Do-more T1H Series PLC User Manual, 1st Edition, Rev. C
A-7
Appendix A: European Union Directives (CE)
Equi–potential Grounding
Adequate site earth grounding must be provided for equipment containing modern
electronic circuitry. The use of isolated earth electrodes for electronic systems is forbidden
in some countries. Make sure you check any requirements for your particular destination.
IEC 1000–5–2 covers equipotential bonding of earth grids adequately, but special attention
should be given to apparatus and control cubicles that contain I/O devices, remote I/O
racks, or have inter-system communications with the primary PLC system enclosure. An
equipotential bond wire must be provided alongside all serial communications cables, and
to any separate items of the plant which contain I/O devices connected to the PLC. The
diagram shows an example of four physical locations connected by a communications cable.
Communications and Shielded Cables
Good quality 24AWG minimum twisted-pair shielded cables, with overall foil and braid
shields are recommended for analog cabling and communications cabling outside of the PLC
enclosure. To date it has been a common practice to only provide an earth ground for one
end of the cable shield in order to minimize the risk of noise caused by earth ground loop
currents between apparatus. The procedure of only grounding one end, which primarily
originated as a result of trying to reduce hum in audio systems, is no longer applicable to the
complex industrial environment. Shielded cables are also efficient emitters of RF noise from
the PLC system, and can interact in a parasitic manner in networks and between multiple
sources of interference.
Serial Communication Cable
Equi-potential Bond
Key
Screened
Cable
Equi-potential
Bond
Control Cubicle
To Earth
Block
Conductive
Adapter
Serial
I/O
