PUBLICATIONS A111D7

=

- - I IMBS1R 84-28S5

Calibration Check of Bonneville

Power Administration SO-Hz

Electric Field Exposure Monitor

and Measurement of Its Surface

Field Enhancement for Various

Uniform and Nonuniform

Operating Configurations

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and Electrical Engineering

Eiectrosystems Division

Gaithersburg Maryland 20899

March 1 984

Issued June 1 984

" ad for: ^

jville Power Administration

100 nd, Oregon 97208

. 1)56

84-2335

NBSIR 84-2885

CALIBRATION CHECK OF BONNEVILLE

POWER ADMINISTRATION 60-Hz

ELECTRIC FIELD EXPOSURE MONITOR

AND MEASUREMENT OF ITS SURFACE

FIELD ENHANCEMENT FOR VARIOUS

UNIFORM AND NONUNIFORM

OPERATING CONFIGURATIONS

P. Michael Fulcomer

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and Electrical Engineering

Electrosystems Division

Gaithersburg, Maryland 20899

March 1984

Issued June 1 984

Prepared for: Bonneville Power Administration

Portland, Oregon 97208

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

TABLE OF CONTENTS

Page

LIST OF FIGURES iv

LIST OF TABLES iv

Abstract 1

SUMMARY 1

1. INTRODUCTION 2

2. BACKGROUND: SELECTION OF ELECTRIC FIELD MEASUREMENT METHOD,

THE EFEM, AND SURFACE FIELD ENHANCEMENT 2

3. TEST PROCEDURE AND APPARATUS 3

3.1 Calibration Check 3

3.2 Real Time EFEM (OLEFEM) 6

3.3 Surface Field Enhancement 7 3.4 Application of Uniform Field Data to

Nonuniform Field Conditions 7

4. RESULTS 11

4.1 EFEM Uniform Field Calibration Check 11

4.2 Applicability of Flush OLEFEM Uniform Field Calibration to

Flush Nonuniform Field Situations 13

4.3 Surface Field Enhancement - Uniform Field 14

4.4 Applicability of Surface Field Enhancement Factors Obtained

in Uniform Field to Nonuniform Field Situation 15

5. DISCUSSION 16

6. REFERENCES 18
LIST OF FIGURES

Page

Figure 1. Electric field sensor patch (10 x 10 cm), and

EFEM located in holder flush with bottom plate

of parall el -pi ate structure 6

Figure 2. Metal tower used to produce nonuniform fields

between the parallel plates. The OLEFEM is

mounted as a "bump" on the top surface 8

Figure 3. Parallel plate arrangement with plates spaced

at approximately 60 cm and the metal tower

installed for nonuniform measurements 8

Figure 4. Four section sensor to be used on tower is

mounted in bottom plate of parallel plate

structure for calibration 10

Figure 5. Surface field enhancement as a function of

EFEM sensor distance above grounded surface

for uniform and nonuniform field configurations 17

LIST OF TABLES

Page

Table 1. BPA Design Electric Field Ranges for EFEM Bins 4

Table 2. NBS Determined Electric Field Values at the

Boundaries Between EFEM Bins 11

Table 3. Electric Field vs Frequency Readout for OLEFEM #R-8 12

Table 4. Comparison of Sensor Patch and OLEFEM Electric

Field Indications When Mounted Flush in Equivalent

Nonuniform Fields 13

Table 5. Surface Field Enhancement Determined for OLEFEM

in Uniform Field 14

Table 6. Surface Field Enhancement Determined for EFEM's

in Uniform Field 15

IV
CALIBRATION CHECK OF BONNEVILLE POWER ADMINISTRATION

60-Hz ELECTRIC FIELD EXPOSURE MONITOR

AND MEASUREMENT OF ITS SURFACE FIELD ENHANCEMENT

FOR VARIOUS UNIFORM AND NONUNIFORM OPERATING CONFIGURATIONS

P. Michael Ful comer

Abstract

This report presents the results of tests requested by the

Bonneville Power Administration (BPA) on a 60-Hz Electric Field

Exposure Monitor (EFEM) developed by their Instrumentation and

Standards Branch. The unit is designed to be worn on the body, such

as in a shirt pocket or attached to the clothing. The calibration of

two sample units is examined, information on surface field enhancement

(which results from the EFEM sensors' elevated position relative to

the surface, of the body) is presented, the effect of material covering

the sensor is specified, and the applicability of calibration and

operational information obtained in uniform fields to nonuniform

fields is investigated.

Keywords: calibration accuracy; Electric Field Exposure Monitor (EFEM);

enhancement; flush mounted; nonuniform field; parallel plates; sensor;

surface field enhancement; uniform field.

SUMMARY

This report presents the results of tests requested by the Bonneville Power

Administration (BPA) on a 60-Hz Electric Field Exposure Monitor (EFEM) developed

by their Instrumentation and Standards Branch. The unit is designed to be worn

on the body, such as in a shirt pocket or attached to the clothing. The

calibration of two sample units is examined, information on surface field

enhancement (which results from the EFEM sensors' elevated position relative to

the surface of the body) is presented, the effect of material covering the sensor is specified, and the applicability of calibration and operational

information obtained in uniform fields to nonuniform fields is investigated.

The uniform field calibration is found, with the exception of one point for

each sample unit, to be within 4% of the design value for one unit and within 5%

for the other unit. Surface field enhancement in a uniform field varies from

approximately 54% with the EFEM case in contact with a grounded surface down to

approximately 31% with the case separated 2 to 4 cm from the surface by a material with low (<2) dielectric constant. The comparable figures for two

different nonuniform field configurations are 51% to 52% enhancement with the

EFEM case in contact with the surface, down to 10% at an EFEM sensor distance of

6 cm from the surface. Enhancement caused by cotton and/or wool cloth over the sensor is relatively independent of field type and averages between 1.5% and

2.5% for one layer and between 4% and 5% for two layers.

The report shows that, with the exceDtion of enhancement caused by cloth over the EFEM sensor, surface field enhancement information determined in a uniform field cannot be applied to a nonuniform field without some modification. The nonuniform field information was obtained using a rectangj'^r

tower between parallel plates. Although it might be argued that such data : r

. / : be extrapolated to estimate enhancement for an E C EM placed on a human, the hu^an

form is not the same as that of the tower. Further testing employing a mor^

realistic geometry would be necessary to provide rigorous information.

1
1. INTRODUCTION

For a number of years, there has been concern over the possible biological

effects from long-term exposure to electric fields arising from the transmission

of electric power across the country via high voltage transmission lines. Of concern has been the level of exposure received both by workers in the field and

by people living near the transmission lines. A great deal of research has been

and is being done on the subject, but to date results are inconclusive and

sometimes contradi ctory. One failing of studies involving human subjects up to

the present has been the lack of instrumentation to obtain accurate electric

field exposure data. This may be due in part to the lack of agreement

concerning the manner in which the electric field interacts with the body (which

necessarily affects the method by which exposure is measured). In any event,

various electric field monitors have recently been developed in response to this

need, and one of them, the 60-Hz Electric Field Exposure Monitor (EFEM) from the

Bonneville Power Administration (BPA) Instrumentation and Standards Branch is

the subject of this report. Its calibration is checked and a determination made

as to the applicability of the calibration performed under uniform field

conditions for subsequent use in nonuniform field conditions.

The BPA Electric Field Exposure Monitor is in the shape of a small

rectangular box, approximately 7 cm x 5.7 cm x 1.6 cm, and designed to be worn

on the body, such as in a shirt pocket or attached to the clothing in some fashion. The box consists of a steel case with a copper foil sensing plate

attached to, but insulated from, one of the large sides to form a small parallel

plate arrangement. Every four seconds, analog circuitry detects the average

current induced between the plates by the electric field and places a "count" in one of eight memory locations or "bins", according to the magnitude of the

induced current. The induced current's relationship to the electric field is

determined by calibration and the memory locations can be adjusted to store

counts for different field strength intervals. At the end of a measurement

period (which can be as long as nine hours), the EFEM is inserted into a separate instrument for readout of the number of counts stored in each

bin [1,2].

Because of the field enhancement resulting from the sensors' elevated

position relative to the surface of the body, the electric field strength

indicated by the EFEM is higher than the actual field strength at the surface of

the body where the EFEM is placed. This effect will vary with mounting

position, distance from the body, material between the EFEM and the body (if

any), and material covering the EFEM (if any).

This report examines the uniform field calibration of two sample units,

determines the "surface field enhancement" for various EFEM configurations, and

provides information on how valid the uniform field calibration is for

measurements made under nonuniform field conditions.

2. BACKGROUND: SELECTION OF ELECTRIC FIELD MEASUREMENT METHOD,

THE EFEM, AND SURFACE FIELD ENHANCEMENT

The selection of the measurement method to best indicate electric field

magnitude in biological effect studies depends upon how the electric field

interacts with the biological systems. At the present time, the mechanisms for

the biological effects that have been reported in the literature are not well

known. There has been speculation by biologists that two possible mechanisms

may be (1) surface interactions (i.e., surface field mechanism), and

2
(2) interactions at cell membranes (which might be related to internal currents

induced by the field) inside the body. The EFEM makes measurements that are more closely related to the first mechanism. But again, there is more than one measurement approach that could be used to characterize the electric field. For

example, the unperturbed electric field to which the biological system (person)

is exposed, i.e., the field which existed prior to the entrance of the person,

could be measured, or the electric field at the surface of the person could be

measured. Although the EFEM indication is much closer to the second example, it measures neither one directly and its indication must be mathematically adjusted

to arrive at the desired information.

Previous studies [1,3,4] have determined factors (enhancement factor,

activity factor, etc.) by which the EFEM reading can be multiplied to arrive at an approximate value for the unperturbed electric field at the EFEM location.

The numerical values of the factors depend on such variables as where on the

body the EFEM is attached, the activity being engaged in during measurement, the

body impedance to ground, etc. This report, in addition to checking the

calibration accuracy of two sample EFEM units, determines a surface field

enhancement factor, i.e., the amount that the EFEM reading exceeds the actual

surface field at the location of the EFEM, and examines the variation of this

factor with uniformity of the field, the location of the EFEM on the body, the

distance from and the angle formed with the body surface, the materials (if any)

between it and the body surface, the materials covering the sensor, etc. The

EFEM reading will almost always be higher than the true body surface field

because its sensor is located above the body surface.

3. TEST PROCEDURE AND APPARATUS

3.1 Calibration Check

The eight memory locations or bins of the two EFEMS sent to NBS for

evaluation have been designed by BPA to register electric field levels as indicated in Table 1. For example, exposure to an electric field of 4 kV/m

should cause a count to be placed in bin #2 every four seconds. Determination

of the actual bin edges is a relatively lengthy process because, for each edge,

the EFEM must be subjected to a range of electric field values which will be

certain to contain that bin edge. Since the actual boundary might differ from

the value given in the table, the initial tests must cover a relatively wide

range of electric field values. Once the general area of the bin edge is

determined, further testing can locate the edge value to an acceptable narrow

electric field range. The problem is that to locate the boundary, counts must

always be obtained from fields that are both higher and lower than the

boundary. Narrowing the range too quickly may miss the boundary entirely,

thereby requiring additional testing.

Initial calibration of the EFEM by BPA was accomplished in a uniform field

generated between two parallel plates (bottom plate grounded) with the EFEM sensor flush with the bottom plate, and the EFEM case electrically connected to

the bottom plate. This is also the configuration which NBS used to check the

accuracy of the calibration for the two sample EFEM units.

The NBS parallel plates are approximtely 168 cm square and were spaced

40.0 cm apart for all uniform field measurements except those required to

determine the boundary between the bins representing the two highest elect'- 1 : field levels. The plate spacing was reduced to 33.15 cm for this latter

measurement in order to reduce the voltage required from the supply

transformers

3
Table 1. BPA Design Electric Field Ranges for EFEM Bins

EFEM sensor flush with surface, EFEM case connected to surface

Bin Bin Range Median Bin Value

Number (kV/m) (kV/m)

0 0 to 1 0.5

1 1 to 3 2

2 3 to 6 4.5

3 6 to 10 8

4 10 to 15 12.5

5 15 to 21 18

6 21 to 28 24.5

7 28 to 36 32

8

a >36 plus total running time

aThe number of counts in the overrange bin equals the difference between the

reading in bin 8 and the sum of readings in all the other bins (bins 0 to 7).

Previous studies [5] have shown that in the absence of nearby ground

planes, the electric field between parallel plates, where the shortest plate

dimension is at least twice the plate spacing, d, to be uniform and equal to V/d

within 0.4% (where V is the voltage between the plates) for areas on the bottom

plate which are at least one plate spacing away from the plate edge. The

parallel plate setup at NBS thus provides a relatively large central area with

good field uniformity. Since control and measurement instrumentation which is

located within 30 to 60 cm of the plate edges at one side is a possible source

of nonuniformity, an independent check of electric field over the central area

of the bottom plate was made with a flat plate probe. The field was uniform

within the above-stated 0.4%.

The electric field between the parallel plates can be calculated as the

quantity V/d where V is the voltage between the plates and d is the distance

between them, as noted above. The voltage is measured with a mirror-backed

analog electrostatic voltmeter with four ranges. Each electrostatic voltmeter

range was calibrated against an accurate (less than 0.1% uncertainty) digital

voltmeter which measured voltage scaled down by a high voltage divider system,

with an uncertainty of less than 0.01%. In spite of this, the combined effects

of calibration and scale reading errors produce an electrostatic voltage

measurement uncertainty between scale marks of 0.6%. Combined with the

±0.5% uncertainty in determining the distance between the parallel plates and

the possible 0.4% variation in electric field at the bottom plate from the

theoretical, the overall electric field uncertainty as determined by V/d is

approximately ±0.9%. The root-sum-of-squares (RSS) is used to arrive at this

overall uncertainty.

4
To reduce this uncertainty, and at the same time improve the repeatabi 1 i ty

of measurements, a sensor patch approximately 10 cm square was installed flush

with, but insulated from, the surrounding bottom plate at a location 73 cm from

each of the two nearest plate edges. The electric field, E, induces a current,

i, in the patch according to the relation

i = 2rfe AE , (1)

o where A is the area of the patch, e 0

is the permittivity of free space

(3.8542 x 10

-12

C

2 /N«m 2

) and f is the frequency (60 Hz). The current is

subsequently converted into a voltage by an operational amplifier circuit

according to the relation

where V0

is the operational amplifier output voltage and the value of B is a constant controlled by the circuit feedback resistor. Its value was measured by

inserting different levels of current into the amplifier and measuring the

correspondi ng output voltages. Combining equations (1) and (2) gives the

rel ation

E =

B 2nf £ A

o

(3)

where f and e 0 are known with negligible uncertainty. The uncertainties in B,

A, and V0 are less than ±0.3, ±0.4, and ±0.08% respectively so that the

uncertainty of E obtained by this method is less than ±0.5%.

As mentioned earlier, an independent check of electric field was made with

a flat plate probe which could be moved to different areas on the bottom plate.

Its dimensions are 8.5 cm square with a central sensor area of 5 cm square

surrounded by a 1.75 cm guard band. Its measurement uncertainty is less than

±0.5%. The flat plate probe measures the field in the same manner as the

stationary patch but uses a different operational amplifier. The difference

between the patch and the flat plate probe readings was less than 0.2%.

The total harmonic distortion of the electric field generated between the

plates was checked by means of a spectrum analyzer connected to the operational

amplifier output from the 10 cm square patch on the bottom plate. Current

induced into the sensor patch is proportional to the derivative of the electric

field, causing each harmonic at the operational amplifier output to be

multiplied in value by its harmonic number. Corrections to the individual

harmonic amplitudes were thus necessary before total harmonic distortion could

be computed. The total harmonic distortion is less than 0.6% at electric field

values to 25 kV/m and less than 1.0% for values up to 36 kV/m.

For the calibration check, the EFEM was inserted into a holder designed so that the EFEM sensor could be adjusted flush with the bottom plate of the

parallel plate apparatus (see fig. 1). The holder also permitted electrical

isolation of the EFEM case from the bottom plate, if desired, in order to see what effect this had on the calibration. The holder is located 72 cm and QQ cm from the two nearest plate edges, the opposite edges from which the sensor paten

is located. The shortest distance between the sensor patch and EFEM (edae to

edge) is approximately 19 cm.

5
Figure 1. Electric field sensor patch (10 x 10 cm), and EFEM located in holder

flush with bottom plate of parallel plate structure.

The EFEM begins registering counts in its various memory locations as soon

as it is removed from the charger/reader unit. The procedure followed for each

measurement was to start a timer as soon as the EFEM was removed (thereby

keeping track of time spent at zero or near zero field), insert it into its

holder in the bottom plate, energize the electric field (which had already been

adjusted to the next desired value), leave at that value for 60 seconds, and

then quickly change it to a somewhat higher value for another 60 seconds, etc.

The EFEM would be exposed to a series of perhaps five to seven different

electric field values, each for 60 seconds, and hopefully to values which fall

on either side of the bin edge being determined. The EFEM was then inserted

into the reader and notation made of the time spent in each of the bin ranges.

If, for example, a determination of the edge between bins #2 and #3 was underway, the EFEM might first be subjected to fields of 5.7, 5.8, 5.9, 6.0,

6.1, 6.2, and 6.3 kV/m, each for 60 seconds. When placed in the reader at the

conclusion of that series of measurements, there would be a certain number of

counts in bin #0 corresponding to the time spent in near zero field, and a certain number of counts in each of bins #2 and #3 (assuming 5.7 to 6.3 kV/m

encompassed the actual bin edge), which would indicate between which two

electric field values the actual bin edge fell, e.g., if approximately 75 counts were in bin #2 and 30 counts were in bin #3, it would indicate that the boundary

was between 6.1 and 6.2 kV/m. The EFEM would then be subjected to a series of

electric fields between 6.1 and 6.2 kV/m to further narrow the range and finally

determine the boundary to a precision of +0.2% or better.

3.2 Real Time EFEM (0LEFEM)

Measurements designed to determine the effect of different configurations

and locations on the EFEM indication would be very time consuming, as the

foregoing description of the EFEM calibration check procedure indicates. For

6
this reason, the BPA provided a modified EFEM in which the electric field

information is converted to light pulses and transmitted by fiber optics to a separate receiver which converts the information to a frequency that can be

measured by an ordinary counter in real time. This modified unit is referred to

in this report as the optic link electric field exposure monitor or OLEFEM.

The OLEFEM was also calibrated in the parallel plate structure and a relationship derived from which the electric field value could be calculated

from the frequency readout with an imprecision of less than ±0.3%.

The OLEFEM is nearly the same size as the regular EFEM. The nonconducting

optic cable is connected at the center of one of the short edges of the device.

Measurements were made to determine if the optic cable had any effect on the

electric field by placing the cable in various locations near and on top of the

electric field sensor plate. No effects were discernable.

3.3 Surface Field Enhancement

After calibration in the ground plane, the OLEFEM was placed on top of the

bottom plate, in the same location, and its indication recorded for two or three

of the same electric field values that were used in the initial calibration.

The increase in OLEFEM indication when located as a "bump" on the ground plane,

rather than being flush with the surface at the same location, is a measure of

the surface field enhancement. This procedure was repeated with the OLEFEM

separated from the bottom plate by various thicknesses of foam, which has a dielectric constant similar to that of air, and for one or two layers of cotton

and/or wool cloth. From this data a graph of surface field enhancement vs. distance from the ground plane is derived.

Determination of the electric field was made with the sensor patch before

the OLEFEM was placed in position because its presence as a bump has a small

(<0.5%) but measurable effect on the electric field indication from the sensor

patch. This is because the EFEM projection above the bottom plate tends to

distort the uniform field.

3.4 Application of Uniform Field Data to Nonuniform Field Conditions

In use, the EFEM will almost always be measuring nonuniform electric

fields. Even if the field over a certain volume is relatively uniform, the

entrance of a person into that volume causes the field at the surface of the

person to be nonuniform. For this reason, the applicability of calibrations and

surface field enhancement factors obtained under uniform field conditions (the

only easily reproducible way to obtain such data) to various nonuniform field

conditions must be examined.

To obtain nonuniform field information, a metal tower approximately 30.5 cm high, and 33.3 cm by 18.5 cm on its sides, was constructed with provision for

flush mounting of an electric field sensor patch either on top or on either of

the larger sides, and for mounting of the OLEFEM either flush or as a bump,

again either on top or on one of the larger sides. The parallel plate spacing

was increased to approximately 60 cm and the tower placed in the center between

the plates. To prevent corona, the edges of the tower were rounded by means of

wood quarter rounds set flush with the sides and top and painted with conduct '/e

paint (see figs. 2 and 3)

7
Figure 2. Metal tower used to produce nonuniform fields between the parallel

plates. The OLEFEM is mounted as a "bump" on the top surface.

Figure 3. Parallel plate arrangement with plates spaced at approximately 60 cm

and the metal tower installed for nonuniform measurements.
To compare the OLEFEM indication with the field at the surface of the

tower, an electric field sensor patch the same size as either the OLEFEM or EFEM

sensor was fabricated and placed in the same relative position on the tower as the OLEFEM. Thus if measurements were being made on the top of the tower, the

sensor patch was located flush with the top surface with its center

approximately 9.5 cm from one end and equidistant between the sides, while the

OLEFEM was located with the center of its sensor approximately 9.5 cm from the

other end and again centered between the sides. This arrangement can be

observed in figure 2. The same arrangement was used for measurements on the

sides of the tower; i.e., the electric field sensor was mounted flush on one of

the larger sides with its center approximately 9.5 cm down from the top edge and

centered between the short sides, while the OLEFEM was mounted on the opposite

side in the same relative position. To confirm that the positions were exposed

to equal electric fields, readings were taken with the sensor patch mounted in

each of the two comparable positions. The results agreed within experimental

error.

To obtain some indication of the degree of nonuniformity across the OLEFEM

sensor, the tower electric field sensor patch was constructed from four separate

smaller patches whose total area adds up to that of the OLEFEM sensor. The

dimensions of each small patch are the same as the full sensor in one direction,

but only one fourth of the full sensor in the other direction. The small patch

outputs can be measured separately, combined in pairs or all combined to give a total patch reading. The tower was also designed so that the long dimension of

the patch sections could be placed in either direction, i.e., parallel to either

the short or long dimension of the tower top plate when mounted on top, or either horizontal or vertical when mounted on the side.

Before use on the tower, the four-section sensor patch was checked in the

uniform field parallel plate structure using the large 10 x 10 cm patch as a reference (see fig. 4). An electric field would be set using eq (3) with V0

being the operational amplifier output for an input from the 10 x 10 cm patch.

When using the same operational amplifier circuit (thereby keeping B the same)

and same electric field, each small sensor patch would be connected to the

operational amplifier in turn, and the resulting V0 ]_, V0 2, V0 3 , and V 0 4

recorded. In addition, operational amplifier output readings were taken with sensor sections #1 and #2 combined, with sections #3 and #4 combined, and with

all four combined. This was done for five different electric fields between 3.0

and 36 kV/m. The results were compared with the V0 calculated for each

configuration based on a rearrangement of eq (3)

where E, B, f, and e 0 are all known and A was determined by measuring the

total sensor area and dividing by four. The value used for A would of course

depend on the number of sensor sections combined to achieve the reading.

The measured Voa n (all four sensor sections combined) agreed with the

calculated Voa n to within ±0.3%. Because of slight variations in size among

the four sensor sections, the measured V0 for three of the sections when

checked individually differed from the calculated V0 by an amount exceeding

9
the above ±0.3%. It averaged 0.0% for section #1, +0.4% for section #2, +0.6%

for section #3, and -1.2% for section #4, and remained fairly consistent at each

electric field calibration point, thus providing a reliable correction factor

for the reading from each section. (None was needed for section #1.)

Figure 4. Four section sensor to be used on tower is mounted in bottom plate of

parallel plate structure for calibration.

A holder was designed for the 0LEFEM, similar to that used for the EFEMs in

the parallel plate structure, so that the OLEFEM could be adjusted flush with

the surface of the tower, either on top or on the side. For measurements with

the OLEFEM sitting as a bump on the surface, a plain flat metal plate was substituted for the holder. Metal conductive tape on the back (inside the

tower) was used to hold the four section sensor and OLEFEM mounting plate or plain metal plates, as the case may be, in place on the sides of the tower.

Measurements were also made in which the OLEFEM was separated from the

tower surface by the same thickness of foam material or cloth as used in the

uniform field measurements. Double stick tape was required to accomplish the

desired configurations on the side of the tower and on occasion additional tape

was necessary across the top of the OLEFEM to the tower sides. The effect of

this tape on the measurements was checked by measuring the same setup on the

tower top, both with and without tape. Some types of tape did produce a measurable difference but a type was found which produced no discernable effect,

and this is the type used for all of the reported measurements.

To avoid effects from the OLEFEM, measurements of the electric field at the

surface of the tower were in all cases made with the four section sensor patch

before the OLEFEM was placed on or in the tower. In place of the OLEFEM and/or

its holder was a plain metal plate level with the tower surface. The electric

field was then switched off to allow placement of the OLEFEM in its desired

position on the tower. The electric field was then switched back on, restoring

it to its original value.

10
4. RESULTS

4.1 EFEM Uniform Field Calibration Check

Table 2 lists the bin edge electric field values, as determined in the NBS

uniform field parallel plate structure, for the 3PA EFEM units #R - 11 and #R— 15

mounted flush with the bottom plate. The first column designates the bin edge.

The second column lists the design electric field for that bin edge or boundary

and the final columns list the actual electric field determined at that boundary

for each EFEM, and the percentage difference between the actual and design

fields

Table 2. NBS Determined Electric Field Values at the

Boundaries Between EFEM Bins

EFEM sensor flush with surface, EFEM case connected to surface

Design EFEM #R-11 Percentage A EFEM #R - 15 Percentage A

Bin E-Field Actual E-Field Between Actual Actual E-Field Between Actual

Edge (kV/m) (kV/m) and Design (kV/m) and Design

0/1 1.0 0.883 + 0.3% -11.7 0.942 ± 0.3% -5.8

1/2 3.0 2.962 + 0.4% - 1.3 3.047 + 0.2% 1.6

2/3 6.0 6.037 + 0.2% 0.6 6.20 ± 0.3% 3.3

3/4 10.0 10.187 + 0.1% 1.9 10.374 ± 0.15% 3.7

4/5 15.0 15.337 + 0.1% 2.3 15.615 -U 0.15% 4.1

5/6 21.0 21.575 + 0.1% 2.7 21.805 ± 0.1% 3.8

6/7 28.0 28.925 + 0.1% 3.3 29.29 ± 0.2% 4.6

7/8 36.0 37.28 + 0.1% 3.6 37.60 ± 0.1% 4.4

The plus and minus percentage following the actual electric field boundary

value is the uncertainty caused by the spacing between applied electric field

values during the final series of readings at each boundary (see the discussion

under Calibration Check). This percentage could have been reduced further by

making another series of measurements, but it was felt that the marginal

improvement involved did not justify the extra time required.

The above calibration checks were performed with the EFEM sensor flush wit n the bottom plate and the EFEM case electrically connected to the grounded bottom

plate. Some of the boundaries were also redetermined with the EFEM case separated from the grounded bottom plate by an impedance consisting of a specific resistance value in parallel with the approximately 15 dF capacitance

inherent between the EFEM holder and the bottom plate. The boundary electric

field determined did not change from the original value by more than 1.0% jntl

the resistor value was increased to about 50 Mn. Since the impedance of *:re 1~ pF capacitance is approximately 180 Mn at 60 Hz, this result suggests that t*e

impedance between case and surface must be above approximately 40 Mr. ;50 M

parallel with 180 Mft) before a 1.0% change in EFEM indication occurs.

11
It should be noted that an increase in value of the electric field at the

boundary between EFEM bins is equivalent to a decrease in the resultant EFEM

electric field indications, or vice versa. For example, bin #3 of EFEM Unit

R - 15 had a range of 6.20 to 10.374 kV/m when its calibration was checked with

the sensor mounted flush with the bottom plate in a uniform field and with the

unit case electrically connected to the plate (the normal method). When the

upper boundary was checked again with the case electrically separated from the

bottom plate by approximately 100 Mfi, the boundary value had increased to

11.42 kV/m. 'This means that any electric field between 10.374 and 11.42 kV/m

which previously registered in bin #4 would now register in bin #3. The EFEM

indications for those fields have thus apparently decreased under the new condition.

In order to obtain information on the surface field enhancement phenomena, some additional tests were made with each EFEM unit mounted as a bump on the bottom

plate, directly above its previous flush mounted location. These results are discussed in a following section.

Table 3 lists the calibration results for the 0LEFEM mounted with its sensor flush with the grounded bottom plate and its case electrically connected

to the plate. Intermediate electric field values of 0.5 kV/m or higher can be

calculated by using one of the three linear equations shown below the table or by interpolation between points. Three separate straight line segments are used

for improved accuracy. The maximum calculation error when using these equations

to determine E from the frequency listed in the table is 0.23%.

Table 3. Electric Field vs Frequency Readout for 0LEFEM #R-8

0LEFEM sensor flush with surface, 0LEFEM case connected to surface

Electric Field (E) Frequency Readout (f)

Poi nt kV/m kHz

0 0 0.0462

1 0.5 0.0818

2 1.0 0.142

3 2.0 0.264

4 4.0 0.510

5 9.0 1.124

6 12.0 1.492

7 16.0 1.981

8 20.0 2.471

9 25.0 3.073

10 30.0 3.680

11 36.0 4.441

E = 8.306 f - 0.179 from points #1 through #2 1 where E is in kV/m

E = 8.146 f - 0.155 from poi nts #2 through #6

| and f is in kHz.

E = 8.237 f - 0.320 from poi nts #6 through #11 )

12
The electric field was set using the 10 x 10 cm sensor patch on the bottom

plate. The frequency readout is the midpoint of a ranae which averaged about

0.2% of the reading, e.g., readings between 3070 and 3076 Hz were obtained at 25.0 kV/m.

4.2 Applicability of Flush 0LEFEM Uniform Field Calibration to

Flush Nonuniform Field Situations

Before discussing results involving the EFEM mounted on or above a surface,

something must be said concerning the applicability of the flush mounted uniform

field calibrations to flush mounted nonuniform field conditions. Even though

the flush mounting method is not usually possible in a real situation, it serves

as a base for later experimental comparisons with situations where the unit is

mounted on or above the surface.

Table 4 shows the four-section sensor total electric field indication as calculated from V0 (operational amplifier output) when the sensor is mounted

flush with the tower top or tower side. Next to this is the electric field

indication calculated from the OLEFEM frequency readout, using the linear

equations of table 3. The OLEFEM was mounted in an equivalent position on the

tower with its sensor also flush.

Table 4. Comparison of Sensor Patch and OLEFEM Electric Field

Indications When Mounted Flush in Equivalent Nonuniform Fields

Four Section Sensor Patch OLEFEM

Mounting

Position

Vo

Vrms

Calculated E Vo x 6 . 28195 a kV/mFkHz

Calculated E kV/mAE

<V

To

Top of tower 0.6351 3.99 0.5103 4.002 0.3

II 1.9166 12.04 1.500 12.064 0.2

II 4.521 28.401 3.529 28.748 1.2

Side of tower 1.7001 10.68 1.304 10.467 2.0

dThis number is the calculated value of B/(2-rfe 0A) from equation (3) in the

Test Procedure and Applications section. The area. A, of the four section

sensor patch is 5.476 cm

2

.

A measure of the field nonuniformity on the top surface of the tower can be

obtained by noting the difference in readings between the outermost sections of

the four section sensor. When the four-section sensor was aligned so that the

long dimension of each section was parallel to the short side of the tower, the

difference in field readings was approximately 4%. When mounted on the tower

side with the section long dimension horizontal, the difference in reading

between the top and bottom sections was approximate ly 33%.

Examination of table 4 shows that, as expected, the flush mounted

calibrations hold for nonuniform field situations up to 33%. Maximum deference

between the sensor and OLEFEM indications is 2.0%.

13
4.3 Surface Field Enhancement - Uniform Field

Raising the EFEM or OLEFEM so that it rests either on the surface or above

the surface (as it must in most real situations) causes an i ncrease in the

OLEFEM indication or a decrease in the indicated boundary between EFEM bins.

The latter, as explained in a previous section, is equivalent to an increase in

EFEM indication. The amount of increase is given first for a uniform field

situation, and then results from further testing will indicate whether surface

field enhancement correction factors derived from the uniform field situation

are applicable to nonuniform field situations.

Results are also presented from an investigation of the effect of cotton

and/or wool placed over and behind the OLEFEM sensor and the applicability of

the measured effects in a uniform field to nonuniform field conditions.

In discussing the following results, the OLEFEM unit is mentioned much more

often than either of the EFEM units. However, it must be remembered that the

two are approximately equivalent, the only difference being that the EFEM units

are a slightly thicker 1.63 cm as opposed to 1.61 cm for the OLEFEM. The OLEFEM

is used in most of the testing because of the advantage it has in speed of

obtaining data. This was discussed more fully in the section on Test Procedure

and Apparatus.

The OLEFEM was placed on the surface of the bottom plate in the parallel

plate structure with sensor facing upwards, directly over the spot it occupied

when it was calibrated in the flush configuration. Table 5 shows that the

increase in indication registered is consistent over a range of electric field

values, and that the surface field enhancement factor (the increase in electric

field reading when the OLEFEM or EFEM is located as a bump on a surface over the

field actually present at that surface) is between 54% and 55%.

Table 5. Surface Field Enhancement Determined for OLEFEM in Uniform field

Surface

Electric

Field

kV/m

OLEFEM

Indication

Flush

kHz

Cal cul ated

E kV/m

OLEFEM

Indication

As Bump

kHz

Calcul ated

Eb

kV/m

Surface Field

Enhancement

[(Er-E)/E]%

1.0 0.142 1.0005 0.2075 1.544 54.3

9.0 1.124 9.026 1.724 13.89 53.9

25.0 3.073 24.992 4.736 38.69 54.8

The higher the OLEFEM sensor is raised above the grounded surface while

keeping the OLEFEM case electrically connected to the surface, the higher will

be the indication. For example, the OLEFEM was raised an additional 1.3 cm above the bottom plate surface by means of a metal spacer for a total sensor height of 2.91 cm, but the case remained connected to ground. Under these

conditions, the surface enhancement factor was determined to be 88.6%. This

shows that surface field enhancement is a function of the electric field sensor height above the surface.

14
If the OLEFEM case is separated from the surface, as it will be when

carried in a shirt pocket inside of its protective case, the surface field

enhancement factor decreases as shown by the solid line in figure 5. An

asymptote is eventually reached so that beyond about 5 cm from the surface

little further decrease is noted.

As a double check on surface field enhancement, the two EFEMs were each

mounted in turn as a bump in the uniform field, and a new determination made for

two of the eight bin edge electric field values for each unit. Table 6 shows

the results. It should be noted that the bin edge electric field values remain

fixed but that the surface electric field required to produce a particular value

of electric field at the EFEM sensor will vary depending upon the EFEM sensor location in relation to the surface. In the situation described here, the EFEM

is mounted as a bump on the bottom plate surface and hence the surface field is

enhanced, i.e., less surface electric field is required to produce a count in a parti cu 1 ar EFEM bin than would be required if the EFEM were mounted flush.

Many of the OLEFEM measurements described above were also repeated with a section of cotton cloth, wool cloth, or both placed over the electric field sensor to determine what effect this would have on the indication. In all

cases, the indication was increased, i.e., the electric field reading obtained

by an EFEM covered by shirt pocket, coat, etc., would need to be decreased an additional small amount to obtain the correct surface field value. A layer of

either wool or cotton by itself over the sensor adds between 1.5% and 2.5% to

the indication, while a layer of both adds between 4.0% and 5.0%.

Table 6. Surface Field Enhancement Determined for EFEMs in Uniform Field

Bin

Edge

EFEM

Checked

Boundary

E-Field Value

Flush (F)

kV/m

Boundary

E-Field Value

As A Bump (B)

kV/m

Calcul ated

Surface Field

Enhancement

[ (F-B)/B]%

1/2 R—1

2.962 1.88 57.6

2/3 R -15 6.20 3.938 57.4

5/6 R-ll 21.575 13.725 57.2

6/7 R - 15 29.29 18.675 56.8

The plastic pouch placed over the OLEFEM increased indications about 1.5%.

The pouch plus a layer of wool caused an increase of about 3.5%. The effects

appear to be additive.

4.4 Applicability of Surface Field Enhancement Factors Obtained

in Uniform Field to Nonuniform Field Situations

Many of the measurements described in the previous section usinq the 0 Ll : : under uniform field conditions were also repeated under controlled nonuniform

15
field conditions. The purpose of this is to see if the results of tests

performed in a parallel plate uniform field could be used as an indication of

what will happen in nonuniform field situations. Much of the data obtained is

summarized in figure 5 where percentage surface field enhancement is plotted

against the distance which separates the EFEM sensor from the surface upon which

a measure of electric field is desired. With the EFEM case in contact with the

surface (the sensor therefore being approximately 1.6 cm above the surface), the

surface field enhancement is approximately 54% in a uniform field and 51% to 52%

for two different nonuniform fields. The dielectric constant of the material

normally used to separate the EFEM case from the surface is close to that of air

or just slightly larger than one. Cotton and/or wool cloth was used behind the

OLEFEM for the very small spacings and the dielectric constants of these

materials are approximately 1.8 and 1.3 respectively. A higher dielectric

constant for the separating material causes a larger surface field enhancement

for an equal spacing.

The solid curve relates surface electric field enhancement to distance

above the surface for a uniform electric field. The dashed and dotted curves represent the same information for two different nonuniform field

configurations. The data for the dashed curve was taken with the OLEFEM mounted

on top of a 30 cm high grounded tower sitting between parallel plates spaced at about 60 cm, while the dotted curve data were taken with the OLEFEM mounted on the side of the same tower near its top.

Two additional measurements of electric field were made with the OLEFEM

mounted at a slight angle with respect to the side of the tower with the top of

the unit farther away from the tower surface than the bottom. This is a configuration which could easily occur when an EFEM is carried in a shirt

pocket. When making an angle of approximately 10° with the tower surface

(bottom of the sensor about 2.2 cm away and the top about 3.05 cm away), the

surface field enhancement was calculated as 30.5%. When making an angle of

approximately 15° (bottom of the sensor about 2.2 cm away and the top about

3.45 cm away), the surface field enhancement was calculated as 27%.

No measurable difference from the uniform field results was observed when

the OLEFEM sensor was covered by cotton and/or wool cloth in either of the

nonuniform configurations. As before, either cloth alone caused an increase of

between 1.5% and 2.5% in the indications that would have been observed without

the cloth, and both together caused an increase of between 4.0% and 5.0%.

Results with the OLEFEM enclosed in the plastic pouch were also similar to the

uniform field measurements, i.e., the pouch caused an increase in reading of

about 1.5% by itself and of about 3.5% when combined with a layer of wool.

5. DISCUSSION

The results show that calibration of an EFEM to measure surface electric

field when mounted flush in a uniform electric field is also applicable to that

EFEM when mounted flush in a nonuniform field. When mounted on or above the

surface, the EFEM indication is higher than the actual surface electric field by

a surface field enhancement factor. This factor can be determined for specified

conditions in a uniform field but, as figure 5 shows, cannot be applied without

some modification to a nonuniform field situation.

16
17

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

EFEM

SENSOR

DISTANCE

ABOVE

GROUNDED

SURFACE

(cm)

—
Figure 5 shows that the surface field enhancement for a given EFEM sensor- to-surface spacing is less for nonuniform fields and least for a top-mounted

unit as opposed to a side-mounted unit. Although it might be argued that the

information in figure 5 could be extrapolated to estimate enhancement for an EFEM placed on a human, the human form is not the same as that of the

rectangular tower used in tests to provide nonuniform field information.

Further testing employing a more realistic geometry would be necessary to

provide rigorous information.

The enhancement caused by the plastic pouch, cotton cloth, and/or wool

cloth over the EFEM sensor is relatively independent of field type and thus the

additional enhancement can be estimated depending upon the number of layers of

cloth, etc.

6. REFERENCES

[1] L. Anderson and M. Stevenson, "Static and Dynamic Laboratory Testing of BPA

Personal Electric Field Exposure Monitors in Shirt Pocket and Lanyard

Positions," BPA Report No. ER-81-08, June 1981.

[2] M. M. Stevenson, "The Hard Hat Mounted Electric Field Exposure Monitor:

Laboratory Performance Evaluation," BPA Report No. ER-82-13, April 1982.

[3] P. S. Wong and G. E. Gill, B. C. Hydro and Power Authority Report prepared

for Bonneville Power Administration, "Electric Field Exposure Program - BPA

Electric Field Exposure Monitor Enhancement Factor," R & D Project 7222-83,

File F130E3.4.2, June 1983.

[4] M. M. Stevenson, "Method of Calculation of Enhancement Factor for BPA

Electric Field Exposure Monitor (EFEM)," BPA Report No. ER-82-15, May 1982.

[5] F. Ralph Kotter and Martin Misakian, NBS report prepared for

U.S. Department of Energy," AC Transmission Line Field Measurements,"

Report No. HCP/T-6010/E1, Nov 1977. Available from NTIS,

Springfield, VA 22161.

18
NBS-114A REV. 2-80

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET i See nstructions)

1. PUBLICATION OR

REPORT NO. 84-2885/' (L)

2. Performing Organ. Report NoJ 3. Publication Date

June 1984

4. TITLE AND SUBTITLE

CALIBRATION CHECK OF BONNEVILLE POWER ADMINISTRATION 60-HZ ELECTRIC FIELD

EXPOSURE MONITOR AND MEASUREMENT OF ITS SURFACE FIELD ENHANCEMENT FOR

•VARIOUS UNIFORM AND NONUNIFORM OPERATING CONFIGURA I IONS

5. AUTHOR(S)

P. Michael Ful comer

6. PERFORMING ORGANIZATION (If joint or other than N BS, see in struct ion s)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

S. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

10. SUPPLEMENTARY NOTES

T Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most si gn i fi cant information . If document includes a si gn i fi cant

bibliography or literature survey, mention it here)

This report presents the results of tests requested by the Bonneville

Power Administration (BPA) on a 60-Hz Electric Field Exposure Monitor (EFEM)

developed by their Instrumentation and Standards Branch. The unit is designed to

be worn on the body, such as in a shirt pocket or attached to the clothing.

The calibration of two sample units is examined, information on surface field

enhancement (which results from the EFEM sensors' elevated position relative to

the surface of the body) is presented, the effect of material covering the sensor

is specified, and the applicability of calibration and operational information

obtained in uniform fields to nonuniform fields is investigated.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate <ey wo re. r , -.r— calibration accuracy; Electric Field Exposure Monitor (EFEM); enhancement; *1 jsf

mounted; nonuniform field; parallel plates; sensor; surface field enhancement;

uniform field

13. AVAILABILITY

J Uni imi ted

|X | For Official Distribution. Do Not Release to NTIS

] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

i Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGE:

15. Pr.ce

J t c OMM 4 )*»«