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Industrial Ethernet Book Issue 72 / 40
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When fibre optic cabling breaks out to the factory

With Ethernet switches and device communications becoming ubiquitous for industrial control and monitoring systems, the use of fibre optic cable for creating longer haul, high reliability communication links in industrial environments has also grown dramatically. How best to engineer and install these industrial fibre optic networks is a lively topic of discussion. Stephen Woram teases out the passive elements of effective fibre optic network design.

FIBRE OPTIC NETWORKS deserve wider consideration for general industrial network design. It goes beyond a simple argument about reliability; a financial case can be made for following the structured cabling standards pioneered by the telecommunications and IT communities (albeit with tweaks to address the unique realities of industrial control system environments.) But first, an overview of the materials used in IT and industrial fibre optic networks is certainly warranted and includes the following cast of characters¡­

Sold on long reels, Distribution optic cable is the most commonly used fibre cable type for pulling between end points of an installed or ducted physical path. Distribution cable comprises a number of individual fibre strands tightly bundled into the smallest possible overall diameter, at the lowest price per unit length. To accomplish this goal, distribution cable provides all necessary protection through its jacket against the environment in which the entire cable resides, but each fibre strand within the cable is not afforded much physical protection. A cable run would normally be expected to sit within cable trays or conduit. The protection of the overall cable jacket would normally be sufficient protection provided that the distribution cable is not physically disturbed. Only at the end-points of the fibre run will the individual fibre optic strands be exposed for connection.


Distribution cable bundles a number of individual fibre strands into the smallest possible overall diameter, at the lowest price per foot. However, when fibre connectors are attached to the end of a distribution cable, the strand is isolated from much of the protective sheath.

Distribution cable is generally sold on spools, and the individual fibre optic strands once fanned out from each end of the cable, must be terminated and minimally handled after that point.

Also available on long reels, Breakout cable is designed for greater individual fibre strand protection than distribution cable. The outer sheath of the cable bundle provides environmental protection just as with distribution cable but, in addition, each strand of fibre within the cable is also individually protected by its own sheath. By peeling back the outer sheath of the breakout cable, individually protected fibre strands can be ¡®broken out' of the bundle and run to intermediate drops along the path of the entire fibre run. While the practice of breaking out fibre strands is used in building networks, fibre breakout is rarely used for industrial applications.


Breakout cable is designed for greater individual fibre strand protection than distribution cable.Most importantly, the protected, teased out fibres will stand rougher handling after the connector has been fitted allowing repeated insertion/removal cycles.

In practice, most industrial fibre optic cable installers purchase breakout cable for point to point cabling because they believe it is more rugged, or because they are more comfortable direct connecting fibre connectors to the bulkier fibre strands. The added construction elements of breakout cables carry a significant price premium and possess larger outer diameter (and weight per unit length) than straight distribution cable.

Enter the patch

Patch panels are passive interconnection devices designed to provide separation of each of the individual strands of a fibre cable at the end of a cable run. Field bulk cable enters the rear of the enclosure and the individual fibre strands of the field cable are separated and terminated using fusion slicing to short pigtails within the enclosure, or direct connection of the fibre optic adapters on the rear of the faceplate. Once the field fibre has been channelled into the patch panel, slack bulk fibre can be spooled within the patch panel body and the patch panel closed. At this point, the field cable is protected from accidental jostling or breaks caused by someone working in the cabinet.


Patch Panels are passive interconnection devices designed to provide separation of each of the individual strands of a fibre cable at the end of a cable run. Field bulk cable enters the rear of the enclosure and the individual strands of the cable may be separated. Once the field fibre has been landed within the patch panel, slack bulk fibre can be spooled in the patch panel body and the patch panel closed.

The pass–through fibre optic adapters on the front of the panel faceplate (ST, SC, LC or other) can be labelled to ensure ready identification of each strand. Unused fibre strand connectors (often referred to as 'dark cables') on the faceplate can be covered with a protective cap to protect against dust and human contact.

Fibre optic patch cables are used for connection between the fibre optic adapters on the patch panel and the communication ports on field equipment installed in the enclosure. Patch cables are typically short (1 to 10m), preterminated and tested lengths of fibre cable and – of critical importance – are manufactured with enough environmental protection to survive in enclosures and to be repeatedly handled in connection/disconnection cycles.

Why use patch panels?

When designing an industrial fibre optic network for control/monitoring applications, installers might wonder why they need to heed the bulk patch–patch, panel-patch cable precedent of the front–office IT structured cabling world. Why not simply land distribution fibre optic cable directly on Ethernet–enabled equipment in field enclosures, and forego the expense of patch panels and patch cables? Surely it is a case that those IT guys are simply over–designing¡­In fact, there are two solid reasons: first to ensure plant uptime and second, to reduce cost.

Let's first address the system failure risks. What many industrial network engineers may not appreciate is the fragility of distributiongrade fibre optic cable. Connectors attached to this cable are intended to be handled as little as possible, a case which is rarely met when hooking directly to an Ethernet device. When fibre connectors are attached to the end of a distribution cable, the strand is isolated from much of the protective sheath around the cable. The weight of the connector may [unintentionally] hang from the fibre strand and over-bend or break while inserting the cable strand connector into the electronic equipment within the enclosure.


Distribution Cable (12-strand) with Patch Panels

A couple of decades later, as Ethernet application technology evolved to 10Gbps transmit rates, a marked resurgence in the specification of screened and fully–shielded twisted–pair cabling systems has occurred. The practical benefits of screens and shields can enhance the performance of traditional UTP cabling designs to support high bandwidth transmission. But it is also necessary to dispel a few common myths and misconceptions regarding the behaviour of screens and shields.

In a patch panel, the delicate connection from the bulk cable to the field connector is protected from contact by the patch panel case. Further, damage to the connection at the end of a distribution cable means that the damaged connector must be cut off, and a new connector glued, polished and tested on the nowshortened fibre strand. Most facilities do not have the specialised equipment and skilled technicians to be able to re–fit a connector to a bulk fibre optic cable. This can lead to longer repair times, and associated network downtime. Of course, in a patch panel/patch cable installation, it is also possible to damage a patch cable. In that case, however, a spare patch cable or two can easily be kept in reserve. Disconnecting a damaged patch cable and replacing it with a spare takes seconds, and require no special skills.

A fistful of dollars¡­

So is there a financial benefit to using the patch panels and patch cables as part of fibre runs? As illustrated in Table 1, the use of patch panels can actually reduce the overall cost of the installation. The installation savings can be realised by the use of distribution fibre cable rather than breakout cable in the system. If the end-points of the fibre run can be protected by a patch panel, then the added expense of using breakout cable may be eliminated. This leaves ample budget to purchase the patch panels and patch cables. In addition to the installation savings, should a patch cable be damaged during the life of the system, repair costs will be reduced. Patch cables can be kept as spares, or purchased inexpensively from a multitude of vendors. By contrast, repairing a broken connector on a direct field fibre connection often entails a fibre technician, a fistful of dollars and hours of downtime.


Breakout Cable (12-strand) without Patch Panels
Table 1. There is a financial benefit to using the patch panels and patch cables as part of fibre runs. As these tables indicate, the use of patch panels can actually reduce the overall cost of the installation in the first place. The savings may be realised by the use of distribution fibre cable rather than cabling the entire system with breakout cable.    (DINSspace)

Industrial patch panel installers face an obstacle: the typical commercial–grade fibre patch panels found in IT closets assume a few luxuries that most industrial enclosures are not blessed with. The first is space. IT racks are predominantly 19in racks that have the depth and height to accommodate spacious equipment. Because of this, the largest IT-oriented patch panel vendors make panels intended to fit standard IT 19in enclosures.

While many control rooms may have the space to house these rack mounts (or the wall room to house a wall-mount panel), the cost justification for using them can be blown out if an already expensive field enclosure has to be upsized to allow an IT patch panel to be housed within it. Also, few IT patch panel manufacturers have made accommodation in their product offerings for the ubiquitous DIN-Rail mounting for industrial enclosures.


Patch panels can include DIN-Rail mounting as used with Industrial Ethernet switches fitted in field enclosures

The difficulty in finding patch panels that provide the ST connectors that are so loved by industrial users is a further drawback. Twist and lock ST fibre adapters are far more resilient for high vibration usage, but are less desired by IT folks who prefer the ease of SC press-connect adapters.


Twist and lock ST fibre adapters are far more resilient in the high vibration environments

In the past couple of years, a number of industrial control marketplace manufacturers have come to realise the benefit of patch panel cabling – and the shortcomings of IT vendors for this type of installation. When the right product is available, why choose anything else?

These industrial patch panels include features such as DIN-Rail mounting, size in keeping with industrial Ethernet switches, rugged metal construction, and importantly, the UL certification required for many industrial enclosure components. There is no excuse for not 'doing it right'.

Perhaps not easy to admit, but occasionally those IT guys do get something right! The use of patch panels in industrial applications can be proven to not only make the industrial communication networks we install more reliable, but also make them less expensive to install and maintain. Co-opt the IT guy's methods, but please don't tell him¡­ His ego is too big already!

Stephen Woram has worked in the industrial Ethernet market since founding Industrial Networking Solutions (INS) in Dallas, TX in 1998. Woram's latest venture is DINSpace, an INS subsidiary that manufactures passive network connectivity equipment for industrial use.

www.dinspace.com


Source: Industrial Ethernet Book Issue 72 / 40
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