The original transfer switches were installed in approximately 1965. 

THEY WERE  replaced with ASCO bypass isolation transfer switches. 

bypass isolation SWITCHES ARE NOW THE STANDARD THROUGHOUT THE INDUSTRY

What is a bypass isolation switch?

It is a single unit consisting of an automatic  AND  a manual transfer switch

 WHAT IS THE PURPOSE OF A BYPASS ISOLATION SWITCH? 

TO ALLOW FOR PROPER MAINTENANCE AS WELL AS AND REDUNDANCY. the two switch system ALLOWS FOR PROPER MAINTENANCE WITHOUT SHUTTING OFF A critical load.  it is often difficult to IMPOSSIBLE TO PERFORM THE PROPER  maintenance ON CRITICAL CIRCUITS WITHOUT BYPASS ISOLATION TRANSFER SWITCHES 

Three standard automatic transfer switches were replaced with solid state controlled ASCO  Bypass Isolation Switches in the existing cabinets