A new test to characterise low voltage cables subjected to thermal and mechanical stresses

During the service, electric cables for low voltage (450/750 V) systems can be subjected both to thermal stress due to the high current caused by short circuits in the system and to mechanical stresses due to the laying conditions. The behaviour of two different kinds of cables, when subjected to such thermal and mechanical stresses, was considered: a PVC insulated cable not propagating fire and a very low (smoke and toxic gasses) emission cable insulated by a LSOH compound. An "ad hoc" test cell was designed to study the above insulating materials subjected to the combined stresses. As to the thermal stress, the specimens (a part of a 1.5 mm² cable) were subjected to sinusoidal current waves (hundreds of Ampere amplitude, lasting 100 ms) sequentially applied every 5 seconds. The mechanical stress was obtained applying a high pressure crushing the cable to four suitable cylindrical supports. When any cylinder got in contact with the specimen cable copper, an electrical current flew through that cylinder to ground and a max current relay was tripped, stopping the aging test. The proposed test cell and procedure, could be considered as a new "type test" useful to characterize low voltage cables insulation in presence of both thermal and mechanical stresses.