LG D4470 - Manual (Page 33)

Appendix E
EU Directives
A–7
European Union Directives
The recommendation is to use shielded cables as electrostatic “pipes” between
apparatus and systems, and to run heavy gauge equi-potential bond wires
alongside all shielded cables. When a shielded cable runs through the metallic wall
of an enclosure or machine, it is recommended in IEC 1000–5–2 that the shield
should be connected over its full perimeter to the wall, preferably using a conducting
adapter, and not via a pigtail wire connection to an earth ground bolt. Shields must be
connected to every enclosure wall or machine cover that they pass through.
Providing an earth ground for both ends of the shield for analog circuits provides the
perfect electrical environment for the twisted pair cable as the loop consists of signal
and return, in a perfectly balanced circuit arrangement, with connection to the
common of the input circuitry made at the module terminals. RS232 cables are
handled in the same way.
RS422 twin twisted pair, and RS485 single twisted pair cables also require a 0V link,
which has often been provided in the past by the cable shield. It is now
recommended that you use triple twisted pair cabling for RS422 links, and twin
twisted pair cable for RS485 links. This is because the extra pair can be used as the
0V inter-system link. With loop DC power supplies earth grounded in both systems,
earth loops are created in this manner via the inter-system 0v link. The installation
guides encourage earth loops, which are maintained at a low impedance by using
heavy equi-potential bond wires. To account for non–European installations
using single-end earth grounds, and sites with far from ideal earth ground
characteristics, we recommend the addition of 100 ohm resistors at each 0V
link connection in network and communications cables.
RXD
Master
RXD TXD 0V
+ – + –
Slave n
TXD 0V
+ – + –
Last Slave
RXD TXD 0V
+ – + –
Termination
100 W 100 W
Termination
100 W
When you run cables between PLC items within an enclosure which also contains
susceptible electronic equipment from other manufacturers, remember that these cables
may be a source of RF emissions. There are ways to minimize this risk. Standard data
cables connecting PLCs and/or operator interfaces should be routed well away from other
equipment and their associated cabling. You can make special serial cables where the
cable shield is connected to the enclosure’s earth ground at both ends, the same way as
external cables are connected.
For safety reasons, it is a specific requirement of the Machinery Directive that a keyswitch
must be provided that isolates any network input signal during maintenance, so that
remote commands cannot be received that could result in the operation of the machinery.
The FA–ISONET does not have a keyswitch! Use a keylock and switch on your enclosure
which when open removes power from the FA–ISONET. To avoid the introduction of
noise into the system, any keyswitch assembly should be housed in its own earth
grounded steel box and the integrity of the shielded cable must be maintained.
Again, for further information on EU directives we recommend that you get a copy of
our EU Installation Manual (DA–EU–M). Also, if you are connected to the World
Wide Web, you can check the EU Commision’s official site at:
http://eur–op.eu.int/
Analog and RS232
Cables
Multidrop Cables
Shielded Cables
within Enclosures
Network Isolation