The normal power supply of the PLC system is powered by the power grid. Due to the wide coverage of the grid, it will be subject to all space electromagnetic interference and induced voltage and circuit on the line. In particular, the changes in the power grid, the incoming switch operation surge, the start and stop of large power equipment, the harmonics caused by AC and DC transmission devices, the short-circuit transient impact of the power grid, etc., are transmitted to the original side of the power supply through the transmission line. PLC power supply usually uses isolated power supply, but its mechanism and manufacturing process factors make its isolation is not ideal. In fact, absolute isolation is not possible due to the presence of distributed parameters, especially distributed capacitors.

b Interference from signal lines

All kinds of signal transmission lines connected with the PLC control system, in addition to the effective transmission of all kinds of information, there will always be external interference signal intrusion.

There are two main ways of this interference: one is through the transmitter power supply or the power supply of the shared signal instrument into the grid interference, which is often ignored; Second, the signal line is interfered by electromagnetic radiation induction in space, that is, the external induction interference on the signal line, which is very serious. The interference introduced by the signal will cause the abnormal work of the I/O signal and the measurement accuracy will be greatly reduced, and the components will be damaged in serious cases. For the system with poor isolation performance, the signals will interfere with each other, causing the bus backflow of the common ground system, resulting in logical data changes, misoperation and crash. PLC control system due to signal interference caused by I/O module damage is quite serious, resulting in a lot of system failures.