How to Prevent Lightning Effects

What is Lightning?

At times when heavy rains occur, you might have seen a flash of light in the sky and of course you are always advised to stay safe at homes. Along with the flash of light, you can also hear a large thunder sound. This flash of light is nothing but the discharge of electricity or lightening as we call it.  So let us see what actually causes lightning, its effects and how we can prevent our electrical appliances from getting damaged.

What Causes Lightning?

When the earth surface gets heated up, it heats the air above it. As this hot air comes in contact with any water body, it heats up the water which vaporizes and as the air rises with the water vapor, the latter cools down and forms clouds. As the clouds raise further up, their size increases and when the liquid particles in the cloud reaches the higher altitude get frozen to ice particles. When these ice particles and liquid particles collide with each other, they get charged with positive polarity. The smaller ice particles get positively charged whereas the larger particles get negatively charged and get pulled down to earth due to earth’s gravitational pull. Thus there forms an electric field between these two charges. As this electric field intensity increases, there comes a point when static electricity starts flowing through the electric field lines, resulting in a spark between them.  The lightning can be within a cloud between the positive charged particles at top and negative charged particles at bottom. The lightning can also be between the negatively charged cloud and the positively charged things on ground such as humans, trees or any other conductors. Thus as electric charge flows between the cloud and the person on the ground; he/she gets a shock. This is the reason why during thunderstorm, it is advised not to go out or stand under a tree or touch any conductive material like the iron rods for your window. Also the temperature of the lightning bolt can be at a higher temperature range of 27000 degree Celsius, which is about six times more than that at the surface of sun. As this electricity passes through the air, it increases the temperature of the air in a short span of time and after some time the air cools off. As the air gets heated up, it expands and as it gets cooled down, it gets contracted. This expansion and contraction of air causes the production of sound waves.

Now since light travels faster than sound, we can see the lightning first and then hear the thunderstorm.

How Lightning Effects the electric supply systems at homes

Measure the AC voltage between Earth and neutral terminal in the three pin plug in your house. Everybody will be surprised to find that it varies from 1 to 50 volt or more even. Ideally it should be zero. Earth open will also show zero which is dangerous. Then what should we do to be in safe side? Shorting earth and neutral is dangerous and it is never done.

Why lightning damages your electrical system?

Neutral at the substation feeding your house has a definite resistance, say 1 ohm with respect to ground. Because of unbalanced voltage in 3 ph, current flows in this resistance. This current may be 1 A to 50 A or more even. So IR varies from 1 V to 50 volts. Thus at your home, between earth to neutral the same voltage appears, on which you have no control. The worst happens if a lightning strikes on the sub station that can force kilo amps through this resistance. Imagine that voltage. This causes catastrophic damage to an electronic circuit that also uses the earth of the house wiring. Companies have lost millions of rupees in the past until a solution to that was implemented. Home electric appliances like TV, Computer, etc often get damaged from high voltage spikes that appear in the power lines. Very high voltage spikes and transients develop for a fraction of a second in the supply lines when lightning occurs. Such short duration high voltage spikes get super imposed on the mains also when high capacity loads are switched on or off. Also it happens when power resumes after a power failure due to high magnetic field in the distribution transformer. The heavy inrush current flows when power resumes after a power failure. This is due to the generation of high magnetic field in the distribution transformer of the power distribution system. This can cause instant breakdown of the devices like TV if it is kept switched on state during power failure. Hence it is mostly advisable to switch off the appliances during power failure. Even though the spikes are too short in short in time period, they can cause permanent damage to the appliances.

How damage from lightning is prevented?

The best solution is where one can short circuit the earth to an isolated neutral by using an isolation transformer of 1:1 primary to secondary ratio. Mind it one cannot short circuit the neutral supplied by the utility company to your house earth.

2 ways to protect your electrical devices from getting damaged due to lightning effects

1. Using MOVs (Metal oxide Varistor)

Few MOVs can be added in the existing switch board to protect the appliances from high voltage spikes. If heavy transients develop in the mains, the MOV in the circuit will short circuit the lines and the fuse/ MCB in the house will blow out.

MOV Protection:

Metal Oxide Varistor (MOV) contains a ceramic mass of zinc oxide grains, in a matrix of other metal oxides such as small amounts of bismuth, cobalt, manganese, etc. sandwiched between two metal plates which forms the electrodes. The boundary between each grain and its neighbor forms a diode junction, which allows current to flow in only one direction. When a small or moderate voltage is applied across the electrodes, only tiny current flows caused by reverse leakage through the diode junctions.

When a large voltage is applied, the diode junction breaks down due to a combination of thermionic emission and electron tunneling, and large current flows. Varistor can absorb part of a surge. The effect depends on the equipment and details of the selected Varistor.

The Varistor remains non-conductive as a shunt mode device during normal operation when voltage remains well below its “clamping voltage”. If a transient pulse is too high, the device may melt, burn, vaporize, or otherwise be damaged or destroyed.

Here three MOVs are used, one between the Phase and Neutral, another one between the Phase and Earth and the third one between the Neutral and Earth.10 Amps fuses or MCBs can be provided both in the Phase and Neutral lines for total protection. This set up can be arranged in the existing switch board from which the appliance gets power.

2. Delaying Switching time of relays

The basic idea is be to delay the switching time of the relays which are electromagnetic switches to power on the electronic devices.

This simple circuit solves the problem. It gives power to the device only after a delay of two minutes when switched on or power resumes after a power failure. During this interval, the mains voltage will stabilize.

Basically the switching of the relay is controlled by the SCR, whose switching is in turn controlled by the rate of charging and discharging of the capacitor.

The circuit works like the delay circuit in stabilizers. It uses only a few components and can be assembled easily. It works on the principle of charging and discharging of the capacitor. A high value capacitor C1 is used to get the required time delay. At power on, C1 charges slowly through R1. When it gets fully charged, the SCR triggers and the relay switches on. Power to the device is provided through the NO (Normally Open) and the Common contacts of the relay. So when the relay triggers, the device will switches on. The SCR has the latching property. That is, it triggers and the current flows from its Anode to Cathode when the gate gets a positive pulse. The SCR continues to conduct, even if its gate voltage is removed. The SCR switches off only if its anode current is removed by switching off the circuit.

An LED indicator is provided to indicate the activation of the relay. Resistor R3 limits the LED current and resistor R2 discharge the capacitor.

How to Set

Setting of the circuit is easy. Assemble it on a common PCB and enclose in a case. Fix an AC socket in the case. Connect the phase line to Common contact of the relay and the NO contact to the AC socket. Neutral line should go directly to the other pin of the socket. So the phase line continues when the NO contact of the relay makes the contact with the common contact.