Mistake proofing...

Mistake proofing is when you take precautions to ensure that there is no possibility of error occurring. In England, it was found that people mistook between petrol and diesel fillers while fueling their cars at gas stations. (They have to fill it by themselves) . Manned gas stations did not solve the problem completely because there was always the possibility of human failure. Ultimately the car manufacturers fitted the petrol cars with positive polarity fuel tank mouth, and diesel cars with negative polarity. The gas stations had exactly the opposite polarity - petrol dispensers were fitted with negative polarity, and diesels dispensers with positive polarity. (Like charges repel, unlike attract!!!).