In this case, the electrical circuit is not protected, due to a fault in isolation, in any case in its primary stage, which can last for hours or days. Vector insulation error can be caused by damaged wires at the time of installation of lines, insulated wires that have been damaged by rodents or moisture ... etc. In all these cases, there is a loss of some current (0.3 amperes or some amperes), which is not sufficient for the fuse to be lost, but it is sufficient to cause a fire.