Transcript effects of concrete on touch and step voltages in substations

EFFECTS OF CONCRETE ON TOUCH
AND STEP VOLTAGES IN SUBSTATIONS
IEEE SUBSTATIONS COMMITTEE ANNUAL
MEETING
CHICAGO, IL
MAY 16, 2011
PROJECT SPONSORS
 INITIAL 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 PROJECTS
 CONCRETE: FOUNDATIONS, DRIVEWAYS, ADJACENT
SIDEWALKS
 IEEE STD 80 SUGGESTS VALUES BETWEEN 30 AND 100
Ω-M FOR WET CONCRETE
 THIS SURFACE MATERIAL RESISTIVITY RESULTS IN
EXTREMELY LOW TOLERABLE TOUCH ANS STEP
VOLTAGES
THE ISSUES
 HOW 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 RESISTIVITY
 Seven 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 RESISTIVITY
Slab
A
B
C
D
E
F
G
Cylinder
REINFORCEMENT
NA
NA
NA
Rebar
Rebar
Wire Mesh
Wire Mesh
SUBSTRATE
Conductive
Non-conductive
Conductive
Non-conductive
Conductive
Non-conductive
Conductive
DIMENSIONS
ELECTRODE PLACEMENT
152mm diameter
229mm high
(6in diameter
9in high)
NA
A
B
C
D
E
127mm (5in) from bottom
F
152mm (6in) from bottom
G
178mm (7in) from bottom
H
203mm (8in) from bottom
I
J
K
152mm diameter
229mm high
(6in diameter
9in high)
NA
RESISTIVITY MEASUREMENTS
 VOLUME RESISTIVITY METHOD
 FOIL ELECTRODES ON TOP AND BOTTOM OF SLAB
 INJECTED CURRENT, MEASURED VOLTAGE
  R  A/ L
 PROVED TO BE BAD TECHNIQUE, DUE TO HIGH RESISTANCE
FILM
 4-PIN METHOD
 SMALL BOLTS EMBEDDED IN CONCRETE TO IMPROVE
CONTACT RESISTANCE
 SPACINGS OF 51, 102, 203 and 305mm (2, 4, 8 and 12 inches
MEASURING RESISTIVITY
CONTROLLING MOISTURE
 SLABS AND CYLINDERS CURED FOR 158 AND 227
DAYS, RESPECTIVELY
 SLABS RESUBMERGED FOR 31 DAYS, THEN LIFTED TO
ALLOW CONTROLLED DRYING
 WEIGHED PERIODICALLY TO DETERMINE MOISTURE
CONTENT
RESISTIVITY VS. MOISTURE
NO REINFORCEMENT, NON-CONDUCTIVE BOTTOM
RESISTIVITY VS. MOISTURE
NO REINFORCEMENT, CONDUCTIVE BOTTOM
RESISTIVITY VS. MOISTURE
NO REINFORCEMENT, CONDUCTIVE BOTTOM
RESISTIVITY VS. MOISTURE
REBAR, NON-CONDUCTIVE BOTTOM
RESISTIVITY VS. MOISTURE
REBAR, CONDUCTIVE BOTTOM
RESISTIVITY VS. MOISTURE
WIRE MESH, NON-CONDUCTIVE BOTTOM
RESISTIVITY VS. MOISTURE
WIRE MESH, CONDUCTIVE BOTTOM
MOISTURE WICKING EFFECTS
 CYLINDERS 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
METHOD
 ELECTRODES EMBEDDED – SHOULD NOT BE SUBJECT
TO ERROR FROM HIGH RESISTANCE FILM
MOISTURE WICKING EFFECTS
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 RESISTIVITY
 THOROUGHLY WET CONCRETE (AFTER FLOODING?)
RANGES FROM 50 Ω-M (REINFORCED) TO 100 Ω-M ( NONREINFORCED)
 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 INSTALLED
 THREE CONCRETE 6x6 FT SLABS POURED IN
MESHES, ONE LEFT AS SOIL, 4TH SLAB POURED
OUTSIDE GRID
 ONE SLAB JUST CONCRETE
 ONE SLAB WITH REINFORCING MESH
 ONE SLAB WITH REBAR
 REINFORCEMENT COULD BE CONNECTED OR
DISCONNECTED FROM GRID
 ~ 20A INJECTED FROM REMOTE SOURCE
TOUCH VOLTAGE ON CONCRETE
0.91m
0.91m
1.83m
0.91m
0.91m
Ground Grid
1.83m3m
- 4/0 bare copper
- Buried 0.457m deep
0.91m
- exothermic connections
1.83m
Slab 1
Slab 2
1.83m x 1.83m x 0.254m
1.83m x 1.83m x 0.254m
No reinforcement
Rebars
Rebar to grid connection
All Slabs
- 27 580kpa
- ~ 19mm gravel
0.91m
- Slabs stick out 0.102m
above earth
0.91m
Slab 3
1.83m
1.83m x 1.83m x 0.254m
Wire mesh to
grid connection
0.91m
Wire mesh
Soil 1
Slab 4
2.44m x 2.44m Plastic Cover
for Controlled Soil Surface
1.83m x 1.83m x 0.254m
No reinforcement
(Cover removed during
measurements)
0.91m
TOUCH VOLTAGE ON CONCRETE
 SOIL RESISTIVITY MEASURED
 ~ 20A CURRENT INJECTED INTO GRID
 TOUCH 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
TOUCH VOLTAGE ON CONCRETE
Pad 1
Pad-1
Pad-2
V11
1.24’
1.24’
Pad 2
BC1
V21
V12
1.5’
1.5’
V22
BC2
V13
V23
V14
1.5’
1.5’
1.5’
1.5’
Nine pins to measure concrete
9 pins to measure concrete
resistivity at 2”, 4”, 8”
resistivity at 2”, 4”, 8” and
2” @
12” spacing
and 12” spacing
4” @
8”
1’1’
V24
V15
V25
1.5’
1.5’
1.24’
Soil 1
Pad 3
2.7’
VC5
V35
VC4
V34
BC3
Pad-4
VC3
V33
1’
Pad 4
Soil-1
BC5
BC4
VC2
V32
V41 V42 V43 V44 V45
VC1
V31
1’
2.7’
2.7’
3’3’
3’
4’
3’
VS1
Soil 2
VS2
BC6
VS3
VS4
VS5
Each
Each1’
1’spacing
spacing
TOUCH VOLTAGE ON CONCRETE
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
TOUCH VOLTAGE ON CONCRETE
TOUCH VOLTAGE ON CONCRETE
The circuit looking from the
two contact points C1 and
C2/C3
Vtoc
or Vtcc
Vtoc or Vtcc
1000 Ω
C1
Vtoc
Rb=
Iexp
C2
Iexp
C3
1000 Ω
Rcontact
Rthev
Ifault
Rmutual
Igrid
C2/C3
TOUCH VOLTAGE ON CONCRETE
FROM STD 80:
Rthev  1.5 s
FROM THE TEST CIRCUIT:
Vtoc  I exp Rthev  1000 
Vtcc  I exp  1000
COMBINING THESE EQUATIONS:
Rthev
 Vtoc  Vtcc
 1000
 Vtcc



TOUCH VOLTAGE ON CONCRETE
LARGE PIN SPACINGS
INFLUENCED BY
UNDERLYING SOIL – USE
2”-4” SPACINGS
AVG WET ρ=196 Ω-M
AVG DRYρ=264 Ω-M
SOIL MODEL:
UPPER ρ=195 Ω-M
UPPER ρ=1244 Ω-M
H=26 ft
NOTE: WET CONCRETE
NEARLY SAME AS UPPER
LAYER SOIL
TOUCH VOLTAGE ON CONCRETE
TOUCH VOLTAGE ON CONCRETE
TOUCH VOLTAGE ON CONCRETE
• 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
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
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
Etouch 50  1000  1.5Cs   s IB
• THIS IS COMPARED TO COMPUTED (FROM EQUATIONS OR
PROGRAMS) OPEN CIRCUIT TOUCH VOLTAGE TO
DETERMINE IF DESIGN IS SAFE
WHAT DOES ALL THIS MEAN?
• USING TEST RESULTS FOR SLAB 1 (NO REINFORCEMENT),
WET ρs = 150 Ω-M ρsoil =195 Ω-M
Etouch 50  1000  1.5 *1.0 *150* 0.03236  39.6V
• 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)
WHAT DOES ALL THIS MEAN?
• USING TEST RESULTS FOR SLAB 3 (UNGROUNDED WIRE
MESH), WET ρs = 50 Ω-M ρsoil =195 Ω-M
Etouch 50  1000  1.5 *1.0 * 50* 0.03235  34.8V
• MEASURED Vtoc = 53V
• 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
WHAT DOES ALL THIS MEAN?
• USING TEST RESULTS FOR SLAB 3 (GROUNDED WIRE
MESH), WET ρs = 50 Ω-M ρsoil =195 Ω-M
Etouch 50  1000  1.5 *1.0 * 50*.00173  1.86V
• MEASURED Vtoc = 2V
• GREAT AGREEMENT!!!
• TOTAL EQUIVALENT BODY CIRCUIT RESISTANCE IS ONLY 7% LOW
• WOULD GET OK AGREEMENT WITH ρs = 200 Ω-M (42V)
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