Lightning Rod Attraction: Blunt Tip Vs. Pointed Tip

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Procedure:

In this system three types of corona discharges are involved; onset streamers, negative glow, and pre-breakdown streamers. For this experiment the pre-breakdown streamer mechanism was studied. The standard pointed shape, concave, blunt, flat, and conical LATs were all placed under the high-voltage side plane electrode one at a time. The variable voltage regulator controlled the output HDVC by adjusting the power frequency main voltage. The voltage is stopped once a continuous discharge of the pre-breakdown streamers starts. The peak corona discharge was observed and recorded at this point.

A close loop equation is obtained through the use of dimensional analysis. The corona discharge current relies on numerous parameters that include permittivity, time, area of the tip that produces a corona, distance, and input voltage. The four fundamental dimensions, length, mass, time, and electric charge, may represent these parameters in their respective equations. These equations can also be represented in a dimensional matrix by using dimensional analysis to determine the relationships of the quantities. The relationships then provide a general form for equations that describe the occurrence. In experiments there are often variables that are more cared about than others. The Buckingham theorem states that the dimensionless parameters can be related. The theorem allows researchers to solve for the most relevant quantities in a particular experiment thus, providing a trusted general model of corona discharges.

Purpose of Experiment:

The purpose of these experiments is to study the effect of lightning on the different tip geometries. Until recently, it was thought that the standard pointed rod was the best protection for structures. As lightning research became more and more in depth this proved not to be the case. In field tests the blunt rod appeared to be the preferred path for lighting. Scientists wanted to try and replicate these results in a lab. Testings done in the High Voltage Labratory of Universiti Teknologi Malaysia were able to generate an electric pulse in a lab since attracting real lighting to a specific spot is not possible with current technology. Researchers had to assume that the air electrification at the rod tip caused by the electric impulse was similar to that of real lightning. Without this assumption the experiment would not yield usable results. Many variables had to be considered during the experiments. Two important variables studied are the rods being ionized or not and they performed competitive test between different rod tips.

Results

Table 1 Correlation among various LAT tips with electric field strength

Standard

Concave

Blunt

Flat

Conical

E(kV/cm)

I(mA)

E(kV/cm)

I(mA)

E(kV/cm)

I(mA)

E(kV/cm)

I(mA)

E(kV/cm)

I(mA)

5.65

5.83

5.35

4.31

7.35

≈0

8.16

14.75

8.00

14.75

5.32

5.53

4.70

4.00

7.12

≈0

7.41

14.17

7.18

13.83

4.94

5.07

4.39

3.86

6.78

≈0

6.92

14.00

6.57

13.08

4.53

4.39

4.17

3.76

6.53

≈0

6.39

12.33

6.17

11.50

4.38

4.13

3.95

3.65

6.31

≈0

6.03

11.08

5.79

10.42

Table 2: Type of comparison

Type of Rod

Number of strikes

Percentage of strikes (%)

Standard and Blunt

Standard

8

40

Blunt

12

60

Conical and blunt

Conical

6

30

Blunt