Clearance Requirements In EHV AIS Substation You MUST Respect

Planning EHV AIS substation

Starting point for planning a substation is its single line diagram (SLD) which relates to circuit configuration, number of bus bars and its type and other associated equipment.

However from erection and installation point of view layout of any substation is the most vital and key engineering because the single line diagram, bus switching scheme is to be translated into a layout of appropriate bay widths, section and ground clearances so as to physically achieve the feeder switching required for ease in erection and maintenance.

Clearance requirements you MUST take into account when planning EHV AIS substation
Clearance requirements you MUST take into account when planning EHV AIS substation (on photo: High voltage transformation substation of the Kimanis Power, in Malaysia; credit: Wikipedia)
Substation layout consists essentially in arranging a number of switchgear components in an ordered pattern governed by their function and rules of spatial separation.

Spatial separation consists of following types of separation:

  1. Earth clearance
  2. Phase clearance
  3. Creepage
  4. Ground clearance
  5. Sectional safety working clearance (will be explained in 2nd part)

Creepage Distance

Creepage (Leakage distance) is the shortest path between two conductive parts (or between a conductive part and the bounding surface of the equipment) measured along the surface of the insulation. A proper and adequate creepage distanceprotects against tracking, a process that produces a partially conducting.

Insulators in substation are provided to avoid any leakage current from live electrical conductors to flow to the earth through supports. The atmospheric dust sticks to the insulator surface forming a conducting layer.

The leakage current flows from the live conductor to the earth through such surface layers. The leakage properties (creepage properties) of an insulator s in substation are characterized by the length of the leakage path. While designing the insulator sheds, the leakage distance for insulators requirement should be satisfied.

Damages the insulating material normally occurs because of one or more of the following reasons:

  • Humidity in the atmosphere.
  • Presence of contamination.
  • Corrosive chemicals.
  • Altitude at which equipment is to be operated.


Clearance is the shortest distance between two conductive parts (or between a conductive part and the bounding surface of the equipment) measured through air. Clearance distance helps prevent dielectric breakdown between electrodes caused by the ionization of air.

The dielectric breakdown level is further influenced by relative humidity, temperature, and degree of pollution in the environment.

Clearance is the shortest distance between two conductive parts
Clearance is the shortest distance between two conductive parts

Dry arcing distance (or arc distance)

Dry arcing distance is the shortest distance outside the insulator along the air not along the insulator body and between those parts which normally have the operating voltage between them. Dry arcing distance also called arc distance, and it means direct discharge distance, also IEC 61109 standard can be referred.

Dry arcing distance and creepage distance: Dry arcing distance means the shortest path which the voltage can puncture the air outside the insulator. Creepage distance means the shortest path between two conductive parts measured along the surface of the insulation.

Dry arcing distance
Dry arcing distance

Power frequency dry flashover voltage test on insulator
Power frequency dry flashover voltage test on insulator

Strike Distance (Dry Arcing Distance) is the principal constituent to electrical values
Strike Distance (Dry Arcing Distance) is the principal constituent to electrical values

Arcing distance is also called as flash over distance. Below illustration will make it clearer. The corrugation below the insulator is for the purpose of obtaining longer creepage path between the pin and cap.

The corrugation increases the creepage length so consequently increasing resistance to the insulator leakage current. The leakage current that flows through the surface of insulators should be as little as possible.

HV insulator
HV insulator

Important Notes:

  • Flashover distance – It is the shortest distance through air between the electrodes of the insulator. For a pin type insulator shown in above figure the double headed red arrow line is flashover distance.
  • Flashover voltage – The voltage at which the air around insulator breaks down and flashover takes place shorting the insulator.
  • Puncture voltage – The voltage at which the insulator breaks down and current flows through the inside of insulator.
  • Creepage is related with leakage current and puncture voltage whereas arcing distance is related to flashover voltage or BIL level of that voltage level. That’s why height/length of insulator depends upon arcing / flashover distance.

An insulator may fail due to excessive electrical stress, excessive thermal and mechanical stress or degradation due to environmental chemical action ofsurface of the insulator. The electrical failure can happen between conductor and earth through air or through the volume of insulating material.

In one case due to excessive electric stress the insulator may fail when a flashover takes place through the air between the conductor and tower. In other case the insulator may be punctured through the volume. The insulating material like porcelain has high dielectric strength in comparison to air. The insulators are designed so that it will be flashover before it gets punctured.

Failure due to flashover is generally temporary and self-restoring. But failure due to insulation puncture is permanent and the insulator is damaged and required to be replaced. An insulator which has internal defects like voids and impurities reduces the electrical strength of the insulator.

The flashover may results in damage of insulator glaze which can be repaired.

In polluted regions contaminants deposit on the surface of the insulator those results in reduction of the flashover voltage of the insulator in wet condition.

For example if the power frequency flashover voltage of a 33 kV pin insulator is 95 kV in dry then in wet condition the flashover voltage may be reduced to below 80 kV. Insulators are designed to withstand flashover voltage. In this example you can observe that even in the wet condition the flashover voltage (80 kV) is more than twice the insulator working voltage (33 kV).

Ground Clearance

The ground clearance is the distance between ground level and bottom of any insulator in an outdoor substation.

This ensures that any person working in the area cannot touch or damage the insulators accidentally.

This clearance is kept as 2.5 meters for all voltage levels. The minimum vertical distance from the bottom of the lowest porcelain part of the bushing,porcelain enclosures or supporting insulators to the bottom of the equipment base, where it rests onthe foundation pad shall be 2.5 meters.

This means that support structure from plinth up to the bottom of insulator or top of the metallic earthed part of the equipment below insulator should be at 2.5 meters. Refer below sketch of CT and CVT mounted on support structure.

CT and CVT mounted on support structure
CT and CVT mounted on support structure

However in cases, where the vehicles and cranes are allowed inside a substation, the ground clearance for the equipment falling on both sides of the road are to be enhanced as the vehicles and cranes height is generally 3.5 meters.

The minimum ground clearances between the live point and ground at the substation for the different voltage classes in rule no 64 of I.E. Rule 1956. Below is the table for ground clearance, Phase to earth clearance and bus height.

Minimum clearance in air for outdoor substation is as stated in DIN VDE 0101 or specified by IEC 61936. They are also listed as per voltage level in EN 60071-1.

When atmospheric conditions or ambient conditions are different from standardized conditions than appropriate correction factor should be applied by finding the withstand voltage in that condition that is actual BIL testing in new non-standard ambient/atmospheric condition. Than based on dielectric strength of air required phase to phase and phase to earth clearance can be calculated.

Impact of atmospheric condition on dielectric strength of air is dealt in later part of this article.

Ground clearance
in mm
Approximate arcing distance of insulator mm. (Creepage being considered as 31 mm/kV)Bus Height in mm = Ground clearance + dry arcing distance of equipment insulator + Height of tank terminal connector etc… 
400 kV2500
  • Post insulator: 3650
  • CT insulator with end casting: 3350
  • CB support insulator: 3850
  • Tension & Suspension insulator string: 3625
2500 + 3650 + 1000 = 7150
which can be rounded of to 8000 mm. In order to meet 8000mm, structure height is adjusted making ground clearance more than 2500mm
220 kV2500
  • Post insulator: 2300
  • CT insulator with end casting: 2000
  • CB support insulator: 2100
  • Tension & Suspension insulator string: 2175
2500 + 2300 + 800 = 5600
which is considered as 5500mm. For equipment with lowest arcing distance 2100mm, 5500mm is met by adjusting the structure height which makes ground clearance more than 2500mm
132 kV2500
  • Post insulator: 1500
  • CT insulator with end casting: 1190
  • CB support insulator: 1770
  • Tension & Suspension insulator string: 1450
2500 + 1500 + 600 = 4600
For equipment with lowest arcing distance 1190mm, 4600mm is met by adjusting the structure height which makes ground clearance more than 2500mm
66 kV2500 6304000
33 kV2500 3203700

Will be continued in friday 26th of may 2016.


  • UNCERTAINTIES IN THE APPLICATION OF ATMOSPHERIC AND ALTITUDE CORRECTIONS AS RECOMMENDED IN IEC STANDARDS: Paper Published on the 16th International Symposium on High Voltage Engineering, Cape Town, South Africa, 2009
  • IEC 61936
  • CBIP Manual 299

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About Author //


Asif Eqbal

Asif Eqbal - Bachelor of Engineering in Electrical & Electronics engineering, from Manipal University, (Karnataka), India in 2006. Presently involved in the design of EHV outdoor substation and coal fired thermal power plants for more than seven years. Motto of joining EEP as a contributor is to share my little engineering experience and help the budding engineers in bridging the conspicuous gap between academics and Industrial practice. “If you have knowledge, let others light their candles with it, so that people who are genuinely interested in helping one another develop new capacities for action; it is about creating timeless learning processes".


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