*Continued from first part:*

* How to Determine Correct Number of Earthing Electrodes (Strips, Plates and Pipes) – part 1*

## 2. Calculate Number of Plate Earthing

**The Earth Resistance of Single Plate electrode is calculated as per IS 3040:**

R=ρ/A√(3.14/A)

**Where:**

**ρ** = Resistivity of Soil (Ω Meter),

**A** = Area of both side of Plate (m2),

**Example:** Calculate Number of CI Earthing Plate of 600×600 mm, System has Fault current 65KA for 1 Sec and Soil Resistivity is 100 Ω-Meters.

**Current Density At The Surface of Earth Electrode (As per IS 3043):**

**Max. Allowable Current Density I = 7.57×1000/(√ρxt) A/m2**- Max. Allowable Current Density = 7.57×1000/(√100X1)=757 A/m2
- Surface area of both side of single 600×600 mm Plate= 2 x lxw=2 x 0.06×0.06 = 0.72 m2
**Max. current dissipated by one Earthing Plate = Current Density x Surface area of electrode**- Max. current dissipated by one Earthing Plate =757×0.72=
**545.04****Amps** **Resistance of Earthing Plate (Isolated)(R)=ρ/A√(3.14/A)**- Resistance of Earthing Plate (Isolated)(R)=100/0.72x√(3.14/.072)=290.14 Ω
- Number of Earthing Plate required =Fault Current / Max.current dissipated by one Earthing Pipe.
- Number of Earthing Plate required= 65000/545.04 =
**119 No’s.** **Total Number of Earthing Plate required = 119 No’s.**- Overall resistance of 119 No of Earthing Plate=290.14/119=
**2.438 Ω.**

## 3. Calculating Resistance of Bared Earthing Strip

### 1) Calculation for earth resistance of buried Strip (As per IEEE)

The Earth Resistance of Single Strip of Rod buried in ground is:

R=ρ/Px3.14xL (loge (2xLxL/Wxh)+Q)

**Where:**

**ρ** = Resistivity of Soil (Ω Meter),

**h** = Depth of Electrode (Meter),

**w** = Width of Strip or Diameter of Conductor (Meter)

**L** = Length of Strip or Conductor (Meter)

**P** and **Q** are Koefficients

### 2) Calculation for earth resistance of buried Strip (As per IS 3043)

The Earth Resistance of Single Strip of Rod buried in ground is:

R=100xρ/2×3.14xL (loge (2xLxL/Wxt))

**Where:**

**ρ** = Resistivity of Soil (Ω Meter),

**L** = Length of Strip or Conductor (cm)

**w** = Width of Strip or Diameter of Conductor (cm)

**t** = Depth of burial (cm)

**Example: **Calculate Earthing Resistance of Earthing strip/wire of 36mm Diameter, 262 meter long buried at 500mm depth in ground, soil Resistivity is 65 Ω Meter.

**Here:**

**R** = Resistance of earth rod in **W**.

**r** = Resistivity of soil(Ω Meter) = **65 Ω Meter**

**l** = length of the rod (cm) = 262m = **26200 cm**

**d** = internal diameter of rod(cm) = 36mm = **3.6cm h** = Depth of the buried strip/rod (cm)= 500mm =

**50cm**

- Resistance of Earthing Strip/Conductor (R)=ρ/2×3.14xL (loge (2xLxL/Wt))
- Resistance of Earthing Strip/Conductor (R)=65/2×3.14x26200xln(2x26200x26200/3.6×50)
- Resistance of Earthing Strip/Conductor (R)=
**= 1.7****Ω**

## 4. Calculate Min. Cross Section area of Earthing Conductor

Cross Section Area of Earthing Conductor As per IS 3043

(A) =(If x√t) / K

**Where:**

**t** = Fault current Time (Second).

**K** = Material Constant.

**Example: **Calculate Cross Section Area of GI Earthing Conductor for System has 50KA Fault Current for 1 second. Corrosion will be 1.0 % Per Year and No of Year for Replacement is 20 Years.

Cross Section Area of Earthing Conductor **(A) =(If x√t) / K**

**Here:**

**If** = **50000 Amp**

**T** = **1 Second**

**K** = **80** (*Material Constant, For GI=80, copper K=205, Aluminium K=126*).

- Cross Section Area of Earthing Conductor (A) =
**(50000×1)/80** - Cross Section Area of GI Earthing Conductor (A) =
**625 Sq.mm** - Allowance for Corrosion = 1.0 % Per Year & Number of Year before replacement say =
**20 Years** - Total allowance = 20 x 1.0% =
**20%** - Safety factor =
**1.5** - Required Earthing Conductor size = Cross sectional area x Total allowance x Safety factor
- Required Earthing Conductor size = 1125 Sq.mm say 1200 Sq.mm
**Hence, Considered 1Nox12x100 mm GI Strip or 2Nox6 x 100 mm GI Strips**

## Thumb Rule for Calculate Number of Earthing Rod

The approximate earth resistance of the Rod/Pipe electrodes can be calculated by:

**Earth Resistance of the Rod/Pipe electrodes:**

R= K x ρ/L

**Where:**

**ρ** = Resistivity of earth in Ohm-Meter

**L** = Length of the electrode in Meter.

**d** = Diameter of the electrode in Meter.

**K** = 0.75 if 25< L/d < 100.

**K** = 1 if 100 < L/d < 600

**K** = 1.2 o/L if 600 < L/d < 300

**Number of Electrode if find out by Equation of R(d) = (1.5/N) x R**

**Where:**

* R(d)* = Desired earth resistance

*= Resistance of single electrode*

**R***= No. of electrodes installed in parallel at a distance of 3 to 4 Meter interval.*

**N****Example: **Calculate Earthing Pipe Resistance and Number of Electrode for getting Earthing Resistance of 1 Ω ,Soil Resistivity of ρ=40, Length=2.5 Meter, Diameter of Pipe = * 38 mm*.

**Here:**

L/d = 2.5/0.038=65.78 so **K = 0.75**

- The Earth Resistance of the Pipe electrodes R= K x ρ/L = 0.75×65.78 =
**12 Ω** - One electrode the earth resistance is
.**12 Ω** - To get Earth resistance of 1 Ω the total Number of electrodes required = (1.5×12)/1 =
**18 No**

## Calculating Resistance & Number of Earthing Rod

**Reference:** As per EHV Transmission Line Reference Book page: 290 and Electrical Transmission & Distribution Reference Book Westinghouse Electric Corporation, Section-I Page: 570-590.

**Earthing Resistance of Single Rods:**

R = ρx[ln (2L/a)-1]/(2×3.14xL)

**Earthing Resistance of Parallel Rods:**

R = ρx[ln (2L/A]/ (2×3.14xL)

**Where:**

* L* = length of rod in ground Meter,

*= radius of rod Meter*

**a***= ground resistivity, ohm-Meter*

**ρ***= √(axS)*

**A***= Rod separation Meter*

**S**## Factor affects on Ground resistance

The NEC code requires a minimum ground electrode length of 2.5 meters (8.0 feet) to be in contact with the soil. But, there are some factor that affect the ground resistance of a ground system:

**Length / Depth of the ground electrode**: double the length, reduce ground resistance by up to 40%.**Diameter of the ground electrode**: double the diameter, lower ground resistance by only 10%.**Number of ground electrodes**: for increased effectiveness, space additional electrodes at least equal to the depth of the ground electrodes.**Ground system design**: single ground rod to ground plate.

## The GI earthing conductor sizes for various equipment

No | Equipment | Earth strip size |

1 | HT switchgear, structures, cable trays & fence, rails, gate and steel column | 55 X 6 mm (GI) |

2 | Lighting Arrestor | 25 X 3 mm (Copper) |

3 | PLC Panel | 25 X 3 mm (Copper) |

4 | DG & Transformer Neutral | 50X6 mm (Copper) |

5 | Transformer Body | 50×6 mm (GI) |

6 | Control & Relay Panel | 25 X 6 mm (GI) |

7 | Lighting Panel & Local Panel | 25 X 6 mm (GI) |

8 | Distribution Board | 25 X 6 mm (GI) |

9 | Motor up to 5.5 kw | 4 mm2 (GI) |

10 | Motor 5.5 kw to 55 kw | 25 X 6 mm (GI) |

11 | Motor 22 kw to 55 kw | 40 X 6 mm (GI) |

12 | Motor Above 55 kw | 55 X 6 mm (GI) |

**Selection of Earthing System:**

Installations/Isc Capacity | IR Value Required | Soil Type/Resistivity | Earth System |

House hold earthing/3kA | 8 ohm | Normal Soil/ up to 50 ohm-meter | Single Electrode |

Sandy Soil/ between 50 to 2000 ohm-meter | Single Electrode | ||

Rocky Soil/ More than 2000 ohm-meter | Multiple Electrodes | ||

Commercial premises,Office / 5kA | 2 ohm | Normal Soil/ up to 50 ohm-meter | Single Electrode |

Sandy Soil/ between 50 to 2000 ohm-meter | Multiple Electrodes | ||

Rocky Soil/ More than 2000 ohm-meter | Multiple Electrodes | ||

Transformers, substation earthing, LT line equipment/ 15kA | less than 1 ohm | Normal Soil/ up to 50 ohm-meter | Single Electrode |

Sandy Soil/ between 50 to 2000 ohm-meter | Multiple Electrodes | ||

Rocky Soil/ More than 2000 ohm-meter | Multiple Electrodes | ||

LA, High current Equipment./ 50kA | less than 1 ohm | Normal Soil/ up to 50 ohm-meter | Single Electrode |

Sandy Soil/ between 50 to 2000 ohm-meter | Multiple Electrodes | ||

Rocky Soil/ More than 2000 ohm-meter | Multiple Electrodes | ||

PRS, UTS, RTUs, Data processing centre etc./5KA | less than 0.5 ohm | Normal Soil/ up to 50 ohm-meter | Single Electrode |

Sandy Soil/ between 50 to 2000 ohm-meter | Multiple Electrodes | ||

Rocky Soil/ More than 2000 ohm-meter | Multiple Electrodes |

## Size of Earthing Conductor

* Ref IS 3043 and Handbook on BS 7671:* The Lee Wiring Regulations by Trevor E. Marks.

Size of Earthing Conductor | ||

Area of Phase Conductor S (mm2) | Area of Earthing conductor (mm2) When It is Same Material as Phase Conductor | Area of Earthing conductor (mm2) When It is Not Same Material as Phase Conductor |

S < 16 mm2 | S | SX(k1/k2) |

16 mm2<S< 35 mm2 | 16 mm2 | 16X(k1/k2) |

S > 35 mm2 | S/2 | SX(k1/2k2) |

K1 is value of Phase conductor,k2 is value of earthing conductor | ||

Value of K for GI=80, Alu=126,Cu=205 for 1 Sec |

## Standard earthing strip/plate/pipe/wire weight

**GI Earthing Strip:**

Size (mm2) | Weight |

20 x 3 | 500 gm Per meter |

25 x 3 | 600 gm Per meter |

25 x 6 | 1/200 Kg Per meter |

32 x 6 | 1/600 Kg Per meter |

40 x 6 | 2 Kg Per meter |

50 x 6 | 2/400 Kg Per meter |

65 x 10 | 5/200 Kg Per meter |

75 x 12 | 7/200 Kg Per meter |

### GI Earthing Plate

Plate | Weight |

600 x 600 x 3 mm | 10 Kg App. |

600 x 600 x 4 mm | 12 Kg App. |

600 x 600 x 5 mm | 15 Kg App. |

600 x 600 x 6 mm | 18 Kg App. |

600 x 600 x 12 mm | 36 Kg App. |

1200 x 1200 x 6 mm | 70 Kg App. |

1200 x 1200 x 12 mm | 140 Kg App. |

### GI Earthing Pipe

Pipe | Weight |

3 meter Long BISE | 5 Kg App. |

3 meter r Long BISE | 9 Kg App. |

4.5 meter (15′ Long BISE) | 5 Kg App. |

4.5 meter (15′ Long BISE) | 9 Kg App. |

4.5 meter (15′ Long BISE) | 14 Kg App |

### GI Earthing Wire

Plate | Weight |

6 Swg | 5 meter in 1 Kg |

8 Swg | 9 meter in 1 Kg |