On March 28, 1979 there was a cascading failure in reactor number 2 at Three Mile Island. This failure allowed large amounts of nuclear reactor coolant to escape. The accident coalesced the anti-nuclear movement and ultimately caused the decline in nuclear plant construction in the United States.

Three Mile Island

As described by Wikipedia, the accident “began with failures in the non-nuclear secondary system, followed by a stuck-open pilot-operated relief valve in the primary system”. Analysis also showed that the valves of two backup pumps were left closed by maintenance. A warning light, which would have alerted the operator of the closed valves, was obscured by a maintenance tag. While the problem was partly blamed on human error and a lack to training, it was also a result of the complexity of the control panel. There were over 750 lights, each with a letter code, some of which had the related flip switches nearby, while others are far away.

Because of the incident at Three Mile Island, the control panels have been modified with color codes. The purpose of the color coding is to help operators, in a moment of extreme stress, flip the correct switches.  This mistake proofing is the idea behind Poka-yoke. Color coding is a common Poka-yoke practice.

Poka-yoke

Poka-yoke is the Japanese term for mistake proofing. It is a fundamental quality tool. Poka-yoke actions are designed to: eliminate the opportunity for error; detect the potential for error and prevent the error. This is fundamentally the same objective as the Risk Analyst.

The common objective means that the Poka-yoke frame work, while quality oriented, can be useful to the Risk Analyst. Under Poka-yoke mistakes are classified into four categories. These are listed below.

Information Errors

• Information is ambiguous.
• Information is incorrect.
• Information is misread, misinterpreted or mismeasured.
• Information is omitted.
• There’s inadequate warning.

Misalignment

• Parts are misaligned.
• A part is misadjusted.
• A machine or process is mistimed.

Omission or commission

• Material or part is added.
• Prohibited and/or harmful action is performed.
• Operation is omitted.
• Parts are omitted, so there’s a counting error.

Selection errors

• A wrong part is used.
• There’s a wrong operation.
• There’s a wrong orientation.

As with all methods or schemas, application needs to be adjusted to the specific situation or circumstance. However, this schema does provide a useful frame work for analyzing both quality and risk. Poka-yoke is one example of the overlap between Risk and Quality Analysis.

Bio:

James Kline PhD is an ASQ certified Six Sigma Green Belt and Manager of Quality/Organizational Excellence. He has over ten years supervisory and managerial experience. He has consulted on economic, quality and workforce development issues for the City of Corvallis, Benton County Oregon, the State of Oregon and the League of Oregon Cities. He has also published numerous articles related to quality in government.