Two Full Capacity Generators - Why is the Calculated Emergency Power System PFD so High?

The probability of failure on demand (PFD) for an emergency generator system is important when electrical power is needed to protect humans from harm or to prevent equipment damage. Emergency lighting is needed for emergency responders during a power outage. Pumps, compressors, and blowers need to operate during the power outage to safely shut down the facility.

To evaluate the PFD for an emergency power system, we must consider more than just two generators. We must also consider all the components for the generator fuel, the generator controls, the transfer switches, and the circuit breakers in the feeders to the emergency load. In addition, it is important to consider the capability of the weekly, monthly, semiannual, annual, and three-year inspections and proof test to detect all the failure modes that can prevent the generator system from operating correctly. There may be many common cause events that can prevent both generators from starting or running. For example, the fuel storage system, the generator control system including over-voltage and overload protection, the downstream electrical system including the transfer switches and circuit breakers may have single points of failure affect the power supply from both generators. In addition, human action during maintenance and testing and introduce points of failure, such as leaving the transfer switches in test mode instead of automatic. While the emergency power system may be designed and operated according to NFPA 110, it is critical to evaluate and eliminate single points of failure. The paper will suggest opportunities to provide redundancy, to manage human error, and to improve inspections and proof testing to detect more failure modes.