Detection of Poor Contacts in Low-Voltage Electrical Installations

Effects of a reduced torque on heating and the electrical resistance of wire-screw terminal connections in an old (used) receptacle and two new receptacles with intentionally corroded connections were experimentally investigated in the laboratory. In one of the new receptacles, the corrosion process was accelerated by treating the line connection with a mixture of nitric acid and hydrochloric acid, and in the other by treating the neutral connection with sulfuric acid. It was shown that by using these acids, both the temperature rise and electrical behavior of long-serving receptacles can be simulated. The possibility to detect a poor contact in an electrical circuit by measuring both the line to earth (L-PE) resistance and the line to neutral (L-N) short circuit loop resistance during a periodic verification of low-voltage electrical installations was also analyzed. In the laboratory conditions, a test electrical circuit with a poor contact in each of the tested receptacles was formed and the influence of that contact on both of the mentioned resistances was observed during long-lasting heating processes. Based on the results of both the laboratory experiments and periodic verifications of low-voltage electrical installations in public and commercial buildings with the total of 11 243 receptacles, a new procedure for detecting poor contacts in low-voltage electrical installations, valid for the terra neutral protection system, is recommended. The procedure, intended for periodic verification, provides information about poor contacts and their location in the tested receptacle (on the phase, neutral, and/or equipment grounding conductor) by comparing both the L-PE resistance and the L-N short circuit loop resistance to the limit values obtained during the verification.