Why Salt and Charcoal are added in Earthing Pit?&

PS Class CT Specifications & its applications

Salt and charcoal increase the conductivity of the soil in the earth pit. As a result, the fault current finds it easy path through equipment to the earth. The alternate layers of charcoal and salt maintain low resistance for earth fault currents.

Adding charcoal, salt, and watering in earthing pit decreases soil resistivity.

We use conventional pipe and plate earthing for earthing purposes. Two types of material are used in the earth pit to fill the earth pit.

  • Salt
  • Charcoal

Before understanding the concept behind the use of salt and charcoal for earth pit, let’s first understand the properties of these materials.

Earthing Pit Materials


The wood, other plants, and animal materials are volatile materials and these can be burned. On strongly heating up these materials, we can easily remove all water and volatile constituents. After removing water & volatile contents, the black carbon residuals are left is known as charcoal.

The properties of charcoal are,

  • It is porous material
  • Absorbs moisture from air- Hygroscpic
  • charcoal is an allotrope of carbon and acts as a conductor of electricity


It is an ionic compound. It has a strong attraction for polar water molecules. Thus, the salt absorbs moisture from the soil and keeps the earth pit moist.

The charcoal and salt perfectly make the ionic bonding with moisture in the soil. Thus, the water content in the earth pit increases. The moisture increases the conductivity of the earth pit.

The earth resistance of the earthing pit is the combined resistance of the earth pit and the surrounding soil. The soil conductivity differs with types of soil. Therefore, it is a must to check the conductivity of the soil before making an earth pit.

Why are salt and charcoal used in earth pit?

  • The charcoal and soil both absorbs the moisture from the soil. The salt can make a good bonding with charcoal,soil and water. Thus, the both materials used the earth pit increase the conductivity of the earth pit.
  • The charcoal and coal can retain the absorbed moisture for a longer period of time.
  • Salt and charcoal minimize the earth pit resistance. Thus, it provides the least resistance path to the fault current.
  • The low value earth resistance leads to easier passing of the fault current to ground. Thus, it provides protection to machines and humans.

Maintenance of Earthing Pit

We must take the following steps to ensure the healthiness of the earth pit.

  • Measure the earth pit resistance on the most dry day once in a year.
  • Record the resistance
  • If resistance is high take necessary action to lower the resistance by watering the pit.
  • Again measure the resistance
  • If resistance OK, no action required
  • If resistance not OK, discard the earth pit
  • Salt and Charcoal have leaching properties. It means these materials washed away after some time. Therefore, periodical checking and maintenance is must for its healthiness.

The high resistance of the earth pit may cause serious accidents because the step voltage and touch potential increase with an increase of the earth resistance. Therefore, it is of utmost importance to keep the earth pit resistance as lower as possible.

We can perform the soil treatment to lower the resistance as per IEEE80-2013 standard.

What is the ratio of salt & Charcoal in Earthing?

A ratio of 3:1 of charcoal and salt is good for earthing pit.

PS Class CT Specifications & its applications

In this post, we will discuss the PS class CT specifications & its application. Without the use of PS class CT, differential protection can not be reliably made. We will also discuss what is differential protection & how PS( Protection Special) class CT is important for differential protection.

The differential protection also called zone protection or unit protection. It is used for sensing the fault in a piece of particular equipment or a particular section. We use differential protection scheme for the protection of large rating alternators, transformers, and HT motors.

The differential protection system measures the incoming and outgoing current and calculates the difference of current. If the difference of the current is zero, it means the equipment is healthy and, so there is no fault inside the equipment. Any difference in the incoming and outgoing current clearly shows that there is fault inside the equipment. Under this condition the the differential relay trips the upstream breaker to isolate the faulty section.

The main criterion of reliable operation of the differential relay is that it must trip only in the case when there is a fault in particular equipment, section, or zone. And, the differential protection relay should not trip in the case of through fault condition.

Application of PS Class CT

The differential protection scheme is as given below.

Two current transformers CT1 and CT2 are mounted at the incoming and outgoing sides of the equipment to measure the current. Both the current transformers are wired in a differential protection relay. The differential protection relay measures the incoming & outgoing current and calculates the difference of the current. The relay output a trip command to breaker if found difference in the current.

For feeder protection, protection class CTs 5P10, 5P20, 10P10 are used. The protection class CT can not be used in place of the PS class current transformer.

Why is PS Class CT suitable for Differential Protection?

Let us understand Why protection class CT can not be used for differential protection?

If the 5P20 protection class CTs are used for differential protection at the incoming and outgoing sides of the transformer. In case of fault, If 20 times of rated current of CT flows through it, the current transformer will not saturate because the accuracy limit factor of the current transformer is 20. The CT may function well above the 20 times current, however, it is difficult to predict at what current the CT will get saturated.

The protection class CT installed at the incoming side of the zone protection let’s say saturate at 21 times current and the CT installed at the outgoing side of the zone protection saturate at 22 times current of the rated current of CT. In a faulty condition, the difference of current will be sensed once the relay senses the saturation of one of the CTs. Thus, the differential protection can not be reliably ensured with the use of protection class CTs.

The PS class CT is special protection class CT. Both the PS class CTs used for differential protection have identical saturation characteristics and their knee point voltage is the same. The designer takes the data like lead resistance, relay burden, and system fault level from the user before the design of PS class CT. In through fault condition, both the CTs operate at the same operating points of the BH curve and thus differential relay remains inoperative under through fault condition.

The knee point voltage (KPV) of the current transformer depends on.

  1. System fault level
  2. Total burden of current transformer

The knee point voltage of the protection class CT is;

PS Class CT Specification

Primary TerminalP1-P2
Secondary Terminal3S1-3S2
Burden (VA)Specify
KPV(min) Voltage600
iexc(max) at Vk/250 mA
Rct @ 75o C< 6 Ohm