Introduction to Lightning Protection System Design- Part Two

In Article " Introduction to Lightning System Design- Part One ", I listed all terms, abbreviations and Symbols used in lightning field and which will be used throughout Course EE-5: Lightning Design Calculations.

Today, I will continue explaining the introduction to Lightning Protection System Design.

What is Lightning?

Lightning is the visible discharge of static electricity within a cloud, from cloud to cloud, or between the earth and a cloud.

 Fig(1): Lightning phenomena

• Lightning is one of nature’s most powerful and destructive phenomena. Lightning discharges contain awesome amounts of electrical energy and have been measured from several thousand amps to over 200,000 amps which is enough to light half a million 100 watt bulbs. Even though a lightning discharge is of a very short duration, typically 200 microseconds, it is a very real cause of damage and destruction.

• The damage from lightning comes from electrocution, human burns, burning buildings, exploding bricks and mortar, melted electrical equipment, damaged electrical equipment and stresses on electrical equipment that are responsible for failures months later.

 Types of Lightning Flashes In the lightning phenomena, the lightning flashes can be divided to: Lightning discharges, Lightning Strokes.

1- Lightning Discharges

Lightning flashes to earth (discharges) lead to a neutralization of charge between the cloud charges and the electrostatic charges on the ground. We distinguish between two types of lightning flashes to earth:

1. Downward flashes (cloud-to-earth flashes),
2. Upward flashes (earth-to-cloud flashes).

1.1  The Downward flashes (cloud-to-earth flash)

 Fig (2): cloud-to-earth flash

The Downward flashes (cloud-to-earth flash) (see fig.2) in turn, are divided to two types (see fig.3):

1. Negative lightning discharges,
2. Positive lightning discharges.

 Fig (3): Negative and Positive lightning discharges

1.1.A Negative lightning discharges

 Fig (4): Lightning Cloud Formation and Lightning Types

• In Fig.4, The clouds has a negative charge center located in the lower part of the cloud where temperature is about -5 C°, and the main positive charge center is located several kilometers higher up, where the temperature is usually below -20 C°. In the majority of storm clouds, there is also a localized positively charged region near the base of the cloud where the temperature is 0 C°.

• The earth beneath a charged cloud becomes charged to the opposite polarity. As a negatively charged cloud passes, the excess of electrons in the cloud repels the negative electrons in the earth, causing the earth’s surface below the cloud to become positively charged. Conversely, a positively charged cloud causes the earth below to be negatively charged.

• The air between cloud and earth is the dielectric, or insulating medium, that prevents flash over. When the voltage withstand capability of the air is exceeded, the air becomes ionized. Conduction of the discharge takes place in a series of discrete steps, as follows (see fig.5):

 Fig (5): Lightning Flash Formation

1. First, a low current leader of about 100 amperes extends down from the cloud, jumping in a series of zigzag steps, about 100 to 150 feet (30 to 45 m) each, toward the earth.
2. As the leader or leaders (there may be more than one) near the earth, a streamer of opposite polarity rises from the earth or from some object on the earth.
3. When the two meet, a return stroke of very high current follows the ionized path to the cloud, resulting in the bright flash called lightning.
4. One or more return strokes make up the flash.
5. Lightning current, ranging from thousands to hundreds of thousands of amperes, heats the air which expands with explosive force, and creates pressures that can exceed 10 atmospheres. This expansion causes thunder, and can be powerful enough to damage buildings.

1.1.B Positive Lightning discharges

• Positive flashes to earth can arise out of the lower, positively charged area of a thundercloud.
• The difference between positive and negative lightning discharges is that the leader in the case of positive lightning is generally not stepped and there are rarely multiple strokes. There is typically only one return stroke, after which a continuous current flows to discharge the cloud.

Notes:

• Clouds can be charged with ten to hundreds of millions of volts in relation to earth. The charge can be either negative or positive, although negative charged clouds account for 98% of lightning strikes to earth.
• While only about 2% of the lightning strikes to earth originate from positively charged clouds, these strikes usually have higher currents than those from negatively charged clouds.

1.2 Upward flashes (earth-to-cloud flashes)

 Fig(6): earth-to-cloud flashes

• On very high, exposed objects (e.g. radio masts, telecommunication towers, steeples) or on the tops of mountains, upward flashes (earth-to-cloud flashes) can occur. It can be recognized by the upwards-reaching branches of the lightning discharge (see Fig.6).

 fig(7): Upward flashes

• Upward flashes occur with both negative polarity and also with positive polarity (see Fig.7). Since, with upward flashes, the leaders propagate from the exposed object on the surface of the earth to the cloud, high objects can be struck several times by one lightning discharge during a thunderstorm.

• Earth-to-cloud flashes are extremely rare and generally only occur from high mountain tops or tall man-made structures they are typically positive strokes (positive lightning). In these situations a positive leader channel may start upward from the mountain peak due to the intense concentration of positive charge at that point.

Note:

• Objects struck by lightning are subject to higher stress by downward flashes (cloud-to-earth flashes) than by upward flashes (earth-to-cloud flashes). The parameters of downward flashes are therefore taken as the basis when designing lightning protection measures.

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 2- Lightning Strokes Lightning strokes are Cloud-to-cloud flashes (see fig.4) result in charge neutralization between positive and negative cloud charge centers, and do not directly strike objects on the ground in the process. Air discharges emerge from the cloud but do not reach the ground. They can run horizontally for many kilometers. Sometimes they re-enter the cloud base further on, in which case they are regarded as cloud to- cloud discharges. Cloud flashes take place inside the thundercloud so that only a diffused flickering is seen. These are more numerous than flashes to the ground and a ratio of 6:1 or more is thought probable. Approximately 90% of all lightning flashes are cloud-to-cloud with the other 10% being cloud-to-ground flashes.

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What is the shape of The Lightning Waveform?

A maximal lightning event begins with a high current pulse of up to 200,000 amps that can last 500 microseconds. A commonly used lightning waveform (see fig.8) illustrating this is presented in figure. Examining the waveform, we can correlate different effects of the waveform applied to lightning conductors.

 fig(8): Lightning Waveform

1- The A and D Components

These components contribute to electromagnetic forces and the development of high voltages due to the fast rise time of the pulse and the high peak current. The construction of the conductors and their associated installation practices have to account for these effects. Electromagnetic forces can damage or even break conductors. The inductive reactance of conductors, usually ignored in most power system considerations becomes a major contributing factor in conductor failure.

2- The B and C components

Far more charge is transferred during the B and C components of the lightning event compared to the A and D components. While lightning conductors are of robust construction or heavy enough gauge of low resistance material, which minimizes liberation of heat, any point of high resistance can cause melting and failure. For example, a corroded or loose connection or a frayed conductor can cause a failure from ohmic heating.

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How Lightning strikes can affect the electrical and/or electronic systems of a building?

Lightning strikes can affect the electrical and/or electronic systems of a building in two ways:

 Fig (9): Points of Impact for Lightning Strikes

1- By direct impact of the lightning strike on the building (direct lightning strike) (see Fig.9 a),

2- By indirect impact of the lightning strike on the building (indirect lightning strike):

• A lightning strike can fall on an overhead electric power line supplying a building (see Fig.9 b). The overcurrent and overvoltage can spread several kilometres from the point of impact.
• A lightning strike can fall near an electric power line (see Fig.9 c). It is the electromagnetic radiation of the lightning current that produces a high current and an overvoltage on the electric power supply network.
• A lightning strike can fall near a building (see Fig.9 d). The earth potential around the point of impact rises dangerously.

Note:

• In the two cases (b &c), the hazardous currents and voltages are transmitted by the power supply network.

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 What are the main effects of Lightning? However, the main effects of lightning strikes are as follows: 1- Thermal effects:  These effects are linked to the quantity of charges involved when lightning strikes. For materials with high resistivity, they cause various melting points at large amount of energy is released the form of heat. The moisture they contain causes a sudden overpressure that may result in explosion. 2- Effects Due To Arching: The resistivity of the soil makes earthing resistant and therefore unables to prevent a sudden rise in the facility’s potential when lightning current passes through it. This creates differences in potential between the various metal parts . Earthings and connections between the metal parts must therefore be carefully designed to down conductors. 3- Electrodynamic Effects: These effects are produced if part of the path along which the lightning current travels is within the magnetic field of another part. This may produce repulsion and attraction forces when lightning travels through conductors close to each other. 4- Electrochemical Effects: These are negligible and have no effect on the earthing (compared with stray current in the soil). 5- Acoustic Effects  (Thunder): Thunder is due to the sudden pressure rise (2 to 3 atmospheres) in the discharge channel subject to electrodynamics forces during the lightning strike. The duration of the thunder depends on the length of the ionized channel. The propagation of the spectral components produced by the shock wave is at right angles to the channel for the higher frequencies but Omni directional for lower frequencies. The results are a series of rumbling and crackling sounds that vary according to the distance of the observer from the lightning chamels and the direction taken by the channels. 6- Induction Effects: Induction effects are often the biggest challenge to protection systems. When lightning approaches a site and flows through its conductors, it creates a magnetic flux that produces high and sometimes destructive induced voltages. Electromagnetic loops may be formed between lightning conductor down leads and electrical circuit. This is why protection systems must be very carefully designed and must include any necessary additional protection devices. 7- Luminous Effects:  A lightning strike creates an image on the observer’s retina which may leave him dazzled for several seconds before regaining sight. 8- Indirect Effects:  Offset potential or pace voltage. Dispersion of lightning currents in the soil depends on the nature of the terrain. A heterogeneous soil may create dangerous differences of potential between two neighboring points. So, Lightning protection is essential for the protection of humans, structures, contents within structures, transmission lines, and electrical equipment from thermal, mechanical, and electrical effects caused by lightning discharges. Lightning cannot be prevented, but it can with some success be intercepted, and its current can be conducted to a grounding system without side flashes where it is harmlessly dissipated and.

In the next Article, I will explain the Lightning Protection System LPS types and components. Please, keep following.