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 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.
Communications and Shielded Cables
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a result of trying to reduce hum in audio systems, is no longer applicable to the complex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.
DL205 Installation and I/O Manual, 2nd Edition
A-6
Appendix A: European Union Directives (CE)
A
2
3
4
5
H
7
10
A
10
11
12
13
14
A
B
C
D
Serial Communication Cable
Equi-potential Bond
Key
Screened
Cable
Equi-potential
Bond
Control Cubicle
To Earth
Block
Conductive
Adapter
Serial
I/O