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Cable hygiene for better EMC immunity

This article was originally written in the period 1995-2000

Mitsubishi has produced a booklet to be issued with the FX range of PLCs that explains the EMC regulations and CE marking requirements in simple terms. This section deals with “Cable Hygiene”

What can be done to minimise EMC problems at installation? There is no magic solution for EMC problems. There are only two secrets to good EMC hygiene: Avoid the problem in the first place Conduct remedial work.

Always try to keep cable lengths as short as possible. it is often the practice of installation engineers to cut cables a couple of metres longer than they need. The spare cable is then looped into trunking to allow for any future system modifications. This is very nice, but at what cost? It will increase the chances of an EMC problem, as the available antenna or aerial has just got longer, allowing greater “access” to lower frequency areas. Long wavelengths and low frequencies need long antennae.

Always try to use a good quality cable. The lower the inductance the better. If a lower grade of cable is used (with a high inductance) it will be cheaper, but the system will be open to a lower EMC immunity within its local environment. You will also increase your ability to emit greater EMC levels. Remember, inductance is a measure of the ability of the device or cable to store energy in the form of either a charge or a magnetic field. You are trying to maximise your Electromagnetic Compatibility, so you must lower the chances of a magnetic field being present or causing a problem.

There are two things which will improve a wiring loom’s EMC potential:

  • (a) if you are using plain cables, which have equal and opposite currents, twist them together. By doing this, the circuit inductance is lowered and the effective area available for EMC radiation is reduced to approximately the twist pitch. This is because, as the cable extends, each alternative twist acts to cancel any EMC effects of the previous twisted cables. This method will not reduce all of the EMC noise, but it will help to reduce any that is present.

    There are pre-made versions of twisted cables and even some variations which have both twist and screened properties. This last style of cable is ideally suited to delicate signal types, such as analog.

  • (b) The use of shielded/screened cables is another good way to reduce EMC problems. This type of cable can be expensive, but with a little planning, the use of screened multicore cables can actually save money.

    The benefits of using screened multicore cables come from ease of installation. hence installation time and cost go down. Not only are the contained fibres of the multicore screened in one go, but the screen can also offer a small degree of mechanical protection. This is useful when long cable runs are laid and the cables have to be drawn through many different conduits and cable trays. It’s easier to damage a single cable than it is to damage a well-screened multicore. It is usual to try to keep “like” signal” types/strengths in the same multicore and not to mix the signals carried by any single multicore. So with a little planning, multicore cables can be utilised to their optimum to provide good EMC protection and value for money.

High and low voltage cabling should always be separated as far as possible, This will both make a safe installation and reduce any EMC noise pick-up coming from the high voltage cables onto the lower voltage cables. In addition, voltage types should also be separated as far as possible. AC and DC circuits can couple, causing a transfer of EMC noise. Again, there are also safety implications from mixing voltage types which are not reliably separated.

An ideal situation is to have AC and DC cabling running in different trunking or conduit and then to split this again to high and low voltage cabling runs.

If for any reason, different voltage types (as described) meet through their respective cable runs, then the meeting point should be kept as small as possible. Ideally a perpendicular (90deg) crossover is required. This will minimise the contact area, reducing the chances of any EMC problems. In addition, it becomes easier for safety considerations to be added to a system, maintaining reliable separation of cabling.

The Mitsubishi booklet on EMC Regulations for PLCs is available free of charge by entering the enquiry number below.

Added on 9 May 2009 at 12:22 pm | Read more Drives articles