NBSIR 75-731

A Study of Air-Gap Breakdown at

28.5 Kilohertz

F. Ralph Kotter

Electricity Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

June 20. 1975

Final Report

Requisition Number N00039-3316-7105

Prepared for U.S. Naval Electronic Systems Command

Code PME-117-1022

Washington, D. C. 20360

NBSIR 75-731

A STUDY OF AIR-GAP BREAKDOWN AT

28.5 KILOHERTZ

F, Ralph Kotter

Electricity Division

Institute for Basic Standards

National Bureau of Standards

Washington, D. C. 20234

June 20, 1975

Final Report

Requisition Number N00030-331 6-71 05

Prepared for U.S. Naval Electronic Systems Command

Code PME-117-1022

Washington, D, C, 20360

U.S. DEPARTMENT OF COMMERCE. Rogers CB. Morton. Secretary

NATIONAL BUREAU OF STANDARDS, R, chard ft. Roberts. Director

TABLE OF CONTENTS

____

I. Introduction 1

II. Literature Search 3

III. Characteristics of Typical Protective Gap Assemblies 7

IV. VLF Performance of "Standard Gaps" 12

V. Conventional Protective Gap Systems 19

VI. Multiple-Gap Assemblies 21

A. The Rod-plane Array 21

B. The Rod-rod Array 25

C. Impulse Tests 27

VII. Summary 29

References 30

Appendix A. Annotated Bibliography 31

Appendix B. Data Tabulation 35

Appendix C. Gap Settings 39

I. Introduction

Air plays an important role as an electrical insulating medium in any

application involving use of high voltage at frequencies from DC to UHF.

While the influences of such variables as the air composition, temperature,

density, the humidity, the shape, size and spacing of the electrodes, as veil as parameters associated with the applied voltage such as waveform,

pulse polarity, and others, have been studied extensively at power

frequencies during the past several decades, such studies still command a sizable share of the research efforts in the electrical power engineering

field. The influence of frequency on gas breakdown has been studied to a more limited extent and then, with few exceptions, only at relatively low

voltages. Most such studies have been academic in nature, motivated by a desire to understand the phenomena involved rather than directed toward the

provision of data useful in engineering design.

The unexpected behavior of a protective gap in a VLF (15-30 kHz) radio

antenna system prompted the study herein reported. It was initially directed

toward the development of engineering criteria which could be used in the

design of a satisfactory lightning and overvoltage protective gap system

for the tower base insulator. The first effort was a literature search

(summarized in Appendix A) to determine whether sufficient performance data

already had been published to permit satisfactory engineering design. The

literature search provided a useful insight into the problem and established

a basis for a better understanding of the phenomena involved but it also

revealed the need for additional experimental data. The VLF performance of

several protective gap assemblies proposed by the contractor (Section III)

provided further evidence of this need. The second major effort of this study was then directed toward the acquisition and analysis of data on the breakdown

characteristics of air as a function of electrode geometry to provide a more

solid basis than has heretofore been available for the engineering design of

equipment planned for use at high voltages at VLF, as well as toward the more

immediate objective—the development of a satisfactory lightning and over- voltage protective gap system for the existing VLF tower base insulator

assemblies. In addition to a documentation and evaluation of these two efforts,

this report discusses the performance of the protective gap system which was designed using the results of the study and explores briefly an alternative

system.

The work herein reported was made possible by the existence of the

excellent test facility constructed adjacent to the helix house of the VLF

transmitter at the Naval Radio Transmitting Facility (NRTF), Lualualei , Hawaii.

Designed by A. N. Smith of the Navy Electronics Laboratory Center (NELC),

San Diego, California, and constructed and operated under his direction, this

versatile facility utilizes the transmitter as a power source and the inductors

and variometers in the helix house as the fine tuning elements of a resonant

circuit which also includes the capacitor and inductor shown in Figure 1.

A detailed description of this facility is to be given in a forthcoming NELC

report

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II . Literature Search

The effect of frequency on the dielectric strength of air and of othergases has been of interest to many investigators since the early part of

this century. The earliest reference of significance found is contained

in Dielectric Phenomena in High Voltage Engineering by F. W. Peek (1915) [l].

Figure 2 taken from that reference shows a reduction of the order of 10% in

the breakdown voltage of a sphere gap over the range 0 to 1.3 cm spacing

at hO kHz relative to the 60 Hz value. Peek attributed the phenomena to

the formation of needle-like streamers from rough places on the sphere as

a result of the higher energy loss at the higher frequency and concluded

that with highly polished spheres there would be no effect of frequency.

This conclusion came into question as the results of other studies became

available

.

The technical literature prior to 1953 is well summarized in Meek and

Craggs Electrical Breakdown of Gases (1953) [2]. The effect of frequency

on the breakdown voltage of an air-gap had been shown to depend upon the

electrode geometry and upon the gap separation. Figures 3 and k present

representative results. It is to be noted that the frequency influence is

much larger if the electrode geometry is such as to produce a highly

non-uniform field—in which case a corona discharge precedes the breakdown.

The explanation for the frequency influence now generally accepted is

as follows: At a low frequency, ions in the gap are swept to the electrodes

during each half cycle of the applied voltage; however, as the frequency

is increased a point is reached where there is insufficient time for an ion

to cross the gap in half a cycle. Under these conditions a space charge may

build up in the gap with a resulting field distortion and consequent lowering

of the breakdown voltage.

The investigations summarized by Meeks and Craggs were intended, for the

most part, to contribute to a better understanding of the phenomenon and were

nearly all concerned with frequencies of the order of 100 kHz or higher and

with air gaps from a few millimeters to a few centimeters in length. The

fact that there appears to have been little interest in extending the studies

to larger gaps and correspondingly lower frequencies may well be explained by

the difficulty of meeting the requirement for a high-voltage power source at

frequencies considerably above the power frequency range. However, there have

been several studies which bear on the overall question, which in general tend

to confirm the explanation offered above for the frequency influence and to

justify extrapolation to the larger-gap, lower-frequency case.

Hepworth et at. (1972) [U] studied the effect of charged particles on impulse corona and breakdown. With non-uniform field gaps both short (l cm)

and long (l 1/2 meters), a significant influence of ionic space charge on

corona, formation and on breakdown voltage was observed. The effects were of

relatively long duration- -from a few seconds to several minutes.

By blowing air across a discharge gap into a charge detector, Fatehchand

(1957) [5] demonstrated the existence of a positive space charge prior to

breakdown when a high-frequency alternating voltage was applied across the

gap. He also investigated the average space-charge density as a function of the

frequency of the applied voltage. Zhukov (1965) [6] improved on Fatehchand'

s

experimental techniques and was led by the result of his measurements in the

vicinity of the "first critical frequency" to an empirical formula for

See also Appendix A.

•5
cm

Figure 3 . Breakdown voltage in air at atmospheric

pressure as a function of gap length between plates

and between points for frequencies of 50c./s., 0-5 Mo./s. 2 and 1-0 Mc./s.(Prom Meek and Cragga )

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calculating the breakdown voltage as a function of gap length and frequency

which agreed very well with his experimental data over the frequency range

.! to 10 MHz. It appears that in his work sufficient space charge to achieve

a reduction in breakdown voltage with frequency required an external source

of ionization.

While Fatechand and Zhukov worked with gaps at most a few centimeters in

length and with frequencies in the megahertz region, the fact that space charges

associated with corona discharges at 60 Hz on power transmission lines are significant at appreciable distances from the lines themselves had been

demonstrated by Carroll and Lu sign an [7] in 1927. Their work suggests that even at power frequencies one might anticipate a reduction of the voltage

breakdown of a long air gap as a result of space-charge distortion of the

electric field between the electrodes under circumstances where disturbing

factors such as wind are not significant.

The mobility of "small" ions at atmospheric pressure is accepted as about 2.5 cm/s per volt /cm [8]. Using this value one calculates 3.1 cm as the 28.5 kHz critical gap spacing for which an ion could just cross the gap

during a half cycle of an impressed voltage of 250 kV. Thus one would expect

that in somewhat larger uniform-field gaps the likelihood of space-charge

produced field distortion exists and that for these larger gaps the breakdown

voltage at this frequency might be expected to be significantly lower than

at 60 Hz.

6
III. Characteristics of Typical Protective Gap Assemblies

In the course of evaluating the performance of several protective gap

assemblies proposed by the contractor for protection of the NRTF Annapolis

and NRTF Lualualei VLF tower base Insulator assemblies, a limited amount of

data on both the 60 Hz and VLF (28.5 kHz) voltage breakdown as a function of

gap length for two different electrode geometries as well as some VLF

performance data . for several variations of a third arrangement were obtained.

In obtaining the values of breakdown voltage reported in this and in

the following sections, efforts were made to follow closely the procedure

outlined in American National Standard C68.1 Techniques for Dielectric Tests

which reads:

"h.3 Test Procedure. The voltage should be applied to the test

object starting at a value low enough to prevent overvoltages

due to switching transients. It should be raised sufficiently

slowly to permit accurate reading of the measuring instrument

,

but not so slowly as to cause unnecessary prolongation of the

stress of the test object near the test voltage. These requirements are, in general, met if the rate of rise above 75 percent

of the estimated final test voltage is about 2 percent of this

voltage per second."

Relative air density corrections were applied but none were made

for humidity variations. See Appendix B.

In one protective-gap arrangement both electrodes were spheres of nominally

equal diameter; the second was a horn-gap structure similar in general form to

that which has seen extensive use in the electric power industry; and in the

third, both electrodes were spheres but one was much smaller than the other.

The results obtained with an l8" (U5-7 cm) sphere gap mounted as shown

in Figure 5 (a) are given in Figure 5 (b). The solid curve was obtained by

interpolation from the internationally accepted standard sphere gap tables

(ANSI Standard C68.1 and IEC Pub. 52). The agreement with the standard curve

is somewhat better than might have been expected since the restrictions on the

proximity of the supporting structures specified in the ANSI Standard were not

met.

The 60 Hz data points represent the mean of at least three successive

trials for which the spread in the values obtained was less than 1%. At the

VLF frequency (28.5 kHz) the scatter in the breakdown values at a given spacing

was quite large. The data points plotted show the extreme values observed.

Of significance is the fact that at the 25-cm spacing two of the five trials

resulted in flashovers from the base-insulator top rainshield about 2.5 meters

to the ground plane without involving the sphere gap. The other three were

flashovers in the gap. At the 36-cm spacing all except two of the five trials

resulted in flashovers from the rainshield to ground rather than in the gap.

Only gap flashover values are shown in Figure 5.

There is no pre-breakdown corona discharge in a relatively small gap

between two large spheres. Thus the development of sufficient space charge

to significantly distort the field must depend upon stray ionizing radiation.

The experimental results indicate that the probability of such space charge

development during the time interval in which the voltage is being raised

toward the undistorted-field breakdown value is significant although not high.

7
(A)

BREAKDOWN

VOLTAGE

(kV-RMs)

GAP LENGTH (CM)

(B) Figure 5 Protective Gap. 45.7-cm (18-inch) Diameter Spheres

(a) Mounting arrangement, (b) Breakdown characteristics

8
These tests appear to indicate that there was a greater probability of spacecharge development initiated by corona discharges from electrode imperfections

or from nearby structures or by other stray ionization sources in the 2.5-m

gap between the rainshield and the ground plane than in the 0.36-m gap between

the spheres

.

A horn-gap configuration was also studied both at 60 Hz and at 28.5 kHz.

The electrodes were constructed of h" (10-cm) diameter aluminum tubing bent

and mounted as shown in Figure 6 (a). The results obtained are plotted in

Figure 6 (b). Each data point shown for the 60 Hz experiments represents the

average of at least three trials .with mean deviations of less than 3%. The

scatter in the dry 28.5 kHz test data was quite large; however, all the data

obtained for flashovers in the gap fell within the ranges shown on the figure.

Not indicated on the figure were several flashovers which occurred from the

high voltage electrode or the top rainshield to the ground plane or ground

electrode rather than in the gap during dry 28.5 kHz testing at gap settings

of 75 cm and larger.

The behavior of the horn-gap configuration was investigated under simulated

rain conditions with the results also shown in Figure 6 (b). Flashover seemed

always to develop from corona streamers originating at water drops on the highvoltage electrode surface and at both frequencies with the larger spacings

there were several flashovers to the ground plane rather than in the gap.

However, for flashovers which occurred in the gap the consistency was quite

comparable with that of the dry 60 Hz data.

In each of these gap arrangements the performance at 28.5 kHz relative

to that at 60 Hz , where the data permit such comparisons , can be explained on the basis of space-charge produced field distortion at the higher frequency,

Under dry conditions, the smooth surfaces of the electrodes are not distorted

by water droplets and the breakdown voltage appears to be independent of

frequency—except for a rather significant percentage of what might be called

anomalous flashovers which occur at 28.5 kHz. Under wet conditions the water

droplets on the electrode surfaces act as corona sources and provide a copious

supply of ionizing electrons and the breakdown voltage at 60 Hz as well as at

28.5 kHz is reduced. However, at the larger gap spacings the space-charge

field distortion discussed in Section II is significant only at VLF and the

breakdown voltage is very much lower at 28.5 kHz than at 60 Hz. The anomalous

flashovers which occur at 28.5 kHz under dry conditions are to be explained as indicated above.

Several modifications of an asymmetrical sphere-gap system in which the

high voltage electrode was one of the U5.7~cm spheres and the ground electrode

a much smaller sphere were investigated at 28.5 kHz only. Figure 7 (a) shows

a typical arrangement and Figure 7 (b) presents the results.

It is believed that the quite consistent performance exhibited when the

grounded sphere was 2.5 or 5 centimeters diameter resulted from an ample

supply of Ions in the gap. Audible corona from the small ball prior to

breakdown was always present. Note that under dry conditions with grounded

sphere diameters of 7-5 and 15 centimeters, the scatter in values was comparable

to that observed with the symmetrical sphere gap. Under wet conditions where

water droplets on the spheres distorted the surfaces, the breakdown voltages

appeared to be very nearly independent of the diameter of the grounded sphere.

9
(A)

10
(A)

400

300

BREAKDOWN

VOLTAGE

(kV-RMs)

200

;00

__

28.5 KHz

GROUNDED SPHERE

1.0 1.5 GAP LENGTH (METERS)

A/1

DIAMETER DRY WET

2.5CM A A

5 CM V

7.S CM

IS CM o

2.0

Figure 7 Protective Gap. Asymmetric spheres, (a) Counting

arrangement. <b) breakdown characteristics

11
IV. VLF Performance of "Standard" Gaps

In the design of apparatus and structures for use in power-frequency

high-voltage systems, engineers rely heavily upon characteristic curves showing "breakdown voltage as a function of gap spacing for guidance in

deciding upon safe spacings "between components. In addition to this use,

the well-documented data for sphere and so-called "standard" rod gaps

make them useful as voltage measuring devices.

In order to provide a comparable "basis for facility design at VLF,

several electrode arrangements for which the 60-Hz performance is well

documented were studied at 28.5 kHz. These included a 25-cm sphere gap,

a horizontal rod gap and a rod-plane gap.

The 25-cm spheres used were heavy brass spheres with sparking surfaces

in excellent condition. Care was taken to make certain the supporting

structure met the requirements of the standards documents which apply to the

use of sphere gaps as standards for voltage breakdown measurements (ANSI C68.1

and IEC Wo. 52). The test procedures recommended in the standards were followed as closely as possible. Corrections were applied for relative air

density but none for humidity. (See Appendix B). Figure 8 (a) provides the

details of the mounting arrangement used and Figure 8 (b) shows the results

obtained.

It is significant to note that except for a few anomalous flashovers

,

attributed to chance space-charge-induced field distortion, the breakdown

voltage is the same at 28.5 kHz as at 60 Hz for spacings up to a sphere

radius. The radius spacing is the limit for which a sphere gap is recommended

as a measuring instrument. The significance of the difference between the

60 Hz and 28.5 kHz curves for larger gap spacing is open to question, since

as the spacing is increased the influence of the surroundings becomes more

important, and the extent of that influence is not known in either case.

This good agreement between the VLF and 60-Hz performance was not found

in the case of "standard" rod gaps as Figure 9 (b) indicates. Figure 9 (a)

shows the physical arrangement which met the clearance specifications for a standard rod gap (C68.1, p. 2k) up to 1 meter gap length. The reduction in

breakdown strength at 28.5 kHz relative to that at 60 Hz increases markedly

with increasing gap spacing for gaps more than a few centimeters in length.

It appears that the copious supply of ionizing electrons resulting from the corona discharge at the ends of the rods is adequate to produce significant

space-charge field distortion.

Additional data were obtained for a sphere gap using the U5.T-cm diameter

aluminum spheres previously tested as a protective gap. The sparking surfaces

of these spheres were in good condition, however, it was not feasible to meet

all the standards specifications for mounting them. The mounting arrangement

is shown in Figure 10 (a) and the results of the testing are plotted in

Figure 10 (b). In view of the fact that the dimensions and spacing of the

supporting structures used for the VLF tests did not comply with the standards,

the significance of the apparent reduction in the maximum values of breakdown

strength at VLF relative to 60 Hz for gap lengths greater than the radius of one of the spheres is open to question. Unfortunately, a test series at 60 Hz

with the same physical arrangement was not feasible. However, the results

shjwn in Figure 5 obtained with a mounting arrangement where the influence of

the proximity of the supporting structures should have been even more disturbing,

lends confidence to the assumption that the interpolated curve for 60 Hz in

12
(A)

13
(A)

BREAKDOWN

VOLTAGE

(kV- rms)

.5 1.0 1.5 GAP LENGTH (METERS)

(B)

Figure 9 "Standard" (1/2-inch Square) Rod-rod Gap: (a) Mounting

ARRANGEMENT, (b) BREAKDOWN CHARACTERISTIC

ik
(A)

15
Figure 10 (b) is not seriously in error.

VLF "breakdown data for several rod-plane and sphere-plane gap configurations

were obtained. The physical arrangements and the results obtained are shown in

Figures 11 and 12. In those cases where 60 Hz data are available for essentially

identical geometries, they are included for comparison.

The rod-rod characteristic could not be extended above about 170 kV since

at higher voltages the flashover was preferentially over the longer path from

the high voltage rod to the ground plane rather than to the grounded rod.

Similar problems set an upper limit to the range of sphere-plane spacings for

which data could be obtained. At higher voltages than those shown in Figure 12,

the flashover path terminated on the sphere support rather than on the sphere

itself. These results, together with those shown in Figure 13 for round rods with

hemispherical ends which showed approximately sphere-gap characteristics up to

spacings and voltages at which significant pre-breakdown discharges developed,

and the "standard" rod-gap results at the higher voltages, are in accord with

the space-charge field-distortion theory already discussed.

16
(A)

BREAKDOWN

VOLTAGE

(kV-RM3/

IT
200 --

BREAKDOWN

VOLTAGE

(kV-RMs)

100 --

25 CM SPHERE- PLANE

7.5 CM SPHERE- PLANE V

60 Hz

28.5 kHz

ROD- PLANE

GAP LENGTH (METERS)

Figure 12 Sphere-plane characteristics (Rod-plane characteristic replotted

from Figure 11 for comparison. 60 Hz data from Bellaschi and Teague^)

100

VOLTAGE BREAKDOWN

(kV-RMs)

I.9CM ROUND ROD

WITH HEMISPHERICAL END'

1.27 CM ROUND ROD

WITH HEMISPHERICAL END

'STANDARD" ^ INCH (I.27CM) SQUARE ROD

DRY

_!_ 10 20 GAP LENGTH

30 (CENTIMETERS)40

Figure 13 Effect of rod-end geometry on rod-rod gap breakdown characteristics

18
V. Conventional Protective Gap Systems

The protective gaps supplied with the original "base insulator assemblies

for the NRTF Annapolis and. NRTF Lualualei VLF antenna towers were found to

flash over occasionally at VLF voltages substantially below the design values.

It was not possible to increase the spacing sufficiently to prevent anomalous

flashovers without sacrificing completely the protective aspects. The highvoltage electrode of the gap was the outer rim of the top rainshield which

had a major diameter of about h.3 meters and a minimum radius of curvature

in the vertical plane of about 7-5 centimeters. The ground electrode was a 10-cm sphere supported at the horizontal level of the rainshield by a 7-5-cm

vertical rod.

Efforts to improve the performance by substituting electrodes with larger

radii of curvature were only partially successful.

In the course of the redesign of the base insulator assembly, several

different protective gap configurations were studied. Two of them, the

^5.8-cm sphere gap and the horn gap were investigated both at 60 Hz and at

VLF with the results indicated in Figures 5 and 6. Several gaps in which the

electrodes were spheres but of unequal diameters were also investigated at

VLF (see Figure 7)

Consideration of the performance of these various gap assemblies at

28.5 kHz in view of the conclusions regarding space-charge field distortion

which could be drawn from the literature search led to the further conclusion

that for the application at hand it was probably not possible to design a single

gap operating at normal atmospheric pressure which would provide overvoltage

protection and yet withstand the desired operating voltages under both dry and

wet weather conditions without occasional anomalous flashovers.

The performance of the gap assembly consisting of a h'J . 5-cm diameter sphere

as one electrode and a 2. 5-cm or 5-cm sphere as the other was such as to nearly

meet the requirements and suggested investigation of two such gaps in series.

The dry flashover results of this investigation are shown in Figure ih. The

limitation appeared to be the radius of curvature of the high voltage electrode

which under rain conditions resulted in streamer formation from water droplets.

However, all flashovers were in the gaps and, as shown in the figure, it was successfully demonstrated that the breakdown voltage for two such gaps in

series is significantly higher than for a similar single gap of equivalent total

length. It is believed the fact that the effect on breakdown voltage of the same incremental change in gap length was different for the two gaps resulted from

lack of equal division of the total voltage between them. Further study of

this configuration was discouraged by the results of a wet test during which

there was extensive streamer development from water droplets on the high-voltage

sphere at about 250 kV with flashovers occasionally to the ground plane as well

as across the gap assembly.

19
(A)

i L i I i l_

.5 1.0 1.5 TOTAL GAP LENGTH (METERS)

<B>

Figure 11 Double asymmetric gap assembly: (a) Mounting

ARRANGEMENT . IB) BpEAKDOWU CHARACTERISTICS

20
VI . Multiple-gap Assemblies

A. The Rod-plane Array

Several versions of a multiple-gap assembly have been studied. The first

of these, shown in Figure 15 consisted of short gaps between hemispherical

electrodes. Under dry conditions the performance of this array with VLF

appeared to be entirely satisfactory; however, rain droplets on the hemispheres

seriously reduced the flashover level. An array which exhibited very nearly

the same flashover values either wet or dry is that shown in Figure l6. The

performance of such a system is highly dependent on the degree of uniformity

achieved in distributing the total voltage over the individual gaps in the array.

However, even when these efforts were only partially successful the overall

performance of the assembly proved satisfactory at VLF.

The improved version of this rod-plane multiple-gap assembly (Figure IT),

which had more nearly uniform voltage distribution and in which some adjustments

were made in individual gap lengths in an attempt to compensate for the

nonuniformity which remained, provided quite acceptable performance as shown

in Table 1.

TABLE 1

Flashover Voltage of

Rod-plane Multiple-Gap Assembly

Shown in Figure 17

(See Appendix C for voltage distribution

and gap settings)

Diameter of

hemispherical

rod ends

(cm)

1.3

1.6

1.9

Dry

Flashover

(kV)

268

290

327

Wet

Flashover

(kV)

23h

2U2

2l+3

The effect of variations in the radius of curvature of the rod end was

also studied using a single section (Figure 16) with the results shown in

Table 2.

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TABLE 2

Effect of Rod-End Curvature On Breakdown Voltage

Rod Gap Average Breakdown Voltage

(cm) (kV)

60 Hz 28.5 kHz

1.3-cm cap nut on a 10.9 1+7 1+9 (1+7 wet)

. 95-cm diameter round rod 8.3 38 1+0 1.1 33 33

1.3-cm round rod with 10.9 ^9 51

hemispherical end

1.9-cm round rod with 10.9 60 6l (1+7 wet)

hemispherical end 8.3 1+8 1+9

The data at 28.5 kHz were obtained at the NRTF Lualualei test facility

while the 60 Hz data were obtained in the laboratory at the National Bureau

of Standards. The values given are averages of at least six trials, and are probably correct within 3% under the precise conditions of the test. The

small differences between the corresponding 60 Hz and 28.5 kHz values are probably the result of systematic errors arising from the effect of nearby

conducting objects (which could not be made identical at the two locations)

on the field distribution between the electrodes. However, these systematic

errors remained constant in either location and thus at either frequency so that the variations with gap distance and with rod geometry are significant.

Over this range of gap settings, the breakdown voltage is demonstrated

to be independent of frequency as could have been predicted from the results

plotted in Figure 9 which show a marked effect of gap length on the frequency

influence

.

These results and those obtained with the complete assembly support the

conclusion that the breakdown is initiated by corona discharge from the rod.

Thus the breakdown voltage increases with increasing radius of curvature of

the electrode surfaces when dry but, since the water droplets on the electrode

surfaces act as corona sources, the breakdown voltage is very nearly independent

of the radius of curvature under rain conditions.

It appears that a breakdown voltage nearly independent of weather conditions

can be achieved with a multiple gap arrangement of this type if the radius of

curvature of the rod end and its spacing from the plane is such that a prebreakdown corona discharge develops under dry as well as wet conditions.

Of passing interest because of the contrast it demonstrates between 60 Hz

and VLF corona phenomena is the result of an experiment with a single section

of a wire-plane gap structure. This was prompted by work reported by Cookson

and Farish in 1972 [10] which took advantage of the "ultra-corona" or "Hermstein

glow" phenomena on a fine wire to greatly increase the breakdown strength of an air gap. In their experiments a 1.6 -mm diameter wire, in the form of a 0.125-meter

2k
diameter hoop was suspended over a plane conductor and the 60 Hz dry breakdown

voltage for the wire-plane gap so formed was found to be well over twice that

of a standard rod-plane configuration at the same spacing. This advantage

when dry disappeared under simulated rain conditions since then flashover was initiated by corona streamers off water droplets on the wire and the behavior

was essentially that of a rod-plane gap.

Since a degree of shielding from the rain is provided by the gap arrangements of Figure l8 and since water would drop from the bottom of a ring mounted

as shown, whereas the expected sparking point is at the top, a 8.U-cm gap of that

design (Figure 18 ) was tested dry both at 60 Hz and at 28.5 kHz with the results

shown in Table 3.

TABLE 3

Rod-ring Gap Performance

Frequency Breakdown Voltage

(kV)

60 Hz ^ 100

28.5 kHz 38

As shown in Table 2, the breakdown voltage for the same length gap with

a 1.3-cm hemi spherically ended rod is about ho kV. Thus the ring-plane gap

exhibits a breakdown value at 60 Hz well over twice that for the rod-plane

gap in confirmation of the findings of Cookson and Farish. However, since the

breakdown voltages for the rod-plane and ring-plane gaps are not significantly

different at 28.5 kHz it appears that the ultra-corona type of discharge either

does not exist or simply does not influence the breakdown at VLF.

B. The Rod-rod Array

Anticipating a significant polarity effect in lightning impulse testing

of a rod-plane structure, several rod-rod configurations were investigated to

the extent time would permit since a rod-rod gap assembly would be expected

to exhibit an impulse flashover characteristic nearly independent of the polarity

of the impulse.

The rod-rod gap assembly shown in Figure 19, consisting of ten 6.U-cm gaps,

exhibited at 28.5 kHz a dry flashover value of 26l kV and a wet flashover of

2l8 kV. However, the flashovers under wet conditions were all from the grading

ring to ground. It appeared that had time permitted, an assembly of such gaps

on a taller structure with more care taken in the design of the grading ring

would have shown satisfactory performance at VLF.

An attempt to evaluate the rod-rod concept at VLF with improved field

grading was made by installing a rod gap such as shown in Figure 20 in each

section of the assembly of Figure IT- This, however, proved unsuccessful in

that with the fourteen gaps required, the spacing of each gap could be only

of the order of 2.5 centimeters. At that spacing it appeared that periodic

bridging of one or more of the gaps by a water stream occurred in the heavy

rain tests and the performance became erratic.

25
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bO

a

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Ph •H

Oh

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a

O P cti bO

rH CD H o £ Cm u

CD •H i CO

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CM

CDM

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rH

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00 6

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50

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26
C. Impulse Tests

While it was not feasible to attempt impulse testing of the gap assembly

shown in Figure 17, a quite similar assembly which used most of the same components and for which very nearly identical VLF performance could be

expected was tested at the Patuxent River Naval Air Test Center facility in

March 1975 following the procedures outlined in ANSI C68.1. The results

of those tests together with comparative data for the Continental Base

Insulator Assembly (CBIA) obtained at the A. B. Chance Laboratory in August 197^, are shown in Table h .

TABLE k

Flashover Voltages

Rod-Plane Array

(kV)

Positive

Impulse

Assured

Flashover

Gap System

700

Withstand

CBIA

1^90

(2!

Assured

Flashover

Gap System

660

Withstand

CBIA

870

(2:

Negative

Impulse

U80 1250 390 1070

(1) Probability of flashover approaching 100$.

(2) Probability of flashover approaching 0%

.

Tests of the gap system were made with it mounted adjacent to and in parallel

with a r.iockup of the CBIA. In the CBIA tests its two protective gaps were opened

sufficiently wide as to render them largely ineffective. Corrections for relative

air density and humidity as called for in ANSI C68.1 were applied. The raw data

and atmospheric conditions are recorded in Appendix B.

The impulse voltage wave shape was that specified by American National

Standard C68.1: 1.2 us wave front and 50 ps tail.

As the table indicates, the gap system would provide lightning protection

for the base insulator assembly.

Impulse tests of a rod-rod gap assembly were also conducted. The assembly

tested differed significantly from those for which limited performance data

were obtained at VLF. However, as shown in Table 5 the rod-rod structure does

exhibit the advantage over the rod-plane structure in that the flashover voltage

is very nearly the same for positive and negative polarity impulses and almost

independent of weather conditions.

27
TABLE 5

* Rod-Rod Array Flashover Crest Voltage;

Dry Wet

Positive

Impulse 685 kV 675 kV

Negative

Impulse 695 kV 680 kV

Eleven 6.U-cm gaps in series

28
VII . Summary

A careful study of the literature suggests that one might expect a significant reduction in the dielectric strength of an air gap at a frequency of 28.5 kHz relative to its strength at 60 Hz for gaps in excess

of several centimeters if the ion density is sufficient to establish a "trapped" space charge in the gap. Experimental studies of several electrode

geometries at 60 Hz and 28.5 kHz have confirmed this expectation.

It has been shown that a gap structure in which there is corona development prior to breakdown exhibits very nearly the same VLF flashover

voltage under wet as under dry conditions. It has further been demonstrated

that a protective gap array consisting of several short, non-uniform- field

gaps in series can be set to protect an insulator assembly, under either

wet or dry conditions, from VLF overvoltage and from lightning impulse

voltages without the risk of occasional anomalous flashovers during continuous

operation at rated VLF voltage. It has not proven possible to achieve such

protection and reliability with a single open-air gap.

29
REFERENCES

I . !' . W. Peek, Jr. , Dielectric Phenomena in High Vol tape Engineering . McGraw-Hill

Book Co., Inc., New York, p. 10?, 1915.

2. J. M. Meek and J. D. Craggs, Electrical Breakdown of Gases . Oxford University

Press, London, Chapter IX, 1953-

3. Josef Kampschulte , "Luftdurchsehlag und (Jbersehlag Mit Weehselspannung Von

50 Und 100000 Hertz". Arch. F. Elektro. , Vol. XXIV, p. 525-552, 1930.

k. J. K. Hepworth, R. C. Klewe, and B. A. Tozer, "The Effect of Charged Particles

on Impulse Corona and Breakdown in a Divergent Field Gap". Proc. 2nd Int. Conf. on Gas Discharges, p. 227, 1972.

5. R.R.T. Fatehchand, "The Electrical Breakdown of Gaseous Dielectrics at High

Frequencies". Proc. l.E.E. (London), vol. C10U, pp. U89-^95, 1957.

6. A. A. Zhukov, "Formation of a Space Charge in a High-frequency Electric

Field in Air". Soviet Physics —Tech. Phys . (Trans from Zhurnal Teknichesko,

Fiziki), vol. 10, pp. Il6-ll8, 1965.

7. J. S. Carroll and J. T. Lusignan, "The Space Charge That Surrounds a Conductor

in Corona". Trans. A.I.E.E., vol. U6, pp. 50-58, 1927.

8. L. B. Loeb, Electrical Coronas . U. of California Press, Berkeley, p. Ul 5

-

9- P. L. Bellaschi and W. L. Teague , "Impulse and 60 Cycle Strength of Air"..

Elect. Eng., vol. 53, pp. I.638-I6U5, 193^.

10. A. H. Cookson and 0. Farish, "Breakdown of Wire-Plane, Disc-Plane, and Rod-Plane

Gaps in Air". Proc. Int. Symp. on High Voltage Technology, Munich, p. 273, 1972.

30
Appendix A

ANNOTATED BIBLIOGRAPHY

ON

AIR-GAP BREAKDOWN AT FREQUENCIES ABOVE 60 Hz

(Arranged Chronologically)

Books

"Dielectric Phenomena in Electrical Engineering," F. W. Peek, Jr. McGraw-Hill

Book Co., Inc., New York (1915) pp. 106-107 . Mention of the effect of frequency.

"Fundamental Processes of Electrical Discharge in Gases," L. B. Loeb. John Wiley

and Sons, Inc., New York (1939). Qualitative interpretation of experimental

data showing reduction in breakdown voltage with increasing frequency.

"Electrical Breakdown of Gases," J. M. Meeks and J. D. Craggs. Clarendon Press

(Oxford), (1953) pp. 37^-382. A brief summary of the literature to 1953-

"Electrical Coronas," L. B. Loeb, U. of Cal . Press (Berkeley), (1965) pp. 569-571.

A brief discussion of the physics involved leading to a reduction in flashover

voltage with increasing frequency.

Journal Articles

Clark, J. C. and Ryan, H. J., "Sphere Gap Discharge Voltages at High Frequencies."

Proc . Am. Inst. EE 33 (191*0 II, 937- Data for 7-inch spheres at gap spacings

up to 1 inch and at frequencies of 123, 255, and 6l2 kHz. In the frequency

range covered the high frequency breakdown was about U.5 kV below the 25 Hz

values over the range 20-50 kV.

Goebeler, E. , "Uber die dielectrischen Eigenschaften der Luft und einiger

fester Isoliermaterialien bei hochgespannter Hochfrequenz . " Arch. Eleck ik (192U)

^91. No difference was observed between dc and 100 kHz for 2 cm sphere and plate

electrodes at spacings up to O.h cm. With a needle gap a significant difference

between 500 Hz and 105 Hz (at 1.5 cm spacing) was recorded.

Reukema, L. E. , "The Relation Between Frequency and Spark-over Voltage in a Sphere-Gap Voltmeter." J. Am. Inst. EE U6 (1927) 13lk . With 6.2 cm spheres

spaced from 0.5 to 2.5 cm, some decrease in sparking voltage was observed

when the frequency was changed from 20 to 60 kHz. kHz values did not

differ from those at 60 kHz. U25 kHz values did not differ from those at 60 kHz.

Gutton, C. and H., "Sur la d'echarge electrique en haute frequence." Comp.

Rendus 186 (1928) 303. Coaxial cylindrical electrodes at 0.5 mm Hg pressure

at frequencies of 5 x lCT to 10° Hz.

Kampschulte, J., "Luftdurchschlag und Uberschlag mit Wechselspanuung von 50

und 100,000 Hertz." Arch Elektrotech 2h (1930) 525- Data for 1, 2.5, 5,

10, and 15 cm spheres as well as 30° points at spacings up to 3.*+ cm and

frequencies of 50 Hz and 75 and 110 kHz. Also given are data for point-to-point,

rod-to-rod, and point-to-plane gaps up to 20 cm spacings at 50 Hz and 75 kHz

(differences between the 50 Hz and 75 kHz values were of the order of a factor

of 2 at the wider spacings). Data are also given for 1-5 cm diameter round

31
platen with round and also with sharp edges at both frequencies as well as results Of flashover teats of insulating solid materials mounted between

iueh electrodei.

Lassen, H. , "Frequenzabhlngigkeit der Funkenspannung in Luft . " Arch, llektroteeh

25 (1931) 322. Uniform field gaps using 2.5 em spheres and 3 em plates up to

0. 5 em spacing. Demonstrates the existence of a transition region both above

and below which breakdown voltage is independent of frequency. (Transition

was to a lower breakdown voltage at the higher frequencies).

MiserS, F. , "Luftdurehsehlag bei Niederfrequenz und Hochfrequenz an verschiedenen Elektroden." Arch. Elektrotech 26 (1932) 123. 5, 10, and 15 cm spheres at spacings up to 7 cm and with frequencies of 50 Hz and ^28 and

850 kHz. The effect of frequency was larger with the smaller spheres amounting

to about 20$ at 6 cm spacing with the 5 cm spheres. Data are also given for

1.6U and 2.26 cm spheres to 18 cm spacing and for points and plates to 35 cm spacing. (A factor of 3 in breakdown voltage at 20 cm spacing is shown for

points )

Muller, F. , "Der elektrische Durschlag von Luft bei sehr hohen Frequenzen."

Arch . Elektrotech 28 (193M 3^1. 1 cm spheres up to 1/2 cm spacing at

frequencies of 9 x 10^ up to 2.5 x 10^ Hz. Data agree with Lassen except,

that evidence of a gradual increase in breakdown voltage following the

transition region is shown.

Fucks, W. , "Zundung bei Wechselspannung und bei pulsierenden Bestrahlung.

Zeit. Phys. 103 (1936). 709. A purely theoretical paper without presentation

of experimental data.

Thompson, J., "Sparking Potentials at Ultra-High Frequencies." Phil. Mag. 23

(1937) 1. Results for frequencies in the range 2 x 10" to 10 Hz at pressures

below 10 mm Hg.

Luft, H. , "Uberschlagspannungen bei Hochfrequenz mittlerer WellenlSnge an einfachen Anordnungen . " Arch. Elektrotech 31 (1937) 93. 5 and 10 cm sphere

gaps to 6 cm spacing at 50 and h.6 x 10^ Hz; points and point-to-plane gaps

to 30 cm spacings at frequencies of 50 and 3.8 x- 10^ Hz. (With the point

electrodes at 25 cm spacing a factor of about 3 in breakdown voltage was recorded )

Bocker, H. , "Die Durchschlagsenkung bei Hochfrequenz." Arch. Elektrotech 31

(1937) 166. A purely theoretical paper based on space charge considerations

leading to what the author calls satisfactory agreement with the experimental

work of Lassen.

Seward, E. W. , "The Electric Strength of Air at High Frequencies." J. I.E.E.

(London) 8U (1939) 288. 1/2 cm spheres at 50 Hz and 109, 600, and 900 kHz.

1. h cm spheres at 50 and 600 kHz maximum spacings of ho cm. An explanation

of the results is offered on the basis of space-charge accumulation.

Jacottet, P., "Zur Frage der Messung von Hochfrequenz spannungen und

Stoszspannungen kurzester Dauer mit der Kugelfunkenstrecke . " Electrotech Z. 92 (1939) 60. This appears to be simply a summary of uniform field data from

a number of authors.

32
Ganger, B. , "Der Hochfrequenzdurchschlag verdichteter Gase." Arch. Elektrotech

37 (19^3) 267. 1/2 and 5 cm spheres and 25° points vs. plate at frequencies

of 120 and 3^5 kHz. Measurements are given for several gases- at hO atmospheres

pressure

.

Hale, D. H. , "The Breakdown of Gases in High Frequency Electrical Fields."

Phys. Rev. 73 (19^8) 10k6. Theoretical study which is claimed to he in good

agreement with experimental data for frequencies greater than 10^ Hz at very

low pressures.

Fatehchand, R.R.T., "Positive-ion Formation in Air Prior to High-Frequency

Breakdown." Mature 167 (1951 ) 566. Demonstrates that a large positive ion

current can he collected by a probe in an air stream flowing through a gap

subjected to high frequency voltage prior to breakdown. Gap widths less than

1 mm and frequencies in the 2-20 MHz range.

Fatehchand, R.R.T., "The Electrical Breakdown of Gaseous Dielectrics at High

Frequencies." Proc. I.E.E. (London) ClOU (1957) ^89. Gaps of the order of

0.1 mm at frequencies between 3.5 and 30 MHz. Demonstrates that at pressures

below atmospheric the high-frequency breakdown voltage does not fall below

the 50 Hz value until the amplitude of the + ion oscillation has become much

smaller than the gap width. Explanation given in terms of positive ion

accumulation based on measurements of ion currents as outlined in his 1951 paper

( above )

.

Zhukov, A. A. , "Formation of a Space Charge in a High-Frequency Electric Field

in Air." Soviet Phys. —Tech. Phys. (Translation from Zhur . Tekh. Fiz. 35 (1965)

151). Study similar to that of Fatehchand (above) but based on what the author

considers an improved technique. Gaps of 0.6 to 1.8 mm and frequencies of

1.5 to 10.0 MHz.

Rasquin, W. , "Einfluss von uberlagerten hochfrequenten Wechselspannungen auf

die niederfrequenten Durchschlagspannungen von Luft . " ETZ - A, 87 (1966) 272.

The results of experiments in which 50 Hz and 150 kHz voltages were superimposed

on coaxial cylindrical electrode systems are reported. With a 9 cm outer

cylinder and a 3 mm inner electrode breakdown apparently dependent on the peak

voltage reached, but with a 1 mm center electrode breakdown occurred with a

2 kV, 150 kHz signal superimposed on a 50 kV, 50 Hz voltage while without the

150 kHz signal 80 kV at 50 Hz were required for breakdown.

Zhukov, A. A., "Positive Space Charge and Reduction in Breakdown Voltage in the

First Critical Frequency Region." Soviet Phys. —Tech. Phys. (Translation

from Zhurnal Tekhnich. Fiz. 37 (1967) 710). Discussion of mechanism of

electrical breakdown in parallel-plane gap 1 mm) at atmospheric pressure

and frequencies between 0.1 and 20 MHz — based on space-charge considerations.

Feser, K. , "Influence of Corona Discharges on the Breakdown Voltage of Air

Gaps." Proc. IEE (London) 118 (1971) 1309- Results are reported with 50 Hz

and with lightning and switching surge voltages applied to Uo, 50, and 70 cm rod-plane and rod-rod gaps. In some cases a large scatter in breakdown voltage

(up to 30%) was observed which was a function of the electrodes' shape (a

point or 2 cm sphere) spacing and waveform. An explanation for the results

is offered on the basis of the corona mode at the gap electrodes.

33
Kormal Reports

Bright, A. W. , "Corona and Breakdown at Frequencies up to 12 Megacycles per

Second." British Elect, and Applied Industries Res. Assoc. Report,

Ref. L/T 229 (1950). Not readily available in this country and therefore

not reviewed.

Muehe, C. I., "AC Breakdown in Gain." Lincoln Laboratory, Manaehusetts

Inititute of Technology, Technical Report No. 380, §6 Feb. 196" 5. Deali with

eleetrodiltii breakdown at microwave ffrequeneiei but eentaini a bibliography

of nearly 200 entriei (1891-19^3) which coven the entire frequency ipeetrum.

Feurnell, H. D. , "Das Entladungsvtrhalttn Von Luft In Koaxialen

Zylinderelektrodenanordnungen Bei Gleiehzeitiger Beanspruchung Mit GleichUnd (SinusfSrmigen) Wechselspannungn . " Doctoral Thesis, RheinisehWest fSlischen Technischen Hochschule Aachen (Germany) 1968. An extensive

study of corona and breakdown in a coaxial cylinder geometry with superimposed

direct and alternating voltages of frequencies: 50,500, 13 x 103

, 30 x 103

, 15k x 10 3

, 760 x 10 3 and 1.7 x 106 Hz.

3h
%>pendix B

DATA TABULATION

The data plotted in the figures in the body of the report were corrected for relative air density in accord with the recommendations

of ANSI C68.1; however, no corrections for humidity were made since none

are available for VLF and those recommended for 60 Hz apply only to

electrode geometries which differ significantly from those used in this

study. For this reason, the uncorrected data obtained in the more significant series of measurements, together with the atmospheric

conditions as recorded are tabulated in this appendix.

35
"STANDARD" (1/2" SQ.UARU) BOD-HOD GAP

Barometric Wet Dry

)bserved

breakdown standard

Date Pressure Bulb Bulb Gap Voltage Deviation

Tmillibars

(°f) (op]( em) (kvf (unbiased %)

Dry, 28.5 kHz

5/17/7^ AM 1011+.3 69 80.5 19.9 1.3

1013.

o9o Ao n Od . U 50 1 1 ft I1U • 9

90 Ikl i.i

120 159 .9* 1U0 166 l'.i

30 86.5 1.0

PM 1012.6 69 79 10 ^9.9 2.0

k.9 29.

2.1+

5/18/7U 1015.5 67.5 75.5 10 51.0 1.6

h.9 30.2 1.3

90 1U0 .7

Wet, 28.5 kHz

5/lS/T^ 1015.5 10 52.1 3.1

h.9 28.6 l.l* 19.9 73.7 1.5

30 86.7 2.2

90 1U3 • 9

NOTE: Each of the first six values of breakdown voltage tabulated above is

the mean of 10 observations. Each of the others is the mean of at

least 5 observations.
25-cm Sphere Gap

Dry, 28.5 kHz

Observed

Barometric

Date Pressure

Wet

Bulb

Dry

Bulb Ga.p

Breakdown

Voltage

Standard

Dev iat, i on (millibars

)

(°F) (°F) (cm) (kV) (unbiased, %)

5/15/7U AM 1013 2 6q 5 2

.

5 5? 6 . H 1015 .

0

70 76 n 6nUU. 7I s 1

h J_ 83.7 ] .2

-

)4 . 9 9 j • y 1.0

7 0 • 9

n

I • 110*

10. 0 xoy J. . d.

10.,0 l^o

i m )i <^ oo . 5 in(

9

12. 1+5 195 -\ )i

•

PM 1013.8 73 82 15. 0 •7 Late PM 1013.8 69 78 15. 0 219 .8

19. 9 ** 261 • 9

19. 9 180

19- 9 238

19- 9

* 193

5/16/T^ AM 101U.6 70.2 80-. 2 19. 9 26U .5

2k. 95 293 • 5 2k. 95 228 —

29. 9 312 .7

29- 9 260

PM 1012.6 69 75 35. 0 333 .5

ko.,0 3^0 1.0

ko.,0 302*

U5-, l 357 .14 • k%,1 308*

50.,0 362 1.1+

* Single measurement not included in the mean.

Mean of 6 observations. Each of the other values is the mean

of 10 observations.

37
MULTIPLE-GAP IMPULSE TESTS

Hod-plane Array

Positive

Impulse DryNegative

Impulse Dry

Positive

Impulse Wet

Negative

Impulse Wet

1018

1018

1018

1018

Wet

Bulb

56

56

Dry

Bulb

68

68.5

68.5

Observed "1005&"

Flashover Voltage

(kV crest ) .

kkO

klO

Rod-rod Array

Positive

Impulse Dry

Negat ive

Impulse Dry

Positive

Impulse Wet

Negative

Impulse Wet

986.5

985.5

985

985

62

61

67.5

65

63.5

63.5

665

680

675

680

38
Appendix C

GAP SETTINGS

IN TEST OF

MULTIPLE ROD-PLANE GAP ASSEMBLY

Gap Number Percent of Total Gap Length

(#1 is bottom gap) Voltage Across Gap (cm)

1 8.8 3.8

2 7.1 3.8

3 8.2 3.8

k 8.0 3.8

5 7.5 3.8

6 8.U 3.8

7 8.7 3.8

8 8.5 3.8

9 11.1 6.h

10 12.1 6.U

11 11.7 6.U

39
NBS:1MA (REV, 1-1 -I

ti ,. Bt f*T. ©f c©MM,

ilBUOGRAPHIC BATA

SMIHT

1, I'UHUt ATION OK REPORT NO,

NBSIR T5-T31

2. Gov't AtM eHNIOIl No,

3. Htu- ipitviu

f H Af-iM'fcHiun No,

4. TI 1 1 1 A N H SI M i l 11,1

A Study of Air-Gap Breakdown at 28.3 Kilohertz,

§• PiiM ivsil ion Dine July 20, 1975

6. Performing Organisation Code

7. AUTHORISE

F. Ralph Kotter

8. Performing Orgttn. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

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1 1 . ( lontrut't /Grunt No. NESC Req. No.

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12. Sponsoring Organisation Nairn1 and Complete Address (Street, City, State, ZIP)

Naval Electronic Systems Command

Washington, D.C. 20360

13. Type of Report & Period Covered 11-6-73 to

Final , present

.

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less (actual summary ot most significant information. If document includes a significant

bibliography or literature survey, mention it here.) Measurements of the electrical breakdown of both quasi-uniform and highly nonuniformfield air gaps at a frequency of 28.5 kHz are reported. Gaps between a variety of

electrode geometries ranged from a few centimeters to over two meters in length.

Breakdown voltages significantly below the corresponding 60 Hz values were observed

with electrodes for which appreciable pre-brea.kdown discharges occurred, and a pattern

of "anomalous" flashovers at considerably lower than the normal breakdown voltages

was noted with quasi-uniform field gaps. The results obtained appear to correlate

well with the data found in the literature for higher frequencies but lower voltages.

On the basis of the gap behavior observed, a prototype protective gap system designed

for application to the tower base insulator assembly of a VLF (15 to 30 kHz) radio

transmitter was tested and found satisfactory.

17. KEY WORDS (nix to twelve entries; alphabetical order; capitaltge only the first letter of the first key word unless a proper name; separated by semicolons)

Air gap; electrical breakdown; insulator flashover; lightning protection;

VLF antenna; voltage breakdown

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