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THE EUROPEAN directive on electromagnetic compatibility requires that equipment and electrical installations do not emit unacceptable levels of electromagnetic interference and will not be affected by interference from their surroundings.

The directive aims to prevent electromagnetic interference (EMI) on systems. These might include air traffic control, essential healthcare services, military establishments and data services. The consequences of such interference can sometimes be disastrous.

For electromagnetic interference to occur, three elements must exist: an electrical noise (EMI) source, a coupling path and a victim receptor. The coupling path from a source to a receptor can be conducted (electric current), inductively coupled (magnetic field), capacitively coupled (electric field) and radiated (electromagnetic field).

Cabling can be a significant EMI concern. Cables can act as coupling paths, or they can act as EMI-radiating sources if they are used as radiating antennas. They can also be susceptible to EMI if they are performing as receiving antennas.

The proper layout and routing of cables can contribute to the electromagnetic compatibility (EMC) of an installation and should be considered at the design stage of a project.

Power cables often need screening to reduce the level of emissions into the installation鈥檚 surroundings or to reduce coupling to parallel runs of cables connected to sensitive service equipment.

Placing cables in screened flexible or pliable conduit can be a useful solution to this problem, while this type of cable can also offer mechanical protection.

Screened flexible or pliable conduit systems contain screening layers of metal, steel, stainless steel or tinned copper and use nickel-plated brass fittings. Such products should be independently tested to confirm their screening effectiveness, which is measured in decibels (dB) of attenuation against EMI over a range of frequencies. This can be plotted on a graph (see below).

As an illustration, Flexicon manufactures 12 types of conduit, with screening performance ranges from 49 dB to 74 dB at 1 MHz. Each system鈥檚 performance from 0.1 MHz to 1000 MHz is also shown in the form of charts. These performance indicators can significantly reduce the impact of EMI and therefore increase the likelihood of achieving electromagnetic compatibility.

Some manufacturers test only at one frequency or give no screening performance figures at all, so make sure that you are comparing like for like. Perhaps it is time for industry standardisation of reporting such results, but in the meantime you may need to check the EMC screening performance graphs when comparing alternatives.

The Guide on EMC for Functional Safety from the Institution of Engineering and Technology (IET) makes the point that any company involved with creating an EMC system for functional safety should take adequate steps to check whether the supplier鈥檚 declarations are true.

The guide goes on to say:鈥淯sing commercial products whose EMC performance specifications are validated by independent assessment bodies is one way of achieving due diligence鈥� Another way is to investigate supplier鈥檚 claims yourself, for example by requesting test certificates or test reports, and checking that they indicate the desired performance and checking with the test laboratory to see how independent they are.鈥�

When considering EMC it is of course important to consider the conduit system as a whole. The conduit and fittings have to be tested as an assembly and the maker鈥檚 instructions should be adhered to.

There are alternatives to conduit. You can use individual screened and insulated conductors, but you will need to terminate the screening on every conductor. It is also possible to use a cable that is screened and oversheathed, but designers normally need specific numbers and types of conductor.

The advantage of using conduit is that it can also protect the cables from mechanical force, water, dust, temperature fluctuations, abrasion and other hazards. It can be a complete solution for protecting cabling.

Another benefit of using individual conductors in a conduit is that if one fails you can pull it out and replace it. Also, if the installation needs upgrading or refurbishing, extra cables can be fitted.

Since 2007 all fixed installations have had to comply with the EMC directive. While the responsibility often lies with the client requesting the work, it can nominate someone else to be the responsible person. Often this is the contractor.

If you are responsible for EMC compliance, make sure you know what is expected and if necessary seek advice about the product鈥檚 performance. Often screened flexible or pliable conduit is the most convenient and effective means of achieving EMC for cabling.

Make sure that you know what you are buying and ask for your supplier鈥檚 test certificate or test report.