It is known that there is an information signal in power supply systems and networks. The presence of information signals in the power elements of power supply systems and networks (electrical signal) in combination with other information allows extracting secondary information from power supply systems and networks. In some cases, this kind of information is confidential, has a high level of significance, and power supply facilities may belong to critical information infrastructure facilities. Thus, auditing and ensuring information security of power supply systems and networks seem relevant. In this regard, the issues of identifying previously unaccounted for channels of possible leakage of confidential information, developing methods for contactless monitoring of information security of generation, transportation, transformation and electricity consumption facilities are important. A contactless method for recording and calculating spurious emissions in established operating modes and during transients in long lines is proposed by solving the inverse problem of calculating the currents of multi-wire long lines based on measuring their magnetic field, taking into account the principle of superposition. To implement the method in application to a Q-wire line, simultaneous measurement of the magnetic field at Q different points with known coordinates is required. It also requires knowledge of the coordinates of the wires with the length of the line. Geometric measurements are proposed to be implemented using laser rangefinders or scanners. When measuring the magnetic field of a long line, the quasi-constant component of the Earth’s magnetic field is taken into account. A method is proposed for determining the direction and delay of reflection of traveling waves in a long line, based on information from two magnetic field sensors located at a sufficient distance from each other along the line. Methods are proposed to ensure the audit and monitoring of the state of power supply systems and networks that are under the influence of threats to information security violations. Mathematical modeling of the proposed method of contactless current measurement in a long line and field experiments of current measurement in a long line and registration of traveling waves are performed. The experimental results show the accuracy of the proposed methods sufficient to solve the tasks. The work develops an idea of methods and means of ensuring audit and monitoring of information security of electrical systems and networks. The results of the work make it possible to identify new, previously unaccounted for channels of information leakage and to develop new contactless methods for registering information signals in power transmission lines.