Lightning arresters are designed to safely channel a lightning strike to ground without damaging equipment. Inside the porcelain housing of an arrester, are a series of spark gaps plus one or more silicon carbide blocks. Silicone carbide has an unusual electrical characteristic. It has a very high resistance to comparatively low-voltage, but a very low resistance to extremely high voltage. When lightning strikes there is a sudden rapid rise in voltage. The silicon carbide resistance breaks down, allowing the current to be conducted to ground. After the surge is passed, the resistance of the blocks increases back to normal levels.
Lightning arresters are generally located on both the high and low side of a substation transformer to protect it from strikes coming in either direction.