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Tuesday, 29 March 2016

Chemical compounds for Electrical Earthing

An electrical installation may face –
1.       Damage because of Lightening,
2.       Electrical leakage,
3.       Short circuit between phases or between phase and neutral,
4.       Surges in the supply.

The above causes may lead to electric fire causing loss of property and loss of human lives. 

Equipments with electronic circuits/PCBs/electronic cards are liable to get damaged due to high neutral currents and unbalanced voltages. Therefore, the voltage between neutral and earth point of any installation should be kept to a minimum. 

Why Earthing or Grounding is needed?

A well designed Earthing or Grounding system is very essential for any electrical installation. All the equipment casings, and neutral of the 3-phase system have to be kept at zero or ground potential. It avoids dangers associated with fault currents; protects both the equipment and the operator against hazardous voltages. 

Whenever there is an insulation failure, there is a tendency that some metallic parts are also energized to the potential of the current carrying part, unless the equipment is effectively earthed. The tolerable value of current through any human being is less than 100 micro-amps. The impact of current flow in a human depends upon the magnitude of current, duration of current flow, and nature of current. Current flow may cause muscular contraction, respiratory nerve blockage, and burning. The most severe is the stopage of heart beat resulting in immediate end of blood circulation. 

A good earthing system protects the installation and equipment by providing low impedance path to fault currents. It also minimizes electromagnetic noise thus preventing unwanted interference with communication signals.

Recommended Values of Earth Resistance:

 The recommended values of earth resistance for various installations are as under:

  1. Large sub-station, generating stations etc less than 1 Ohm,
  2. Transmission sub-stations, primary distribution sub-stations, large industries; 1 to 5 Ohm,
  3. Sub-stations and equipments below 10 kV; 5 to 10 Ohm.

In any case the value of earthing should not exceed 25 Ohm. 

Limitations of  Common Salt and Charcoal when used in an Earthing system:

Resistivity of Soil is important in earthing. It depends on the soil nature, moisture, temperature and content. Clay and black cotton soil have low resistivity as compared to red or rocky soil.

Traditionally we have used charcoal and salt in the earthing pit to reduce the earth/soil resistivity. The common salt is a known corrosive electrolyte which decays the pipe and the conductor used for earthing leading to inconsistent resistive values. Similarly, the soft coke and charcoal used to become ash due to the heavy heat generated by large fault currents in the system, particularly at high voltages.

Recent trend is using Chemical Compounds: 

Now a day advanced chemical compounds are used in the earthing system which lowers the contact resistance of earth electrode significantly (approximately over 60%). It offers low impedance to surges resulting in faster energy dissipation. The earthing system can fail because of inadequate dissipation of heat. These chemical compounds have a high melting temperature of 2500oC and thus helps in dissipating the heat generated due to faults.

These chemicals or mixtures have excellent shelf life, require no maintenance and do not adversely affect soil or ground water. These compounds have very good performance even during dry weather as its working does not require continuous presence of water.

These chemical compounds for earthing, mainly consisting of Aluminum Silicate, are available in easy carry bags of 10/25/50 kg. Its resistivity is less than 0.1 ohm-m. The key features of these compounds are:

1. Absorbs and retains the moisture for long time; in fact they have the property to absorb water 15 times of its weight,
2. Reduces soil resistivity,
3. Keeps the earth resistance same over a wide temperature variations,
4. Dissipate fault currents at a faster rate,
5. Eliminate the need of salt and charcoal around the electrode,