERTAKE SAFE

DESIGNING AND COST EFFECTIVE GROUNDING SYSTEMS FOR TIGA HEP.

TO ADOPT SAFE LIMITS OF TOUCH AND STEP POTENTIAL INSIDE THE POWER HOUSE WITHOUT SATELLITE GRID STATION AND SAVE COST.

TO CONSIDER SURFACE RESISTIVITY VALUE OF CONCRETE AS 10,000 Ω INSIDE THE POWER HOUSE AND THAT OF WASHED QUARTZ AS 5,000 Ω IN SWITCH YARD. WITH THE CHANGE IN THE SURFACE RESISTIVITY VALUES, THE SAFE LIMITS OF E TOUCH AND E STEP OF EARTH MAT WERE ALSO ACHIEVED IN 8MW PORTION OF POWER HOUSE AND THIS ALSO RESULTED IN REDUCTION IN THE MATERIAL REQUIRED FOR EARTH MAT.

FINAL DESIGN OF GROUND MATINPUT DATA

Sl. no

Symbol Descriptions Values PH 8MW & TRP

Values PH-2MW

Values ODY

Values SB & SG room

Units

1 IscFault current (60% of 31.5kA) 18900 18900 18900 18900 Amps

2 ρ Soil resistiviy 670.5 112 112 112 Ω-m

3 ρs

Surface layer resistivity 10000 10000 5000 10000 Ω-m

4 DSpacing between parallel conductor

1.5

3

4

4 Metre

5L

Length of PH, ODY & Service bay including switch gear room as per excavated profile

82.3

12.8

53.5

28.5

Metre

6B

Breadth of PH, ODY & Service bay including switch gear room

22.5

22.5

38.75

22.5 Metre

SUMMARY OF THE RESULTLocation Particulars Tolerabl

e Calculat

edLimits

Power house 8MW Step voltage (ESTEP ) 12619.39 V

> 7886.36 Safe

Touch voltage (ETOUCH )

3319.37 > EMESH calculated

1692.44 Safe

Grid resistance RG 0.42 Ω Safe Power house 2MW Step voltage (ESTEP ) 12611.39

V> 351.90 Safe

Touch voltage (ETOUCH )

3319.37 > EMESH calculated

340.97 V Safe

Grid resistance RG 0.053Ω SafeSwitch yard Step voltage (ESTEP ) 6466.71 > 488.51V Safe

Touch voltage (ETOUCH )

1170.78V > EMESH calculate

d

501.60 V Safe

Grid resistance RG 0.053 Ω SafeService bay & Switch gear room

Step voltage (ESTEP ) 12611.39 V

> 346.56 V Safe

Touch voltage (ETOUCH )

3319.37 > EMESH calculate

d

414.18

Safe

Grid resistance RG 0.053 Ω Safe

ISSUES FOR DELIBERATION OF CONFERENCE

THE DESIGN OF GROUNDING MAT OF POWER HOUSES AND EHV SUBSTATIONS IS CARRIED OUT LARGELY BASED ON IEEE80-2000 INCLUDING POWER HOUSE PROVISIONS OF IEEE 665-1997. THE DESIGN OF GROUND MAT ATTAINS CRITICALITY WHEN SOIL RESISTIVITY IS HIGH AND INHERITED SMALL GROUND AREA IN CASE OF SMALL POWER HOUSES (SPH). THE FOLLOWING TOPICS ARE LISTED FOR CONSIDERATION OF THE INTERNATIONAL CONFERENCE

SOIL RESISTIVITY UNIFORM SURFACE RESISTIVITY MEASURED AT THE SURFACE

OF POWER HOUSE AND SWITCH YARD WITH CONVENTIONAL METHOD OF MEASUREMENT (80 OHM-M)]

AS THE EXCAVATION AT HIGHER LEVEL FOR 2MW GEN UNIT AND 8MW GEN UNIT WAS COMPLETED AND SOIL RESISTIVITY WAS MEASURED AT BOTH THE GEN UNITS LEVELS, SWITCHGEAR CONTROL & SERVICE BAY AND ODY

THE SOIL RESISTIVITY AT ALL LOCATIONS EXCEPT 8MW UNIT WAS MEASURED AS 120 OHM-M WHEREAS SOIL RESISTIVITY AT 8MW UNIT WAS MEASURED AS 670 OHM-M.

THIS WAS COMPARATIVELY HIGH SOIL RESISTIVITY AND SMALL POWER HOUSE SURROUNDED AREA

DESIGN OF GROUND MAT IN IEEE 665, THE SAID STANDARD DOES NOT INSIST FOR

MEETING TOUCH AND STEP POTENTIAL REQUIREMENT INSIDE POWER HOUSE.

OPERATORS DURING WORKING IN THE POWER HOUSE DONOT STEP OUT OF POWER HOUSE AND DANGER OF DIFFERENT VOLTAGE GRADIENT CAN BE AVOIDED BY PROVIDING SOME ROUNDS OF GROUNDING MATERIAL (COPPER) AROUND THE POWER HOUSE.

THIS IS SIMILAR TO EXTENDING THE GROUND MAT OF S/S OUTSIDE THE FENCE TO PROTECT FROM DANGEROUS VOLTAGES WHEN UN-AUTHORIZED PERSONNEL TOUCH THE FENCE FROM OUTSIDE.

IT WAS CONSIDER TO USE SURFACE RESISTIVITY OF CONCRETE AS 10,000 OHM-M INSTEAD OF 5000 OHM-M. ON REVISION OF CALCULATION IT HAS BEEN OBSERVED THAT TOUCH AND STEP POTENTIAL AT ALL THE LOCATIONS MEET THE SAFE REQUIREMENT.

IT IS CONVENIENT FOR DESIGNER TO ADOPT 5000, 10,000OHM-M OR MORE AS THE SURFACE RESISTIVITY OF CONCRETE TO ACHIEVE TOUCH AND STEP POTENTIAL LIMITS THOUGH CONCRETE IS PROVIDED IN EVERY POWER HOUSE

GROUND RESISTANCE VALUE WHEN CALCULATED FOR EACH PORTION OF POWER HOUSE AND SWITCHYARD ARE WITHIN PERMISSIBLE LEVEL. WHEN CONNECTED TOGETHER AND RESISTANCE MEASURED, THE AVERAGE VALUE MAY BE FURTHER LESS.

REFERENCE IEEE 80-2000: GUIDE FOR SAFETY IN AC SUBSTATIONS GROUNDING IEEE 665-1995 : GIDE FOR GENERATING STATIONS GROUNDING BS 7430-1998 : CODE OF PRACTICE FOR EARTHING IS 3043 AS AMENDED : CODE OF PRACTICE FOR EARTHING

COVERING ELECTRICAL INSTALLATIONS CBI&P PUBLICATION NO 223 : DESIGN OF EARTHING MAT OF HIGH

VOLTAGE SUBSTATIONS NATIONAL ELECTRICAL SAFETY CODE ( NESC) -1990, 1997,2002

AND 2012 IE RULES 1956 AND ELECTRICITY ACT 2003 IACSIT International Journal of Engg Techn. Vol 4 No.3, June 2012, Earthing System Design of Small Hydro Power (SHP) Stn – A Rewiew By M.K.Singhal, S.N Singh

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