Page 305 - Programmable Logic Controllers, Fifth Edition - Mobile version

electrode
Enclosure Grounding
system
Ground for slot Ground bus
power supply

CPU CPU Equipment
or I/O or I/O grounding To grounding
rack rack conductor electrode
system
Equipment
grounding
conductor
Star washer
CPU CPU
or I/O or I/O
rack rack Chassis
mounting tab
Figure 13-8 PLC grounding system.

of conductors, color codes, and connections necessary
for safe grounding of electrical components. In addition,
Equipment grounding
most manufacturers provide detailed information on the conductor (ground lug To ground bus
proper grounding methods to use in an enclosure. with (8 AWG) wire)
Figure 13-8 illustrates a PLC grounding system. A Figure 13-9 Make ground connections using a star
properly installed grounding system will provide a low- washer.
impedance path to earth ground. The complete PLC in-
stallation, including enclosures, CPU and I/O chassis, and
power supplies are all connected to a single low- impedance
ground. These connections should exhibit low DC resis- Source Source Source
tance and low high-frequency impedance. A central ground
bus bar is provided as a single point of reference inside the No ground loops Ground
loop
enclosure to which all chassis and power supply equipment formed
grounding conductors are connected. The ground bus is
then connected to the building’s earth ground. Ground bus
In the event of a high value of ground current, the tem-
perature of the conductor could cause the solder to melt,
resulting in interruption of the ground connection. There- Figure 13-10 Formation of ground loops.
fore the grounding path must be permanent (no solder),
continuous, and able to conduct safely the ground-fault
current in the system with minimal impedance. Paint or Ground loops can cause problems by adding or sub-
other nonconductive material should be scraped away tracting current or voltage from input signal devices.
from the area where a chassis makes contact with the A ground loop circuit can develop when each device’s
enclosure. The minimum ground wire size should be ground is tied to a different earth potential thereby allow-
No. 12 AWG stranded copper for PLC equipment grounds ing current to flow between the grounds, as illustrated in
and No. 8 AWG stranded copper for enclosure backplane Figure 13-10. If a varying magnetic field passes through
grounds. Ground connections should be made with a star one of these ground loops, a voltage is produced and cur-
washer between the grounding wire and lug and metal rent flows in the loop. The receiving device is unable to
enclosure surface, as illustrated in Figure 13-9. differentiate between the wanted and unwanted signals
and, thus, can’t accurately reflect actual process condi-
• Any protective ground wires must have a resistance tions. Certain connections require shielded cables to help
value of less than 0.1 Ω. reduce the effects of electrical noise coupling. Each shield
• The resistance from the system ground to the earth should be grounded at one end only, as a shield grounded
ground must have a value of less than 0.1 Ω. at both ends forms a ground loop.

286 Chapter 13 PLC Installation Practices, Editing, and Troubleshooting

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