EXAMPLE:
Calculate Earthing Strip / Cable Size for Electrical Equipment’s / Panels in Power Distribution Networks.
- At RMU
- At Transformer
- At D.G Set
- At Main Distribution Panel
- At Sub Panel-1
- At Sub Panel-2
CALCULATION:
(1) Earthing Strip Size at RMU:
- Shot circuit capacity at RMU is 18.37 KA for 1 Second.
- Corrosion in Strip is 1% per year
- Earthing Strip shall be replaced after 25 Years.
- Safety Factor for Strip is 1.5
- Earthing Strip Material is GI
Calculation
- As per IS: 3043, clause 17.2.2.1:
- Cross section area of Earthing Strip (A) =(Isc x√t)/k
- Where Isc= Shot circuit current capacity in Ampere.
- t= Time for Shot circuit current in Second.
- K= Material Constant
| Bare Conductor Material with No Risk of Fire or Danger to any Other Touching or Surrounding Material | ||
| TABLE 11A (IS:3043) | ||
| Material | K value (1 second) | K value (2 second) |
| Steel | 80 | 46 |
| Aluminum | 126 | 73 |
| Copper | 205 | 118 |
- Cross section area of Earthing Strip (A) = (Isc x√t)/k
- Cross section area of Earthing Strip (A) = (18.37×1000 x√1)/80
- Cross section area of Earthing Strip (A) = 229.69 Sq.mm
- Allowable corrosion =1% per Year
- No of Year for replacement = 25 Year
- Allowable corrosion in 25 Years = 229.69x1x25% =57.40 Sq.mm
- Allowable Safety Factor = 229.69×1.5%=3.44 Sq.mm
- Required Earthing Strip size = Cross sectional area + Total Corrosion allowance + Safety factor
- Required Earthing Strip size=229.69+57.40+3.44 Sq.mm
- Required Earthing Strip size=290.47 Sq.mm
- Proposed GI Earthing Strip shall be 50×6 mm = 300 Sq.mm.
- Here Proposed Earthing Strip Size > Required Earthing Strip Size
- Proposed Earthing Strip is OK
(2) Earthing Strip Size at Transformer:
- Shot circuit capacity at Transformer is 25.32 KA for 1 Second.
- Corrosion in Strip is 1% per year
- Earthing Strip shall be replaced after 25 Years.
- Safety Factor for Strip is 1.5
- Earthing Strip Material for Transformer Neutral is Copper
- Earthing Strip Material for Transformer Body is GI
Calculation
- For Neutral
- As per IS: 3043, clause 17.2.2.1:
- Cross section area of Earthing Strip (A) =(Isc x√t)/k
- Where Isc= Shot circuit current capacity in Ampere.
- t= Time for Shot circuit current in Second.
- K= Material Constant
| Bare Conductor Material with No Risk of Fire or Danger to any Other Touching or Surrounding Material | ||
| TABLE 11A (IS:3043) | ||
| Material | K value (1 second) | K value (2 second) |
| Steel | 80 | 46 |
| Aluminum | 126 | 73 |
| Copper | 205 | 118 |
- Cross section area of Earthing Strip (A) = (Isc x√t)/k
- Cross section area of Earthing Strip (A) = (25.32×1000 x√1)/205
- Cross section area of Earthing Strip (A) = 125.51 Sq.mm
- Allowable corrosion =1% per Year
- No of Year for replacement = 25 Year
- Allowable corrosion in 25 Years = 125.51 x1x25% =30.87 Sq.mm
- Allowable Safety Factor = 125.51 x1.5%=1.85 Sq.mm
- Required Earthing Strip size = Cross sectional area + Total Corrosion allowance + Safety factor
- Required Earthing Strip size=125.51+30.87+1.85 Sq.mm
- Required Earthing Strip size=156.24 Sq.mm
- Proposed Cu Earthing Strip shall be 32×6 mm = 192 Sq.mm.
- Here Proposed Earthing Strip Size > Required Earthing Strip Size
- Proposed Earthing Strip is OK
- For Body
- As per IS: 3043, clause 17.2.2.1:
- Cross section area of Earthing Strip (A) =(Isc x√t)/k
- Where Isc= Shot circuit current capacity in Ampere.
- t= Time for Shot circuit current in Second.
- K= Material Constant
| Bare Conductor Material with No Risk of Fire or Danger to any Other Touching or Surrounding Material | ||
| TABLE 11A (IS:3043) | ||
| Material | K value (1 second) | K value (2 second) |
| Steel | 80 | 46 |
| Aluminum | 126 | 73 |
| Copper | 205 | 118 |
- Cross section area of Earthing Strip (A) = (Isc x√t)/k
- Cross section area of Earthing Strip (A) = (25.32 x1000 x√1)/80
- Cross section area of Earthing Strip (A) = 316.5 Sq.mm
- Allowable corrosion =1% per Year
- No of Year for replacement = 25 Year
- Allowable corrosion in 25 Years = 316.5 x1x25% =79.12 Sq.mm
- Allowable Safety Factor = 316.5 x1.5%=4.74 Sq.mm
- Required Earthing Strip size = Cross sectional area + Total Corrosion allowance + Safety factor
- Required Earthing Strip size=316.5+79.12+4.74 Sq.mm
- Required Earthing Strip size=400.37 Sq.mm
- Proposed GI Earthing Strip shall be 75×6 mm = 450 Sq.mm.
- Here Proposed Earthing Strip Size > Required Earthing Strip Size
- Proposed Earthing Strip is OK
(3) Earthing Cable Size at D.G Set:
- Shot circuit capacity at D.G Set is 10KA for 1 Second.
- Corrosion in Strip is 1% per year
- Earthing Strip shall be replaced after 25 Years.
- Safety Factor for Strip is 1.5
- Earthing Wire Material is Copper, XLPE Insulated
Calculation
- As per IS: 3043, clause 17.2.2.1:
- Cross section area of Earthing Strip (A) =(Isc x√t)/k
- Where Isc= Shot circuit current capacity in Ampere.
- t= Time for Shot circuit current in Second.
- K= Material Constant
| Insulated Protective Conductors Not Incorporated in Cables or Bare Conductors Touching Other Insulated Cables | ||
| TABLE 11B (IS:3043) | ||
| Material | K value (1 second) | K value (3 second) |
| Copper , PVC Insulated | 136 | 79 |
| Copper, Rubber Insulated | 150 | 92 |
| Copper, XLPE Insulated | 170 | 98 |
| Aluminum, PVC Insulated | 90 | 52 |
| Aluminum, Rubber Insulated | 106 | 61 |
| Aluminum, XLPE Insulated | 112 | 65 |
| Steel, PVC Insulated | 49 | 28 |
| Steel, Rubber Insulated | 58 | 33 |
| Steel, XLPE Insulated | 62 | 36 |
- Cross section area of Earthing Strip (A) = (Isc x√t)/k
- Cross section area of Earthing Strip (A) = (10×1000 x√1)/170
- Cross section area of Earthing Strip (A) = 58.82 Sq.mm
- Allowable corrosion =1% per Year
- No of Year for replacement = 25 Year
- Allowable corrosion in 25 Years = 58.82x1x25% =14.70 Sq.mm
- Allowable Safety Factor = 58.82 x1.5%=0.88 Sq.mm
- Required Earthing Strip size = Cross sectional area + Total Corrosion allowance + Safety factor
- Required Earthing Strip size=58.82+14.7+0.88 Sq.mm
- Required Earthing Strip size=74.41 Sq.mm
- Proposed Earthing Single Core Copper, XLPE Cable=95 Sq.mm
- Here Proposed Earthing Cable Size > Required Earthing Cable Size
- Proposed Earthing Cable is OK
(4) Earthing Strip Size at Main Panel :
- Shot circuit capacity at Main Panel is 21 KA for 1 Second.
- Corrosion in Strip is 1% per year
- Earthing Strip shall be replaced after 25 Years.
- Safety Factor for Strip is 1.5
- Earthing Strip Material is GI
Calculation
- As per IS: 3043, clause 17.2.2.1:
- Cross section area of Earthing Strip (A) =(Isc x√t)/k
- Where Isc= Shot circuit current capacity in Ampere.
- t= Time for Shot circuit current in Second.
- K= Material Constant
| Bare Conductor Material with No Risk of Fire or Danger to any Other Touching or Surrounding Material | ||
| TABLE 11A (IS:3043) | ||
| Material | K value (1 second) | K value (2 second) |
| Steel | 80 | 46 |
| Aluminum | 126 | 73 |
| Copper | 205 | 118 |
- Cross section area of Earthing Strip (A) = (Isc x√t)/k
- Cross section area of Earthing Strip (A) = (21 x1000 x√1)/80
- Cross section area of Earthing Strip (A) = 262.5 Sq.mm
- Allowable corrosion =1% per Year
- No of Year for replacement = 25 Year
- Allowable corrosion in 25 Years = 262.5 x1x25% =65.62 Sq.mm
- Allowable Safety Factor = 262.5 x1.5%=3.93 Sq.mm
- Required Earthing Strip size = Cross sectional area + Total Corrosion allowance + Safety factor
- Required Earthing Strip size=262.5+65.62+3.93 Sq.mm
- Required Earthing Strip size=332.06 Sq.mm
- Proposed GI Earthing Strip shall be 75×6 mm = 450 Sq.mm.
- Here Proposed Earthing Strip Size > Required Earthing Strip Size
- Proposed Earthing Strip is OK
(5) Earthing Strip Size at Sub Panel-1 :
- Shot circuit capacity at Sub Panel-1 is 11 KA for 1 Second.
- Corrosion in Strip is 1% per year
- Earthing Strip shall be replaced after 25 Years.
- Safety Factor for Strip is 1.5
- Earthing Strip Material is GI
Calculation
- As per IS: 3043, clause 17.2.2.1:
- Cross section area of Earthing Strip (A) =(Isc x√t)/k
- Where Isc= Shot circuit current capacity in Ampere.
- t= Time for Shot circuit current in Second.
- K= Material Constant
| Bare Conductor Material with No Risk of Fire or Danger to any Other Touching or Surrounding Material | ||
| TABLE 11A (IS:3043) | ||
| Material | K value (1 second) | K value (2 second) |
| Steel | 80 | 46 |
| Aluminum | 126 | 73 |
| Copper | 205 | 118 |
- Cross section area of Earthing Strip (A) = (Isc x√t)/k
- Cross section area of Earthing Strip (A) = (11 x1000 x√1)/80
- Cross section area of Earthing Strip (A) = 137.5 Sq.mm
- Allowable corrosion =1% per Year
- No of Year for replacement = 25 Year
- Allowable corrosion in 25 Years = 137.5 x1x25% =34.37 Sq.mm
- Allowable Safety Factor = 137.5 x1.5%=2.06 Sq.mm
- Required Earthing Strip size = Cross sectional area + Total Corrosion allowance + Safety factor
- Required Earthing Strip size=137.5+34.37+2.06 Sq.mm
- Required Earthing Strip size=173.93 Sq.mm
- Proposed GI Earthing Strip shall be 32×6 mm = 192 Sq.mm.
- Here Proposed Earthing Strip Size > Required Earthing Strip Size
- Proposed Earthing Strip is OK
(5) Earthing Strip Size at Sub Panel-2 :
- Shot circuit capacity at Sub Panel-2 is 6 KA for 1 Second.
- Corrosion in Strip is 1% per year
- Earthing Strip shall be replaced after 25 Years.
- Safety Factor for Strip is 1.5
- Earthing Strip Material is GI
Calculation
- As per IS: 3043, clause 17.2.2.1:
- Cross section area of Earthing Strip (A) =(Isc x√t)/k
- Where Isc= Shot circuit current capacity in Ampere.
- t= Time for Shot circuit current in Second.
- K= Material Constant
| Bare Conductor Material with No Risk of Fire or Danger to any Other Touching or Surrounding Material | ||
| TABLE 11A (IS:3043) | ||
| Material | K value (1 second) | K value (2 second) |
| Steel | 80 | 46 |
| Aluminum | 126 | 73 |
| Copper | 205 | 118 |
- Cross section area of Earthing Strip (A) = (Isc x√t)/k
- Cross section area of Earthing Strip (A) = (6 x1000 x√1)/80
- Cross section area of Earthing Strip (A) = 75.0 Sq.mm
- Allowable corrosion =1% per Year
- No of Year for replacement = 25 Year
- Allowable corrosion in 25 Years = 75 x1x25% =18.75 Sq.mm
- Allowable Safety Factor = 75 x1.5%=1.12 Sq.mm
- Required Earthing Strip size = Cross sectional area + Total Corrosion allowance + Safety factor
- Required Earthing Strip size=75+18.75+1.12 Sq.mm
- Required Earthing Strip size=94.87 Sq.mm
- Proposed GI Earthing Strip shall be 25×6 mm = 150 Sq.mm.
- Here Proposed Earthing Strip Size > Required Earthing Strip Size
- Proposed Earthing Strip is OK
CONCLUSION:
- At RMU=GI Earthing Strip: 50x6MM
- At Transformer Neutral=CU Earthing Strip: 32x6MM
- At Transformer Body=GI Earthing Strip: 75x6MM
- At D.G Set=CU, XLPE Earthing Cable: 1Cx95 SQ.MM
- At Main Distribution Panel=GI Earthing Strip: 75x6MM
- At Sub Panel=1=GI Earthing Strip: 32x6MM
- At Sub Panel-2=GI Earthing Strip: 25x6MM.
