What should be safe distance of electric transformer from a residence building

A transformer is a static device comprising coils coupled through a magnetic medium connecting two ports at different voltage levels (in general) in an electric system allowing the interchange of electrical energy between the ports in either direction via the magnetic field. It essentially consists of two wingdings, the primary and secondary,wound on a common laminated magnetic core. The winding connected to the a.c. source is called primary winding (or primary) and the one connected to load is called secondary winding (or secondary). The most important tasks performed by transformers are: 
    (i) changing voltage and current levels in electric power systems,
    (ii) matching source and load impedances for maximum power transfer in electronic and                 control circuitry
    (iii) electrical isolation (isolating one circuit from another or isolating dc while maintaining ac             continuity between two circuits).
There are various types of transformers e.g. step-up transformer, step-down transformer, isolation transformer etc. If the secondary voltage is greater than the primary value, the transformer is called a step-up transformer; if it is less, it is known as a step-down transformer; if primary and secondary voltages arc equal, the transformer is said to have a one-to-one ratio. One-to-one transformers are used to electrically isolate two parts of a circuit. Any transformer may be used as a step-up or step-down depending on the way it is connected.


Electricity is transferred from power plants via high voltage lines (100-500kV) to substations. There, the high voltage is reduced by transformers and the electricity is transferred to medium voltage power lines (20-40 KV). Again, the medium voltage is reduced by transformers and the electricity is then transferred to low voltage power lines (110-480 V) which finally bring it to our buildings electricity meter. From there it goes into our individual electrical panel and the cables in our walls, lights, wall outlets and electrical equipment. 

Due to the flow of current in the conductive parts of electrical network electric field and magnetic field are produced. Electric fields are blocked by grounded conductive objects like tress, building materials while the magnetic field penetrates the building materials. 


According to statistics, the most common cause of high values of radiation resulting from magnetic fields are low voltage power lines that bring electricity to every house. Only 23% of these values are due to high voltage lines. This is because the magnetic fields depend on the amount of Current that flows through the cables (ampere) and not on the voltage (Watt = Volt * Ampere). The low voltage cables can be overloaded particularly in densely populated areas
. Moreover, the low voltage cables are usually found closer to residencies (high voltage cables usually have a minimum distance of 20 meters). with the use of non-linear devices such as fluorescent lamps, AC adapters, dimmer electronic switches, inverters, air conditioners, plasma TVs, photovoltaic systems etc. results in the deformation of the plain sinusoidal signal of 50-60Hz mains with high frequency harmonics. This phenomenon is called "Dirty Electricity" because it causes overheating of the neutral conductor and premature aging of equipment. Some scientists believe that the new waveform of the power grid is particularly burden some for humans

To reduce the exposure of magnetic fields due to cable/conductors and substations by increasing the distance between them. Another way of reducing the exposure of the magnetic fields is using magnetic shield materials, but they are very expensive and their use in useful without proper measurement of magnetic fields. Using of underground cables reduces the magnetic field rapidly but they can cause high levels of magnetic field on the ground floors or basement apartments or shops, in yards, gardens etc. 


There are various effect of radiations on human health like miscarriages,  leukemia, skin cancer, multiple sclerosis etc. Due to their connection with childhood leukemia, magnetic fields have been classified as "possible carcinogens" in 1998 by the National Environmental Health Institute, USA (NIEHS) and in 2001 by the International Agency for Research on Cancer (IARC) of the World Health Organization. New research are showing that the various human plagues emerged in 21th century like female breast cancers, malignant malanoma, asthama are linked with the use of electricity. A recent study  states that "Maternal exposure to high magnetic fields during pregnancy may be a new and previously unknown factor contributing to the world-wide epidemic of childhood obesity/overweight. 

Now the question arises what are  the safe limits of exposure to low frequency magnetic fields? the various safe distance of transformers from buildings according to various standards are given below.

Clearance from outdoor liquid  installed  Transformers for building (as per National Electric Code)

Liquid

Liquid Volume (m3)

Fire Resistant Wall

Non-Combustible Wall

Combustible Wall

Vertical Distance

Less Flammable

NA

0.9 Meter

0.9 Meter

0.9 Meter

0.9 Meter

<38 m3

1.5 Meter

1.5 Meter

7.6 Meter

7.6 Meter

>38 m3

4.6 Meter

4.6 Meter

15.2 Meter

15.2 Meter

Mineral Oil

<1.9 m3

1.5 Meter

4.6 Meter

7.6 Meter

7.6 Meter

1.9 m3 to 19 m3

4.6 Meter

7.6 Meter

15.2 Meter

15.2 Meter

> 19 m3

7.6 Meter

15.2 Meter

30.5 Meter

30.5 Meter

Clearance from Indoor installed Dry Type Transformers (as per NES 420.21)


Voltage

Distance (min)

Up to 112.5 KVA

300 mm (12 in.) from combustible material unless separated from the combustible material by a heat-insulated barrier.

Above 112.5 KVA

Installed in a transformer room of fire-resistant construction.

Above 112.5 KVA with Class 155 Insulation

separated from  a fire-resistant barrier not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically


Clearance from outdoor installed Dry Type Transformers (as per NES 420.22)


Voltage

Distance (min)

Above 112.5 KVA with Class 155 Insulation

separated from  a fire-resistant barrier not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically


Clearance from Oil insulated indoor Transformer (as per NES 420.25) 


Voltage

Distance (min)

Up to 112.5 KVA

Installed indoors Vault (With construction of reinforced concrete that is not less than 100 mm (4 in.) thick.

Up to 10 KVA & Up to 600V

Vault shall not be required if suitable arrangements are made to prevent a transformer oil fire from igniting

Up to 75 KVA & Up to 600V

Vault shall not be required if where the surrounding Structure is classified as fire-resistant construction.

Furnace transformers (Up to 75 kVA)

Installed without a vault in a building or room of fire resistant construction


Clearance of Transformers from building ( As Per IEEE)


Transformer

Distance from Building (min)

Up to 75 KVA

3.0 Meter

75 KVA to 333 KVA

6.0 Meter

More than 333 KVA

9.0 Meter

 
 Read Also

01.     Introduction to Electrical Power System

02.Principle and Construction of DC generator  

03.Electric Circuit | Terms Related to Electric Circuit | What is Electric Circuit

04.Electric Charge | Electric Current | Voltage

05.Series and Parallel Combination of Resistances

06.Resistors

07.Electrical Thumb Rules

08.Kirchhoff’s Laws

09.Where does the reactive power Go?

10.Why Transformer is not Connected to DC

11.Series and Parallel combination of Capacitors

12.Capacitor

13.Series and parallel Combination of Capacitors

14.Types of Resistors

15.Voltage divider Rule and Current divider Rule

16. Lightning Arrester

17. Losses in Transmission and distribution

18.Charging and Discharging of Capacitor

19.Electrical Earthing

20. Methods of Electrical Earthing

21.Calculating the Number of earthing rods/Pipes required

22.Inductor


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