Under normal conditions for the summation current transformer the magnetising effects of current carrying conductors,in accordance with kirchhoff's low,cancel each other out. There is no residual magnetic field remaining, which could induce a voltage in the secondary winding. However if a defect in insulation causes a fault current, the balance becomes disturbed, and a residual magnetic field remains in the transformer core. this produces a voltage in the secondary winding, which via the release and the contact latching mechanism disconnects the circuit with the insulation defect.

(2)The properties demand of nanocrystalline cores for voltage independent RCCBs.
　　a. High permeability μ. The fault current given by the summation transformer will directly feed to the release, without any amplifying of signal through an electronic unit. Consequently it is important for higher permeability μ to obtain a higher voltage value in the secondary winding with low fault current.
　　b. Lower coercivity Hc. There are two respects for Hc to influence the operating properties of the summation current transformer. First, it is necessary for low Hc to obtain high permeability, and most importantly, Coercivity Hc has a great influence on the impulse current withstand capability for voltage independent RCCBs. after a surge current occurs to the summation current transformer, the cores will be quickly magnetized to saturation. When the surge current is eliminated, theoretically the saturate induction Bs should equal to zero, but practically the remance Br exists. when the undemagnetized cores are magnetized again, the permeability will decrease ,the secondary winding voltage value for summation current transformer will also decrease. This will increase the rated fault current of the voltage independent RCCBs whose value is dependent on the coercivity of the croes,e.g. the higher the value of Hc, the more reduction of permeability and the secondary winding voltage value. For a small reduction of permeability of the cores under surge current, it is necessary to reduce the coercivity of the cores.
      c. Excellent stress stability.Viberation will occur during delivery and assembly for the voltage independent RCCBs,which will result in the worse of magnetic properties for soft magnetic cores.In particularly the permeability will seriously reduce while Hc increase when the nanocrystalline cores are broken.consequently good stress stability is necessary for nanocrystalline cores to guarantee the excellent magnetic stability.
      d. Excellent temperature and time stability.When the soft magnetic cores are under long-term service at high temperature and rated operating temperature range,the magnetic properties will decrease slowly.The value of reduction can not influence the operating properties of the voltage independent RCCBs.

(3) Nanocrystalline cores for type AC voltage independent RCCBs.
Magnetic properties of nanocrystalline cores for type AC voltage independent RCCBs.

Fig.2 shows the static hysteresis loops and magnetization cures of nanocrystalline cores for type AC voltage independent RCCBs.
      a .Hysteresis loop and magnetization cure for type AC 10-30mA


      b. Hysteresis loop and magnetization cure for type AC 100mA