Grounding of PLC system



##What is grounding?

Proper grounding is an important safety measure in all electrical installations. Grounding or earthing is reffered as connecting the electrical equipment body with earth.

PlC grounding:

The authoritative source on grounding requirements for a PLC installation is the National Electrical Code. The NEC specifies the types of conductors, color codes, and connections necessary for safe grounding of electrical components.

A properly installed grounding system will provide a lowimpedance path to earth ground. The complete PLC installation, including enclosures, CPU and I/O chassis, and power supplies are all connected to a single lowimpedance ground. These connections should exhibit low DC resistance and low high-frequency impedance.

A central ground bus bar is provided as a single point of reference inside the enclosure to which all chassis and power supply equipment grounding conductors are connected. The ground bus is then connected to the building’s earth ground.

In the event of a high value of ground current, the temperature of the conductor could cause the solder to melt, resulting in interruption of the ground connection. Therefore the grounding path must be permanent (no solder), continuous, and able to conduct safely the ground-fault current in the system with minimal impedance. The conection technique is shown below, which is a chassis connection:

The minimum ground wire size should be No. 12 AWG stranded copper for PLC equipment grounds and No. 8 AWG stranded copper for enclosure backplane grounds.

Ground loop:

Ground loops can cause problems by adding or subtracting current or voltage from input signal devices. A ground loop circuit can develop when each device’s ground is tied to a different earth potential thereby allowing current to flow between the grounds. Illustration of ground loop:

If a varying magnetic field passes through one of these ground loops, a voltage is produced and current flows in the loop. The receiving device is unable to differentiate between the wanted and unwanted signals and, thus, can’t accurately refl ect actual process conditions. Certain connections require shielded cables to help reduce the effects of electrical noise coupling. Each shield should be grounded at one end only, as a shield grounded at both ends forms a ground loop