Underground utilities are dangerous when struck by lightning! E. M. Bazelyan

Introduction

Lightning protection specialists are well aware of this issue, as underground electrodes are commonly used to divert lightning currents into the ground. Any grounding arrangement is organized by a network of such electrodes. Their calculation method is well known in the field of lightning protection. Here we will discuss a completely different function of grounded electrodes, which are typically quite long. They provide an excellent channel for transporting lightning overvoltages from the point of strike to the protected structure. Such channels can include metal pipes for water or hydrocarbon fuel transportation systems, heat supply systems from thermal power plants, cable lines, and any other sufficiently long underground conductors. By contacting their metal surface with the soil, they transfer some of the lightning current injected into them, but not all of it. Part of it is still delivered to the structure protected from lightning, posing a risk to both people and low-voltage internal electrical and electronic circuits.

1. Calculation scheme for estimating the transported voltage

Рис 1.

It is quite obvious that the share of the transmitted voltage depends on the length of the metallic communication, its conductor resistance, and the leakage conductivity of the pulse current into the soil, and therefore on the soil specific resistivity. In general, the problem has to be solved within the framework of a substitution scheme with distributed parameters Such a solution is necessary if the inductance of communication of l and radius r₀

(1)

and its total leakage resistance into the soil at the laying depth t 

 (2)

are characterized by the time constant T = L/R, the value of which is commensurate with the duration of the lightning current pulse. In general case the propagation process is described by the system of equations in partial derivatives

(3)

with linear values of communication inductance L₀, its capacitance С₀,, longitudinal resistance R₀ and leakage into the soil G₀.

Simplifying the calculations with qualitative preservation of the basic regularities, it is reasonable to limit ourselves to a private but practically significant situation when the capacitive leakage of current is significantly less than the leakage through the soil conductivity and the inductive voltage drop along the length of the communication is many times greater than its loss on the longitudinal resistance. This simplifies the system of equations (3), which is now reduced to a more computationally convenient expression

(4)

with linear values of inductance and ohmic leakage into the soil

A numerical solution was performed for a normalized pulse lightning current with time parameters of 10/350 μs. This current was assumed to be injected into the ground terminals of the lightning arrester, which has a concentrated grounding resistance R_Н and is connected to the extended communication, which is closed at its remote end to the grounding resistance R_к.

2. Results of computer modeling

The computational data in Fig. 2 show the stress dependence delivered to the site by an underground communication with a length of 500 m and radius of 0.1 m. It was assumed that it was laid at a depth of 1 m in soil with specific resistivity ρ, the value of which varied in the calculations from 100 to 2000 ohm m.

Fig. 2. Dependence of computer modeling results on the soil specific resistivity for a 500 m long communication with grounding resistance of 10 Ohm at its ends.

U/I м Ом U/I m Ohm
Время, мкс Time, mcs
P=2000 Ом м P=2000 Ohm m

On the axis of ordinates of the calculated graph is plotted the reduced value of voltage related to the amplitude of the pulse current of lightning. One can see a very strong dependence of this parameter on the specific resistivity of the soil ρ. The value of U/IM decreases from 2.5 to 0.2 Ohm as ρ changes from 2000 to 100 Ohm m. For the voltage transported across the communication, we can speak here of a dependence on ρ that is quite close to linear. Thus, at the standardized lightning current of 100 kA (III level of protection) in the soil with specific resistivity of 2000 Ohm m to the protected facility will be delivered voltage close to 250 kV (U/IM ρ 2,5 Ohm), and at ρ = 100 Ohm m only 20 kV.

Do not be fooled by a sharp drop in the value of delivered voltage in the soil with low specific resistivity. A value of tens of kilovolts is quite significant. It will be demonstrated below that it must be considered when ensuring employee safety and protecting electronic equipment.

It is obvious that the magnitude of the transported voltage decreases as the communication length increases, which is especially noticeable in high conductivity soils. To demonstrate this, Fig. 3 shows the results of computer modeling for a soil with a specific resistivity of 200 ohm m.

Fig. 3. Dependence of computer modeling results on the length of communication in the soil with specific resistivity 200 Ohm m.
All other communication parameters are similar to those shown in Figure 2.

Длина коммуникации, м - Utility length, m

It is easy to see that voltage transport through hundreds of meters of communications can be a serious threat even in well-conducting soils, delivering extremely dangerous overvoltages to the protected facility whenever lightning strikes the lightning arrester.

Finally, the result of transportation also depends on the value of the resistance of earthing of the communication ends. This is supported by computer calculations of a share of lightning current delivered via communication to the grounding arrangement of the protected facility (Fig. 4). The calculations were performed for earthing resistances of 2 and 10 Ohm at both ends of the communication. The figure shows other basic calculation parameters, and the lightning current impulse time parameters are taken as 10/350 µs/.

Fig. 4. Calculated parameters of the current pulse delivered to the protected facility through the communication grounded at its ends by 2 or 10 Ohm resistances.
Calculation is performed for the pulse current of lightning (10/350 μs).

Удельное сопротивление 1000 Ом м Specific resistance: 200 Ohm m.
Длина коммуникации 500 м Utility length: 500 m
Радиус коммуникации 0,1 м Utility radius: 0.1 m
Глубина в грунте 1 м Depth in soil: 1 m
Время, мкс Time, mcs

It can be seen that, all other things being equal, a fivefold reduction in a communication's grounding resistance results in a 2.3-fold increase in the current delivered. As a result, the facility's earthing arrangement experiences a voltage reduction, but only by a factor of 2.17.

Fig. 5. Gas pipeline entry into the gas distribution station with electrochemical protection device

In a number of practically significant conditions, the resulting situation proves to be highly expensive. Figure 5 shows the underground gas pipeline entrance to the gas distribution station, complete with an electrochemical corrosion protection system and grounding arrangement. An insulating pad is used to separate the flanges at the underground communication point of entry. To prevent an emergency breakdown of the pad, which would result in gas release into open space and fire in the event of a thunderstorm exposure, it is bypassed by surge protectors, the increased capacity of which must be determined in accordance with the connection of the grounding arrangement to the gas pipeline of the gas distribution station.

Characteristics of works on extended communication

At facilities with long communications, extra care must be taken to guarantee the safety of operating personnel. If the communication is not connected to the facility's grounding arrangement, touching it exposes the person to the full voltage delivered through the communication from the point of lightning strike. As demonstrated above, the value of this voltage can range in the tens of kilovolts even in well-conducting soils.

What is the main danger of such an exposure lasting a few hundred microseconds?

Fig. 5. Fig. 5. Dependence on the current along the hand-foot path of the voltage exposure time for excitation of cardiac fibrillation (solid curve - probability up to 5%, middle - up to 50%, right - more than 50%)

Длительность импульса, мс Pulse duration, ms
Ток через тело человека (неразборчиво) Current through the human body (illegible)

With known human body resistance Rh, these values allow us to estimate the dangerous value of the voltage delivered by the extended communication to the protected structure as follows

U_fib = R_ChI_{h (5)}

The only difficulty with this assessment is that because of the electrical breakdown of the skin layer, the resistance of the human body changes depending on the voltage applied to it. However, this value becomes stable at voltages greater than 1000 V. Most people have a 50% chance of not exceeding 750 ohms. According to (5), this means that the voltage capable of causing cardiac fibrillation in at least 5% of service personnel does not exceed 1500 V, but a voltage of 4500 V increases the probability of such an event to 95%.

Fig. 6. Computer modeling of the extended communication on grounded supports at different specific soil resistivity.

Длина коммуникации 1000 м - Utility length: 1000 m
радиус 0,1 м - Radius: 0.1 m
Шаг локального заземления 10 м - Local grounding spacing: 10 m
Одиночный стержень длиной 3 м и радиусом 1 см - Single rod of 3 m length and 1 cm radius
время, мкс - Time, mcs

It goes without saying that touching lengthy communication lines carries a very high risk! The problem is exacerbated by the fact that the communication may not be installed underground but rather on the surface of the grounded supports, making it easily accessible to touch along its entire length. Furthermore, it might not be permanently attached to grounding arrangement of the protected facility.

The results of the computer calculation in Fig. 6 are performed for such a 1000 m long communication with grounded supports every 10 m of length. It was assumed that a 3 m vertical rod would serve as the support's ground terminal. The calculations were performed on soil with resistivity values of 1000 and 200 ohm m. It is evident that at a ground resistivity of 200 ohm m, a lightning with a current of 13 kA can easily generate a dangerous voltage of 4500 V, and at 1000 ohm m, even the weakest lightning with a current of only 3 kA is sufficient.

It is crucial to remember the unique circumstances surrounding human exposure to such low voltages. According to studies, fibrillation is only occasionally caused by mildly hazardous voltages that align with the unique heart valve cycle. Higher voltages have even more peculiar effects. There may be a delay of several hours before they happen.

That is why, regardless of the affected person's current condition, he/she should be immediately placed under qualified medical supervision. Naturally, any work involving extended communications in a stormy environment should be avoided.

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