If you have once had your TV, audio system and other electronic equipment destroyed by a voltage surge during a thunderstorm, you will surely know how to prevent this from happening once again. For preventing such drastic accidents, it is common to use a device called the surge protector, and to have the maximum protection, it is important to know why it is required and how it works.
Most people know of a surge protector as a long strip of electrical power connectors, which power sensitive electronic gadgets. However, two components inside the strip provide the actual protection. One of them is the Metal Oxide Varistor (MOV), and the other is the familiar fuse. The combination of an MOV and the fuse protects your electronic gadgets by limiting the voltage delivered.
Normally, all households and offices experience power surges many times during the day, including at night. The surges are generated when nearby appliances are switched on or off. Appliances such as microwave ovens, air conditioners, refrigerators and pumps switch on and switch off periodically. When they switch, they create a disturbance in the electrical supply lines, causing either a voltage dip or a voltage spike, or both. Since all electronic gadgets have a limit to the level of voltage they can withstand, any spike over and above the limit will have a damaging effect.
A thunderstorm is another factor generating a power surge. Even if lightning does not strike a home directly, it is enough if it hits a power line nearby. The power lines feeding a home can carry this surge in and can cause massive damages. Using a surge protector largely prevents all this.
The MOV inside a surge protector has a special property. As long as the voltage across it does not cross its specified limit, the MOV remains a passive device, with a very high resistance. When a surge arrives, and is above the voltage limit, the MOV lowers its resistance immediately. This causes a massive current to flow through the MOV. The increased current also flows through a fuse, which precedes the MOV, causing the fuse to blow and cutting off any further supply to the MOV and any connected gadget. In the absence of a fuse, or the fuse not blowing because of improper rating, the MOV may burn out allowing further spikes to be passed on to the gadget.
An MOV has a specific voltage rating and the spike expected at the point of use defines the rating selected. The telephone industry uses a special type of surge protection, known as Gas Discharge Tube or GDT, at specific points where the telephone lines enter a building. A GDT operates at a much higher voltage as compared to an MOV, and offers protection from higher voltage surges.
For working satisfactorily, an MOV and a GDT both need a good electrical earthing and a proper earth-wire connection.