IEEE SUBSTATIONS COMMITTEE ANNUAL MEETING CHICAGO, IL MAY 16, 2011 EFFECTS OF CONCRETE ON TOUCH AND...

IEEE SUBSTATIONS COMMITTEE ANNUAL MEETING

CHICAGO, ILMAY 16, 2011

EFFECTS OF CONCRETE ON TOUCH AND STEP VOLTAGES

IN SUBSTATIONS

PROJECT SPONSORSINITIAL PROJECT ON RESISTIVITY OF

CONCRETE FUNDED BY SOUTHERN COMPANY (PROJECT MANAGER – LANE GARRETT)

FOLLOW-UP PROJECT TO MEASURE TOUCH AND STEP VOLTAGES FUNDED BY EPRI (1020031) (PROJECT MANAGER – GEORGE GILA)

BOTH PROJECTS PERFORMED BY NEETRAC (PRINCIPAL INVESTIGATOR – SHASHI PATEL)

REASON FOR PROJECTSCONCRETE: FOUNDATIONS, DRIVEWAYS,

ADJACENT SIDEWALKSIEEE STD 80 SUGGESTS VALUES BETWEEN

30 AND 100 Ω-M FOR WET CONCRETETHIS SURFACE MATERIAL RESISTIVITY

RESULTS IN EXTREMELY LOW TOLERABLE TOUCH ANS STEP VOLTAGES

THE ISSUESHOW WET IS WET?DOES THE CONCRETE WICK ENOUGH

MOISTURE FROM THE EARTH TO BE CONSIDERED WET?

WHAT ARE THE EFFECTS OF GROUNDED OR UNGROUNDED METALLIC REINFORCEMENT IN THE CONCRETE

IS 30-100 Ω-M CORRECT? IF NOT, WHAT IS THE CORRECT VALUE? HOW DO YOU MEASURE IT?

CONCRETE RESISTIVITYSeven slabs and eleven cylinders poured from same

mix of concrete, with strength rating of 4000 psi and aggregate approximately 19mm (3/4 in) gravel

Some slabs poured on a conductive substrate, while others poured on non-conductive substrate to simulate either highly conductive or highly resistive underlying soil

Some slabs reinforced with rebar, some with welded-wire mesh and some poured without reinforcement.

Some cylinders poured with wire mesh electrode placed horizontally at various heights within cylinder.

SLABS IN TANK, NO WATER

CONCRETE RESISTIVITYSlab REINFORCEMENT SUBSTRATE

A NA Conductive B NA Non-conductive C NA Conductive D Rebar Non-conductive E Rebar Conductive F Wire Mesh Non-conductive G Wire Mesh Conductive

Cylinder DIMENSIONS ELECTRODE PLACEMENT

A152mm diameter

229mm high(6in diameter

9in high)

NAB

C

D

E 127mm (5in) from bottom

F 152mm (6in) from bottom

G 178mm (7in) from bottom

H 203mm (8in) from bottom

I 152mm diameter229mm high(6in diameter

9in high)

NAJ

K

RESISTIVITY MEASUREMENTSVOLUME RESISTIVITY METHOD

FOIL ELECTRODES ON TOP AND BOTTOM OF SLABINJECTED CURRENT, MEASURED VOLTAGE

PROVED TO BE BAD TECHNIQUE, DUE TO HIGH RESISTANCE FILM

4-PIN METHODSMALL BOLTS EMBEDDED IN CONCRETE TO

IMPROVE CONTACT RESISTANCESPACINGS OF 51, 102, 203 and 305mm (2, 4, 8

and 12 inches

LAR /

MEASURING RESISTIVITY

CONTROLLING MOISTURESLABS AND CYLINDERS CURED FOR 158

AND 227 DAYS, RESPECTIVELYSLABS RESUBMERGED FOR 31 DAYS, THEN

LIFTED TO ALLOW CONTROLLED DRYINGWEIGHED PERIODICALLY TO DETERMINE

MOISTURE CONTENT

RESISTIVITY VS. MOISTURENO REINFORCEMENT, NON-CONDUCTIVE BOTTOM

RESISTIVITY VS. MOISTURENO REINFORCEMENT, CONDUCTIVE BOTTOM

RESISTIVITY VS. MOISTUREREBAR, NON-CONDUCTIVE BOTTOM

RESISTIVITY VS. MOISTUREREBAR, CONDUCTIVE BOTTOM

RESISTIVITY VS. MOISTUREWIRE MESH, NON-CONDUCTIVE BOTTOM

RESISTIVITY VS. MOISTUREWIRE MESH, CONDUCTIVE BOTTOM

MOISTURE WICKING EFFECTSCYLINDERS SUBMERGED TO SATURATE,

THEN PLACED VERTICALLY IN 4 INCHES OF WATER

DETERMINE IF FOUNDATIONS, ETC. CAN ABSORB ENOUGH MOISTURE FROM EARTH TO BE CONSIDERED “WET”

RESISTIVITY MEASURED USING VOLUME RESISTIVITY METHODELECTRODES EMBEDDED – SHOULD NOT BE

SUBJECT TO ERROR FROM HIGH RESISTANCE FILM

MOISTURE WICKING EFFECTS

MOISTURE WICKING EFFECTS

NOTE DARKER (WETTER) REGION 5-6 INCHES ABOVE WATER

DRY REGION AT TOP INDICATES LIMIT ON HOW FAR MOISTURE CAN WICK

CONCLUSIONS ON RESISTIVITYTHOROUGHLY WET CONCRETE (AFTER

FLOODING?) RANGES FROM 50 Ω-M (REINFORCED) TO 100 Ω-M ( NON-REINFORCED)

WITH REALISTIC MOISTURE CONTENT AND NO REINFORCEMENT, RANGES FROM 150 Ω-M (CONDUCTIVE UNDERLYING SOIL) TO 300 Ω-M (HIGH RESISTIVITY SOIL

WITH REALISTIC MOISTURE CONTENT AND WITH REINFORCEMENT, RANGES FROM 50 Ω-M (CONDUCTIVE UNDERLYING SOIL) TO 100 Ω-M (HIGH RESISTIVITY SOIL

NOTE: USING THIS, ALONE, MIGHT NOT ACCURATELY PREDICT TOUCH AND STEP VOLTAGE

TOUCH VOLTAGE ON CONCRETE

24x24 FT GRID WITH 4 MESHES INSTALLEDTHREE CONCRETE 6x6 FT SLABS POURED

IN MESHES, ONE LEFT AS SOIL, 4TH SLAB POURED OUTSIDE GRIDONE SLAB JUST CONCRETEONE SLAB WITH REINFORCING MESHONE SLAB WITH REBAR

REINFORCEMENT COULD BE CONNECTED OR DISCONNECTED FROM GRID

~ 20A INJECTED FROM REMOTE SOURCE

TOUCH VOLTAGE ON CONCRETE0.91m0.91m0.91m Ground Grid

- 4/0 bare copper

- Buried 0.457m deep

- exothermic connections

Slab 1

1.83m x 1.83m x 0.254m

No reinforcement

Slab 3

1.83m x 1.83m x 0.254m

Wire mesh

Slab 2

1.83m x 1.83m x 0.254m

Rebars

Soil 1

2.44m x 2.44m Plastic Cover for Controlled Soil Surface

(Cover removed during measurements)

Slab 4

1.83m x 1.83m x 0.254m

No reinforcement

0.91m

1.83m 1.83m3m

All Slabs

- 27 580kpa

- ~ 19mm gravel

- Slabs stick out 0.102m above earth

0.91m

1.83m

1.83m

0.91m

0.91m

0.91m0.91m

Rebar to grid connection

Wire mesh to grid connection


SOIL RESISTIVITY MEASURED~ 20A CURRENT INJECTED INTO GRIDTOUCH VOLTAGES MEASURED ALONG

DIAGONALS OF EACH GRID (RELATIVE TO GRID POTENTIAL)

GRIDS WITH REINFORCEMENT MEASURED WITH AND WITHOUT GRID CONNECTION

LAWN SPRINKLERS USED TO WET CONCRETE AND SOIL


Pad-1 Pad-2

Soil-1

Pad-4

Soil 2

9 pins to measure concrete resistivity at 2”, 4”, 8” and 12” spacing 2” @

4” @8”

1’

1’

1.24’

1.5’

1.5’

1.5’

1.5’

V11

V12

V13

V14

V15

V31

V32

V33

V34

V35

V25

V24

V23

V22

V21

1.24’

VC5

VC4

VC3

VC2

VC1

2.7’

2.7’ 1’

3’3’

Each 1’ spacing

3’

V41 V42 V43 V44 V45

VS1

VS2

VS3

VS4

VS5

Nine pins to measure concrete resistivity at 2”, 4”, 8”

and 12” spacing2” @

4” @8”

1’

1’

1.24’

1.5’

1.5’

1.5’

1.5’

V11

V12

V13

V14

V15

V31

V32

V33

V34

V35

V25

V24

V23

V22

V21

1.24’

VC5

VC4

VC3

VC2

VC1

2.7’

2.7’

3’

4’

3’

Each 1’ spacing

V41 V42 V43 V44 V45

BC2

BC1

BC4BC5

BC3

BC6

Pad 1 Pad 2

Pad 3Pad 4

Soil 1


Measured Iexp between grid and boots with Al foil placed at BC1, BC2, BC3, BC4, BC5 and BC6. Boots were worn by 200 lbs man.

Concrete pins are embedded ¼”Wx3/4”L threaded anchors. Iexp measured across 1000 Ω resistor as a voltage. GRIDS WITH REINFORCEMENT MEASURED WITH AND WITHOUT GRID CONNECTION


Ifault

Iexp

Rcontact

Rmutual

Igrid

Vtoc or Vtcc

C1

C2C3

1000 Ω

C2/C3

Iexp

Vtoc or Vtcc

Rthev

Vtoc

Rb=

1000 Ω

The circuit looking from the two contact points C1 and C2/C3


FROM STD 80:

FROM THE TEST CIRCUIT:

COMBINING THESE EQUATIONS:

sthevR 5.1

1000exp thevtoc RIV 1000exp IVtcc

tcc

tcctocthev V

VVR 1000


LARGE PIN SPACINGS INFLUENCED BY UNDERLYING SOIL – USE 2”-4” SPACINGSAVG WET ρ=196 Ω-MAVG DRYρ=264 Ω-M

SOIL MODEL:UPPER ρ=195 Ω-MUPPER ρ=1244 Ω-MH=26 ft

NOTE: WET CONCRETE NEARLY SAME AS UPPER LAYER SOIL


• Voltages increase as drying of slabs and soils occur. Voltages on concrete slabs increase at higher rate compared to those over soil areas.

• For a given environmental condition, voltages on Pad 1 (no reinforcement) and Pad 2 (rebars not grounded) are mostly higher compared to those on Soil 1 (controlled soil). Voltages on Pad 3 (ungrounded wire mesh) are close to those on Soil 1.

In wet conditions, ungrounded rebars and wire meshes tend to equalize voltages spatially (along diagonal). In the process, touch voltages are reduced in comparison with slab having no reinforcement (Pad 1). As concrete dries, ungrounded rebars become less effective with characteristics similar to Pad 1. Voltage equalizing characteristics of wire meshes remain the same.

Voltages reduce significantly when rebars or wire meshes are connected to grid. Due to their close spacing, wire meshes are more efficient in reducing these voltages.

EXPOSURE CURRENT ON CONCRETE

EXPOSURE CURRENT ON CONCRETE

• Between wet and dry conditions, the wet condition (8/3/09) causes the maximum exposure current at each location.

• The exposure currents reduce at a dramatic rate as the concrete continue to dry.

• Ungrounded rebars or wire meshes have little influence on exposure currents.

• However, grounding of rebars and wire meshes reduces the exposure current significantly.

• Wire meshes is more efficient than rebar in reducing exposure current.

• Overall, the exposure currents are higher on soil areas compared to concrete locations. Also, as drying occurs, the exposure currents reduce at a much slower rate compared to concrete areas.

THEVENIN EQUIVALENT RESISTANCE

THEVENIN EQUIVALENT RESISTANCE

• Wet conditions generally caused the minimum resistances in series with the feet.

• Thevenin’s resistances increase at a dramatic rate as the concrete dries.

• Slabs with ungrounded rebars or wire meshes do not show a definite advantage over the slab with no reinforcement.

• However, the influence of grounded rebars and wire meshes is mostly to increase the resistances in series with the feet.

• Thevenin’s resistances are lower for the soil areas compared to concrete locations. Also, as drying occurs, the resistances increase at a much slower rate compared to concrete areas.

WHAT DOES ALL THIS MEAN?

• STD 80 DERIVES EQUATION FOR EQUIVALENT BODY CIRCUIT RESISTANCE, THEN MULTIPLIES THIS BY ALLOWABLE BODY CURRENT TO GET ALLOWABLE TOUCH VOLTAGE

• THIS IS COMPARED TO COMPUTED (FROM EQUATIONS OR PROGRAMS) OPEN CIRCUIT TOUCH VOLTAGE TO DETERMINE IF DESIGN IS SAFE

Bsstouch ICE 5.1100050


• USING TEST RESULTS FOR SLAB 1 (NO REINFORCEMENT), WET ρs = 150 Ω-M ρsoil =195 Ω-M

• MEASURED Vtoc = 46V

• PRETTY GOOD AGREEMENT• WAS CONCRETE ALREADY DRYING?• ACTUAL FOOT RESISTANCE FACTOR IS 1.64, NOT 1.5• TOTAL EQUIVALENT BODY CIRCUIT RESISTANCE IS

13.9% LOW• WOULD GET BETTER AGREEMENT WITH ρs = 200 Ω-M

(42V)

VEtouch 6.3903236.0*150*0.1*5.1100050


• USING TEST RESULTS FOR SLAB 3 (UNGROUNDED WIRE MESH), WET ρs = 50 Ω-M ρsoil =195 Ω-M


• NOT SO GOOD AGREEMENT• WOULD GET BETTER AGREEMENT WITH ρs = 200 Ω-M

(42V)• ACTUAL FOOT RESISTANCE FACTOR IS 1.64, NOT 1.5• TOTAL EQUIVALENT BODY CIRCUIT RESISTANCE IS

34.3% LOW

VEtouch 8.3403235.0*50*0.1*5.1100050


• USING TEST RESULTS FOR SLAB 3 (GROUNDED WIRE MESH), WET ρs = 50 Ω-M ρsoil =195 Ω-M


• GREAT AGREEMENT!!!• TOTAL EQUIVALENT BODY CIRCUIT RESISTANCE IS ONLY

7% LOW• WOULD GET OK AGREEMENT WITH ρs = 200 Ω-M (42V)

VEtouch 86.100173.*50*0.1*5.1100050

CONCLUSIONS• REASONABLY CONSERVATIVE VALUE FOR WET

CONCRETE (WITH OR WITHOUT REINFORCEMENT) = 200 Ω-M

• UNGROUNDED REINFORCEMENT GIVES ABOUT SAME BODY CURRENT AS PLAIN CONCRETE

• GROUNDING THE REINFORCEMENT IN CONCRETE SUBSTANTIALLY REDUCES BODY CURRENT

• BASED ON THESE TESTS, STD 80 EQUATION SLIGHTLY UNDERESTIMATES THE EQUIVALENT BODY CIRCUIT RESISTANCE

• COMPUTATION OF OPEN CIRCUIT VOLTAGE MUST USE MODEL THAT DEPICTS EFFECTS OF GROUNDED REINFORCING MATERIAL – VOLTAGE ON SOIL WILL GROSSLY OVERESTIMATE Vtouch

IEEE SUBSTATIONS COMMITTEE ANNUAL MEETING CHICAGO, IL MAY 16, 2011 EFFECTS OF CONCRETE ON TOUCH AND...

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