Wiring recommendations

From Electrical Installation Guide
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Signal classes

(see Fig. R39)

Fig. R39 – Internal signals can be grouped in four classes

Four classes of internal signals are:

  • Class 1
Mains power lines, power circuits with a high di/dt, switch-mode converters, power-regulation control devices.
This class is not very sensitive, but disturbs the other classes (particularly in common mode).
  • Class 2

Relay contacts.

This class is not very sensitive, but disturbs the other classes (switching, arcs when contacts open).
  • Class 3
Digital circuits (HF switching).
This class is sensitive to pulses, but also disturbs the following class.
  • Class 4
Analogue input/output circuits (low-level measurements, active sensor supply circuits). This class is sensitive.
It is a good idea to use conductors with a specific colour for each class to facilitate identification and separate the classes. This is useful during design and troubleshooting.

Wiring recommendations

Cables carrying different types of signals must be physically separated

Fig. R40 – Wiring recommendations for cables carrying different types of signals

Disturbing cables (classes 1 and 2) must be placed at some distance from the sensitive cables (classes 3 and 4)

(see Fig. R40 and Fig. R41)

In general, a 10 cm separation between cables laid flat on sheet metal is sufficient (for both common and differential modes). If there is enough space, a distance of 30 cm is preferable. If cables must be crossed, this should be done at right angles to avoid cross-talk (even if they touch). There are no distance requirements if the cables are separated by a metal partition that is equipotential with respect to the ECPs. However, the height of the partition must be greater than the diameter of the cables.

Fig. R41 – Use of cables and ribbon cable

A cable should carry the signals of a single group

(see Fig. R42)

If it is necessary to use a cable to carry the signals of different groups, internal shielding is necessary to limit cross-talk (differential mode). The shielding, preferably braided, must be bonded at each end for groups 1, 2 and 3.

Fig. R42 – Incompatible signals = different cables

It is advised to overshield disturbing and sensitive cables

(see Fig. R43)

The overshielding acts as a HF protection (common and differential modes) if it is bonded at each end using a circumferential connector, a collar or a clampere However, a simple bonding wire is not sufficient.

Fig. R43 – Shielding and overshielding for disturbing and/or sensitive cables

Avoid using a single connector for different groups

(see Fig. R44)

Except where necessary for groups 1 and 2 (differential mode). If a single connector is used for both analogue and digital signals, the two groups must be separated by at least one set of contacts connected to 0 V used as a barrier.

Fig. R44 – Segregation applies to connectors as well!

All free conductors (reserve) must always be bonded at each end

(see Fig. R45)

For group 4, these connections are not advised for lines with very low voltage and frequency levels (risk of creating signal noise, by magnetic induction, at the transmission frequencies).

Fig. R45 – Free wires must be equipotentially bonded

The two conductors must be installed as close together as possible

(see Fig. R46)

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.

Fig. R46 – The two wires of a pair must always be run close together

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. R47)

For example: Covers, metal trunking, structure, etc. In order to take advantage of the dependable, inexpensive and significant reduction effect (common mode) and anti-cross-talk effect (differential mode).

Fig. R47 – Run wires along their entire length against the bonded metal parts

The use of correctly bonded metal trunking considerably improves internal EMC

(see Fig. R48)

Fig. R48 – Cable distribution in cable trays