Typical Earthing Resistance Value

TYPICAL EARTHING RESISTANCE VALUE

The resistance offered by the earth electrode to the flow of current into the ground is known as the earth resistance or resistance to earth. * Ideally a ground resistance should be of zero ohms but It is always greater than Zero .System ground resistances can be reduce by the use of a number of individual electrodes connected together. * Total earthing resistance is the sum of the resistance of earth lead wires, Contact resistance between the surface of the earth electrode and the soil and The resistance of the body of the soil surrounding the earth electrode. * The value of earthing resistance varies on the Type of Soil, Soil characteristic, soil resistivity and the climatic condition. Moisture content in soil plays a vital role in the soil resistivity. value of individual earthing pit resistance is not so important. Different codes specifies the required value of earthing system. * Electrical Systems can work with earth resistance of 20 ohms, though generally 10 ohms is the specified Maximum limit. * But communication systems need very stringent limit, typically one ohm. This is because the higher the ground resistance, higher would be noise interference in the systems.

USAID

a) Power stations (generating station)

0.5 ohms

b) EHT Sub-station

1.0 ohms

c) 33 KV Stations

2.0 ohms

d) D/t Structure

5.0 ohms

e) Tower Foot resistance

10.0 ohms

IEEE STANDARD 142

Chapter: 4.1.3 , page 164 For industrial plant substations and buildings and large commercial installations.

1Ω to 5 Ω

Resistances of less than 1 ohm may be obtained using a number of individual electrodes connected together. Such a low resistance is only required for large substations, transmission lines, or generating stations.

National Electric code (NEC) 2011, (IS SP30 Chapter 14 -India)

Chapter: 3.0.9 unless otherwise specified ,It is recommended that the value of any earth system resistance shall not be more than

5Ω

IS 3043 (India)

Chapter: 22.2.3 The continuity resistance of the earth return path through the earth grid should be maintained as low as possible and in no case greater than

1Ω

This applicable for main earth grid connected with the transformer/return path

Oil Industry Safety Directorate Government of India (OISD STANDARD – 137)

Chapter: (7. ii. b) Allowable earth-Resistance Values Allowable earth-Resistance Values The resistance value of an earthing system to general mass of the earth should not exceed. For electrical systems and metallic structures.

4Ω

For storage tanks.

7Ω

for main earth grid, and bonding connections between joints in pipelines and associated facilities.

1Ω

for each electrode to the general mass of the earth

2Ω

IS 2309 (india) / BS 7430:1998

Clause:12.3.1 Page 32,Resistance to Earth Lightning arrestors ground resistance for Protection of buildings and allied structures is

10Ω

An earth electrode should be connected to each down conductor. Each of these earths should have a resistance not exceeding the product given by 10 a multiplied by the number of earth electrodes to be provided. The whole of the lightning protective system, including any ring earth, should have a combined resistance to earth not exceeding 10 Ω without taking account of any bonding. If the value obtained for the whole of the lightning protective systems exceeds 10 Ω, a reduction can be achieved by extending or adding to the electrodes or by interconnecting the individual earth terminations of the down conductors by a conductor installed below ground, sometimes referred to as a ring conductor

IS 2689:1989

Table 4 page 28 (Reaffirmed March 2010) Lightning arrestors ground resistance for Protection of buildings and allied structures is

10Ω

NEC 250.56

Clause: 250.53 Grounding Electrode System Installation. The maximum resistance for a single electrode consisting of a rod, pipe, or plate.

25Ω

If a higher resistance is obtained for a single electrode, a second electrode of any of the types specified in the NEC is required. This should not be interpreted to mean that 25 ohm is a satisfactory resistance value for a grounding system.

IEEE Std 80-2000 (Revision of IEEE Std 80-1986)

the evaluation of ground resistance for the most transmission and other large substations, the ground resistance is usually about

1Ω or less

In smaller distribution substations, the usually acceptable range is

1Ω to 5Ω

NFC 17-102, July 1995

that the resistance value measured using conventional equipment should be

1Ω or less

This resistance should be measured on the earthing termination insulated from any other conductive component.

IEC 62305-1

edition 2.0 – 2010-12 the conventional earthing impedance related to the earth termination system is (*for the soil resistivity less than or equal to 100 Ω)

4 Ω

Ministry of Railways- Government of India

The acceptable Earth Resistance at earth MEEB b…