Hello from San Diego: A few weeks ago, I asked for anecdotes of failure of basic insulation after passing the production-line hi-pot test. Here is a summary of the responses. I've classified the failures into three broad classes, mechanical failures, thermal failures, and electrically- caused failures. MECHANICALLY-CAUSED FAILURES. PHYSICAL DAMAGE. Leads breaking through sheet insulator. Heat sink extrusion flaw penetrated insulation. Cord abrasion at sharp edges and strain-relief. Motor winding failure due to mechanical damage. Diode stud cut wire insulation. Cord insulation failure due to crush under furniture. Pinched between appliance inlet and metal frame. Magnet wire outside slot insulation. Caught in metal-to-metal joint. (Some of the physical damage incidents were manifested over time due to the cold flow of plastic material as a function of time. Initially, the insulation passed hi-pot, but failed at a later time.) BRIDGING. Bridged by leaking capacitor electrolyte (2 reports). Bridged by loose screw. Bridged by metal shavings (2 reports). THERMALLY-CAUSED FAILURES. Overheated components caused carbonized current paths. Overheated transformer caused insulation failure. Transformer thermal protector installed with shorted leads. Arcing in circuit-breaker heated the terminal insulation. ELECTRICALLY-CAUSED FAILURES. (No reports.) My motive in requesting this information was to determine whether or not the electric strength of basic insulation is adequate. A follow-on question is whether hi-pot testing is contributing to early failure of insulation. HP has studied the electric strength of insulation as a function of time and temperature in our optocouplers. The results of this study are published in the HP Optocoupler Application Note 1074. This note can be found on the web: http://www.hp.com/HP-COMP/isolator/app_index.html (This is an Adobe Acrobat file.) There is lots of circumstantial evidence to conclude: Hi-pot test voltage, hi-pot test waveform, and hi-pot test time are not critical to solid insulation or Y-capacitor lifetime. Electric strength requirements in safety standards are at least adequate, and may be quite conservative. The electric strength of modern insulating materials is very much greater than the electric strength required by safety standards. On the other hand, we don't do a good job of identifying mechanical threats to basic insulation. Best regards, Rich