Internal layout

An enclosure must be designed so that coupling with the outside is not compromised by internal layout, particularly that of cables, which may not be as well controlled as circuit boards. This means that it is vital to control the physical layout of the cables inside a unit.

Location

Wherever possible cables should be located close to the adjacent metal work of a chassis, preferably in corners, and where the exposure to the external environment is minimised. Care should be taken to ensure that cables do not run adjacent and parallel to seams or apertures. Electric fields will couple capacitively through the aperture, and magnetic fields will couple inductively.

Lengths

Cables should always be kept as short as is practical. Excess length tends to allow and even encourage uncontrolled layout and routing; it also reduces the unavoidable frequencies of resonance of a cable, which may lead to greater coupling problems.

Installation

A relevant standard for controlling EMC at the installation level is IEC 61000-5-2, "Electromagnetic compatibility (EMC) - Part 5: Installation and mitigation guidelines - Section 2: Earthing and cabling". IEC 61000-5-2 describes the use of parallel earth conductors (PECs), which as well as providing other useful functions are common-mode return paths for the cables that are run against them. To function as a PEC, a structure must be RF bonded at all joints along its path, and also be RF bonded to the enclosure shield of the equipment at both ends of all of the cables that are routed along it (if no enclosure shield exists, connect the PEC to the chassis, frame or RF reference plane).

Cables must always be routed very close to their PECs, preferably with their insulation touching it.

In commercial and industrial systems and installations the PECs are often the metal cable trays used to support the cables. Metal structures such as girders can also be used. It is commonplace in some EU countries to always route Ethernet cables strapped to large-diameter conductors used as PECs – these cables aren’t very good PECs at radio frequencies, but they help provide a common mode return path for low frequencies and also help protect equipment from damage during thunderstorms or power faults.

Classification

IEC 61000-5-2 identifies five classes of cable, of which just four are used in general:

Class 1: cables carrying very sensitive signals (using shielded cables and connectors along their entire length)

Class 2: cables carrying slightly sensitive signals

Class 3: cables carrying slightly interfering signals

Class 4: cables carrying strongly interfering signals

Spacings between cable classes

Inside equipment it is best to maintain at least 100mm distance between each class of cable when run in parallel. So the spacing between parallel runs of class 1 and class 4 would be at least 300mm.

Outside equipment the minimum spacings recommended for all parallel cables routed over a PEC and up to 30 metres in length are:

Class 1 to 2: 150mm

Class 2 to 3: 300mm

Class 3 to 4: 150mm

So the spacing between parallel routes of classes 1 and 4 should be at least 600mm for cable lengths up to 30 metres. For longer runs of paralleled cable classes, the spacings should be increased proportionally (e.g. for 60 metres use double the spacings that were used for lengths of up to 30 metres).

Where the recommended minimum spacings cannot be met, it is sometimes possible to provide external shielding between classes (e.g. with a tall metal ridge, or, for magnetic shielding, a mu-metal insert, along a cable tray). Another approach is to run each class in its own metal trunking or conduit; a conduit acting as a PEC will effectively remove the need for spacing. Alternatively, use higher-performance shielding for cables which are already shielded, and add shields to cables that were unshielded. Other alternatives are heat shrink or zip-on screening tubing.

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