5 key factors to the correct cable selection and application

Cable selection and application

It is essential to know cable construction, characteristics, and ratings to understand problems related to cable systems. However, to correctly select a cable system and assure its satisfactory operation, additional knowledge is required. This knowledge may consist of service conditions, type of load served, mode of operation and maintenance, and the like.

The key to the successful operation of a cable system is to select the most suitable cable for the application, make a correct installation, and perform the required maintenance.

In this technical article, discussion is based on the correct cable selection and application for power distribution and utilization.

Cable selection can be based upon the following five key factors:

1. Cable installation
2. Cable construction
3. Cable operation (voltage and current)
4. Cable size
5. Shielding requirements

1. Cable installation

Cables can be used for outdoor or indoor installations depending upon the distribution system and the load served.

A good understanding of local conditions, installation crews, and maintenance personnel is essential to assure that the selected cable system will operate satisfactorily! Many times cable insulation is damaged or weakened during installation by applying the incorrect pulling tensions.

Designs of conduit systems not only should minimize the number of conduit bends and distances between manholes but also should specify the pulling tensions.

The inspection personnel should ensure that installation crews do not exceed these values during installations. It is also important that correct bending radius be maintained in order to avoid unnecessary stress points. Once a correct installation is made, routine inspection, testing, and maintenance should be carried out on a regular basis to chart the gradual deterioration and upkeep of the cable system.

Cable systems are the arteries of the electric power distribution system and carry the energy required for the successful operation of a plant. Following is a brief discussion on cable installation and maintenance.

There are several types of cable systems available for carrying electrical energy in a given distribution system. The selection of a particular system may be influenced by local conditions, existing company policies, or past experience.

No set standards or established guidelines can be given for the selection of a particular system.

2. Cable construction

Selection and application of cable involves the type of cable construction needed for a particular installation. Cable construction involves conductors, cable arrangement, and insulation and finish covering.

2.1 Conductors

Conductor materials such as copper and aluminum should be given consideration with regard to workmanship, environmental conditions, and maintenance. The requirements for aluminum conductors with regard to these factors are more critical than for copper conductors.

Cable conductors should be selected based upon the class of stranding required for a particular installation.

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Sizing of power cables for circuit breaker controlled feeders (part 3)

3. Criteria Starting and running voltage drops in cable

This criterion is applied so that the cross sectional area of the cable is sufficient to keep the voltage drop (due to impedance of cable conductor) within the specified limit so that the equipment which is being supplied power through that cable gets at least the minimum required voltage at its power supply input terminal during starting and running condition both.

Cables shall be sized so that the maximum voltage drop between the supply source and the load when carrying the design current does not exceed that which will ensure safe and efficient operation of the associated equipment. It is a requirement that the voltage at the equipment is greater than the lowest operating voltage specified for the equipment in the relevant equipment standard.
So before starting with calculation for voltage drop let us first analyze that what is the permissible voltage drop as per relevant standards and guidelines and what is the possible logic behind selecting these values as the permissible values.

Indian standard 1255- CODE OF PRACTICE FOR INSTALLATION AND MAINTENANCE OF POWER CABLES UP TO AND INCLUDING 33 kV RATING in its clause 4.2.3.4 mentions the permissible value for different cross sectional sizes of Aluminium conductor in volts/kM/Ampere for cables from voltage grade of 1.1kV till 33kV. Since we calculate voltage drop in terms of percentage of source voltage, this clause is not very widely used in basic as well as detailed engineering fraternity.
Its complex unit requires to be multiplied by cable length and ampacity. However one can definitely check for any cable size and length, what value is obtained in terms of percentage?

IEEE standard 525 – Guide for the Design and Installation of Cable Systems in Substations in its annexure C, clause number C3 mentions that Voltage drop is commonly expressed as a percentage of the source voltage. An acceptable voltage drop is determined based on an overall knowledge of the system. Typical limits are 3% from source to load center, 3% from load center to load, and 5% total from source to load. These values are indicated diagrammatically below.

6.6kV substation layout

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Sizing of power cables for circuit breaker controlled feeders (part 2)

2. Criteria-2 Continuous current capacity (Ampacity)

This criterion is applied so that cross section of the cable can carry the required load current continuously at the designed ambient temperature and laying condition. Ampacity is defined as the current in amperes a conductor can carry continuously under the conditions of surrounding medium in which the cables are installed. An ampacity study is the calculation of the temperature rise of the conductor in a cable system under steady-state conditions.
Cable ampacity, if required to be calculated than it is calculated as per the following equation givenin IEEE -399, section 13.

This equation is based on Neher-McGrath method where,

• Tc’ – allowable conductor temperature (ºC)
• Ta’ – ambient temperature (either soil or air) (ºC)
• ∆Td – temperature rise of conductor due to dielectric heating (ºC)
• ∆Tint – temperature rise of the conductor due to interference heating from adjacent cables (ºC)
• Rac – electrical ac resistance of conductor including skin effect, proximity and temperature effects (µ_/ft)
• R’ca – effective total thermal resistance of path between conductor and surrounding ambient to include the effects of load factor, shield/sheath losses, metallic conduit losses, effects of multiple conductors in the same duct etc (thermal- Ωft, ºC-cm/W).

From the above equation it is clear that the rated current carrying capacity of a conductor is dependent on the following factors:

1. Ambient temperature (air or ground)
2. Grouping and proximity to other loaded cables, heat sources etc.
3. Method of installation (above ground or below ground)
4. Thermal conductivity of the medium in which the cable is installed
5. Thermal conductivity of the cable constituents

However please note that while sizing a power cable we never calculate the ampacity. The above equation is used to analyze the cable ampacities of unique installations. Standard ampacity tables are available for a variety of cable types and cable installation methods and can be used for determining the current carrying capacity of a cable for a particular application.

These standards provide tabulated ampacity data in manufacturers catalog for cables installed in air, in duct bank,  directly buried or in trays for a particular set of conditions clearly defined.
It is because of this reason that we need to give the reference of manufacturers catalog from where the ampacity  values are picked up.

Now once the current carrying capacity of a cable is found from standard catalog; we convert that rated capacity (Ampacity) into actual laying condition. The standard current ratings for cables are modified by the application of suitable multiplying factors to account for the actual installation conditions. Hence we define one more term here called ampacity deration factor.

Ampacity duration factor is defined as the product of various factors which accounts for the fraction decrease in the ampacity of the conductor. Those factors and physical condition deriving them are as follows:

1. K1= Variation in ambient air temperature for cables laid in air / ground temperature for cables laid underground.
2. K2 = Cable laying arrangement.
3. K3 = Depth of laying for cables laid direct in ground.
4. K4 = Variation in thermal resistivity of soil.

Ampacity Deration factor = Product of applicable multiplying factors among 1 to 4 listed above.

K = K1 x K2 x K3 x K4

Now from where do we get these multiplying factors to find the overall ampacity deration factor? Againwe get these values from manufacturers catalog because manufacturer of the cable is in best position to conduct thepractical experiments and test on the cables and find the percentage/fractional decrease in current carrying capacity of the cable in various conditions.

For better understanding of the ampacity deration factor the following pictorial representation is provided below.

Table for ampacity deration factor along with pictorial representation is provided below.

However readers to note that ampacity deration factor table provided in this article is to verified from the manufacturers catalog which is intended to be used for project.

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Sizing of power cables for circuit breaker controlled feeders (part 1)

The following three criteria apply for the sizing of cables for circuit breaker controlled feeders:

I. Short circuit current withstand capacity

 This criteria is applied to determine the minimum cross section area of the cable, so that cable can withstand the short circuit current.

Failure to check the conductor size for short-circuit heating could result in permanent damage to the cable insulation and could also result into fire. In addition to the thermal stresses, the cable may also be subjected to significant mechanical stresses.

II. Continuous current carrying capacity

 This criteria is applied so that cross section of the cable can carry the required load current continuously at the designed ambient temperature and laying condition.

III. Starting and running voltage drops in cable

This criteria is applied to make sure that the cross sectional area of the cable is sufficient to keep the voltage drop (due to impedance of cable conductor) within the specified limit so that the equipment which is being supplied power through that cable gets at least the minimum required voltage at its power supply input terminal during starting and running condition both.

1. Criteria-1 Short circuit capacity

The maximum temperature reached under short circuit depends on both the magnitude and duration of the short circuit current. The quantity I2t represents the energy input by a fault that acts to heat up the cable conductor. This can be related to conductor size by the formula:

A = Minimum required cross section area in mm2
t = Operating time of disconnecting device in seconds
Isc = RMS Short Circuit current Value in Ampere
C = Constant equal to 0.0297 for copper & 0.0125 for aluminum
T2 = Final temp. ° C (max. short circuit temperature)
T1 = Initial temp. ° C (max. cable operating temperature – normal conditions)
T0 = 234.5° C for copper and 228.1° C for aluminum

Equation-1 can be simplified to obtain the expression for minimum conductor size as given below in equation-2:

 


Now K can be defined as a Constant whose value depends upon the conductor material, its insulation and boundary conditions of initial and final temperature because during short circuit conditions, the temperature of the conductor rises rapidly. The short circuit capacity is limited by the maximum temperature capability of the insulation. The value of K hence is as given in Table 2.
Boundary conditions of initial and final temperature for different insulation is as given under in Table 1 below.

Table 1

Insulation material	Final temperature T2	Initial temperature T1
PVC	160° C	70° C
Butyl Rubber	220° C	85° C
XLPE / EPR	250° C	90° C

 

Table 2

Material →	Copper			Aluminum
Insulation →	PVC	Butyl Rubber	XLPE / EPR	PVC	Butyl Rubber	XLPE / EPR
(K) 1 Second Current Rating in Amp/mm2	115	134	143	76	89	94
(K) 3 Second Current Rating in Amp/mm2	66	77	83	44	51	54

In the final equation-2 we have determined the value of constant K. Now the value of t is to be determined. The fault current (ISC) in the above equation varies with time. However, calculating the exact value of the fault current and sizing the power cable based on that can be complicated. To simplify the process the cable can be sized based on the interrupting capability of the circuit breakers/fuses that protect them.

This approach assumes that the available fault current is the maximum capability of the breaker/fuse and also accounts for the cable impedances in reducing the fault levels.

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5 key factors to the correct cable selection and application

Cable selection and application

It is essential to know cable construction, characteristics, and ratings to understand problems related to cable systems. However, to correctly select a cable system and assure its satisfactory

operation, additional knowledge is required. This knowledge may consist of service conditions, type of load served, mode of operation and maintenance, and the like.

The key to the successful operation of a cable system is to select the most suitable cable for the application, make a correct installation, and perform the required maintenance.

In this technical article, discussion is based on the correct cable selection and application for power distribution and utilization.
Cable selection can be based upon the following five key factors:

1. Cable installation
2. Cable construction
3. Cable operation (voltage and current)
4. Cable size
5. Shielding requirements

1. Cable installation

Cables can be used for outdoor or indoor installations depending upon the distribution system and the load served.
A good understanding of local conditions, installation crews, and maintenance personnel is essential to assure that the selected cable system will operate satisfactorily! Many times cable insulation is damaged or weakened during installation by applying the incorrect pulling tensions.
Designs of conduit systems not only should minimize the number of conduit bends and distances between manholes but also should specify the pulling tensions.


The inspection personnel should ensure that installation crews do not exceed these values during installations. It is also important that correct bending radius be maintained in order to avoid unnecessary stress points. Once a correct installation is made, routine inspection, testing, and maintenance should be carried out on a regular basis to chart the gradual deterioration and upkeep of the cable system.

Cable systems are the arteries of the electric power distribution system and carry the energy required for the successful operation of a plant. Following is a brief discussion on cable installation and maintenance.

 

There are several types of cable systems available for carrying electrical energy in a given distribution system. The selection of a particular system may be influenced by local conditions, existing company policies, or past experience.
No set standards or established guidelines can be given for the selection of a particular system.

Click here to access the full article

4 ways in which noise can enter a signal cable and its control – Part 2

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

 

 

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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.


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Cable spacing as a means of 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.

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Cable Chain Design Guidelines - Part 1

Of all the machine design details to worry about, cable chains probably don’t top your list. Yet if you care about the reliability and uptime of moving machines, it pays to devote extra engineering attention to cable chains and the components within them.

A well-designed cable chain will dramatically extend the life of cables and fluid power supply lines—by protecting them from damaging bends, crimping, abrasive wear and crushing.

Watch the video below or click here to download our full whitepaper

Practices for grounding and bonding of cable trays

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.

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How to wire an HA3/4 Connector

HA Series connectors are used whenever space is limited. They provide the smallest possible footprint in a 10A power connector. Wiring a HA 3 series connector takes less than 10 min. Watch this video to learn how

How to terminate a CAT.6A RJ45 Industrial Ethernet Connector

See for yourself how easy it is to terminate a shielded ETHERLINE CAT.6A continuous flex Industrial Ethernet cable with our CAT.6A RJ45 connector. Lapp's CAT.6A RJ45 insulation displacement connector allows the connection of twisted pair Ethernet cables with solid or stranded wires and a wire-section of up AWG22.

How to wire your HBE series connector from Lapp Group

Lapp Group’s EPIC HBE Series Rectangular Connectors offer three different kinds of wire termination for various applications: crimp, cage clamp, and screw. Each termination type has its own benefits for specific applications.. In this video we will show you how to wire each termination type.

Troubleshooting electrical equipment with insulation resistance test instrument

Troubleshooting of electric motors

Insulation resistance testing is performed when troubleshooting electric motors and related equipment. IEEE Standard 43-2000, Recommended Practice for Testing Insulation Resistance of Rotating Machines, recommends the insulation test voltage to apply, based on winding rating, and minimum acceptable values for electric motor windings.

The IEEE also provides typical guidelines for DC voltage to be applied during an insulation resistance test.

In order to obtain meaningful insulation resistance measurements, the technician should carefully examine the systemunder test.

The best results are achieved when the following conditions are met:

1. The system or equipment is taken out of service and disconnected from all other circuits, switches, capacitors, overcurrent protection devices, and circuit breakers.
   
   Ensure that the measurements are not affected by leakage current through switches and overcurrent protective devices!
2. The temperature of the conductor is above the dew point of the ambient air. When this is not the case, a moisture coating will form on the insulation surface, and, in some cases will be absorbed by the material.
3. The surface of the conductor is free of hydrocarbons and other foreign matter that can become conductive in humid conditions.
4. Applied voltage is not higher than the system capacity. When testing low voltage systems, too much voltage can overstress or damage insulation.
5. The system under test has been completely discharged to the ground. The grounding discharge time should be about five times the testing charge time.
6. The effect of temperature is considered. Since insulation resistance is inversely proportional to insulation temperature (resistance goes down as temperature goes up), the recorded readings are altered by changes in the temperature of the insulating material.

It is recommended that tests be performed at a standard conductor temperature of 68°F (20°C).

When comparing readings to 68°F base temperature, double the resistance for every 18°F (10°C) above 68°F or halve the resistance for every 18°F below 68°F in temperature. For example, a 1 MΩ resistance at 104°F (40°C) will translate to 4 MΩ resistance at 68°F (20°C).

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How to select a variable frequency drive

Many drive choices are available from a variety of vendors, so some of the basics and best practices are important to follow.

When choosing a variable frequency drive (VFD), several decisions must be made besides the obvious voltage and current selections. Even the name should be decided on, as it is often called a variable speed drive, adjustable speed drive, micro drive and inverter.

In general, a VFD takes an ac power source and converts it into dc power. The speed control portion of the drive uses the dc voltage to create dc pulses in varying frequency to drive the output motor at speeds other than the 3,600 rpm or 1,800 rpm or other speed depending upon the number of poles the motor was designed to operate at using a 60 or 50 Hz ac supply voltage.

How big should the VFD be? The size of the VFD should be chosen based on maximum motor current at peak demand and not chosen based upon motor horsepower. Constant starting, stopping and dynamic loads affects the electronics inside the VFD far more than the effect they have upon the local power bus and a full voltage motor starter. Therefore, peak demand current should be used. Manufacturers may continue to list hp ratings more as an historical rating than as a useful one.

Perhaps the first decision to make when choosing a VFD is to pick between a voltage/frequency (V/F or V/Hz) drive and a vector controller. Both control methods may or may not be used with feedback such as a rotary encoder. In general, most VFD-controlled motors are operated in an open-loop scenario but take advantage of the VFD’s soft start and adjustable speed features.



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Why do we need variable speed drives (VSD)?

Reasons for using  variable speed drives

 

There are many and diverse reasons for using variable speed drives. Some applications, such as paper making machines, cannot run without them while others, such as centrifugal pumps, can benefit from energy savings.
In general, variable speed drives are used to:

1. Latch the speed of a drive to the process requirements
2. Latch the torque of a drive to the process requirements
3. Save energy and improve efficiency

The needs for speed and torque control are usually fairly obvious.
Modern electrical VSDs can be used to accurately maintain the speed of a driven machine to within ±0.1%, independent of load, compared to the speed regulation possible with a conventional fixed speed squirrel cage induction motor, where the speed can vary by as much as 3% from no load to full load.

The benefits of energy savings are not always fully appreciated by many users. These savings are particularly apparent with centrifugal pumps and fans, where load torque increases as the square of the speed and power consumption as the cube of the speed.

Substantial cost savings can be achieved in some applications.

An everyday example, which illustrates the benefits of variable speed control, is the motorcar. lt has become such an integral part of our lives that we seldom think about the technology that it represents or that it is simply a variable speed platform. lt is used here to illustrate how variable speed drives are used to improve the speed, torque and energy performance of a machine. It is intuitively obvious that the speed of a motorcar must continuously be controlled by the driver (the operator) to match the trafiic conditions on the road (the process).

In a city, it is necessary to obey speed limits, avoid collisions and to start, accelerate, decelerate and stop when required.

On the open road, the main objective is to get to a destination safely in the shortest time without exceeding the speed limit.

The two main controls that are used to control the speed are the accelerator, which controls the driving torque, and the brake, which adjusts the load torque.

A motorcar could not be safely operated in city traffic or on the open road without these two controls. The driver must continuously adjust the fuel input to the engine (the drive) to maintain a constant speed in spite of the changes in the load, such as an uphill, downhill or strong wind conditions. On other occasions he may have to use the brake to adjust the load and slow the vehicle down to standstill.

Another important issue for most drivers is the cost of fuel or the cost of energy consumption. The speed is controlled via the accelerator that controls the fuel input to the engine.
By adjusting the accelerator position, the energy consumption is kept to a minimum and is matched to the speed and load conditions. Imagine the high fuel consumption of a vehicle using a fixed accelerator setting and controlling the speed by means of the brake position.





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Comparison Of Cable Insulating Materials

Electrical insulation materials are employed over the metallic conductors of underground cables at all voltage ratings. Polymeric materials are employed as the insulation, but the nature of the polymer may vary with the voltage class.
Since paper insulation was used first in the power industry, and was later replaced in low and medium voltage applications, any comparison of properties usually employs the paper-fluid system as the standard.
.
Transmission cables, which are defined as cables operating above 46 kV, have traditionally used paper / oil systems as the insulation. The paper is applied as a thin film wound over the cable core. Some years back, a variation of this paper insulation was developed, the material being a laminate of paper with polypropylene (PPP or PPLP).
Since the advent of synthetic polymer development, polyethylene (PE) has been used as an insulation material, and in most countries (France being the exception) the use of polyethylene was limited to the crosslinked version (XLPE).

 

XLPE is considered to be the material of choice due to its ease of processing and handling, although paper / oil systems have a much longer history of usage and much more information on reliability exists.

Major Differences Between Paper and Polyolefinic Insulations

Paper / Cellulose	Polyethylene
Natural	Synthetic
Carbon / hydrogen/oxygen	Carbon / hydrogen/oxygen
More polar / medium losses	Less polar, low losses
Chains linear	Chains branched
Fibrils	Non-fibrils
Partially crystalline / Relatively constant	Partially crystalline / Varies with grade employed
No thermal expansion on heating	Significant thermal expansion
Not crosslinked	Not crosslinked
Thermal degradation via cleavage at weak link	Degrades at weak links

.

Crosslinked Polyethylene	Ethylene Propylene Rubber
Synthetic	Synthetic
Carbon / hydrogen	Carbon / hydrogen
Less polar, low losses	Losses due to additives
Chains branched, crosslinked	Chains branched, crosslinked
Non-fibril	Non-fibril
Slightly less crystalinevs PE	Least crystaline of all
Same thermal expansion as PE	Slight thermal expansion
Crosslinked	Crosslinked
Degrades at weak links	Same as XLPE

This table provides a comparison of the properties of paper, polyethylene, crosslinked polyethylene, and ethylene propylene rubber insulations. Only the paper is a natural polymer and is therefore processed differently. Paper is obtained fi-om a wood or cotton source.

The synthetic polymers are produced by polymerization of monomers derived from petroleum. All consist of carbon and hydrogen, but paper also contains oxygen. The latter is present as fuctional hydroxyl or ether groups. The contribute a measure of polarity that is absent in the synthetic polymers. (Polarity means increased dielectric losses.)

Of special note is the concept of thermal expansion during heating. While all of the synthetic polymers undergo thermal expansion during heating, this does not occur with cellulose-although the oil will do so. How these insulations respond on aging is a well studied subject since it is directly related to reliability of the cable after installation and energization. When cellulose degrades, it does so at a “weak link,” the region of the oxygen linkage between the rings. When this happens, the DP is reduced.

On the other hand, polyolefins degrade by a completely different mechanism–oxidative degradation at specific sites.

 

Protection against degradation is imparted to  polyolefins by adding an antioxidant to the pellets prior to extrusion. Note that adding antioxidants to oil to prevent it from degradingis rather common. One further point should be noted on the chart: the different response of the insulation types to dc testing. DC testing of cables has traditionally been performed to ascertain the state of the cable at specific times during their use, such as before peak load season. This is a technique that was adopted for PILC cables many years ago.

This was later carried over to extruded dielectric cables. Research and development in the past few years has shown that PE and XLPE may be harmed by the use of a dc test, but this does not occur with paper-oil systems.
EPR cables have not been studied to the same extent and no conclusions can be drawn at this time about the effect of dc testing on the insulation.


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An example how to calculate voltage drop and size of electrical cable

Input information //

Electrical details:

Electrical load of 80KW, distance between source and load is 200 meters, system voltage 415V three phase, power factor is 0.8, permissible voltage drop is  5%, demand factor is 1.

Cable laying detail:

Cable is directed buried in ground in trench at the depth of 1 meter. Ground temperature is approximate 35 Deg. Number of cable per trench is 1. Number of run of cable is 1 run.

Soil details:

Thermal resistivity of soil is not known. Nature of soil is damp soil.

Ok, let’s dive into calculations…

• Consumed Load = Total Load · Demand Factor:
  Consumed Load in KW = 80 · 1 = 80 KW
• Consumed Load in KVA = KW/P.F.:
  Consumed Load in KVA = 80/0.8 = 100 KVA
• Full Load Current = (KVA · 1000) / (1.732 · Voltage):
  Full Load Current = (100 · 1000) / (1.732 · 415) = 139 Amp.

Calculating Correction Factor of Cable from following data:

Temperature Correction Factor (K1) When Cable is in the Air

Temperature Correction Factor in Air: K1
Ambient Temperature	Insulation
	PVC	XLPE/EPR
10	1.22	1.15
15	1.17	1.12
20	1.12	1.08
25	1.06	1.04
35	0.94	0.96
40	0.87	0.91
45	0.79	0.87
50	0.71	0.82
55	0.61	0.76
60	0.5	0.71
65	0	0.65
70	0	0.58
75	0	0.5
80	0	0.41

Ground Temperature Correction Factor (K2)

Ground Temperature Correction Factor: K2
Ground Temperature	Insulation
	PVC	XLPE/EPR
10	1.1	1.07
15	1.05	1.04
20	0.95	0.96
25	0.89	0.93
35	0.77	0.89
40	0.71	0.85
45	0.63	0.8
50	0.55	0.76
55	0.45	0.71
60	0	0.65
65	0	0.6
70	0	0.53
75	0	0.46
80	0	0.38

Thermal Resistance Correction Factor (K4) for Soil (When Thermal Resistance of Soil is known)

Soil Thermal Resistivity: 2.5 KM/W
Resistivity	K3
1	1.18
1.5	1.1
2	1.05
2.5	1
3	0.96

Soil Correction Factor (K4) of Soil (When Thermal Resistance of Soil is not known)

Nature of Soil	K3
Very Wet Soil	1.21
Wet Soil	1.13
Damp Soil	1.05
Dry Soil	1
Very Dry Soil	0.86

Cable Depth Correction Factor (K5)

Laying Depth (Meter)	Rating Factor
0.5	1.1
0.7	1.05
0.9	1.01
1	1
1.2	0.98
1.5	0.96

Cable Distance correction Factor (K6)

No of Circuit	Nil	Cable diameter	0.125m	0.25m	0.5m
1	1	1	1	1	1
2	0.75	0.8	0.85	0.9	0.9
3	0.65	0.7	0.75	0.8	0.85
4	0.6	0.6	0.7	0.75	0.8
5	0.55	0.55	0.65	0.7	0.8
6	0.5	0.55	0.6	0.7	0.8

Cable Grouping Factor (No of Tray Factor) (K7)

No of Cable/Tray	1	2	3	4	6	8
1	1	1	1	1	1	1
2	0.84	0.8	0.78	0.77	0.76	0.75
3	0.8	0.76	0.74	0.73	0.72	0.71
4	0.78	0.74	0.72	0.71	0.7	0.69
5	0.77	0.73	0.7	0.69	0.68	0.67
6	0.75	0.71	0.7	0.68	0.68	0.66
7	0.74	0.69	0.675	0.66	0.66	0.64
8	0.73	0.69	0.68	0.67	0.66	0.64

According to above detail correction factors:
– Ground temperature correction factor (K2) = 0.89
– Soil correction factor (K4) = 1.05
– Cable depth correction factor (K5) = 1.0
– Cable distance correction factor (K6) = 1.0
Total derating factor = k1 · k2 · k3 · K4 · K5 · K6 · K7
– Total derating factor = 0.93

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Cable spacing as a means of 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.

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What is the difference between Bonding, Grounding and Earthing?

Introduction

One of the most misunderstood and confused concept is difference between Bonding, Grounding and Earthing. Bonding is more clear word compare to Grounding and Earthing, but there is a micro difference between Grounding and Earhing.

Earthing and Grounding are actually different terms for expressing the same concept.

Ground or earth in a mains electrical wiring system is a conductor that provides a low impedance path to the earth to prevent hazardous voltages from appearing on equipment. Earthing is more commonly used in Britain, European and most of the commonwealth countries standards (IEC, IS), while Grounding is the word used in North American standards (NEC, IEEE, ANSI, UL).

We understand that Earthing and Grounding are necessary and have an idea how to do it but we don’t have crystal clear concept for that. We need to understand that there are really two separate things we are doing for same purpose that we call Grounding or Earthing. The Earthing is to reference our electrical source to earth (usually via connection to some kind of rod driven into the earth or some other metal that has direct contact with the earth).

The grounded circuits of machines need to have an effective return path from the machines to the power source in order to function properly (Here by Neutral Circuit).

In addition, non-current-carrying metallic components in a System, such as equipment cabinets, enclosures, and structural steel, need to be electrically interconnected and earthed properly so voltage potential cannot exist between them. However, troubles can arise when terms like “bonding”, “grounding”, and “earthing” are interchanged or confused in certain situations.

In TN Type Power Distribution System, in US NEC (and possibly other) usage: Equipment is earthed to pass fault Current and to trip the protective device without electrifying the device enclosure. Neutral is the current return path for phase. These Earthing conductor and Neutral conductor are connected together and earthed at the distribution panel and also at the street, but the intent is that no current flow on earthed ground, except during momentary fault conditions.


Here we may say that Earthing and grounding are nearly same by practice.
But In the TT Type Power Distribution System (in India) Neutral is only earthed (here it is actually called Grounding) at distribution source (at distribution transformer) and Four wires (Neutral and Three Phase) are distributed to consumer. While at consumer side all electrical equipment body are connected and earthed at consumer premises (here it is called Earthing).
Consumer has no any permission to mix Neutral with earth at his premises here earthing and grounding is the different by practice.

In both above case Earthing and Grounding are used for the same Purpose. Let’s try to understand this terminology one by one.

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Bonding

Bonding is simply the act of joining two electrical conductors together. These may be two wires, a wire and a pipe, or these may be two Equipments. Bonding has to be done by connecting of all the metal parts that are not supposed to be carrying current during normal operations to bringing them to the same electrical potential.
Bonding ensures that these two things which are bonded will be at the same electrical potential. That means we would not get electricity building up in one equipment or between two different equipment. No current flow can take place between two bonded bodies because they have the same potential.

Bonding itself, does not protect anything. However, if one of those boxes is earthed there can be no electrical energy build-up. If the grounded box is bonded to the other box, the other box is also at zero electrical potential.

It protects equipment and person by reducing current flow between pieces of equipment at different potentials.

The primary reason for bonding is personnel safety, so someone touching two pieces of equipment at the same time does not receive a shock by becoming the path of equalization if they happen to be at different potentials. The Second reason has to do with what happens if Phase conductor may be touched an external metal part.
The bonding helps to create a low impedance path back to the source. This will force a large current to flow, which in turn will cause the breaker to trip.
In other words, bonding is there to allow a breaker to trip and thereby to terminate a fault.

Bonding to electrical earth is used extensively to ensure that all conductors (person, surface and product) are at the same electrical potential. When all conductors are at the same potential no discharge can occur.


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Practices for grounding and bonding of cable trays

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.

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Installing Sensors Incorrectly As a Mistake #2 In Instrumentation

Four Sensor ‘Poors’ To Avoid

Generally speaking, there are four ‘Poors’ that can lead to incorrectly installing instrumentation sensors:

1. Poor placement
2. Poor control
3. Poor protection
4. Poor grounding

Each of above sensor ‘poors’ are described below, and by the way… have you read what is the mistake No.1 in an industrial instrumentation?

1. Sensor Placement

The best sensor can yield disappointing results if not installed correctly. Magmeters, for example, tend to generate noisy signals if the flow they’re measuring is turbulent. Bends, junctions, and valves in a pipe can all cause turbulence, thus magmeters work best when installed in sections of straight pipe.

Temperature sensors are also sensitive to placement. Even a highly accurate RTD tucked in the corner of a mixing chamber will only be able to detect the temperature of its immediate vicinity. If the mixing of the material in the chamber is incomplete, that local temperature may or may not represent the temperature of the material elsewhere in the chamber.

Local temperature issues are the classic mistake that home heating contractors often make when installing household thermostats.

A mounting location closest to the furnace may be convenient for wiring purposes, but if that spot happens to be in a hallway or other dead air space, the thermostat will not be able to determine the average temperature elsewhere in the house. It will only be able to maintain the desired temperature in its immediate vicinity. The rest of the house may end up roasting or freezing.

2. Controller performance // Poor control

Poor control also results when a sensor is installed too far away from the associated actuator. A distant sensor may not be able to measure the effects of the actuator’s last move in time for the controller to make an educated decision about what to do next.

Process of flattening hot steel

For example, consider the process of flattening hot steel into uniform sheets by means of two opposing rollers (see Figure 1 above). A thickness sensor downstream from the rollers gauges the sheet and causes the controller to apply either more or less pressure to compensate for any out-of-spec thickness.





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