Design of Special Lightning Protection System for Opened Sites and Structures under Construction

Dangerous Potentials Avaliation and Mitigation

Ronaldo Kascher Moreira Instituto Politécnico

Pontifícia Universidade Católica de Minas Gerais Belo Horizonte, Minas Gerais, Brazil

rkascher@kascher.com.br

Guilherme Hoffmann Leão Coelho Empresa Brasileira de Aeronáutica SA

Empresa Brasileira de Aeronáutica São J. Campos São Paulo, Brazil guilherme.leao@embraer.com.br

Alexandre Kascher Moreira Departamento Técnico

Kascher Engenharia e Comércio Ltda. Belo Horizonte, Minas Gerais, Brazil

alexandre@kascher.com.br

Gabriela Tavares Kascher Moreira Departamento Técnico

Kascher Engenharia e Comércio Ltda. Belo Horizonte, Minas Gerais, Brazil

gabriela@kascher.com.br

Abstract - This paper studies the effective lightning protection for workers in opened sites and mobilized in construction works focusing dangerous potentials developed when the lightning current is conducted to the grounding system throw natural or designed down-conductors. This issue has increased its importance due to the need of special lightning protection systems enabling the continuity of work in opened sites or in structures under construction even in alert of atmospheric discharge occurrence.

Keywords—ligthning, protection system, opened sites,

I. INTRODUCTION

The occurrence of deaths and injuries of workers in petrochemical industry due to lightning motivated the hire of lightning detection services aiming to reduce the risks to which the workers mobilized in construction and maintenance work are subjected on industrial structures or in open sites.

With the use of a lightning detection system, the workers activities in open sites are paralyzed when this system indicates a nearby discharges possibility. At this situation, the workers are moved to safe locations, where there is protection against lightning system that ensure controlled risk.

This process, besides causing economic losses due to activities delay, requires complex logistical solutions to problems such as: Where can workers be moved? How can they be moved? How to control the risk during displacement? How to restart activities after stopping?

A solution that will not cause this logistical problems and will not require the activities stoppage during incoming lightning strikes alert is the design and installation of a special lightning protection system in the workplace. That guarantees level of risk similar to which workers who are in buildings

equipped with a standardized lightning protection system are submitted. In this solution, there is no need to move workers out of working area.

II. SPECIAL LIGHTINIG PROTECTION SYSTEM

A worker performing services on open sites or structures is subject to the following hazards in case of lightning occurrence:

� Incidence of direct lightning to worker; � Development of hazardous electrical potentials (touch

and step voltages);� Transferred potential (work with powered tools or

handling large metal parts).

A. Direct Lightning to worker The primary stage of the designed protection system must

give to the workers a protected area against direct lightning. This goal is achieved by natural protection (in existing structures) or installation of complementary air termination systems to provide the protection level required in consonance with the risk analysis [1] .

The incidence of direct discharges to workers is prevented by inserting them into a protected area provided by the lightning protection system, whose projections are shown in the "mapping drawings" (see Fig. 2). The protected volume is determined by the rolling sphere method, described in [2] and [4] .

2013 International Symposium on Lightning Protection (XII SIPDA), Belo Horizonte, Brazil, October 7-11, 2013

978-1-4799-1344-2/13/$31.00 ©2013 IEEE 348

Fig. 1. Protected volume according the rolling sphere method

B. Hazardous electrical potentials (touch and step voltages) Touch and step voltages developed in the occurrence of

atmospheric discharge must be below the maximum permissible hazardous voltages for the worker equipped with basic and special PPE.

The procedure to control the risk due to dangerous potential is based on references [4] [5] and in the use of individual safety equipment that allows an appropriate level of electrical insulation.

It was considered, in the occurrence of a lightning, the following maximum impulsive voltages between soles of the work shoes and work gloves:

� 20 kV when worker uses shoes with isolation in accordance to [6] and gloves in accordance to [7] Class 00, complemented by work gloves specified for the job.

� 10 kV when worker uses only the shoes specified above and conventional gloves specified for the job.

� 5 kV when workers uses the conventional PPE specified for the job.

III. DANGEROUS VOLTAGES ROUGH CALCULATION IN SPECIFIC SITUATIONS

The assessments provided on this paper were prepared based on simplified models guided by the following premises. Effects of the electrical resistance and inductance of the conductor, which the current passes trough, contributes to the development of touch and step voltages. As lightning current is impulsive with front time varying from 10 µs (return stroke) to 0,25 µs (subsequent strokes) [2] , and the lightning current conductors are made of low electrical resistance material with large enough cross-section, the inductive effects are more significant for touch voltage development. Therefore, it was considered that step and touch voltages developed are consequences only of partial inductance of the loop equivalent circuit, which consists of the structure and the worker body. The inductive magnetic flux can be caused by the discharge current, which passes through the metallic structure near theloop. Due to the simplification model used in this assessment,

conservative values were taken to the critical parameters, such as the maximum derivative of the current grown and the isolation levels of the considered loops. These assumptions made the assessment results safer to workers.

A. Touch Voltages Calculation in Specific Situations Considering the natural protection against ligthning direct

to workers provided by the large metalical structures (so common in petrochemical workplaces), the possible dangerous voltages which could be developed are: touch voltage, step voltage and transfered voltage.

The probability of transferred potential occurrence is reduced when the worker, due to lightning alert, is not handling long metal pieces (for example metallic conduits or metallic bars longer than 3 m) and electrical tools connected to an electrical system. The electrical tool usage can be separated on the following possibilities:

� use of tools powered by internal batteries, avoiding the connection of tool to electrical lines,

� use of an 60 Hz electrical system powered by diesel generators isolated from the plant (where the structure is located),

� use of the same electrical system of the plant and design special protection measures.

On the following items some dangerous potentials calculation (touch and step voltages) are done:

Situation #1 – Lightning hitting the wall of the structure at a point 20 meters high

The Fig. 2 illustrates this situation.

Fig. 2. Flashes to structure´s wall at a point 20 meters high.

The equation (1) given by [2] relates the peak current (I)with the rolling sphere radius (r), according to the rolling sphere method.

�(�) = � �����,� (� ) (1)

h = 20mi

i = 3 kAH > 20m

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For the level 1 LPS, which the rolling sphere radius is 20 m, is assumed that the maximum peak lightning current is 3kA, as shown at (2).

�(20) = �������,� = 2,904 (� ) (2) Disregarding the current distribution through the structure

(conservative attitude) and considering the registered distances at Fig. 3 it is possible to calculate the induced voltage in open loop or the touch voltage.

Fig. 3. Equivalent circuit of a worker – voltage between foot and hand

The total flux going through the loop plane is:

��(�, �) = ��.������ . �. ∫ ��� ������ � ��� (3)

Considering I = 3 kA, r1 = 0,01 m and r2 = 1 m, the total flux is 2,763.10-3 (T/m) , as shown at (4).

��(�, �) = ��.������ . (3. 10). ∫ ��� ����,�� = 2,763. 10� � ��� (4)

Equations (5) and (6) give the induced voltage and the total induced voltage.

��(�, �) = ��(!,�)"# � $�� (5) ���%#(�, �, &) = ��(!,�)"# . &(�) (6) Considering ��(�, �) = 2,763. 10� � ���, & = 1,5 ' and

dt = 1µs, according to (5), the total induced voltage is 4,145 kV.

Similarly, it is possible to calculate the touch voltage between the workers hands, as shown below (Fig. 4).

Fig. 4. Equivalent circuit of a worker – voltage between hands

Using (3) and (6), the developed voltage between the workers hands is 1,174 kV.

As the value of touch voltage between workers hands is less than the touch voltage between hand and foot, the touch voltages between hands were not calculate to the following situations.

As the touch voltage calculated at Situation #1 was 4,2 kV it was concluded that workers can continue to work using conventional PPEs.

Situation #2 – Lightning hitting the wall of the structure at a point higher than 20 meters

From [1], and applying the calculation method described at Situation #1, we have the Table 1 that relates the strike´s point height with the current value and the maximum touch voltage developed between workers foot and hand.

Table 1 Touch voltage as function of the strike´s point height

Strike point height (m)

Peak current value (kA)

Touch voltage

(kV)30 5,4 7,540 8,4 11,650 11,8 16,360 15,7 21,770 20 27,6

On this situation, for strike´s point height between 20 m and 30 m, a worker equipped with isolating shoes, according to [6] besides the conventional PPEs recommended by staff of work security is properly protected. For strike´s point height between 30 m and 50 m, a worker must be equipped with isolating shoes, according to [6] isolating gloves according to Class 00 [7] , besides the conventional PPEs.

Situation #3 – Lightning hitting the top of the structure with more than 3 floors

The event illustrated by Fig. 5 occur when the analized structure by the perspective of the rolling sphere method [2] [3] allows its use.

V 1,5m

1m

i

V

i

1,5m

1m

350

On this case all high lightning current will be conducted by the structure, distributed by pillars and beams until get to the earthing system at soil level.

In this case the current distribution in each floor is very important for the hazardous potentials evaluation. The current distribution depends on: the structure dimensions, the number of down-conductors, the height between floors and other factors. This current distribution is estimated on [4] .

Fig. 5. Flashes to the top of high structures

The Annex C of [3] presents an evaluation of the partitioning coefficient (kc) of the lightning current amongst down-conductors that depends on the type of air-termination system, on the overall number (n), on the position of the down-conductors, on the interconnecting ring conductors, and on the type of earth-termination system. Fig C.4 of [3] attends the situation shown at Fig. 5. At Table 2 the kc values are shown as function of the different floors of the building.

Table 2 Calculations for kc conform Annex C of [3] Floors/Storeys kc

Higher or last �*� = 12+ + 0,1 + 0,2. / 8ℎ�;

Penultimate �*� = 1+ + 0,1Antepenult �* = 1+ + 0,01

4th from top to bottom and lower floors

�*� = 1+Where: n is the number of down-conductors; c is the

distance between adjacent down-conductors; and h1 is the last floor height.

Table 3 registers the numerical values for kc and their current values, considering n = 50, c = 3 m, h1 = 3 m and a peak current of 100 kA.

Table 3 Values of kc and their effective current values at different floors on the building

illustrated at Fig. 5

Floor / Storey kc Effective current value

Higher or last 0,31 31 kAPenultimate 0,12 12 kAAntepenult 0,03 3 kA

4th from top to bottom and lower floors

0,02 2 kA

Using (3) and (6) touch voltages between hand and foot in each floor of the building (Fig. 5) were calculated. The calculated values are at Table 4.

Table 4 Touch voltages between hands and foot of a worker at different floors of the building illustrated at Fig.

5 Floors/Storeys Touch voltages between

hands and footHigher or last 42,83 kVPenultimate 16,58 kVAntepenult 4,1 kV

4th from top to bottom and lower floors

2,3 kV

Table 5 shows the need for measures for risk control to guarantee the worker safe as function of the floor that he is located.

Table 5 Necessary control measures as function of the floor that the worker is located (building illustrated at Fig.

5) Floors Control measures

Higher or last

It is needed engineering measures or operational action, as only the use of

PPEs is not enough

PenultimatePPEs: shoes conform [6] and gloves conform [7]

Class 00Antepenult Conventional PPEs

4th from top to bottom and lower floors

Conventional PPEs

Note that the calculated values changes as the structure geometry changes, so for structures with different geometry than the used on this example the calculations must be redone.

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Situation #4 – Lightning hitting the top of low structures (one floor structure)

Fig. 6. Flash to an one floor structure

Fig. 6 illustrates the Situation #4. This situation is attended by Fig. C.2 of [3]. The kc expression is the same used at last floor of Situation #3, shown at Table 2:

�*<>#� = ��? + 0,1 + 0,2. @ *A�; (7)

Considering a building with the following characteristics:

height = 5 m, width = 9 m, length = 18 m, distance between down-conductors = 3 m and number of down-conductors = 50. Using the same method used for the others situations the following values were calculated:

� kc = 0,296; � Effective current = 29,65 kA;

� Induced voltage between foot and hand = 40,96 kV.

We conclude that, for this example, the risk can not be eliminated just with the use of PPE, so it is needed the adoption of engineering measures for hazardous potentials control.

B. Metallic tanks of large diameters This situation is illustrated by Fig. 7. The voltages

developed will be function of the current distribution at vertical surface of the tank.

Fig. 7. Flash to the top of a metallic tank of large diameter

At higher parts (closer to the tank tops) we have higher current density, thus the touch voltages that the worker will be submitted will also be higher. The current density becomes lower as the distance from the upper part becomes larger thus lower values of touch voltages are expected at heights closer to the ground. Due the real diameters of industrial tanks, which varies from 15 to 100 meters, we considered the current distribution on a flat surface.

The calculations of the values of touch voltages at the following heights from the tank top were done: 1 m, 2 m, 3 m and 4 m (shown at Fig. 8).

Fig. 8. Calculation points of touch voltages

Using (8) for calculate the total flux per meter which goes through the interest loop, which is the appropriate expression to the physical situation and to the assumption of the uniform distribution of electrical current.

��(�, B, �) = 4 ∗ 10�D. !E . ∫ ∫ FG�HF� �I ��J����,�� � ��� (8) Where: I is the peak current of the lightning; W is the

considered conductor width; and r is the distance between the worker and the tank surface.

The expressions (5), (6) and (10) were used to calculate the induced voltage and the touch voltage.

Table 6 presents the touch voltages calculated values.

Table 6 Calculated touch voltage as function of the vertical distance from the tank top

Vertical distance from the tank top

Calculated touch voltage

1 m 33,5 kV2 m 13,9 kV3 m 8,7 kV4 m 6,4 kV

From the calculated touch voltages and the operational considerations described at this article, Table 7 presents the operational situation where work near the wall of the metallic tank is permitted.

Vti i

Vt

1m2m

3m

flash

Vti

flash

c

I

H

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Table 7 Touch voltage control measures as function of the vertical distance from the tank top

Vertical distance from the tank top

Touch voltage control measures

1 mIt is needed engineering measures or operational action, as only the

use of PPEs is not enough

2 m PPEs: shoes conform [6] and gloves conform [7] Class 00

3 m PPEs: shoes conform [6] 4 m Conventional PPEs

IV. CONCLUSION

The dissemination of use of lightning detection systems, combined with the growing concern with workers safety led to a solution that obligates the work stoppage on lightning alert. This solution involves a great economic loss besides other logistical problems not easily solved.

A more efficient solution for works in open sites or structures is the design and installation of a convenient LPS in the worksite that ensure an acceptable risk level.

The workers risk level can be evaluated by the procedure given in [1]. The risk is attenuated when the control of hazardous potential is taken considering the lighting protection system design. The real situation occurred in construction works not always is represented in the standard recommendations that are obviously written to guide the design on conventional buildings. This paper proposes a simple approach that helps designers to concept protection systems that meet the special cases of works protection against lightning and its effects.

REFERENCES

[1] Protection against lightning – Part 2: Risk Management, IEC 62305-2, 2010.

[2] Protection against lightning – Part 1: General Principles, IEC 62305-1, 2010.

[3] Proteção de estruturas contra descargas atmosféricas, ABNT NBR 5419, 2005.

[4] Protection against lightning – Part 3: Physical damage to structures and life hazard, IEC 62305-3, 2010.

[5] Protection against lightning – Part 4: Electrical and Electronic Systems within Structure, IEC 62305-4, 2010.

[6] Equipamento de proteção individual - Calçado de segurança, ABNT ISO 20345, 2008.

[7] Luvas isolantes de borracha – Especificação, ABNT NBR 10622, 1989.

[8] Mishab, N., R., Kadir, M. Z. A. Ab., Gomes, C., “Modelling and Analysis of Different Aspect of Mechanisms in Lightning Injury”.

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