Abstract: In order to improve the temperature constancy of induction boost in a standard working temperature range for fault current safety switches, a Ni-Mo-Cu-Fe alloy is provided whose nickel and copper content in a binary nickel-copper system is defined by a quadrangle of points A (80.5 wt. % Ni and 0 wt. % Cu), B (82 wt. % Ni and 0 wt. % Cu), C (70 wt. % Ni and 16.5 wt. % Cu) and D (70 wt. % Ni and 14.4 wt. % Cu) and whose molybdenum content, z, in wt. % satisfies the relation:11/30 (x-68).ltoreq.z.ltoreq.11/30 (x-63.5)with a given Ni content, x, in wt. % and the remainder of which essentially consists of iron and minor processing impurities. Such an alloy is worked into a 0.05 through 0.3 mm thick tape and wound into a toroidal tape core which is first annealed for about 30 minutes at temperatures between 900.degree. and 1200.degree. C. and is then tempered in accordance with its Mo content at the temperatures between 450.degree. and 550.degree. C. in such a manner that the magnetic anisotropy, K.sub.