This paper gives the historical background of 250 year old question: should the Franklin rod tip be sharp or blunt ? The earlier works, the new field results from USA and own laboratory experiments are presented. The switching impulse tests were carried out with two different shapes of high voltage electrodes: rod or sphere and without additional dc bias voltage. In the arrangement with rod as high voltage electrode the smaller breakdown voltages were measured with sharp grounded electrode with negative impulse polarity. At the positive polarity the breakdown voltages do not depend on the shape of grounded electrode. The tests with spherical high voltage electrode showed, that breakdown voltages under negative polarity are practically the same for three different grounded electrodes. In spite of that, the remarkable differences in time to breakdown were noted

Chrzan, K L

English

Repozytorium Eny Politechnika Wrocławska

  
Scientific Papers of the Institute of Electrical Engineering Fundamentals 
 of the Wrocław University of Technology, No 40, Conference 15 
 
International Conference on Advances in Processing, Testing 
and Application of Dielectric Materials 
 
 
FRANKLIN RODS – SHARP OR BLUNT? 
 
 
Krystian Leonard CHRZAN   
               
Wrocław University of Technology, Poland   
 
This paper gives the historical background of 250 year old question: should the Franklin rod tip be sharp or blunt ? The 
earlier works, the new field results from USA and own laboratory experiments are presented. The switching impulse tests 
were carried out with two different shapes of high voltage electrodes: rod or sphere and without additional dc bias voltage. In 
the arrangement with rod as high voltage electrode the smaller breakdown voltages were measured with sharp grounded 
electrode with negative impulse polarity. At the positive polarity the breakdown voltages do not depend on the shape of 
grounded electrode. 
The tests with spherical high voltage electrode showed, that breakdown voltages under negative polarity are practically 
the same for three different grounded electrodes. In spite of that, the remarkable differences in time to breakdown were noted. 
         
Key words – lightning rod, protection zone, breakdown voltage  
 
1. INTRODUCTION 
 
The classical lightning conductor was applied first 
in 1752 [1]. Franklin’ discovery was inspired by the 
observation of corona discharges from a sharp grounded 
rod which was moved close to a charged ball. As a 
result of discharges a significant charge decrease on the 
solid state was found. Franklin proposed therefore the 
sharp tip of lightning rod in order to discharge the 
charge accumulated inside the storm clouds. The 
experts’ opinions were shared on this point already in 
the 18th Century. The Royal Society became involved 
and a committee was appointed to study the problem 
and make recommendations. The committee of eminent 
electricians, including Franklin, met and rendered its 
report, proposing sharp rods. One of the five members 
of the committee, Benjamin Wilson, took exception to 
the decision for sharp rods, on the ground that these 
would unnecessarily “invite” strokes, whereas rounded 
ends would not. With the coming of the American 
Revolution the protagonists took on political colour and 
king George III ordered blunts on the rods protecting 
the royal palace [2]. The sharp Franklin rods are shown 
in the drawings from Osinski’ handbook Sposób 
Ubezpieczający śycie i Majątek od Piorunów printed in 
1784 in Warsaw (fig. 1). 
 
  
   
  Fig. 1. Drawings of lightning rods from 18. Century [1] 
It seems, the blunt rods were used not only in 
England but in continental Europe too. Many old, 
representative buildings in Germany have been 
equipped with spherically tipped lightning rods (fig. 2a). 
It is not clear wheather such balls were put to improve 
the lightning protection efficacy or for decorative 
purposes only. There are small balls on many lightning 
rods protecting TV antennas in Germany (fig. 2b).   
 
 
  
                                 a                                        b 
 
Fig. 2. Blunt rods.  a) A house in Wrocław built in 1920s 
(when the town belonged to Germany) b) TV antenna 
protected by blunt rod, Stuttgart, Germany 
 
Prof. Szpor patented the anti-corona tip of lightning 
rod in 1972.  The tip in the form of a hemisphere, a 
basket or a toroidal ring shall decrease the electrical 
field to avoid corona discharges (fig. 3).  
 
 
 
Fig. 3. Tips of lightning rods patented by Szpor [3] 
K.L. CHRZAN 
 
Scientific Papers of the Institute of Electrical Engineering Fundamentals 
 of the Wrocław University of Technology, No 40, Conference 15 
218
2. THE TIP OF HIGH VOLTAGE 
    ELECTRODES 
 
The model lightning experiments carried out in the 
laboratory are conducted at about 100 times smaller 
distances and 10 times smaller voltages than  the 
dimension and voltages of real lightning. Therefore the 
laboratory results have to be treated with a care because 
they cannot be automatically extrapolated to the field 
conditions. 
During the tests carried out in 1930s with scale 
models it was observed that very long upper discharges 
develop from the grounded rod during tests with 
negative impulse voltages [4]. These discharges can 
reach even 40% of the gap distance. Therefore the tests 
carried out at the positive polarity are recognized as 
more representative than tests with the negative polarity.    
The lowest electrical strength is measured in the air 
gap between rod and plate electrodes. A lot of 
experiments conducted in this arrangement show the 
influence of rise time of the impulse voltage and the 
radius of the rod. When the switching voltage is 
switched on to the rod electrode, the first corona burst 
appears. Then, this discharge extinguishes (dark period) 
and ignites again (subsequent coronas). Usually, the 
brighter leader discharge develops before the voltage 
reaches the maximum value. When the streamer 
discharge, moving before the leader, reaches the 
grounded plate, then during the subsequent 10 µs the 
leader bridges the whole gap (final jump).  
It appears that at the early breakdown phases, the 
first or subsequent corona burst are not necessary. They 
depend on the electrode radius and on the rise time of 
the voltage. Even if these discharges totally disappear 
the breakdown voltage does not have to increase. The 
breakdown voltage of the rod – plate arrangement at the 
positive polarity and the critical rise time is given by the 
equation [5]: 
 
                             
d
U 81
3400
%50
+
=
                           (2)   
where:  U50% in (kV);    d  – gap distance in (m). 
 
 
 
 
Fig. 4. Breakdown voltage of the sphere-plate air gap  
as a function of electrode distance. The positive 
switching impulses with the critical rise time [5] 
At the first stage the increase in the rod tip does not 
cause the increase of the breakdown voltage. When the 
radius approaches a critical value, then the breakdown 
voltage increases. The critical radius is greater for 
longer air gap distances (fig. 4). 
 
3. OBJECTS, EXPERIMENTAL 
    PROCEDURE 
 
The measurements at High Voltage Laboratory of 
Darmstadt University of Technology were carried out 
with the electrode arrangements shown in the fig. 5a. 
The high voltage electrode was in the form of a rod 
(diameter of 5 cm) with a conical tip (cone angle of 
45°). Two types of grounded electrodes were chosen: 
the sharp electrode – the rod with a diameter of 2 cm 
and with a conical tip (cone angle of 15°, electrode E2 
in fig. 6) and the sphere electrode with a diameter of 20 
cm (E1). The breakdown voltages were measured at the 
lightning voltage 1.2/50 µs or switching surges 
200/2500 µs for different polarities, distances and 
grounded electrodes. 
In the test performed at High Voltage Laboratory of 
University of Stuttgart the electrode arrangement shown 
in fig. 5b was used. The diameter of high voltage 
electrode amounted to 0.7 m.  Three grounded 
electrodes were chosen as shown in fig. 6 – E2, E3, E4. 
The tip of electrode number E3 was rounded with 
a radius of 2 mm, the hemisphere electrode radius 
amounted to 8 mm.  
The breakdown voltages were measured at the  
negative polarity of switching surges 200/2500 µs for 1 
m distance between the high voltage sphere and the 
grounded electrode.   
 
3 or 4 m 1 m
HV, SI HV, SI ,
+  or
,
 
         a     b 
Fig. 5. Electrode arrangements and distances. Diameter 
of high voltage sphere 0.7 m 
 
 
      20 cm
    
 
            E1        E2             E3         E4 
 
Fig. 6. Grounded electrodes used 
FRANKLIN RODS – SHARP OR BLUNT? 
 
Scientific Papers of the Institute of Electrical Engineering Fundamentals 
 of the Wrocław University of Technology, No 40, Conference 15 
219
4.  RESULTS 
 
The breakdown voltages of air gaps are practically 
the same for both grounded electrodes at the positive 
voltages. Note that at the negative switching voltage the 
electrical strength is lower about 5% –10% for sharp 
grounded electrode than for the grounded sphere. Those 
measurements suggest that the sharp Franklin rod could 
be better than the lightning conductor with a relatively 
big sphere (diameter of 20 cm) at its tip.  
 
Table. 1. The comparison of breakdown voltages for the rod – 
point electrode and the rod – sphere electrode.  
Test arrangement from fig. 4a, grounded electrode E2 or E1 
 
Grounded 
electrode 
 
Gap 
space        
m 
Impulse 
shape, 
polarity 
U0% 
 
kV 
U100% 
 
kV 
Sphere E1 3 − SI 2060 2340 
Point E2 3 − SI 1860 2220 
Sphere E1 3 ⊕ SI 1010  1330 
Point E2 3 ⊕ SI 1045 1285 
Sphere E1 4 ⊕ LI 2200 2270 
Point E2 4 ⊕ LI 2150 2240 
Sphere E1 4 − LI 2500 2590 
Point E2 4 − LI 2470 2550 
 
U0% - withstand voltage, U100% - minimum voltage with the 
100% breakdown probability, explanation in text,  
SI –switching impulse, LI – lightning impulse 
 
Szpor got the similar result. He used two vertical 
rods with a diameter of 1 cm, each 0,5 high. One 
electrode had a sharp point and the second rod had a 
hemispherical end. The high voltage electrode has been 
represented as a rod of 1 cm diameter and 3 m long. At 
the electrode distance of 1,5 m the grounded point 
electrode attracted significantly more negative 
discharges than did the blunt electrode [6]. The 
lightning surge 8/70 µs with the amplitude of -1,75 MV 
was applied. At the positive polarity (breakdown 
voltage of +1,2 MV) more discharges were attracted by 
blunt electrode. 
The test results with switching impulses, especially with 
positive polarity have much greater spread that the 
results with lightning impulses. An example of 
breakdown distribution function under positive 
switching impulses shows fig. 7.  The positive 
breakdown voltage of the arrangement rod-sphere at the 
distance of 3 m was estimated according to the series 
method. The voltage amplitude was increased 25 kV for 
each next series. The distribution function in fig. 7 
suggests that the probability distribution is non-
symmetrical. Note that the estimated breakdown 
probability for voltage greater than 1170 kV is lower 
than 1. 
 
 
 
Fig. 7. Distribution function of breakdowns for the rod- 
grounded sphere arrangement at the distance of 3 m. 
Positive switching impulses 
 
These is due to known unpredictable behavior of 
positive switching discharges. The path of positive 
discharge, as opposite to the negative discharge path, is 
non linear (fig. 8 ). 
 
              
 
negative polarity   positive polarity          positive polarity 
 
Fig. 8. Switching discharge pattern 
 
The higher breakdown voltage measured for sphere 
electrode at negative switching impulse (tab 1) is not 
consistent with model simulations by Akyuz and 
Cooray [7]. Their analysis of conditions necessary for 
the initiation of upward leader from Franklin rod have 
shown that the optimum rod radius is 35 cm. They have 
shown that as the negative stepped lightning leader 
approaches the ground the Franklin rod with a larger 
radius launch a connecting leader first.  
 
The breakdown voltages measured at the test 
arrangement from fig. 5b are listed in table 2. In spite of 
some differences in medium times to breakdown the 
negative breakdown voltages were practically the same. 
The shortest time to breakdown was observed with the 
blunt electrode E4. In this arrangement the discharge 
starts from bottom electrode only. Therefore it does not 
model appropriately the conditions occurring during a 
lightning stroke. 
 
K.L. CHRZAN 
 
Scientific Papers of the Institute of Electrical Engineering Fundamentals 
 of the Wrocław University of Technology, No 40, Conference 15 
220
Table 2. Breakdown negative switching voltages at test 
arrangement from fig. 5b 
 
 
Grounded 
electrodes 
Uw 
(kV) 
U90% 
(kV) 
Tb 
(µs) 
Very sharp E2 550 580 199 
Radius 3 mm E3 550    ~ 580 194 
Blunt E4 550 580 175 
 
 
 
5. INFLUENCE OF D.C. BIAS VOLTAGE 
 
The above tests were carried out in the laboratory at 
the relatively small electrode distances, without the d.c. 
bias and therefore without the corona from the sharp 
electrode. Moore compared the number of strokes to sharp 
rods with that to blunt rods in the Magdalena Mountains 
of central New Mexico at the height of 3200 m [8]. The 
sharp or blunt tips were installed on 6 m high masts 6 m 
apart from each other. During 7 years 13 strokes were 
noted, all of them were attracted by blunt rods. According 
to Moore, this result can be explained by the influence of 
corona space charge. Sharp tips under the electrostatic 
field of a stormy cloud produce the corona.  
The influence of corona from sharp rods on the 
impulse breakdown voltage at negative polarity  was 
studied by Jakubowski [9] and Allen [10]. Jakubowski 
did not observe the influence of intensive corona on the 
impulse breakdown voltage at the electrode distance of 
1 m. At the distance of 1,5 m Allen observed that the 
negative d.c. bias voltage increases the breakdown 
voltage at switching impulses about 4% (fig. 9). 
 
 
Fig. 9. 50% sparkover voltage as a function of delay time [10] 
            
The small effect of corona noted by Allen was 
achieved at the very high d.c. bias voltage of 150 kV. 
The medium electrostatic field amounted about 100 
kV/m.  Under a real cloud the medium electric field is in 
the range of 20 kV/m only. Additionally, the electrode 
arrangement high voltage plate – grounded rod is not 
appropriate for modeling of lightning discharges 
because the breakdown develops here from bottom rod 
electrode only. For modeling of lightning discharge the 
rod-rod arrangement is better. 
6.  CONCLUSIONS 
 
The electrical strength of an air gap consisting of a 
sphere and grounded plate increases if the sphere radius 
is greater than a critical value. This effect was noted for 
positive switching voltage. 
 
Under negative switching impulse the breakdown 
voltage of the 3 m air gap rod – sharp grounded tip is 
smaller than the breakdown voltage of the gap rod – 
grounded sphere with a radius of 25 cm. Under positive 
switching voltage and under lightning impulses the 
sparkover voltage does not depend on the tip of 
grounded electrode. 
 
Intensive corona discharges cause a small increase 
in negative sparkover voltage in the air gap consisted of 
HV plate and grounded rod with a sharp tip.  
 
The above phenomenon could be responsible for the 
observed by Moore better behavior of blunt rods under 
field conditions in New Mexico. 
 
 
 
REFERENCES 
 
[1] CHRZAN K.L. History of lightning protection in Poland. 
Int. Conf. on Lightning Protection, Cracow 2002 
[2]  GOLDE R.H. Lightning, Academic Press 1977,  Vol. 1,  
chapter 1, Prinz H. Lightning in history,  p. 44 
[3] SZPOR S. Zakończenie przeciwulotowe dla zwodu 
pionowego. Polish patent no. 63945, 15. Jan 1972     
[4] GOLDE R.H., The validity of lightning tests with scale 
models. The Journal of the Institution of Electrical 
Engineers, part II (Power Engineering) April 1941, pp. 67-
68 
[5]  CARRARA G., THIONE L., Switching surge strength of 
large air gaps: a physical approach. IEEE Trans. on 
Power Apparatus and Systems, March/April 1976, pp. 
512-524 
[6] SZPOR S., WIŚNIEWSKI A., Series of impulse tests 
showing the efficacy of the surface terminals of lightning 
rods. Archiwum Elektrotechniki,  no 4, 1970, s. 749-755 
[7] AKYUZ  M., COORAY V., The Franklin lightning 
conductor: conditions necessary for the initiation of a 
connecting leader. Journal of Electrostatics 51-52 (2001) 
pp. 319-325 
[8] MOORE C.B., Aulich G.D., Rison W., The case for using 
blunt-tipped lightning rods as strike receptors. Journal of 
Applied Meteorology, July 2003, pp. 984-993 
[9] JAKUBOWSKI J.L., Wpływ świetleń na strefę chronioną    
       piorunochronów prętowych. Archiwum Elektrotechniki,   
       no 1-2, 1953, s. 165-175 
[10]ALLEN N.L., EVANS J.C., New investigations of  the 
early streamer emission principle. IEE Proc. on Science 
Measurement Technology, Sept. 2000, pp. 243-248 
 
Acknowledgements 
 
The work has been supported by the German 
foundation DAAD. The author thanks Prof. 
V. Hinrichsen, Dr. W. Breilmann from Darmstadt and 
Prof. K. Feser, Dr. W. Koehler from Stuttgart for the 
help at experiments carried out at their laboratories. 


Franklin rods - sharp or blunt ?

http://zet10.ipee.pwr.wroc.pl/getfile.py?docid=587&name=APTADM%202004%2042&format=pdf&version=1

Franklin rods - sharp or blunt ?

Abstract

Similar works

Full text

Available Versions

Repozytorium Eny Politechnika Wrocławska