January 2015 Electrical Apparatus

January 2015 Electrical Apparatus

This is a summary of the Electrical Apparatus January 2015 featured technical article,  by Richard L. Nailen, P.E.  

From ancient times, people confronting problems have always looked beyond the immediate “what happened?” and sought to determine “why did this happen?” In the operation of modern machines such as electric motors, the purpose of such questioning is to identify and correct not just the immediate cause of a machine breakdown, but the underlying or “root” cause. Unless this can be done, the same trouble is likely to recur.

Hence, the process of root cause failure analysis has become widely publicized in recent years. Several basic methods are in use, with scores of variations. The simplest is called the “Five-Whys” procedure. As the name implies, it creates a linear cause-and-effect chain beginning with the first “what went wrong” and leading back through successive questions until a true origin is reached. Although simple, this method has two obvious shortcomings.

First, five links in the chain may be too few. Some have suggested a “Seven-Whys” alternative. And on the other hand, an overzealous analyst risks trying to trace everything back to the origin of the universe.

Second, as one well-known machinery failure analyst contends, each effect usually requires two immediate origins: an “event” paired with a “condition.” A detailed investigation should then branch out into an increasing number of steps as it goes back in time, resembling a family tree (which is a second basic form of root cause analysis).

A third procedure uses the “fishbone” diagram. Its multiple branches denote groups or categories of contributing causes, such as manufacturing defects, faulty maintenance procedures, and adverse operating conditions.

In these methods and their many variations, an investigator must be careful to avoid the error of presuming that some mode of failure couldn’t have originated in a particular cause because the design or manufacture would have made that “impossible.”

Electrical circuits and mechanical assemblies thought to be fail-safe have indeed failed—sometimes catastrophically. Also, in dealing with motor failures, the influences of driven load behavior; motor control and protective device selection and operation; power system characteristics; and motor maintenance history cannot be overlooked.

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