The problem of fault line selection and location in non-effective earthed power system has not been perfectly solved because of small value of characteristic of criterion. Concerning enlarging the value of characteristic of criterion, a larger half-wave DC is injected to the fault distribution system through the primary neutral point of earthing transformer, based on the character of neutral voltage changes during pre-fault and post-fault period. The injected half-wave DC is used as the criterion. The principle of half-wave DC injection and its critical fault resistance are analyzed in detail. The analysis result shows the injected half-wave DC will not change the operation of distribution network and the value of injected half-wave DC can be calculated depending on the desired detecting accuracy of half-wave DC. It also shows that the half-wave DC injection method has a high tolerant capacity towards fault resistance both in isolated neutral system and resonant grounded system. Such equipment has been implemented for more than two years in power networks, and the result of detecting fault line is correct.