| 摘要 |
PROBLEM TO BE SOLVED: To specify a breakage start part and a breakage advancement direction of a heating element during device runaway, restrain incorrect control by only a secondary circuit being interrupted, ensure insulation pressure resistance of primary and secondary circuits, and improve reliability by providing a substrate with an isotropic elliptical through hole or groove. SOLUTION: An isotropic disposal hole 26 is employed as an elliptical through hole, and the hole 26 passing through an axial and longitudinal angleαby arbitrary 30 to 90 degrees is arranged so that an axial extension line does not face a DC conductive pattern 24 side and is provided at a position not contact with a resistance heating element of the surface of a heating element substrate 12. Thereby, during device runaway breakage of a heating element 10 starts axially of the rim of the hole 26 in which a tensile thermal stress value is initially a substrate strength limit in a heating element substrate 12, making it possible specify a crack advancement direction. Further, from the position of the hole 26, current-carrying heating can be stopped by breakage of the primary circuit only during device runaway, and the DC conductive pattern 24 is never broken.
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