International Scientific Journals
of Scientific Technical Union of Mechanical Engineering "Industry 4.0"

The previously developed new method for detecting poor electrical contacts in low-voltage electrical installations characterised by the TN protection system has been validated by field measurements performed in residential buildings. The method was developed by the establishment of a correlation between the measured line to earth and line to neutral short circuit loop resistances and the degree of the contact deterioration, i.e. the increase of its electrical resistance. The correlation was established by analysing the data obtained from a large number of documents related to periodic verifications of the quality of low-voltage electrical installations in industrial and administrative facilities (issued by the Laboratory for testing low-voltage electrical and lightning protection installations at the School of Electrical Engineering in Belgrade), as well as the data obtained through a large number of experiments in which the impact of poor electrical contacts on the occurrence of „hot“ spots in low-voltage electrical installations was analysed. In those experiments the influence of an incomplete overlap of the surface of the copper conductor and the contact surface at the electrical component terminal (reduction of the contact surface), a reduced pressure force between the contact surfaces of the copper conductor and the screw of the electrical component terminal (reduction of the torque), and an increased oxide layer at the point of electrical contact (old and/or corrosion-damaged contact) on electrical and thermal behaviour of electrical contacts was investigated. The developed method for detecting poor electrical contacts was applied to the verification of the quality of low-voltage electrical installations in 6 flats of old (10–60 years) residential buildings and the measurement results are presented and analysed in this paper.

During the lifelong maintenance of low-voltage electrical installations in any facility special attention must be given to firecausing failures, which cannot be detected by conventional protection devices (miniature circuit breakers, fuse-links, residual current devices, etc.). Such failures are most often caused by poor electrical contacts. International regulations and standards, which define periodic verification of the quality of low-voltage electrical installations, do not contain procedures by which a poor electrical contact would be detected at an early stage. However, one of the techniques used for this purpose today is performing preventive thermographic inspections (a standard covering this area is available only in the USA). By such inspections a poor electrical contact which creates prerequisites for the occurrence of an initial fire in a low-voltage electrical installation can easily and effectively be detected. Detected failures and irregularities can most frequently be eliminated by simple interventions of electricians or facility technical services. Experience from a large number of periodic verifications of the quality of low-voltage electrical installations in industrial and administrative facilities, conducted by personnel of the Laboratory for testing low-voltage electrical and lightning protection installations at the School of Electrical Engineering in Belgrade, showed that many dangerous failures would not be detected without performing preventive thermographic inspections. Several practical examples of such failures, detected in low-voltage electrical installations in industrial and administrative facilities during preventive thermographic inspections, are presented and analysed. The procedure of performing preventive thermographic inspections within periodic verifications of the quality of low-voltage electrical installations, as well as the explanation for proper interpretation of the measurement results based on the ΔТ and absolute temperature criteria, are also given in this paper.