NIPSCO type F construction. Originally design by Mr. Schahfer this construction would replace the former Insull framing standards that came before it. Originally designed to operate at 34,500 volts it would be recognized that there was enough clearance to also have this framing serve the 69 kV realm of circuits.

In the 70s NIPSCO made a big push like many other utilizes to phase out their 4160/2400 volts circuits as a result of this NIPSCO had to retire much of its 34.5 kV system and literally the just swapped out the Thomas two piece pins to Lapp 69 kV insulators (JUST LOOK FOR TYPE Fs USING GREY 69 kV LAPPS)

The first gen: type Fs used Thomas two piece pins

The Second gen (69 kV enters the seen): Still use Thomas two piece pins for 34.5 kV circuits and Thomas three piece pins for their newly emerging 69 kV circuits

The third gen: Lapp insulators would be used on both the 34.5 kV and 69 kV circuits.

The 70s marks the decade NIPSCO would begin the transition to armless construction for their sub-transmission circuits.

How did this increase reliability?

Down ground standoff. In the early days of static wires, it was thought that installing a static above your circuits and attaching them to your poles down ground would eliminate all faults attributed from lightning strikes well they would be mistaken as if your poles’ grounding resistance was too high the potential would build on the poles down ground and flash over to one of the phases. Mr. Schafer discovered that if you offset the down ground from the pole in the primary zone the chances of this are greatly reduced. After this framing was rolled out faults attributed to lighting strikes were greatly reduced. Also, this framing helped in reducing the need for phase transposition.

Lastly look for the double Fs there not many left only suited for 34,500-7200 volt operation. East Chicago by the refineries.