Selection Of Number Of Cable Cores With Emphasis On Sizing Parameters

Dependance On Installation Site

The selection of number of cable cores basically depends on the type of system where it is going to be installed.

Generally we have two types of systems:

1. A perfectly balanced system and
2. A system with some degree of unbalance (or Unbalanced System).

Generally cable sizing includes below parameters:

1. Cable installation conditions and the load it will carry
2. Continuous current rating of the cable
3. Voltage drop and short circuit considerations
4. Earth fault loop impedance

Here, I am going to describe that how the number of cores can be selected.

3-Core Cables

These cables are used generally for a perfect balanced 3-phase system. When the currents on the 3-live wires of a 3-phase system are equal and at an exact 120° phase angle, then the system is said to be balanced. The 3-phase loads are identical in all respects with no need of a neutral conductor.

An important example of 3-phase load is electric motor and that is why, they are fed through 3-Core cables in most cases.

3.5-Core Cables

A 3-phase system may have a neutral wire. This wire allows the 3-phase system to be used at higher voltages while it will still support lower voltage single phase loads.
It is not likely in such cases that the loads will be identical, so the neutral will carry the out-of-balance current of the system. The greater the degree of imbalance, the larger the neutral current.

3-5-core cable construction (figure by mitesh-raval.blogspot.com)

When there is some degree of unbalance and the amount of fault current is very small, then 3.5 core cables are used. In these types of cables, a neutral of reduced cross section as compared to the 3-main conductors is used, which is used to carry the small amount of unbalanced currents.

4-Core Cables

When there is severe out-of-balance conditions, the amount of fault current will raise to a very high level. Generally in the case of linear loads, the neutral only carries the current due to imbalance between the phases.

4-core PVC insulated and sheathed copper conductor power cable

The non-linear loads such as switch-mode power supplies, computers, office equipment, lamp ballasts and transformers on low loads produce third order harmonic currents (Definition of Harmonics and Their Origin) which are in the phase of all the supply phases.
These currents do not cancel at the star point of a three-phase system as do normal frequency currents, but add up, so that the neutral carries very heavy third harmonic currents.

That is why the neutral of the cable feeding the equipment are not reduced and made with cross sectional area same as that of the main conductor to carry this high amount of current.

5 and 6-Core Cables

Some conditions may arrive when the amount of fault (neutral) current becomes very large than the phase currents. When the load concerned to this type of situation is fed through a multi-core cable, it is necessary to use a 5-Core or 6-Core Cable.

5-core PVC insulated and sheathed copper conductor power cable

In this condition, two (or three) conductors can be used in parallel formation to carry the high amount of generated unbalanced currents.

Keep Your Equipment Up to Date for Efficiency

Many manufacturing companies operate on thin margins, so wringing every drop of efficiency out of your equipment is crucial to profitability. This is especially important in food manufacturing, where consumers are often price sensitive. In addition, unplanned line stoppages or processing delays can result in scrap or spoilage of high cost ingredients or products. Here are eight simple tips to keep your equipment properly maintained and running smoothly.

Plan preventive maintenance

The most important thing you can do to keep equipment running smoothly is to follow the manufacturer’s recommendations for scheduling preventive maintenance. Your team may have to perform some maintenance activities after a period of elapsed time or after a certain level of production. Keep a log for each piece of equipment to be certain that the team is on top of the recommended maintenance.

Part of the maintenance program should be to inspect all components and to replace them at the first sign of wear. Replacing parts before they fail can prevent unplanned downtime. If you can, upgrade components such as wire and cable to higher quality replacements that have a longer expected life or that can withstand more stress.

Stock spare parts

Many manufacturers provide a recommended spares list. If possible, stock the items that fail most frequently. It’s a good idea to stock items such as wires, cables and shear pins so you can get back up and running quickly in the event of a problem.

Choose cables and components that stand up to your environment

Food manufacturing can be a hostile environment for electrical and mechanical equipment. Long hours of continuous operation processing acidic or abrasive foods followed by frequent cleaning with water, heat and chemicals are a normal part of the day in the food industry, but perhaps not for electrical equipment.

When you replace wires and cables, be sure to select materials designed for this harsh environment. The quality and reliability of such simple items as wires and cables directly affects the performance and reliability of your food processing equipment.

Firmware updates

Much of the equipment in use in today’s factories includes computerized components that may require periodic updating. Part of your preventive maintenance program should be monitoring the equipment company’s support website to be on the alert for software or firmware updates. Up-to-date software may improve the equipment’s efficiency and prevent possible problems that could cause disruption during production. It’s worth the time to check and update a machine’s firmware if it can help prevent unplanned downtime.

Calibration

Part of your preventive maintenance program should be calibration of your equipment, but you may also need to adjust your gauges and calibration tools. Scales and measuring devices can get out of alignment or their accuracy may deteriorate over time. You can’t calibrate your equipment if your gauges are off, and lack of calibration may result in improperly mixed or otherwise poor-quality product.

Operator training

Poorly trained operators may cause equipment malfunctions or unnecessary wear and tear on equipment because of improper or inefficient procedures. While it takes time and effort to train operators in proper setup and operation of equipment, it pays off in the long run with higher-quality product, less scrap and rework, more throughput and less unplanned downtime. Properly trained operators can improve efficiency and extend the life of your equipment.

Periodic inspections

Even though you have a preventive maintenance program in place, you might want to add a periodic inspection to your activities list. Inspecting machines for unexpected signs of wear allows you to keep equipment in peak condition. You may want to replace the original equipment wires and cables with higher quality, more reliable counterparts designed to withstand the rigors of your environment. Not only will this prolong the life of the equipment, it may also prevent unplanned downtime due to equipment failures.

Cleanliness and hygiene

Especially in food processing, cleanliness is a manufacturing imperative. Spills and dried-on gunk attract insects and vermin that may choose to snack on your equipment as well as your product. Of course, you do everything you can to stay compliant with government regulations for cleanliness, but cleanliness is more than a regulatory hassle. It can increase productivity and improve quality in addition to prolonging the life of your equipment.

Keeping your equipment in peak condition improves performance. It reduces unplanned downtime due to breakdowns, improves production efficiency and throughput, and improves the quality of your product.

Choosing the right fieldbus

How do you find the ideal fieldbus for your needs? There is no single fieldbus that can fulfill all requirements for every application. There can be considerable differences in communication requirements for different applications, therefore the fieldbus must be selected according to these requirements.

Most users want to select a simple communication method that fulfils the requirements of their application. The selected standard is supposed to have appropriate devices and tools for the application.

Apart from ensuring that the chosen device can accept multi-vendor products, users should consider the following:

* Topology
* Access to the fieldbus
* Physical features
* Protocol
* Access rights in multimaster systems
* Noise immunity
* Transmission speed.

Other consideration needs to be given to the technical, communications and system safety requirements. For a complete list, visit the Online Features section of www.mro-esource.com and click on "Choosing the right fieldbus."

Below is a checklist to consider:

Number of nodes
This is the number of devices connected to the field bus or bus segment. This varies from a maximum of 64 nodes with DeviceNet to 247 nodes with Modbus. If, for example, you have an application with 100 drives and want to control from DeviceNet then you will need two DeviceNet scanners.

Transmission speed
Also referred to as Baud rate. The higher the transmission speed the shorter the bus length. The maximum bus segment cable length is defined in the fieldbus standard, such as DeviceNet, Profibus etc., for the supported Baud rates.

Extent of user data
The extent of user data can be limited, depending on either hardware (ASICs, defined by node manufacturer) or software (protocol, defined by the fieldbus standard). This is usually defined in the fieldbus node User Manual.

Topology
The topology usually comes with rules defining how the nodes are connected to one another. The topolgy also defines bus termination rules. This is defined by the fieldbus standard and can be bus, ring, star topology etc.

Modularity, expansion possibilities
Because there is a single cable, the system can be easily constructed in a modular way. This allows the system to be easily expanded later. Machine manufacturers can build their systems based on a modular concept (flexibility).

Noise immunity
Due to reduced amount of cabling there is much less chance to pick up disturbances. Digital technology means drift is eliminated.

Cable requirements
These are defined by the fieldbus standard. Each fieldbus type has its own cable requirements, for example, cross-sectional area, impedance, stray capacitance, etc

Connection cost
Connecting a single cable is much easier than connecting a variety of analogue signal cables. The method of connection is usually standardised.

Device replacement without bus interruption
In daisy-chain bus topology, the bus cable can be removed from the device without interrupting the bus communication. With point-to-point or ring connection, bus communication will always be lost.

Requirements of the application
Type of fieldbus is selected according to the response time required by the application and/or the amount of nodes to be connected to the system

Communication
Required data transfer capacity
Usually the amount of I/O data to be exchanged via the bus

Guaranteed response time
Depending on the bus cycle time, determined by the amount of connected nodes, selected transmission speed and extent of data frame


SYSTEM SAFETY

System safety in a fault situation
Usually the standard fieldbus topology is not redundant but can be made so in many cases. In none redundant bus systems, the action in case of a communication loss can be configured within the nodes, for example fault or remain in operation etc

Safety of data transfer (error recognition)
The safety of the data transfer differs from fieldbus to fieldbus and is defined within the fieldbus protocol

Special attention towards power supply solutions
Some fieldbus systems supply the nodes via the fieldbus cable (DeviceNet) and others require an external power supply (Profibus) to feed the nodes

Readiness of diagnostic tools
For the open fieldbus standards a large variety of diagnostic tools from several manufacturers are available

Cable Spacing as a Mean to Noise Mitigation

Separation distances

In situations where there are a large number of cables varying in voltage and current levels, the IEEE 518-1982 standard has developed a useful set of tables indicating separation distances for the various classes of cables.

There are four classification levels of susceptibility for cables.
Susceptibility, in this context, is understood to be an indication of how well the signal circuit can differentiate between the undesirable noise and required signal. It follows that a data communication physical standard such as RS-232E would have a high susceptibility, and a 1000-V, 200-A AC cable has a low susceptibility.

IEEE 518 – 1982 standard

The four susceptibility levels defined by the IEEE 518 – 1982 standard are briefly:
Level 1 (High) - This is defined as analog signals less than 50 V and digital signals less than 15 V. This would include digital logic buses and telephone circuits. Data communication cables fall into this category.
Level 2 (Medium) - This category includes analog signals greater than 50 V and switching circuits.
Level 3 (Low) - This includes switching signals greater than 50 V and analog signals greater than 50 V. Currents less than 20 A are also included in this category.
Level 4 (Power) - This includes voltages in the range 0–1000 V and currents in the range 20–800 A. This applies to both AC and DC circuits.

The IEEE 518 also provides for three different situations when calculating the separation distance required between the various levels of susceptibilities. In considering the specific case where one cable is a high-susceptibility cable and the other cable has a varying susceptibility, the required separation distance would vary as follows:

Both cables contained in a separate tray:

• Level 1 to level 2-30 mm
• Level 1 to level 3-160 mm
• Level 1 to level 4-670 mm

One cable contained in a tray and the other in conduit:

• Level 1 to level 2-30 mm
• Level 1 to level 3-110 mm
• Level 1 to level 4-460 mm

Both cables contained in separate conduit:

• Level 1 to level 2-30 mm
• Level 1 to level 3-80 mm
• Level 1 to level 4-310 mm.

The figures are approximate as the original standard is quoted in inches.

Trays and conduits

Cable tray/conduit (photo credit: Legrand)

A few words need to be said about the construction of the trays and conduits. It is expected that the trays are manufactured from metal and be firmly earthed with complete continuity throughout the length of the tray. The trays should also be fully covered preventing the possibility of any area being without shielding.

Briefly galvanic noise can easily be avoided by refraining from the use of a shared signal reference conductor, in other words, keeping the two signal channels galvanically separate so that no interference takes place.

 

Electromagnetic induction can be minimized in several ways. One way is to put the source of electromagnetic flux within a metallic enclosure, a magnetic screen. Such a screen restricts the flow of magnetic flux from going beyond its periphery so that it cannot interfere with external conductors. A similar screen around the receptor of EMI can mitigate noise by not allowing flux lines inside its enclosure but to take a path along the plane of its surface.
Physical separation between the noise source and the receptor will also reduce magnetic coupling and therefore the interference.

Twisting of signal conductors

Twisting of signal conductors is another way to reduce EMI. The polarity of induced voltage will be reversed at each twist along the length of the signal cable and will cancel out the noise voltage. These are called twisted pair cables.

Multishield cable (Foil and Braid) – photo credit: multicable.com

Electrostatic interference can be prevented or at least minimized by the use of shields. A shield is usually made of a highly conductive material such as copper, which is placed in the path of coupling. An example is the use of a shield, which is placed around a signal conductor.

When a noise voltage tries to flow across the capacitance separating two conductors, say a power and a signal conductor (actually through the insulation of the conductors), it encounters the conducting screen, which is connected to ground. The result is that the noise is diverted to ground through the shield rather than flowing through the higher impedance path to the other conductor.

If the shield is not of a high conductive material, the flow of the diverted current through the shield can cause a local rise of voltage in the shield, which can cause part  of the noise current to flow through the capacitance between the shield and the second conductor.

Shielding of Power Cables

Medium and high-voltage power cables, in circuits over 2000 volts, usually have a shield layer of copper or aluminum tape or conducting polymer. If an unshielded insulated cable is in contact with earth or a grounded object, the electrostatic field around the conductor will be concentrated at the contact point, resulting in corona discharge, and eventual destruction of the insulation.

As well, leakage current and capacitive current through the insulation presents a danger of electrical shock. The grounded shield equalizes electrical stress around the conductor, diverts any leakage current to ground. Be sure to apply stress relief/ cones at the shield ends, especially for cables operating at more than 2Kv to earth.

Shields on power cables are connected to earth ground at each shield end and at splices for redundancy to prevent shock even though induced current will flow in the shield. This current will produce losses and heating and will reduce the maximum current rating of the circuit. Tests show that having a bare grounding conductor adjacent to the insulated wires will conduct the fault current to earth quicker. On high current circuits the shields might be connected only at one end.
On very long high-voltage circuits, the shield may be broken into several sections since a long shield run may rise to dangerous voltages during a circuit fault. However, the shock hazard of having only one end of the shield grounded must be evaluated for the risk!
Shielding of an electric power cable is accomplished by surrounding the assembly or insulation with a grounded, conducting medium. This confines the dielectric field to the inside of this shield.

Two distinct types of shields are used:

• Metallic
• Nonmetallic

The purposes of the insulation shield are to:

• Obtain symmetrical radial stress distribution with the insulation.
• Eliminate tangential and longitudinal stresses on the surface of the insulation.
• Exclude from the dielectric field those materials such as braids, tapes, and fillers that are not intended as insulation.
• Protect the cables from induced or direct aver-voltages. Shields do this by making the surge impedance uniform along the length of the cable and by helping to attenuate surge potentials.

Conductor Shielding

In cables rated over 2,000 volts, a conductor shield is required by industry standards. The purpose of the semiconducting, also called screening, material over the conductor is to provide a smooth cylinder rather than the relatively rough surface of a stranded conductor in order to reduce the stress concentration at the interface with the insulation. Conductor shielding has been used for cables with both laminar and extruded insulations.
The materials used are either semiconducting materials or ones that have a high dielectric constant and are known as stress control materials. Both serve the same function of stress reduction.
Conductor shields for paper insulated cables are either carbon black tapes or metallized paper tapes. The conductor shielding materials were originally made of semiconducting tapes that were helically wrapped over the conductor. Present standards still permit such a tape over the conductor. This is done, especially on large conductors, in order to hold the strands together firmly during the application of the extruded semiconducting material that is now required for medium voltage cables.
Experience with cables that only had a semi conducting tape was not satisfactory, so the industry changed their requirements to call for an extruded layer over the conductor.
In extruded cables, this layer is now extruded directly over the conductor and is bonded to the insulation layer that is applied over this stress relief layer. It is extremely important that there be no voids or extraneous material between those two layers.

Presentday extruded layers are not only clean (free from undesirable impurities) but are very smooth and round. This has greatly reduced the formation of water tress that could originate from irregular surfaces. By extruding the two layers at the same time, the conductor shield and the insulation are cured at the same time. This provides the inseparable bond that minimizes the chances of the formation of a void at the critical interface. For compatibility reasons, the extruded shielding layer is usually made from the same or a similar polymer as the insulation. Special carbon black is used to make the layer over the conductor semiconducting to provide the necessary conductivity. Industry standards require that the conductor semiconducting material have a maximum resistivity of 1,000 meter-ohms. Those standards also require that this material pass a long-time stability test for resistivity at the emergency operating temperature level to insure that the layer remains conductive and hence provides a long cable life.

A water-impervious material can be incorporated as part of the conductor shield to prevent radial moisture transmission. This layer consists of a thin layer of aluminum or lead sandwiched between semiconducting material. A similar laminate may be used for an insulation shield for the same reason.
There is no definitive standard that describes the class of extrudable shielding materials known as “super smooth, super clean”. It is not usually practical to use a manufacturer’strade name or product number to describeany material. The term “super smooth, super clean” is the only way at this writing to describe a class of material that provides a higher quality cable thanan earlier version. This is only an academic issue since the older type of materials are no longer used for medium voltage cable construction by known suppliers. The point is that these newer materials have tremendously improved cable performance in laboratory evaluations.

Insulation Shielding For Medium-Voltage Cables

The insulation shield for a medium voltage cable is made up of two components:

• Semiconducting or stress relief layer
• Metallic layer of tape or tap , drain wires, concentric neutral wires, or a metal tube.

They must function as a unit for a cable to achieve a long service life

Stress Relief Layer

The polymer layer used with exbuded cables has replaced the tapes shields that were used many years ago. This extruded layer is called the extruded insulation shield or screen. Its properties and compatibility requirements are similar to the conductor shield previously described except that standards require that the volume resistivity of this external layer be limited to 500 meter-ohms.
The nonmetallic layer is directly over the insulation and the voltage stress at that interface is lower than at the conductor shield interface.. This outer layer is not required to be bonded for cables rated up to 35 kV. At voltages above that, it is strongly recommended that this layer be bonded to the insulation .
Since most users want this layer to be easily removable, the Association of Edison Illuminating Companies (AEIC) has established strip tension limits. Presently these limits are that a 1/2 inch wide strip cut parallel to the conductor peel off with a minimum of 6 pounds and a minimum of 24 pounds of force that is at a 90º angle to the insulation surface.

Metallic Shield

The metallic portion of the insulation shield or screen is necessary to provide a low resistance path for charging current to flow to ground. It is important to realize that the extruded shield materials will not survive a sustained current flow of more than a few milliamperes. These materials are capable of handing the small amounts of charging current, but cannot tolerate unbalanced or fault currents.
The metallic component of the insulation shield system must be able to accommodate these higher currents. On the other hand, an excessive amount of metal in the shield of a single-conductor cable is costly in two ways. First, additional metal over the amount that is actually required increases the initial cost of the cable. Secondly, the greater the metal component of the insulation shield, the higher the shield losses that result h m the flow of current in the central conductor.
A sufficient amount of metal must be provided in the cable design to ensure that the cable will activate the back-up protection in the event of any cable fault over the life of that cable. There is also the concern for shield losses.

It therefore becomes essential that:

• The type of circuit interrupting equipment to be analyzed.What is the design and operational setting of the hse, recloser, or circuit breaker?
• What fault current will the cable encounter over its life?
• What shield losses can be tolerated? How many times is the shield to be grounded? Will there be shield breaks to prevent circulating currents?

Concentric Neutral Cables

When concentric neutral cables are specified, the concentric neutrals must be manufactured in accordance with ICEA standards. These wires must meet ASTM B3 for uncoated wires or B33 for coated wires.
These wires are applied directly over the nonmetallic insulation shield with a lay of not less than six or more than ten times the diameter over the concentric wires.

Shielding Of Low Voltage Cables

Shielding of low voltage cables is generally required where inductive interference can be a problem. In numerous communication, instrumentation, and control cable applications, small electrical signals may be transmitted on the cable conductor and amplified at the receiving end. Unwanted signals (noise) due to inductive interference can beaslargeasthedesiredsignal. This can result in false signals or audible noise that can effect voice communications.
Across the entire frequency spectrum, it is necessary to separate disturbances into electric field effects and magnetic field effects.

Electric Fields

Electric field effects are those which are a function of the capacitive coupling or mutual capacitance between the circuits. Shielding can be effected by a continuous metal shield to isolate the disturbed circuit fiom the disturbing circuit.
Even semiconducting extrusions or tapes supplemented by a grounded dmin wire can serve some shielding function for electric field effects.

Magnetic Fields

Magnetic field effects are the result of a magnetic field coupling between circuits. This is a bit more complex thanfor electrical effects.
At relatively low frequencies, the energy emitted from the source is treated as radiation. This increases with the square of the frequency. This electromagnetic radiation can cause dislxrbancesat considerable distance and will penetrate any “openings” in the shielding. This can occur with braid shields or tapes that are not overlapped. The type of metal used in the shield also can effect the amount of disturbance.
Any metallic shield material, as opposed to magnetic metals, will provide some shield due to the eddy currents that are set up in the metallic shield by the impinging field. These eddy currents tend to neutralize the disturbing field. Non-metallic, semiconducting shielding is not effective for magnetic effects. In general, the most effective shielding is a complete steel conduit, but thisis not always practical.

The effectiveness of a shield is called the “shielding factor” and is given as:

SF = Induced voltage in shield circuit / Inducted voltage in unshielded circuit

Test circuits to measure the effectiveness of various shielding designs against electrical field effects and magnetic field effects have been reported by Gooding and Slade.

Practices for Grounding and Bonding of Cable Trays

Grounding and bonding of cable trays (on photo: Ground wire connected to cable tray; photo credit: solarprofessional.com)

Metallic Cable Trays

Cable tray may be used as the Equipment Grounding Conductor (EGC) in any installation where qualified persons will service the installed cable tray system. There is no restriction as to where the cable tray system is installed. The metal in cable trays may be used as the EGC as per the limitations of table 392.60(A).
All metallic cable trays shall be grounded as required in Article 250.96 regardless of whether or not the cable tray is being used as an equipment grounding conductor (EGC).

The EGC is the most important conductor in an electrical system as its function is electrical safety.

Grounding and bonding of cable trays

There are three wiring options for providing an EGC in a cable tray wiring system:

1. An EGC conductor in or on the cable tray.
2. Each multi-conductor cable with its individual EGC conductor.
3. The cable tray itself is used as the EGC in qualifying facilities.

Correct Bonding Practices To Assure That The Cable Tray System Is Properly Grounded

If an EGC cable is installed in or on a cable tray, it should be bonded to each or alternate cable tray sections via grounding clamps (this is not required by the NEC® but it is a desirable practice). In addition to providing an electrical connection between the cable tray sections and the EGC, the grounding clamp mechanically anchors the EGC to the cable tray so that under fault current conditions the magnetic forces do not throw the EGC out of the cable tray.
A bare copper equipment grounding conductor should not be placed in an aluminum cable tray due to the potential for electrolytic corrosion of the aluminum cable tray in a moist environment. For such installations, it is best to use an insulated conductor and to remove the insulation where bonding connections are made to the cable tray, raceways, equipment enclosures, etc. with tin or zinc plated connectors.

NEC Table 250.122 - Minimum size equipment grounding conductors for grounding raceway and equipment

Table 2 – Minimum size equipment grounding conductors for grounding raceway and equipment

Aluminum Cable Tray Systems

Table 392.60(A) – Metal area requirements for cable trays used as equipment grounding conductors

Metal area requirements for cable trays used as equipment grounding conductors

For Sl units: 1 square inch = 645
* Total cross-sectional area of both side rails for ladder or trough cable trays or the minimum cross-sectional area of metal in channel cable trays or cable trays of one-piece construction.
** Steel cable trays shall not be used as equipment grounding conductors for circuits with ground-fault protection above 600 amperes. Aluminum cable trays shall not be used as equipment grounding conductors for circuits with ground-fault protection above 2000 amperes.

Table 392.60(A) “Metal Area Requirements for Cable Trays used as Equipment Grounding Conductors” shows the minimum cross-sectional area of cable tray side rails (total of both side rails) required for the cable tray to be used as the Equipment Grounding Conductor (EGC) for a specific Fuse Rating, Circuit Breaker Ampere Trip Rating or Circuit Breaker Ground Fault Protective Relay Trip Setting.
These are the actual trip settings for the circuit breakers and not the maximum permissible trip settings which in many cases are the same as the circuit breaker frame size.

If the maximum ampere rating of the cable tray is not sufficient for the protective device to be used, the cable tray cannot be used as the EGC and a separate EGC must be included within each cable assembly or a separate EGC has to be installed in or attached to the cable tray.

For specific areas requiring bonding for electrical continuity, refer to Figures 1-4.

Figure 1 left: Expansion splice plates; Figure 2 right: Horizontal adjustable plates

Figure 3 left: Discontinuos segments; Figure 4 right: Cable tray sections vertical adjustable splice plate

Non-metallic cable trays do not serve as a conductor. It is also recommended that wire mesh cable trays not be used as an equipment grounding conductor.
Although permitted by the NEC, it is recommended due to the unique nature of the wire mesh, fittings are manufactured in the field from straight sections by cutting away the current carrying structural wires, reducing the current-carrying capability of the system. As such, the use of wire mesh cable trays as an equipment grounding conductor is not recommended.

If the wire mesh cable tray is to be used as an equipment grounding conductor, then the installation of a ground wire is recommended.

If a wire mesh cable tray is supporting cable with a built-in equipment grounding conductor or control or signal cables, then the tray should have a low impedance path to a non-system ground to reduce noise and remove induced or stray currents. A separate grounding cable attached to the wire mesh cable tray is not usually required.

4 ways in which noise can enter a signal cable and its control - PART 2

Continued from part 1… Read here.

Electromagnetic induction and RFI

In previous part of this technical article, I wrote about electrical noise occurs or is transmitted into a signal cable system in the following four ways:

1. Galvanic (direct electrical contact) – part 1
2. Electrostatic coupling – part 1
3. Electromagnetic induction
4. Radio frequency interference (RFI)

3. Magnetic or inductive coupling

This depends on the rate of change of the noise current and the mutual inductance between the noise system and the signal wires.
Expressed slightly differently, the degree of noise induced by magnetic coupling will depend on the:

• Magnitude of the noise current
• Frequency of the noise current
• Area enclosed by the signal wires (through which the noise current magnetic flux cuts)
• Inverse of the distance from the disturbing noise source to the signal wires.

The effect of magnetic coupling is shown in Figure 1 below.

Figure 1 – Magnetic coupling

The easiest way of reducing the noise voltage caused by magnetic coupling is to twist the signal conductors. This results in lower noise due to the smaller area for each loop.

This means less magnetic flux to cut through the loop and consequently a lower induced noise voltage. In addition, the noise voltage that is induced in each loop tends to cancel out the noise voltages from the next sequential loop.

Hence an even number of loops will tend to have the noise voltages canceling each other out. It is assumed that the noise voltage is induced in equal magnitudes in each signal wire due to the twisting of the wires giving a similar separation distance from the noise voltage (see Figure 3).

Figure 3 – Twisting of wires to reduce magnetic coupling

The second approach is to use a magnetic shield around the signal wires (refer Figure 4).
The magnetic flux generated from the noise currents induces small eddy currents in the magnetic shield. These eddy currents then create an opposing magnetic flux Φ₁ to the original flux Φ₂. This means a lesser flux (Φ2 − Φ1) reaches our circuit!

Figure 4 – Use of magnetic shield to reduce magnetic coupling

Note: The magnetic shield does not require earthing. It works merely by being present. High-permeability steel makes best magnetic shields for special applications. However, galvanized steel conduit makes a quite effective shield.

4 ways in which noise can enter a signal cable and its control - PART 1

Few words about interference…

Noise, or interference, can be defined as undesirable electrical signals, which distort or interfere with an original (or desired) signal. Noise could be transient (temporary) or constant.

Unpredictable transient noise is caused, for example, by lightning.

Constant noise can be due to the predictable 50 or 60 Hz AC ‘hum’ from power circuits or harmonic multiples of power frequency close to the data communications cable. This unpredictability makes the design of a data communications system quite challenging.

Electrical noise occurs or is transmitted into a signal cable system in the following four ways:

1. Galvanic (direct electrical contact)
2. Electrostatic coupling
3. Electromagnetic induction (in part 2)
4. Radio frequency interference (RFI) (in part 2)

If two signal channels within a single data cable share the same signal reference conductor (common return path), the voltage drop caused by one channel’s signal in the reference conductor can appear as a noise in the other channel and will result in interference. This is called galvanic noise.
Electrostatic noise is one, which is transmitted through various capacitances present in the system such as between wires within a cable, between power and signal cables, between wires to ground or between two windings of a transformer. These capacitances present low-impedance paths when noise voltages of high frequency are present.


Thus noise can jump across apparently non- conducting paths and create a disturbance in signal/data circuits.

Electromagnetic interference (EMI) is caused when the flux lines of a strong magnetic field produced by a power conductor cut other nearby conductors and cause induced voltages to appear across them.

When signal cables are involved in the EMI process, this causes a noise in signal circuits. This is aggravated when harmonic currents are present in the system. Higher order harmonics have much higher frequencies than the normal AC wave and result in interference particularly in communication circuits.


Radio frequency interference involves coupling of noise through radio frequency interference. We will now describe these in some detail.