Screened cable in the industrial environment

B Elliott from Connectix explains why screened cable is a good idea in the industrial environment.

'Careful design of the cable interface may allow you to use unscreened cables and still meet EMC requirements, but this isn't always so.

Radio Communications Agency/Ofcom An unscreened balanced twisted pair data cable, when correctly installed, will keep out electromagnetic interference, but only up to a point.

In mission critical systems, and especially where high levels of electrical noise will be encountered, screened cable should be used.

For severe environments, optical fibre links should be used.

Factories, hospitals, laboratories, airports and even office environments that contain dense amounts of equipment, can be classified as severe environments.

There is a range of European standards available, spanning 2007 and 2008, which give guidance on the correct selection of data cables and their installation requirements in different environments.

BS EN 50173-1: 2007.

Information technology, generic cabling systems, Part 1; general requirements.

BS EN 50173-3: 2007.

Information technology, generic cabling systems, Industrial Premises.

BS EN 50174-2: 2008 (Draft).

Information technology - Cabling installation - Part 3: Installation planning and practices internal to buildings.

Industrial Ethernet is the name given to the use of Ethernet protocol to a range of industrial and process control network applications.

Until recently automation and control systems have tended to use manufacturers' proprietary protocols and cabling.

The advance and lower cost of TCP/IP interfaces means that more and more control, data and voice applications are being integrated together, and this means an integrated cable platform to support them.

But cables destined for benign office environments aren't ideal for harsh industrial and outdoor environments.

Several other standards are in the pipeline to define the requirements of cable put into harsher environments.

TIA 1005, Industrial Cabling, is due out in 2008.

TIA/EIA-862 Building Automation Systems Cabling Standard For Commercial Buildings has been published.

ISO/IEC 24702:2006 Information technology -- Generic cabling -- Industrial premises, specifies the design of a generic cabling infrastructure for industrial premises that stretches from the building entrance points to and including the Telecommunications Outlet.

IEC 61918 specifies profiles covering installation for fieldbus communications media within and between the Automation Islands of industrial sites.

It covers copper and optical media.

Where the media includes options for power transfer to communications entities, the power options are also specified.

It also covers the Automation Outlet (AO), that is the interface between the industrial automation network and a corporate network defined according to generic cabling specified in ISO/IEC 24702.

This International Standard is a companion standard to the communication systems specified in IEC 61158 and IEC 61784.

It provides guidelines that cope with the critical aspects of the industrial automation area (climatic conditions, vibrations, chemical pollution, EMC, safety, etc.) It complements existing standards (IEC61158, IEC61784; IEEE 802.3; IEC11801; EN50174, EN50173).

In particular, it complements defined generic industrial wiring specifications for enhanced shielding and armouring standards.

IEC 61918 addresses:.

- Installation planning - Installation implementation - Installation verification - Installation administration and maintenance - Installation troubleshooting.

The actual list of profiles to be addressed in the first Committee Draft (CD) is:.

- DeviceNet.

- ControlNet.

- EtherNet/IP.

- PROFIBUS.

- PROFIBUS-PA.

- PROFINET.

- INTERBUS REMOTE-BUS.

- INTERBUS RTE.

EN 50173-1:2007 Information technology - Generic cabling systems introduces the concept of the 'MICE' classification.

Typical of the new industrial Ethernet and automation standards is the concept of MICE (Mechanical, Ingress, Climatic and EMC) specifications for differing severities of environment.

The acronym MICE is being adopted by many industrial cabling standards committees as a means of describing levels of harsh environments.

* Mechanical - shock, impact, vibration, tensile force, crush.

* Ingress - particulate ingress and immersion.

* Climatic - temperature, thermal shock, humidity, UV (solar radiation), chemical pollution (liquid and gaseous).

* Electromagnetic - ESD, conducted and radiated RF, EFT, transient ground potential, magnetic field.

MICE levels are degrees of environmental severity within an industrial premise.

MICE 1 - Essentially a description of the commercial office environment.

MICE 2 - Light industrial: assembly, food processing, health care, wash-down etc.

MICE 3 - Industrial: petro/chemical, foundry, automotive, machining etc.

However any industrial facility may contain one, two or all three levels, or may exceed all three.

From Table 2 we can see the electromagnetic environment predicted.

Environments described as E2 and E3 are industrial levels.

One strategy to use if installing unscreened cable is to maintain a very large separation between data and power cables.

This is described currently in BS EN 50174-2 where a figure of 200mm is given as the separation between unscreened power and data cables.

This figure will be increased by the forthcoming edition BS EN 50174-2: 2008.

It should be noted that most cable manufacturers will only guarantee their cabling product performance if they have been installed to the BS EN 50174-2 standard.

Table 3 is an extract from the draft EN 50174-2 showing how cable will be classified from A to D in their capacity for rejecting outside electrical interference.

Only screened cable would qualify as Class A or B.

From the cable classification we can derive the separation required.

For unscreened cable we can see that the separation requirement has gone up to 300mm if no other screening effect is supplied by the cable containment.

However the story does not end there.

The distance undergoes a multiplication effect if the size or quantity of power cables increases.

Although there is a reduction for 1 to 5 small power circuits, for large three phase circuits the multiplication factor increases to 6.

There could then be a requirement to maintain 6 x 300mm, or 1.8 metres separation between data and power cables to guarantee safe operation of unscreened data circuits.

The use of screened cable makes more practical sense when the application is in an electrically noisy environment.

Connectix has engineered the components of their cabling systems to precisely match each other and can thus guarantee a bit error performance rate better than 10-12 and thus exceeds the most demanding requirements of all Ethernet and industrial automation protocols.

To ensure external noise does not become a negative factor Connectix recommends the following:.

- Use unscreened cable up to 1000Mb/s and when the ambient field strength is less than 3V/m from zero to 250MHz.

This is the general expectation of the ordinary office environment.

- Use screened cable up to 10,000 Mb/s and when the ambient field strength is between 3 and 10V/m over 500MHz.

This would be considered as an electrically noisy industrial environment.

- Use optical fibre if the ambient field strength exceeds 10V/m, e.g close to airport radars, radio transmitters and high voltage equipment such as x-ray sources.

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