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Safety Clearance Recommendations for Transformer

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Safety Clearance Recommendations for Transformer
Safety Clearance Recommendations for Transformer (on photo 10/0.4 kV transformer substation by FIMA)

Clearance Tables

  1. Clearance from Outdoor Liquid Insulated Transformers to Buildings (NEC)
  2. Clearance between Two Outdoor Liquid Insulated Transformers (NEC)
  3. Dry Type Transformer in Indoor Installation (NES 420.21)
  4. Dry Type Transformer in Outdoor Installation (NES 420.22)
  5. Non Flammable Liquid-Insulated Transformer in Indoor Installation (NES 420.21)
  6. Oil Insulated Transformer in Indoor Installation (NES 420.25)
  7. Transformer Clearance from Building (IEEE Stand.)
  8. Transformer Clearance Specifications (Stand. Georgia Power Company)
  9. Clearance of Transformer-Cable-Overhead Line (Stand. Georgia Power Company)

Clearance from Outdoor Liquid Insulated Transformers to Buildings (NEC)

LiquidLiquid Volume (m3)Fire Resistant Wall Non-Combustible WallCombustible WallVertical Distance
Less FlammableNA0.9 Meter0.9 Meter0.9 Meter0.9 Meter
<38 m31.5 Meter1.5 Meter7.6 Meter7.6 Meter
>38 m34.6 Meter4.6 Meter15.2 Meter15.2 Meter
Mineral Oil<1.9 m31.5 Meter4.6 Meter7.6 Meter7.6 Meter
1.9 m3 to 19 m34.6 Meter7.6 Meter15.2 Meter15.2 Meter
> 19 m37.6 Meter15.2 Meter30.5 Meter30.5 Meter

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Clearance between Two Outdoor Liquid Insulated Transformers (NEC)

LiquidLiquid Volume (m3)Distance
Less FlammableNA0.9 Meter
<38 m31.5 Meter
>38 m37.6 Meter
Mineral Oil<1.9 m31.5 Meter
1.9 m3 to 19 m37.6 Meter
> 19 m315.2 Meter

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Dry Type Transformer in Indoor Installation (NES 420.21)

VoltageDistance (min)
Up to 112.5 KVA300 mm (12 in.) from combustible material unless separated from the combustible material by a heat-insulated barrier.
Above 112.5 KVAInstalled in a transformer room of fire-resistant construction.
Above 112.5 KVA with Class 155 Insulationseparated from  a fire-resistant barrier not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically

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Dry Type Transformer in Outdoor Installation (NES 420.22)

VoltageDistance (min)
Above 112.5 KVA with Class 155 Insulationseparated from  a fire-resistant barrier not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically

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Non Flammable Liquid-Insulated Transformer in Indoor Installation (NES 420.21)

VoltageDistance (min)
Over 35KVInstalled indoors Vault (Having liquid confinement area and a pressure-relief vent for absorbing any gases generated by arcing inside the tank, the pressure-relief vent shall be connected to a chimney or flue that will carry such gases to an environmentally safe area
Above 112.5 KVAInstalled in a transformer room of fire-resistant construction.
Above 112.5 KVA (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

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Oil Insulated Transformer in Indoor Installation (NES 420.25)

VoltageDistance (min)
Up to 112.5 KVAInstalled indoors Vault (With construction of reinforced concrete that is not less than 100 mm (4 in.) thick.
Up to 10 KVA & Up to 600VVault shall not be required if suitable arrangements are made to prevent a transformer oil fire from igniting
Up to 75 KVA & Up to 600VVault 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

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Transformer Clearance from Building (IEEE Stand)

TransformerDistance from Building (min)
Up to 75 KVA3.0 Meter
75 KVA to 333 KVA6.0 Meter
More than 333 KVA9.0 Meter

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Transformer Clearance Specifications (Stand: Georgia Power Company)

Description of ClearanceDistance (min)
Clearance in front of the transformer3.0 Meter
Between Two pad mounted transformers (including Cooling fin)2.1  Meter
Between Transformer and Trees, shrubs, vegetation( for unrestricted natural cooling )3.0 Meter
The edge of the concrete transformer pad to nearest the building4.2 Meter
The edge of the concrete transformer pad to nearest  building wall, windows, or other openings3.0 Meter
Clearance from the transformer to edge of (or Canopy) building (3 or less stories)3.0 Meter
Clearance in front of the transformer doors and on the left side of the transformer, looking at it from the front (For operation of protective and switching devices on the unit)3.0 Meter
Gas service meter relief vents.0.9 Meter
Fire sprinkler values, standpipes and fire hydrants1.8 Meter
The water’s edge of a swimming pool or any body of water.4.5 Meter
Facilities used to dispense hazardous liquids or gases6.0 Meter
Facilities used to store hazardous liquids or gases3.0 Meter
Clear vehicle passageway at all times, immediately adjacent of Transformer3.6 Meter
Fire safety clearances can be reduced by building a suitable masonry fire barrier wall (2.7 Meter wide and 4.5 Meter Tall) 0.9 Meter from the back or side of the Pad Mounted Transformer  to the side of the combustible wall
Front of the transformer must face away from the building.

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Clearance of Transformer-Cable-Overhead Line (Stand: Georgia Power Company)

Description of ClearanceHorizontal Distance (mm)
to pad-mounted transformersto buried HV cableto overhead HV Line
Fuel tanks7.5 Meter1.5 Meter7.5 Meter
Granaries6.0 Meter0.6 Meter15 Meter
Homes6.0 Meter0.6 Meter15 Meter
Barns, sheds, garages6.0 Meter0.6 Meter15 Meter
Water wells1.5 Meter1.5 Meter15 Meter
Antennas3.0 Meter0.6 MeterHeight of Antenna + 3.0 Meter

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About Author

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Jignesh Parmar

jiguparmar - Jignesh Parmar has completed M.Tech (Power System Control) ,B.E(Electrical). He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 13 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.