PLC system and electrical system is how to ground - - programmable logic

by:Coolmay     2020-07-09
Commonly used in industrial field is divided into shielded cable and cable no cable shielding layer. Normal DCS collecting and distributing control system and PLC control modules for each instrument signal cable is laid in the shielded cable. Like a low-pressure AC220V ac power supply cord not shielded cable. Into the DCS system and PLC module should be paid attention to normal signal cable are all adopt the single point grounding. That is a meter on the shielding layer without doing grounding measures.

control cabinet of the DCS system and PLC controller control module to do ground handling. Also note when shielded cable has relay points, shield also should make joint treatment and insulation. This is to ensure that the shield continuity and reliability of single point grounding. For multi-core cable need to be separated when, should choose every to have separate shield twisted-pair cable ( If the environment is good, the absence of high-voltage electrical interference can also need not choose) 。 Enter the site must wear into the signal cable threading pipe or laying in the bridge.

in the electrical field. According to the power device of relay protection and automation equipment design code 'regulation, when static protection is used, use shielded cable, shielding layer should be grounded at both ends. This and our industrial control signal cable shielding layer in the professional do the rules of the single point grounding, conflict each other. For through capacitive coupling electric field interference, single point grounding can reduce disturbance voltage, play a role of shielding. For through the induction magnetic field coupling interference, shielded cable two online induction signal interference voltage nearly equal and opposite direction.

in the application circuit is offset each other. With the method of grounding on both ends. External interference current of the magnetic field generated in the shield induction to produce a current in the opposite direction of current and external interference, this prayer to offset lower interference current also can produce the effect of shielding.

but if ends earthing, potential inconsistency on both ends, when the potential difference is produced. Will generate an additional current in the shield, the shield voltage signal cable to produce interference. As we the automation equipment is more dispersed, a meter to the shield will receive all public grounding location difficulty is big. How much more like the vast majority of our instrument and control system, signal is low frequency signal. The grounding principle of low frequency signal is single point grounding. To avoid the formation of ground loop.

in order to guarantee the DCS and PLC module card is not affected by potential difference causes interference, now all rules into the signal of DCS card and PLC module adopts single point grounding way.

in this say about our new factory module burned cases caused by disturbance. When installation, due to the scene of the installation environment, put into S7 - 1200 module of on-off signal cables and electrical high voltage cable laid in the same cable tray. As a result, PLC switch input module and a large number of burned. Final inspection all the signal cable in the same cable tray with high voltage. The original signal cable was laid in the middle of the high voltage cable layer. Just formed the strong interference voltage. Finally laid again to solve the problem. The last point in signal cable laid must press a regulation and high voltage cable laying separately.


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