Electric fence insulators allow you to fasten electrified wire to the post without losing energy through the post. It is very important to match the type of wire and the type of fence post. The Fi-Shock Large Screw-In Insulators are made from porcelain and are designed for wooden posts. They can be used with steel wire, aluminum wire, polyearope, and polywire. They come in a durable, white porcelain and have a lifetime guarantee.