Types of Earthing Systems Used in Electrical Installations

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The international standard IEC60364, part 4, and Reference 10 uses a set of diagrams to explain the five basic methods of earthing and providing the neutral of an electrical installation where it is required. The five methods are abbreviated TNC, TNS, TNCS, TT and IT.

The first letter denotes the source of power from a star-connected winding. T denotes that the star point of the source is solidly connected to earth, which is usually at a location very near to the winding.
I denote that the star point and the winding are isolated from earth. The star point is usually connected to an inductive impedance or resistance. Capacitive impedance is never used.

The second letter denotes the consumer. The consuming equipment needs

How to Measure Soil Resistivity

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Wenner’s four-electrode method is popularly used to measure soil resistivity. In this method Four electrodes are driven into the earth along a straight line at equal intervals, S. The depth of the electrodes in the ground is always of the order of 10 to 15 cm. The earth megger is placed on a steady and approximately level base. The four electrodes are connected to the earth megger terminal as shown below:

Ampacity Correction Factors for All Cables

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As already discussed in Ampacity of a Conductor, the ampacity of a conductor depends on temperature. Any change in ambient temperature affects the ampacity of an electrical cable. When this happens, correction factors are applied to get new ratings for such a conductor.
The approximate ampacity correction factors for all cables under different ambient temperature conditions for cables whose insulation is rated 90 degree C are given below:

Thermal Properties of Cable Polymers

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Often times in the application of electrical cables, we neglect the thermal characteristics of  the polymers used in the cable insulation. Here is a list of the temperature range each of the common electrical cable polymers can withstand while in service or operation:

Temperature Classifications for AC Motors Application

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When an electric motor is used in an atmosphere with explosive gas, the motor  surface might become hot enough to cause auto-ignition.  Auto-ignition temperature is the temperature at which a gas will ignite spontaneously without any other source of ignition. When hot surfaces are in contact with an explosive atmosphere, auto-ignition is likely to occur.

The table below gives the classification, which is used to indicate the maximum surface temperature that a given piece of an electric motor (the table is for motors designated as EExd) can reach when it is running normally. In general applications, the maximum surface temperature is based on a surrounding temperature of 40°C. The motor’s T-classification can be compared with the auto-ignition temperature for gases. The T-classification helps in making the right decisions regarding the AC motor’s use in areas with explosive atmospheres:

Classes of Explosive Atmospheres – EU Classification

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An understanding of this classification is required because often motors and other electrical equipment are required to be installed in these hazardous or explosive atmospheres. A clear understanding of the classes of explosive atmospheres will help in the selection and specification of electrical equipment in such environment.

Classes of Explosive Atmospheres
The EU (European Union) ATEX directive 99/92/EC distinguishes between two types of explosive atmospheres: GAS and DUST. Areas subjected to these two kinds of explosive atmospheres are each divided into three zones. Each zone characteristics are identical for gas and dust, but their numbering is different. Zones 0, 1, 2 refer to gas and zones 20, 21, 22 refer to dust. 

The various classes of explosive atmospheres are listed below

Basics of Explosive Atmospheres

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An explosive atmosphere is an atmosphere that develops explosively because of an uncontrollable combustion. Explosive atmosphere consists of air and some sort of combustible material such as gas, vapours, mists or dust in which the explosion spreads after ignition. Typical examples of productions where combustible dust is of major concern, is the handling of cereals, animal feed, paper, wood, chemicals, plastics and coal.

Sources of Ignition

Sources of ignition that can cause the atmosphere to explode are listed below:

How To Test Three - Phase AC Motors

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The basic steps in ascertaining the health of a three-phase AC motor are given below:
(a) General Inspections
(b) Earth Continuity and Resistance Test
(c) Power Supply Test
(d) AC Motor Winding Continuity Test
(e) AC Motor Winding Resistance Test
(f) Insulation Resistance Test
(g) Running Amps Test

General Inspections
For the three-phase motor, do the following:

How to Calculate Conductor Diameter from Wire Diameter

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To calculate the nominal diameter of any concentric-lay-stranded conductor made from round wires of uniform diameters, multiply the diameter of an individual wire by the applicable factors listed below:

How to Read AC Motor Name Plate - Terms & Meanings

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When you see the name plate of an electric motor, you will find many terms which defines the performance characteristics of the motor. These include supply voltage, rated full load current, motor horse power, motor speed in RPM, frequency of power supply, motor frame, motor duty, AC motor insulation class, motor design, service factor, motor efficiency, power factor, number of poles, winding type connection-star or delta and many more. Shown below is a typical

AC Motor Duty Cycle Types as Per IEC Standards

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The IEC Duty Cycle ratings are different from those of that of NEMA. While NEMA specifies continuous, intermittent or special duty (typically expressed in minutes) cycles for AC motors, the IEC uses nine different duty cycle designations  according to IEC 60034-1. The table below lists the nine IEC duty cycles and their individual characteristics:

How To Test and Check Single phase Electric Motors

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There are several types of single phase motors. What is however common to them all is that they have a Start Winding, a Run Winding, and a Common connection between them as shown below:

Testing of single phase motors is pretty easy if certain basic steps are followed. The objective of any AC motor test is to determine the health status of the motor. The basic steps in ascertaining the health of any motor are given below
(a) General Inspections
(b) Earth Continuity and Resistance Test
(c) Power Supply Test
(d) AC Motor Winding Resistance Test
(e) Insulation Resistance Test
(f) Running Amps Test

General Inspections
For the single phase motor, do the following:

How to Identify Three Phase Windings

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A procedure has been adopted by the IEC for identifying three phase winding connections. Letters and numbers are used as follows. The high voltage (HV) terminals have upper-case letters e.g. A-B-C, R-Y-B, U-V-W, L1-L2-L3 and the low voltage (LV) terminals have lower-case letters e.g. a-b-c, r-y-b, u-v-w, l1-l2-l3. Each winding has a start numbered 1 and a finish numbered 2.
The choice of letters and numbers tends to be a national preference, see the table below for a rule of thumb guide:

NEMA Locked Rotor KVA for Three-phase AC Motors

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A NEMA motor nameplate has some supplementary information. The most important ones are: a letter code, a design letter and a service factor.

Letter Code
A letter code defines the locked rotor current kVA on a per horsepower basis. The letter code consists of letters from A to V. The farther away from the letter code A, the higher the inrush current per horsepower. Below are values

How to change the Rotation Direction and Wire Configuration – Star or Delta - of Electric Motors

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The direction of rotation of an electric motor can be changed anytime and easily at the motor terminals. In three-phase motors, the windings configuration can easily be changed from a star or Y connection to a delta or mesh connection at the motor terminal box.

Three-phase Electric Motors

The windings of AC motors can either be connected in star(Y)-connection or in delta(Δ)-connection.The standard that deals with this winding connection and designated marking of the terminal box is IEC 60034-8. According to this standard, the phases of motor windings are defined by W, U, V

Star or Y Connection

For a motor with terminal markings as W1, U1, V1 and W2, U2, V2 respectively, proceed as follows:

How Electric Motors are cooled

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The IEC standard defines suitable methods for efficiently cooling electric motors. The three most frequently used motor cooling methods have the following IC designated codes according to the IEC 60034-6 standard: IC 410, IC 411, and IC 418 are applied.

IC 410
In this cooling method, the electric motor is essentially cooled by free convection current from the air.

IC 411
In this cooling method, the motor is cooled by a fan mounted on the motor shaft. This is the most common method for cooling electric motors you will find in the industry.

The IEC standard for AC Motors

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The table below gives an overview of standards in relation to design, production and use of electric motors. If you are involved with working with motors, this table is very important and could be used as a reference as to what area of AC motors the various units of the standard covers:

Electric Motor Standards

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The concept of standard is used for AC motors which are designed in conformity with international or national standards within a given country or area. The standards generally applied to ÀC motors can be divided into two main categories: IEC/EN and NEMA. 

The IEC/EN Standard

The IEC/EN standards cover what is usually referred to as‘IEC’ motors (Europe, Asia).  In this standard, metric units (SI units) such as metre (m) are applied.

Common Tests Performed on Transformers

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If you are involved in transformer (oil immersed) maintenance in your company, here is a list of common tests required to be done on oil immersed transformers according to IEC 60076-1 :

Transformer Name plate – What information is required

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Transformers are usually provided with a rating plate of weatherproof material situated in a visible position. All entries on the plate are usually indelibly marked. According to the IEC 60076-1 standard for transformers, the following are the minimum information required for a transformer name plate. If you are involved in specifying transformers, purchasing transformers or work with transformers, here is a checklist for typical transformer name plate information:

How to Inspect Electric Motors

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Electric motors are the backbone of modern industry. Available statistics shows that they constitute the bulk of electrical machines in the industry. Electric motors use 70 % of the electricity consumed by industry and therefore deserve special attention and regular inspection.

What to Look for During Electric Motor Inspection
(1) Inadequate ventilation or air flow
(2) Unbalanced voltage or an overload
(3) Bearing failure
(4) Insulation failure
(5) Shaft misalignment
Item (1) – (5) above tend to results in

How Transformers are Cooled

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Increasing the cooling rate of a transformer increases its capacity to handle more power requirements. Various methods exist for cooling oil immersed industrial transformers to guarantee efficient operation. Cooling methods must not only maintain a sufficiently low average temperature, but must prevent an excessive temperature rise in any portion of the transformer. It must prevent hot spots within the transformer. For this reason, working parts of large transformers are usually submerged in high-grade insulating oil. This oil must be kept as free as possible from moisture and oxygen, dissolved combustible gases, and particulates

Transformer Cooling Methods
The efficiency of the cooling is a crucial factor determining the operational safety and the life span of a transformer. Below are the four common cooling methods for industrial transformers. Each method is illustrated with a diagram:

Soil Resistivity Values for Different Types of Soil

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One key determinant of a good earthing installation is the soil resistivity of the soil where the earth electrode is installed. If you are involved  in earthing buildings and electrical installations, here is a list of the typical soil resistivity values for different types of soil that you might encounter:

How to Use Earthing Rods for Earthing Improvement

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Vertically driven conductors called Earthing Rods are commonly used in earthing existing buildings and for improving or reducing electrical resistance of existing earth electrodes. See common terms used in earthing to gain understanding of earthing rod and earth electrode.

Characteristics of Earthing Rods
The rods used for earthing buildings may be:
(a) Copper or more commonly Copper clad steel. Copper clad steel are generally 1 or 2 meters long and provided with screwed ends and sockets in order to reach
considerable depths, if

Types of Protection for Hazardous Areas

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Most electrical equipment consists of live or active static parts, and in some cases such as motors, solenoid valves and relays have moving mechanical parts, encased in an enclosure. The electrically live conductors are kept out of touch to prevent electric shock hazards. The detrimental effects of the environment e.g. rain, sprayed water, fine dust and particles are kept out of contact with the conductors, insulation, bearings and the like.

The design of the enclosure of electrical equipment with regard to hazardous area applications is defined by several lower case letter codes, mostly single digits for electrical power equipment but occasionally two digits for very low energy electronic equipment. The most frequently encountered codes are d, e, n, p and i. The lesser used codes are o, m, s and q. The table below gives a brief description of each code:

How to Convert KVA to Amperes for Single and Three Phase Circuits Using Tables

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The following tables list the current in amperes for different KVA values for both single and three phase circuits for 120, 240, 480 and 600V circuits. To get the corresponding value of AMPS for a given KVA, simply read off the current value at the required voltage for the given KVA rating.

KVA Rating to AMPS for Single Phase Circuits

What does the Basic Impule Level (BIL) Rating of an Electrical Equipment Mean?

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Electrical equipment, including wires and cables, are designed to withstand short-term, but very high-voltage pulses such as those sometimes caused by lightning and switching surges. These “spikes,” as they are sometimes called, typically have a rise time in the range of 1.5 microseconds and a fall time around 40 microseconds.

BIL of Electrical Equipment
BIL means

How to Determine Temperature Correction Factors for Resistance: Copper Conductors

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The DC resistance of copper wire increases with increasing temperature in accordance with the formula:

On the basis of the above formula, we now generate a table of correction factors for copper conductors in operating in the temperature range 25 – 200 degree celsius: