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.