Common Terms Used in Earthing/Grounding of Installations- Standard Practice

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Earthing or Grounding of electrical installation is a common practice. However, some common terms used in the practice could sometimes be tricky. Here, we have attempted to provide explanations for some of the more common terms used when earthing or grounding an installation. These terms are the ones used in the various national and international standards:

Earthing an Electrical Installation

To understand some of these terms, the schematic above will be very helpful:

Instrument Transformers – Basic Operating Principles

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Instrument transformers are used for measuring and control purposes. They provide currents and voltages proportional to the primary, but there is less danger to instruments and personnel.

There are two distinct classes of instrument transformers: the potential transformer and the current transformer. 

Potential transformers (PTs) are used to step down high voltage while current transformers (CTs) used to step current down. The function of a PT is to accurately measure voltage on the primary, while a CT is used to measure current on the primary.

Potential Transformer

Potential (voltage) transformers have primary and secondary windings on a common core:

What are Autotransformers?

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Transformers having only one winding are called autotransformers. This is the most efficient type of transformer since a portion of the one winding carries the difference between the primary and secondary currents.

An autotransformer has the usual magnetic core like a typical transformer but only one winding, which is common to both the primary and secondary circuits. An autotransformer schematic is shown below:

Electrical Power in 3-Phase and 1-Phase Systems

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Power in an electrical circuit or system is given by:

Where:

I = Current in Amps

V = Voltage in volts

The unit of power is the Watt(W). Power can also be expressed in volt amps (VA) usually in Alternating current systems.

In a D.C system, the current and voltage do not vary over time. Hence the product of voltage and current gives us power in Watt.

In A.C systems, the voltage and current entities are constantly changing in a sinusoidal manner as shown below:

NEMA Insulation Classes for Transformers

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The capacity or rating of a transformer is limited by the temperature that the insulation can tolerate.  The life of a transformer can be extended by making sure it is not operated over and above the temperature rating of the insulation system on a continuous basis. A guiding rule of thumb would be that the useful operating life of the transformer halves for every 10°C  rise above its rated temperature. 

The insulation system of a transformer is rated in degrees Celsius at its maximum temperature rating:

The class number  = the maximum °C of the transformer insulation

NEMA (National Manufacturer’s Association) has the following thermal or insulation classification as regards transformers (dry type):

Basics of Transformer Ratings

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Transformers are rated using several methods. Two common ratings of transformers include the:

(a) KVA rating

(b) Impedance rating

KVA Rating

KVA ratings of transformers are obtained by simply multiplying the current times the voltage. The result is a rating in VA or volt amperes. Small transformers are rated in VA. As size increases, the ratings are adjusted accordingly to KVA (kilovolt amperes) or MVA (megavolt amperes)

1KVA = 1,000VA or volt amperes

1MVA = 1,000,000VA

Power transformers are defined as transformers rated 500 kVA and larger. Transformers smaller than 500 kVA are generally called distribution transformers.

How a Voltage Transformer Works

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Transformer function is based on the principle that electrical energy is transferred efficiently by magnetic induction from one circuit to another. Basically, a transformer consists of two or more windings placed on the same magnetic path. The winding being fed electrical energy is called the primary winding while the winding where the load is connected is called the secondary winding. A typical two winding transformer action is shown below:

Basics of Inductors and Inductance

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Any conductor possesses a characteristic called inductance. Inductance is the ability to store energy in the form of a magnetic field. Inductance is symbolized by the capital letter L and is measured in the unit of the Henry (H). Some of the symbols for an inductor in an electric circuit are:

Circuit symbols for inductors

Inductance is a non-dissipative quantity. Unlike resistance, a pure inductance does not dissipate energy in the form of heat; rather, it stores and releases energy from and to the rest of the circuit.

Inductors are devices expressly designed and manufactured to possess inductance. They are typically constructed of a wire coil wound around a ferromagnetic core material. Inductors have current ratings as well as inductance ratings. Due to the effect of magnetic saturation, inductance tends to decrease as current approaches the rated maximum value in an iron-core inductor.

Basics of the Electrical Capacitor

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Capacitors are electrical devices manufactured to possess capacitance. Capacitors oppose changes in voltage over time by creating a current. This behavior makes capacitors useful for stabilizing voltage in DC circuits. One way to think of a capacitor in a DC circuit is as a temporary voltage source, always “wanting” to maintain voltage across its terminals at the same value. A typical capacitor is made up of two parallel conductive plates separated by an insulator called a dielectric as shown below:

Power Factor Improvement With Capacitors

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As we have already seen in the basics of power factor in electrical distribution system, most Industrial loads require both Real power and Reactive power to produce useful work. Typically, inductive loads (motors, transformers etc) require two kinds of power to operate:

(1) Active Power (KW) – This actually performs the useful work

(2) Reactive Power(KVAR) – This helps to maintain the electromagnetic field. 

The vector sum of the active power and the reactive power gives total power often referred to as apparent power in KVA:
KVA = KW + KVA (vector sum)

Low power factor in an electrical system often occur when inductive loads are operated below their full load capacity especially motors. Consistently operating electrical loads at low power factor will result in higher utility bills because of the poor utilization of electrical energy. In fact, a higher power factor means less KVA and KVAR components and a more efficient utilization of electrical energy while a low power factor implies the presence of more KVA and reactive (KVAR) power components and less efficient electrical energy utilization:

How to Test a Diode with a Fluke Multimeter

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You are an electrician, a technician or an engineer and the diode in a circuit board gets bad. How do you test this diode to confirm it is bad?

Well if you have a multimeter, it is a very simple exercise. Let’s find out how to do this. The diode test can be carried out in either forward or reverse bias.

Forward Bias Test

Good Diode

To begin the test in forward bias, switch the knob on the multimeter to the “DIODE” selection and test as shown below:

Ampacity of a Conductor

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Ampacity is the current carrying capacity of a conductor. Ampacity calculation should take into account natural variables such as solar warming, wind and air density, viscosity, and thermal conductivity. Ampacity is a temperature rating. In order words, as temperature changes, the ampacity of a conductor changes. 

Increase in ambient/surrounding/medium temperature can significantly limit the current carrying capacities of cables. As cable temperature increases, its resistance increases thereby reducing the amount of current that can be carried. 

According to the National Electrical Code, article 310.15(C), the ampacities of conductors can be calculated by the following general formula:

AC Resistance of a Conductor

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A conductor offers a greater resistance to the flow of alternating current(AC) than it does to direct current(DC). The magnitude of the increase is usually expressed as an “AC/DC ” ratio. The reasons for the increase include:

1. Skin effect, 
2. Proximity effect, 
3. Hysteresis and eddy current losses in nearby ferromagnetic materials, and
4. Induced losses in short-circuited nearby non-ferromagnetic materials

Skin Effect

Skin Effect describes the phenomena of alternating current flowing more densely near the surface of a conductor. The net effect is a reduction in effective area and an increase in the resistance. To calculate skin effect in tubular conductors made of solid wire to an infinitely thin tube, the curves of Ewan are used.

The parameter is:

DC Resistance of a Conductor

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The DC resistance of a conductor or cable is that defined by ohms law. It is a function of many factors including temperature which greatly affects the resistance of a given material. Copper and Aluminium are the most widely used conductors. Their resistance (DC) increases with increasing temperature. 

The DC resistance of copper wire at 20 degree Celsius(68 degree Fahrenheit) is given below:

American Wire Gauge (AWG)

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Wire size is expressed in circular mils(CM). A mil is one-thousandth of an inch. In the United States, the American Wire Gauge is used. It is a scale of even numbers that start with the number 40 and descend. The cross-sectional area becomes larger as the numbers on this scale get smaller. 

For wires larger than No.2 wire, a scale of 1/0, 2/0, 3/0 and 4/0 is used. For even larger wires, thousands of circular mils is used –MCM or Kcmil

AWG Conversions

Copper conductor size conversions are determined using;

Circular mils = sq in. x 1,273,240 = sq mm x 1,973.5

For conductor cross-sectional forms other than circular, where S is the cross-sectional area in square inches, the conversions are: