Why are Salt and Charcoal Added in Earthing Pit for Grounding?

Electrical Technology

Why We Must Put Charcoal and Salt for Earthing and Grounding?

For proper earthing and grounding, it is very important to maintain low resistance of grounding / earthing conductors by reducing the resistivity of soil and surrounding areas. The earth or ground wire is directly connected between the metallic bodies of the machine connected to the earth plat. For this reason, a mixture of salt and charcoal with a ratio of 3:1 is used in earth pits to increase the conductivity of the soil.

Now the question arises why charcoal and salt only? Why not use other materials? Well, let’s know why the combination of charcoal and salt is the perfect choice to put in the earth pit.

When a wood is burnt at a specific temperature, the heating up process will remove all the moisture (water and volatile contents in it). The remaining black residual in the form of carbon as low ash compound is known as charcoal.

Good to know: Charcoal is also known as Soot, Coke, and Carbon black, and soot.

Table of Contents

Toggle

Charcoal

Charcoal is a material in porous form like a sponge exists in a solid amorphous state
It is a hygroscopic material i.e. it absorbs moisture from the air and sounding
As it is an allotrope of carbon, thus it acts as a conductor of electricity like graphite.
Charcoal acts as a good absorbent and readily absorbs moisture from the soil and air.

Salt

A crystalline compound, sodium chloride “NaCl” in the form of crystals or powder is an ionic in nature, thus, it strongly attracts the polar water molecule. It is a hygroscopic material which absorbs moisture from damp atmospheres above 75% relative humidity. This way, the salt keeps the earth pit moist by absorbing the moisture from soil and surroundings.

Why are Charcoal and Salt Added in the Earth Pit?

The combination of salt and charcoal is the perfect mixture which makes the ionic bonding for moisture in the earth pit. When the moisture increases in the soil, it increases the conductivity of the earth or ground conductor to the grounding rod or earth plat buried in the earth pit. That’s why the mixture of charcoal and salt is the best combination to put in the earth pit to maintain the low resistance.

An alternate layer of salt and charcoal is used to increase the effective area of the earth which leads to decrease the earth’s resistance.

As discussed above, the mixture of salt and charcoal as an alternate layer in the earth pit absorbs the moisture from the soil and surroundings. Additionally, the salt makes a perfect bonding with water, soil and charcoal. Therefore, the combination of charcoal and salt decreases the resistance and increases the conductivity of the earth pit. This way, the fault current can easily flow from the metallic body of the machine through the grounding conductor (earth continuity conductor) to the earthing lead and earth electrode (earth plat) buried in the earth pit.

The mixture of coal and powdered charcoal can maintain the moisture around the soil for a long time period. Hence, it reduces the resistance of earth pit and soil. This way, in case of fault, a less resistive path is available for fault current to flow to the ground. Thus, it provides proper protection to electrical machines as well as against the electric shock to a human body in contact with the metallic body of electrical appliances.

As the resistances may vary according to the different types of soil, thus it is important to check and test the conductivity of the soil before making an earth pit for grounding rods and earth electrodes (generally GI pipe or plate).

Keep in mind that the minimum and ideal resistance of the earthing and grounding system should be at least 1 Ω. If the resistance is more than 1-5+ ohms, you may increase the size of earthing lead and earth continuity conductor. Make sure to put water from time to time in the GI pipe connected to the earth pit which makes sure proper moisture around the earth plat. Hence, the earthing & grounding system for protection purposes works smoothly.

Good to know:

According to the NEC (250.56), the impedance of the grounding system should be less than 25Ω.
According to the IEC, the ideal value of earth resistance should be between 1-10Ω.
According to IEEE and NFPA, the resistance of the grounding system should be less than 5Ω or less.
You may perform the soil treatment of conductivity according IEEE 80-2013 (clause 14.5 (a), (b), (c), (d)) standard to reduce the resistance of earthing system.

Maintenance of Earthing Pit

For proper operation of the earthing and ground system, follow the following simple steps to maintain the earth pit which makes sure the system is working properly.

To keep moisture and reduce the resistance around the grounding rod and earth plate, put a 30cm (1ft) layer of powdered charcoal and salt in the earth pit.
Measure the resistance of the earth pit via earth tester / ground resistance meter on a shiny day and make sure the resistance is not more than 1 Ω.
If it is more than 1 ohms, increase the size of the grounding conductor (earth continuity wire) and earthing lead.
As salt and charcoal are leaching compounds, therefore, put some water from time to time in the earth pit to maintain the required moisture around the earth plate.

Ratio of Charcoal and Salt in Grounding and Earthing System

There is no standard ratio for salt and charcoal in the grounding system as it depends on the type of the soil, its resistivity and the environment of the particular area.

As a rule of thumb, a ratio of 1:3 of salt and charcoal is good enough for general purpose plate earthing and grounding. For example, 1kg of Salt + 3 kg of Charcoal). In conventional pipe earthing, 5kg of salt is enough with 5+ kg of charcoal.

Demerits of Charcoal and Salt in the Earthing System

As charcoal and salt are leaching compounds, it needs frequent applications and proper maintenance. According to IEEE 80-2013 clause 14.5(d), For permanent earthing compounds, TEREC+, CUREC+ is the best alternative as it is independent of moisture and permanent earthing compounds having a lifespan of 25+ years.