Thunder Storms, Lightning and Computers

Electrical surges caused by thunderstorms can be fatal for electronic equipment like computers.

There are about 16 Million Thunderstorms per year worldwide, 1800 at any moment.

Storms typically lasts 30 minutes and billions of lightning bolts (flashes) occur every year.

Voltages associated with lightning flashes are in the range of 10 Million volts to 100 million Volts and currents of 10 kA to 100kA may occur.

The energy associated with a lightning bolt is typically 5 billion Joules.

In this section we shall consider surges on power lines due to lightning discharges.

It is commonly believed that surges on power lines are caused by direct lightning strikes. Surges are also caused on power lines whenever a strike takes place in their vicinity.

The situation in a thunderstorm before a lightning discharge takes place is depicted in Fig 1. The storm cloud becomes electrically polarised: inside the cloud winds and circulating air currents cause particles of rain and ice in the cloud to become charged. This is an example of tribo-charging, charging due to friction. Engineers are not that concerned about the mechanism that causes the cloud to become charged, or for that matter, whether the top of the cloud becomes positive with respect to its base, or negative. We have assumed the top of the cloud becomes positively charged in Fig 1.

Figure 1 The Charge and Electric Field Distribution Associated with a Storm Cloud

Figure 1 The Charge and Electric Field Distribution Associated with a Storm Cloud

Within the cloud a very strong electric field exists between the top of the cloud and its base.

The negatively charged base induces charge in any conducting objects near to it.

Positive charge is induced in both the power cables and ground.

The conductors, which are made of copper are at the same d.c. level- and all the static charge resides on their surface.

The power lines are unaffected by the high electric field in their vicinity.

When the electric field exceeds the breakdown strength of air a discharge occurs- this is an avalanche effect.

We have assumed that the discharge occurs within the cloud.

Cloud to cloud and cloud to ground discharges are possible as well as cloud to power line.

We have assumed a discharge within the cloud to show that this causes voltage surges on the power line: a direct discharge to the lines is not necessary.


Figure 2 Illustrating Surges on Power Lines caused by a Lightning Bolt

Figure 2 Illustrating Surges on Power Lines caused by a Lightning Bolt

When the discharge occurs within the cloud, the electric field collapses in a very short time.

The induced charge on the power lines, which is positive, rapidly moves away from the vicinity of the cloud along the power cables- like charges repel.

A similar situation occurs on the ground.

Large differential voltages appear briefly on the ground.

They are high enough to be harmful to both humans and electronic equipment.

The surges on the power lines are both differential and common mode.

Differential surges exist between lines

Common mode surges exist between the lines and ground.

Electronic equipment like computers can be fatally damaged by surges.

The computer shown should be worried, numerous electronic components within it can be damaged by voltage surges of about 20 volts, even less.

To prevent damage surge protection circuits are included within the electronic equipment.

The computer is connected to the power lines via an electrical sub-station.

Figure 3 A Typical Differential Voltage Surge caused by a Lightning Bolt

Figure 3 A Typical Differential Voltage Surge caused by a Lightning Bolt

Very high voltages can be developed when lightning discharges occur.

The surge is characterised by a rapid rise time and a relatively slow decay.

Engineers can produce voltage surges that closely resemble those produced by a lightning bolt.

These can be used to test surge protection circuits within electronic equipment.

The surge shown is not the voltage between the power line conductors, it is the surge emerging from the substation that is applied to the computer power supply.

Typical values of a test surge are-

Vp= 8kV, the peak value of the surge: 8000 volts

tr= 1us, the rise time of the surge: 1 millionth of a second

Vh= 4kV, the half value of the surge: 4000 volts

th = 50µs, the time for the surge to drop to half of its peak value: 50 millionths of a second

The test surge would be referred to a 1/50 wave.

Surges are not only caused by lightning bolts

When large electrical machines are turned off high voltage surges are caused by inductive effects.

When a person walks across a carpeted floor in a very dry environment (low humidity), friction causes the person to become electrically charged.

The person may be charged to several thousand volts, high enough to cause the breakdown of air when the person touches the metal surface of a computer.

The effect on the person is a very short, but sharp electrical shock, not strong enough to harm her/him, but it could be fatal to a computer.