How To Determine When Shielded Network Cabling Is Required

This article will explain the difference between traditional, controlled environments and harsh, industrial environments as well as the type of cabling and interconnectivity products to specify when designing networks for the latter. Additionally, this article includes information on the two popular standards for measuring and rating harsh environments: IP and NEMA. Shielded cabling and shielded interconnectivity products often are used in harsh environments.

Requirements for cabling and other network infrastructure products vary widely when comparing traditional commercial environments such as office buildings to industrial environments, which include factory floors, processing facilities, and waste water treatment plants. Commercial office environments are generally free from fine particulates, liquids, and extremes in temperature while industrial environments contain some or all of these conditions as well as shock, vibration, radio-frequency interference (RFI), and electromagnetic interference (EMI).

Table Of Contents

1. Traditional Controlled Environments
2. Industrial Uncontrolled Environments
3. EMI/RFI
4. Jacket Types
5. Fiber Optics
6. Harsh Environment Rating Systems
7. Summary
8. References

Traditional Controlled Environments

Traditional environments can include small office/home office (SOHO), small and medium business (SMB), and large enterprise networks where interconnect components aren’t exposed to dust, moisture, chemicals, or RFI and EMI. For these installations, many products are available from a host of manufacturers and suppliers.

Much of the infrastructure cabling is placed in the plenum airspace above ceiling tiles or is run inside walls where it goes untouched for years. Each intermediate distribution frame (IDF) and the main distribution frame (MDF) is typically climate controlled with HVAC systems that keep the premise cabling as well as the active equipment such as switches, routers, and servers at a constant temperature and humidity level.

These types of installations generally use a combination of copper, unshielded twisted pair (UTP), and fiber-optic cabling. Fiber-optic cabling is used for longer runs between floors of a building and sometimes underground either in a conduit or direct burial to connect multiple campus buildings (Fig. 1).

1. In traditional telecommunications wiring in a “controlled” environment, long runs of fiber-optic cable are suitable between building floors and in certain cases for underground runs to connect multiple buildings.

Industrial Uncontrolled Environments

Industrial environments often include manufacturing facilities, both onshore and offshore petroleum processing operations, coal mining operations, waste water treatment facilities, and transportation systems including tunnels and subways.

These harsh uncontrolled environments include both indoor and outdoor installations where temperature extremes, moisture, shock, vibration, and exposure to chemicals are prevalent. When designing a cabling infrastructure for these types of conditions, special consideration is required.

EMI/RFI

Motors and generators often are located right next to or near network cabling. EMI and RFI can interfere with data transmission on a copper cable and cause downtime and data errors. To reduce or eliminate EMI/RFI interference, shielded cables and connectors must be used.

Typically two types of cable shields are suitable: braided and foil. Foil shielding provides 100% coverage of the conductors, but it’s hard to effectively terminate to connectors. It also offers high resistance, which does not provide the best path to ground.

2. There are essentially three types of shielding strategies: braided wires, foil wrapping, and a combination of the two.

Braided shield generally offers 60% to 85% shielding and can only offer about 95% coverage at best. The mass of the braid is higher than a foil shield. As a result, it provides better conductivity and offers a good secure connection to the connectors at the cable ends, providing an excellent ground. Many times shielded cables will utilize both foil and braided shields to ensure the highest level of protection from EMI and RFI (Fig. 2).

Common Shielding Options For Ethernet Cables

When building a shielded cable plant, it’s important to use shielded cable, shielded connectors, and shielded interconnection points such as patch panels for the shielding to be effective against EMI and RFI. Additionally, proper bonded grounding of all components is crucial to attaining effective shielding.

Jacket Types

Another consideration when designing a cabling plant in harsh environments is the cable jacket. In some cases robotic equipment is directly connected via Category 5e/6 cables that are constantly moving. The cable also must be constructed with a hi-flex, low-friction jacket as well as robust connectors to ensure a high mean time between failure (MTBF).

Other environments call for cabling that can be exposed to petroleum-based products or other chemicals. Traditional PVC jacketed cables would break down when exposed to these harsh liquids.

If protection of equipment or people is a design requirement, consider low-smoke zero-halogen (LSZH) jacketed cables. They emit fewer toxic fumes than standard PVC-based cable jackets. Typically, LSZH cabling is used in confined spaces such as mining operations.

Fiber Optics

The use of fiber-optic cabling in industrial installations has grown as an alternative to traditional shielded cabling since fiber cabling is impervious to EMI and RFI. Additionally, fiber-optic cables can extend great distances compared to copper cables without the use of costly and cumbersome repeaters.

Although fiber is more difficult to terminate in the field and costs more than shielded twisted pair (STP) cabling, the price for fiber has continued to decline. With more offshore, lower-cost fiber manufacturing, this trend should continue.

In some cases fiber may only be needed in certain areas of an industrial cable plant near high-EMI areas or between facilities where copper falls short due to distance limitations (Table 1). If this is the case, relatively low-cost media converters can preserve the existing copper infrastructure investment and allow access to fiber runs where required.

Harsh Environment Rating Systems

Many times the areas where shielded cabling should be used are considered harsh environments where other factors come into play such as the presence of fine particulates in the air and moisture. The National Electrical Manufacturers Association (NEMA) and the International Electrotechnical Commission (IEC) are two standards groups that have devised rating systems for products such as cable assemblies and enclosures that define a product’s resistance to dust, moisture, water immersion, and ice. The IEC’s rating is called the IP or Ingress Protection rating (Table 2).

Summary

When designing a network, consider the operating environment, not just for active equipment such as Ethernet switches. Cabling, connectors, patch panels, wall plates, and equipment enclosures all must be considered to ensure optimal network performance.

References

1. Twisted pair cabling described and chronicled: http://en.wikipedia.org/wiki/STP_cable
2. Shielded network cabling and components examples: http://www.l-com.com/content/Article.aspx?Type=N&ID=221
3. Shielded Ethernet cable assemblies: http://www.l-com.com/familylist.aspx?id=68
4. Shielded Bulk Ethernet cable: http://www.l-com.com/productfamily.aspx?id=2259