Changes of insulation resistance of fire resistant cable under fire conditions

Low-fire-hazard cable (LFHC) intended for low voltage applications with a fire barrier made from mica tape was tested under conditions similar to those of the fire test specified in IEC 60331-21. In addition to the other standard requirements, the measurement setup was modified to enable insulation resistance measurement throughout the entire fire test. In this manner, cable insulation as a whole can be tested under fire conditions. Results from insulation resistance measurement are supported by thermogravimetric analysis and by the data obtained from two thermocouples placed near the cable sheath and inside the cable core during the test. As results show, insulation resistance is markedly affected by the melting of the organic components of insulation, by decomposition of the flame retardant and by ignition of the core insulation along the first period of the test (10 minutes). The stable value of the insulation resistance from 10th to 90th minutes of testing, together with the insulation´s behavior during cooling suggests the important role played by conductivity of the flame itself. The suggested procedure can assist in the comparison of the fire-proof functionalities of different cable designs, in the analysis of their failure mechanisms and in cable design optimization.