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Appendix A
Appendix E
EU Directives
A–6
European Union Directives
Key
Serial Communication Cable
Equi-potential Bond
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 equi-potential 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 equi-potential 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.
Screened
Cable
Equi-potential
Bond
Control Cubicle
To Earth
Block
Conductive
Adapter
Serial
I/O
Good quality 24 AWG 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audio systems, is no longer applicable to the complex industrial environment.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.
Equi–potential
Grounding
Communications
and Shielded
Cables