When was the last time you checked the X/r rating of your system:

Engineering calculation over the years has moved through a process of simplifications. The original formulae used to verify the proper application of equipment were simplified from the infinite integration to truncated forms surrounded by generalizations to capture on a computer program. This monotonous routine has been removed from the challenge of making money by not being involved in fundamental application of concepts. The general principles used to make the explanations more understandable have changed. The protective devices used to safeguard the electrical systems which are tested to strict definitions involving the power factor of the circuit at the point application. This affects an item known as recovery voltage that will cause a device not to perform to its expectations.