This is an Ohio Brass (OB) static / shield wire suspension insulator. This is insulator is used to insulate the overhead static wire on HV transmission lines. The insulator has a very low flash over point which allows it to flash (arc over) when lightning hits the shield wire which protects the highline. The insulated shield wire is used to carry a low voltage / frequency for relaying of the transmission lines. The suspension has a high rated mechanical strength of 30,000 pounds which is needed because most static / shield wires are 3/8-inch Extra High Strength steel 7-strand which is runs about 900 foot spans between towers and is very heavy.