I, like many others, was stunned to hear that there was a major power outage covering the Outer Banks. Being an engineer, my instincts were to immediately investigate what happened and what steps occurred to lead to that point. Even before I did research, my mind was already asking “Why wasn’t there a back-up system in place for such an occurrence especially when the results effected so many?”
In functional safety, we would think of redundancy, for example, a 1oo2 or 2oo3 system. When more details came out yesterday, I saw that they did have a back-up system, they did have redundancy. North Carolina had three major underground transmission cables connecting the island’s power supply. The accident that occurred completely sheared one of the cables, while damaging the other two.
Redundant architectures seemed to be the perfect solution to get better reliability while keeping a high availability. However, field reliability studies done primarily in the nuclear industries showed that the redundant systems did not meet calculated predictions. In some cases, common stress failed both sets of equipment in a redundant system. The stress is the same thing that causes all failures; combinations of temperature, humidity, corrosion, vibration, shock, electrical surge, radio frequency interference and more. In this case, it was a steel casing that was driven through the cables.
Common cause failures can manifest itself in a host of ways in addition to stress as indicated above. These include but are not limited to the design, installation, operation, maintenance/repair activities, management of change, etc.
But don’t worry, there are ways to help prevent the effects of common cause failures and reduce the calculated failure rate (beta model).
The easiest ways are:
- Physical Separation
- Diverse Technology
Of course, there are pros and cons in both of those options and each SIF and application are different.
When you are thinking of redundancy, don’t be like Outer Banks and forget about these other two factors or you will be left in the dark!