eidocs.files.wordpress.com · Web viewThe national and international standards suggest a level of balance known as Transverse Conversion Loss (TCL) and Equal Level Transverse Conversion

Grounding of Screened cables for Industrial Applications

There are several noise ingress mechanisms that can degrade the communications performance in industrial control applications. The noise ingress mechanisms are dependent on the type of noise in the environment. Fortunately there is two types of cables to help solve these problems, they are UTP and Screened cables.

While each cable type can help reduce the influence of specific types of noise at the same time may reduce the cabling systems immunity to another noise types. Radiated and coupled noise ingress can be reduced by the use of screened cabling while degrading the immunity to ground offsets. Highly balanced UTP cabling can have excellent immunity to radiated and coupled noises in addition has excellent immunity to ground offsets that are very common in control applications.

PICK YOUR CABLES WISELY

Most cables that are designed for today’s industrial control applications have some type of balance characteristics that help to reduce the influence of noise in the environment. The national and international standards suggest a level of balance known as Transverse Conversion Loss (TCL) and Equal Level Transverse Conversion Transfer Loss (ELTCTL) for industrial channels. However depending on the manufacture of the cable, these levels may not be met. Rockwell Automation Cables exceed the minimum requirements for TCL, ELTCTL and CA set by the standards. Currently there is no way to verify the values of TCL and ELTCTL as part of a cabling installation process. Likewise there is no way to verify the coupling attenuation (CA) of a screened cable in the installation. It is worth noting that just because a cable has a screen, does not mean the screen is effective at reducing noise ingress. It is likely that field testers in the future will have the ability to verify TCL and ELTCTL but unlikely that CA or transfer impedance will ever be field verifiable. Therefore one must rely on the supplier of the cable to meet the specifications and provide robust noise immunity.

HISTORY OF GROUNDING AND BONDING PRACTICES

In the early infancy of Ethernet networks for control (pre 2000), there was no industrial class cables and there were very few Ethernet enabled industrial active devices. To deploy a complete solution customers had to use mixture of commercial office computers and interfaces to build a control network. The term COTS (commercially off the shelf) was used to describe these devices. COTS devices were designed to meet IEEE 802.3 requirements and the cabling used was standard office TIA 568 designed and installed cabling (USA). One attribute of IEEE802.3 compliant node is that shielded Jacks are required to be grounded to the electrical system ground. At the time TIA cabling did not specify balance requirements for commercial cabling, a key parameter in noise immunity for balanced cabling. Noise immunity was rarely considered since the environments in which these cables were being installed office buildings where the noise levels are relatively low.

GROUNDING AND BONDING OF COTS EQUIPMENT.

One attribute of industrial control systems is noise in the local grounds. These ground noises are caused by the large drives and motors used for moving machinery and materials on the factory floors (there are

©Rockwell Automation 2014 (rev2015)Bob Lounsbury – [email protected]

many other noise generating devices common to the industrial environment.) Depending on which country you are in Grounding and Bonding can have a different meaning thus may be installed differently. For example in North America the building codes only require that exposed building conductive surfaces be grounded. In Europe a typical building will have an equalized Earthing and Bonding system. The latter requires far more elaborate earthing and bonding system than in North America. In North America it is more likely that ground offsets may exist, however in any grounding (Earthing) system ground offsets do occur. When screened cables are used the loop is formed by the currents using the screen as a path between the two earth ground offsets see Figure 1. These currents will can increase the error rates and chances of machine outages due to noise coupling between the shield of the cable and the pairs within the cable.

Figure 1 Ground Loop Currents in a Control System

In the Early days of Industrial Ethernet ground noise currents were of huge concerns since other predecessor networks using screened cables experienced the same issues. Since IEEE802.3 required shielded jacks to be grounded, the recommended practice for COTS devices was to not connect (disconnect) the screen at one end of a screened cable as shown in Figure 2. This practice effectively opened up the potential loop currents that otherwise would cause high error rates and unstable control systems.


Figure 2 Single Ground Shield

EMERGING INDUSTRIAL ETHERNET STANDARDS

In the early 2000’s consortia began publishing standards to address the cabling requirements for industrial control applications. One of the requirements was placed on the design of the active devices. These interfaces were required to have a parallel resistor and capacitor between the Jack shields and ground (see Figure 3). This circuit helps to open up ground loops by providing a low impedance at high frequencies and high impedance at low frequencies for the shield termination (HPF). By doing this ground noise currents are reduced. This practice had been in use of other industrial networks for decades. Ethernet uses switches and routers as part of the infrastructure it is expected that these devices have grounded jacks. However by having the end (control devices) isolated with the RC circuit there is a measureable reduction in ground currents between the two “end” connected devices. The device standards were also followed with installation practices primarily focused on noise abatement.


Figure 3 R/C in Ground Path

Figure 4 Hybrid Grounding Concept for Industrial Ethernet

OTHER CHANGES TOOK PLACE IN THE CABLING PLANT.

After 2006, the cabling standards introduced balance specifications for industrial cabling TCL, ELTCTL and Coupling Attenuation (CA) for screened cables. The specific levels of TCL, ELTCTL and CA was dependent on the intended environment that was classified by a new concept termed MICE (Mechanical, Ingress, Climatic/Chemical and Electromagnetic). The MICE concept was published under ISO/IEC TR29106. The concept provided three distinct levels of environments spanning commercial office to harsh industrial. The cabling standards followed with specific levels of TCL, ELTCTL and CA for


the cabling that were referenced to the MICE E levels. All of this was designed to help abate noise in the environments.

What does MICE have to do with Grounding (Earthing) and Bonding? The specifications defined in terms

of MICE, the “E” helps to classify the environment that leads to cabling designs geared towards noise abatement. This was the first step towards providing robust cabling for Industrial control applications. By applying balance to UTP type cabling immunity of coupled and radiated noises can be realized. By applying coupling attenuation specifications to screened cables, a greater immunity to coupled and radiated noises may also be realized, extending both into the high noise environments (MICE E2 and E3).

The intent of adding balance in the form of TCL and ELTCTL was to use UTP cabling as the preferred cabling for industrial applications. This mitigates the issues with ground offsets and ground loops common to factory automation and screened cables. Therefore creating is a smaller set of applications where screened cables might be required.

In 2006 the Installation standards were released under IEC 61918 and IEC 61784-5-n that not only dealt cabling installations but also grounding (Earthing) and Bonding of the screened cables. Two Grounding (Earthing) and Bonding concepts were introduced, 1) Star grounding (Earthing) and bonding and 2) Equipotential Grounding (Earthing) and Bonding. Both concepts are designed to control noise currents in the communications system. Both are separate from the electric and light grounding and bonding building systems provided by the building. The star grounding and bonding system reflected the common practice in North America, while the latter is primarily used in Europe.

THE STAR GROUNDING (EARTHING) AND BONDING SYSTEM

By defining a separate and yet common point of reference for the communications system all the noises currents can be isolated from the noise in the electric light and power system. By establishing a common ground reference all the current are routed away from the device communications system. The entire system rides on the same ground reference point thus the differential currents are minimized in the system. High current devices do not share this ground.


Figure 5 Star Grounding System

THE EQUIPOTENTIAL GROUNDING (EARTHING) AND BONDING SYSTEM

The Equipotential Bonding System work on the principal of minimizing the voltage offsets by providing a low impedance path between all the devices. This requires a large matrix of bonding and equalization conductors (shunts) to carry the currents. There are two factors that determine the copper used in the equipotential grounding system, 1) distance between the two points requiring equalization and 2) expected currents.


Figure 6 Equipotential Grounding System

RULES FOR EARTHING AND BONDING CONDUCTORS.

The size of the conductors is dependent on the length of the conductor. The size of the wire can be reduced by at least one wire size if the conductor is installed in a grounded metallic wire way. The length of a wire vs wire size is shown in both feet and meters below, see Figure 7 and Figure 8. As mentioned in this article the effectiveness of the equalizing conductor is in partial, a function of proximity to an equally grounded metallic raceway, see Error: Reference source not found.


0 5 10 15 20 25 30 35 40 450

200

400

600

800

1000

1200

1400

1600

Conductor Length vs mm2

Gauge in mm2

Cabl

e Le

ngth

(m)

Figure 7 Grounding Conductor Length VS Metric Wire Size

2 4 6 8 10 12 14 160

500

1000

1500

2000

2500

3000

Conductor Length vs AWG

Gauge (AWG)

Cab

le (F

T)

Figure 8 Ground Wire Length VS American Wire Gauge


1 10 1000

500

1 103

1.5 103

2 1036 Ft, #8AWG Wire Impedance (Z), Proximitty to a Ground Plane

Z1 f( )

Z10 f( )

Z100 f( )

Z1000 f( )

Z10000 f( )

fFigure 9 Impedance of Equalization Conductor VS Proximity to GRP (f=MHz)

In close proximity to an equally grounded metallic raceway;1mm,10mm,100mm,1000mm,10000mm.

GUIDELINES FOR GROUNDING OR BONDING SCREENED CABLES

The fundamental criteria for these guidelines is whether or not the devices employ directly grounded Ethernet Jacks. Grounding and bonding is not an absolute science. Each application has it’s own unique environmental noise and grounding situations that requires engineering practices and experience to understand. Thus there are no hard fast rules that apply.

Since most of the ODVA compliant devices have parallel resistor and capacitor components in series with the Jack shields to ground, screened cables usually do not require shunting with equalizing conductors. The following are some guidelines for EtherNet/IP systems.

A minimum of a Star grounding (Earthing) and Bonding network is required in any communications network.

Use Industrial Designed UTP cables unless it has been absolutely determined that Screened cables are needed.

Do not place UTP cables in closed metallic conduit. This may cause the electrical properties to change.

The practice of cutting or not connecting one end of a screened cable is discouraged. This was allowed in the early 2000s in support of COTS products in industrial applications.

If one of the devices is directly grounded, the risk may be greater that an equalizing conductor is needed.

Use equalizing conductors to mitigate ground offsets. Equalizing conductors are most effective when short and in close proximity to a grounded metallic raceway.

If your system does not have a star grounding system and one or both of the jacks (in a channel) are directly grounded the following apply. Or addition if your system is expected to have ground offsets the following applies.

Equipotential Grounding (Earthing) and Bonding network is required.


For screen cables directly grounded at both ends and are longer than 5 meters, an equalizing conductor should be routed in parallel with the communications cable as part of the equipotential system. The size of the cable can be determined by using Figure 7 and Figure 8.

Communications cables routed in cabinet and/or less than 5 meters, do not require equalizing conductors and may be directly grounded at both ends. The potential for a voltage off-set is low.

Supplemental mechanical methods as shown in Figure 10 are highly discouraged. By stripping back the cable jackets and applying a clamp the electrical properties of the transmission line are degraded. If the clamps are loose high levels of noise will be introduced into the cable shields.

Figure 10 Discouraged Screened Bonding Practices


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eidocs.files.wordpress.com · Web viewThe national and international standards suggest a level of balance known as Transverse Conversion Loss (TCL) and Equal Level Transverse Conversion