rolling sphere design

NEEC – National Electrical Engineering Consultancy Design Management Construction 6 – 14 Bringelly Road www.NeecGroup.com Kingswood, Australia [email protected] NSW, 2747 Phone: 0415 77 55 75 ACN: 132586675 ABN: 86132586675

Lightning Protection Design I. INTRODUCTION

Lightning stroke can cause fatality structural damage, and could lead to malfunction of the electric equipment. The lightning stroke will vary by characteristics from area to area. The lightning itself is an emission or discharge of electricity from cloud to ground, from ground to cloud and from cloud to cloud. When the lightning strikes the ground, it chooses a path with low resistance. According to the IEEE standard 998-1996 “the stroke occurs in two steps, the first is ionization of the air surrounding the centre and the development of stepped leaders, which propagate charge from the cloud into the air”. The second step is return stroke, according to the same standard, “the return stroke is the extremely bright streamer that propagates upward from the earth to the cloud following the same path as the main channel of the downward stepped leader”.

II. THEORETICAL STUDY

During the first part, the last step of leader will determine the striking distance (S). Many scientists studied this striking distance and came up with different equations to determine the distance, below is the most commonly used equations:

Darveniza

−+=

−8.61302I

eIS

(1)

Love

65.010IS = (2)

Whitehead

3

2

4.9 IS = (3)

IEEE

65.08IS = (4)

Suzuki

78.03.3 IS = (5)

Where

I is the return stroke current in kA

S is the strike distance in meters


Many leading lightning investigators such as J. G Anderson and Mousa [2] support the usage of IEEE equation. This study will use the IEEE equation. There are two common methods to approach the lightning design:

• the fixed angle • the rolling sphere, •

This paper will discuss the rolling sphere method design. In this method the value of the lightning direct strike current will determine the radius of the circle. Many countries including Australia set in their standards the level of protection based on the level the stroke current, table I shows the four level of protection in Australia and its relevant sphere radius and stroke current.

TABLE I. LIGHTNING CURRENT CAPACITY WITH RESPECT TO THE STRIKE DISTANCE

Protection Level

Sphere radius (m)

Interception Current (kA)

1 20 2.9 2 30 5.4 3 45 10.1 4 60 15.7

To determine what level of protection is needed it is recommended to liaise with the local Meteorology Bureau to determine the probability level of lighting in the desired area. If this information is not available it is recommend to use protection level one in the design.

The idea behind mast is to find a low resistive path for the lightning to discharge into the ground. The ground resistivity should be less than 10 ohms for the lightning system according to many standards such as IEEE and AS/NZS. This resistivity will by the soil resistivity value and the type of grid used.

III. ROLLING SPHERE

Rolling sphere is one of the most used methods of lightning protection. The rolling sphere method can use one or multiple mast to protect the house.

A. Single mast protection

Figure 1 shows the proposed method of using single mast to protect an object; the circle shows the rolling sphere of the lightning strike.


Figure 1. single mast protection

� ( )2

222

+−−−−= TdaaaaH (6)

Where

a: the radius of the sphere

d: the heights of the protected object

T: the distance between the mast and the far corner of the protected object

Knowing the dimension of the house and the location of the mast, equation 8 is used to determine the heights of the required mast.

B. Double masts protection

Sometimes using one mast to protect the house required a very high mast. Reduction of the height is possible by using two masts to protect the house. Figure 2 shows the method of protection using 2 masts:

Where:

a: the radius of the sphere

d: the heights of the protected object

M: the distance between the two masts

Note that this formula (7) will only protect a thin object like a Bus-Bar and will not provide protection for a cubical object like house. More information will be shown in the case study section.


Figure 2. double mast protections

2

2

2

−−+= MadaH (7)

C. Three masts protection

Using three masts to protect the house will lead to further decrease in the height of masts. Figure 3 shows the three masts protections, this will be ideal to protect the house and it doesn’t required high masts to complete the design.


(a)

(b)

Figure 3. three masts protection


)30(2Cos

MR = (8)

M should not be greater than a×7.1

D. Four masts protection

Using four masts as shown in figure 4 to protect the house is possible and the height can be calculated using equation 9:

� ( )222 25.0 GLadaH +−−+= (9)

Figure 4. four mast layout

rolling sphere design

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