It means that the power directly connected into a closed loop by the wire but not through the load. This phenomena caused by the following reasons: the machine is too old that the electronic components are anomalous; the foreign matter such as: insects or human carelessness and so on, With respect to a computer , if the transmission line short-circuit, the result is just the loss of data, but if the power supply short-circuit, less causes the equipment damaged, more leads to the electrical fires. Therefore, there are fuses or breaker and other protection design in the electronic devices, its aim is to prevent the damage caused by short-circuit.