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.
RESISTIVITY MEASUREMENTS • VOLUME RESISTIVITY METHOD • FOIL ELECTRODES ON TOP AND BOTTOM OF SLAB • INJECTED CURRENT, MEASURED VOLTAGE • 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
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
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 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 ( 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 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
0.91m 0.91m 0.91m 0.91m Ground Grid - 4/0 bare copper - Buried 0.457m deep - exothermic connections 1.83m3m 1.83m 0.91m Rebar to grid connection Slab 1 1.83m x 1.83m x 0.254m No reinforcement Slab 2 1.83m x 1.83m x 0.254m Rebars 1.83m All Slabs - 27 580kpa - ~ 19mm gravel - Slabs stick out 0.102m above earth 0.91m 0.91m Soil 1 2.44m x 2.44m Plastic Cover for Controlled Soil Surface (Cover removed during measurements) Slab 3 1.83m x 1.83m x 0.254m Wire mesh Slab 4 1.83m x 1.83m x 0.254m No reinforcement 1.83m Wire mesh to grid connection 0.91m 0.91m TOUCH VOLTAGE ON CONCRETE
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
Pad 1 Pad 2 Pad-1 Pad-2 V11 V11 BC1 1.24’ 1.24’ V21 V21 V12 V12 V22 BC2 V22 1.5’ 1.5’ V13 V13 V23 V23 V14 V14 1.5’ 1.5’ Nine pins to measure concrete resistivity at 2”, 4”, 8” and 12” spacing V24 V24 9 pins to measure concrete resistivity at 2”, 4”, 8” and 12” spacing V15 V15 1.5’ 1.5’ 2” @ 2” @ V25 V25 4” @ 4” @ 1.5’ 1.24’ 1.24’ 1.5’ 8” 8” 1’ 1’ Soil 1 Pad 3 1’ 1’ Pad 4 Soil-1 2.7’ VC5 2.7’ VC5 V35 V35 VC4 VC4 V34 V34 BC3 Pad-4 VC3 VC3 BC4 V33 V33 BC5 VC2 VC2 V32 V32 V41 V42 V43 V44 V45 V41 V42 V43 V44 V45 VC1 VC1 V31 V31 1’ 2.7’ 2.7’ 3’ 3’ 3’ 3’ 3’ 4’ VS1 VS2 VS3 VS4 VS5 Soil 2 Each 1’ spacing Each 1’ spacing BC6 TOUCH VOLTAGE ON CONCRETE
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
Vtoc or Vtcc 1000 Ω C1 Iexp C3 C2 Rcontact Ifault Rmutual Igrid Vtoc or Vtcc Vtoc Rb= 1000 Ω Iexp Rthev C2/C3 TOUCH VOLTAGE ON CONCRETE The circuit looking from the two contact points C1 and C2/C3
TOUCH VOLTAGE ON CONCRETE FROM STD 80: FROM THE TEST CIRCUIT: COMBINING THESE EQUATIONS:
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 • 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 • 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 • 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
WHAT DOES ALL THIS MEAN? • 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)
WHAT DOES ALL THIS MEAN? • USING TEST RESULTS FOR SLAB 3 (UNGROUNDED WIRE MESH), WET ρs = 50 Ω-M ρsoil =195 Ω-M • 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 • 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