Replacement Bus Plugs and Failure Bias
A foundry called MIDWEST for another replacement bus plug. They said this was the third one he bought from us in the last two years. He thought it was strange that he suddenly had so many failures and had to call us for replacement bus plugs. The man that called was a foundry maintenance mechanic. These men are the salt of the earth. They can fix just about anything with just about nothing. So we pay pretty good attention to their problems. In this case he complained that their bus plugs must be junk because they were suddenly having so many failures. He was sure something was wrong right from the manufacturer. They never had any problems and suddenly they were all failing. We checked our records and all the replacement bus plugs were for old equipment manufactured in the 1960s. They actually had very good luck with their bus plugs until the last couple years.
We had a little confirmation bias going. The maintenance mechanic thought there must be something wrong with the bus plugs. Why? Well, because the bus plugs were failing, so there had to be something wrong with them. Makes sense, kind of. The reality was that the equipment was getting very old, it was dirty, was seldom exercised, and therefore was mechanically less reliable. We explained that electrical equipment ages and as it gets older the frequency of failure will increase, sometimes rapidly, depending on the condition. We see a very low failure rate for bus plugs in a facility that isn’t very old and is clean. For similar facilities that are much older or much dirtier, the failure rate is higher, and increases as the facilities age. But all this is trumped by the condition of the facility and the load on the bus duct and bus plugs. In a foundry, all bets may be off. In a foundry, when equipment suddenly starts to fail, the sudden increase in failure rate is usually due to condition and not merely age. The failures are a warning that it is time for cleaning and maintenance.