| Radiocommunications Agency
What this technique is used for
Cable routing techniques are used to reduce the ‘unintentional antenna’ efficiency of cables, and also to prevent low-frequency crosstalk and RF coupling between cables in a system or installation.
Reducing the efficiency of the ‘unintentional antenna’ of a cable helps to reduce emissions and improve immunity. Reducing crosstalk and RF coupling helps to reduce the amount of electromagnetic noise that equipment in the same system or installation is exposed to.
How this technique is used
Replace cables which use metallic conductors with metal-free fibre optic cables, infra-red, wireless, free-space laser or microwave communications.
Route cables in intimate proximity to large areas of metal which are RF bonded at both ends (and all joints) to provide a close-proximity return path for the cables’ common-mode emissions.
Segregate cables into ‘classes’ according to the kinds of signals they carry, then route each class so as to maintain at least minimum spacings from the other classes. The spacings depend on the classes.
Key issues in employing this technique
Using non-metallic communications
Metallic communications (i.e. wires and cables) all suffer from being ‘unintentional antennas’, which means that some of their internal signals leak into the environment as RF emissions. At some frequencies, some cables leak all of their signal into the environment. Matched transmission-line cable types leak the least, but they still have emissions.
A physical structure that can leak signals into the environment can also pick up electromagnetic noise from the environment and inject it into the signal. We use cable shielding and filtering to mitigate these problems – techniques which can be surprisingly costly, especially if not designed-in from the start of a project.
There are in increasing number of non-metallic communications becoming available, especially fibre optic cables, infra-red, wireless, free-space laser or microwave communications. Often, they are available in a ‘cable replacement’ style, with a converter box at each end that converts from and to the copper cable provided by the equipment manufacturer, maybe even plugging into the panel connectors provided on the equipment.
When using fibre-optics, be wary of the fact that many fibre-optic cables use metal wires for pulling, or use metal armouring or metal foil for a vapour barrier. These metal features can act as unintentional antennas and if they penetrate a shielded area or volume they should be bonded like the shield of a cable.
Controlling common mode (CM) currents
At frequencies below 1GHz, most of the emissions from a cable are due to CM voltages and their corresponding currents. CM is principally caused by imperfect cable balance and driver balance.
CM currents follow Kirchoff’s Law, in just the same way as differential (wanted) currents must. They must get back to the source that generated the signal they came from. So if we provide a return path for them that has a lower impedance than alternative routes, we can persuade most of the CM currents to follow our CM return path instead of being radiated (leaking’) into the environment or coupled by stray inductance or capacitance to other cables.
When we provide a low impedance CM return path, the CM voltages that are developed along a cable will be less, leading again to lower radiated and coupled ‘leaked’ energy.
A cable shield that is not used to carry signal return currents can be used as a CM return path, by RF bonding it to the equipment at both ends, as described in the section on cable shielding. But even when a cable shield is a CM return path, its impedance might not be as low as it could be and an additional CM return path may help.
CM return paths which are not cable shields are collectively known as parallel earth conductors (PECs) because this is what they are called in IEC 61000-5-2. It is not the best name for them, because they may have nothing at all to do with ‘earth’ (i.e. with protective safety earthing) and because the acronym PEC is also used for ‘protective earth conductor’ – helping to increase confusion. Parallel bonding conductor would have been a better name. PECs are described below.
Parallel earth conductors (PECs)
IEC 61000-5-2 describes the use of 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 they must be RF bonded at all joints along their 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 the PEC (if no enclosure shield exists, connect the PEC to their chassis, frame or RF reference plane).
Cables must always be routed very close to their PECs, preferably with their insulation touching it.
Inside equipment, cables should always be routed very close to a metal chassis or plate, or the inside of the shielded enclosure, to maintain their CM return path.
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 RF, but they help provide a CM return path for low frequencies and also help protect equipment from damage during thunderstorms or power faults.
IEC 61000-5-2 identifies five classes of cable, of which just four are used in general whilst maintaining the same spacing as recommended by IEC 61000-5-2 (see below for spacings).
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 routes over a PEC and up to 30 metres in length are:
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 along a cable tray). Another approach is to run each class in its own metal trunking or conduit. Alternatively, use higher-performance shielding for cables which are already shielded, and add shields to cables that were unshielded.