Chapter A General rules of electrical installation design

Q22
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Q - EMC guidelines
NO! YES!
Power or disturbing cables
Relay cables
Measurement or sensitive cables
Fig. Q37 : Cable distribution in cable trays
Metal tray
5 Wiring recommendations
The two conductors must be installed as close together as possible
(see Fig. Q35)
This is particularly important for low-level sensors. Even for relay signals with a
common, the active conductors should be accompanied by at least one common
conductor per bundle. For analogue and digital signals, twisted pairs are a minimum
requirement. A twisted pair (differential mode) guarantees that the two wires remain
together along their entire length.
PCB with
relay contact
I/Os
- +
Power supply
Schneider Electric - Electrical installation guide 2008
NO! YES!
Area of
loop too large
PCB with
relay contact
I/Os
Fig. Q35 : The two wires of a pair must always be run close together
- +
Power supply
Group-1 cables do not need to be shielded if they are filtered
But they should be made of twisted pairs to ensure compliance with the previous
section.
Cables must always be positioned along their entire length against the bonded
metal parts of devices (see Fig. Q36)
For example: Covers, metal trunking, structure, etc. In order to take advantage of the
dependable, inexpensive and significant reduction effect (common mode) and anticross-talk
effect (differential mode).
Power
supply
NO! YES!
Chassis 1
Chassis 2
Chassis 3
I/O interface
Power
supply
Chassis 1
Chassis 2
Chassis 3
I/O interface
All metal parts (frame, structure, enclosures, etc.) are equipotential
Fig. Q36 : Run wires along their entire length against the bonded metal parts
The use of correctly bonded metal trunking considerably improves
internal EMC (see Fig. Q37)