POWER SYSTEM GROUNDING Presented By H JAYAKUMAR WHY

POWER SYSTEM GROUNDING Presented By H. JAYAKUMAR

WHY GROUNDING IS REQUIRED? GROUNDING PLAYS A VITAL ROLE IN POWER SYSTEM OPERATION. EFFECTIVE GROUNDING IS NECESSARY FOR THE PROPERATION AND CO-ORDINATION OF PROTECTION SCHEMES IN THE POWER SYSTEM AS WELL AS FOR PERSONNEL SAFETY.

PURPOSE OF EARTHING: • PROTECTION OF INSTALLATION. • IMPROVEMENT IN QUALITY OF SERVICE. • SAFTY OF PERSONNEL.

GENERAL REQUIREMENTS: • LOW VALUE OF EARTH RESISTANCE. • ACCEPTABLE SURFACE POTENTIAL GRADIENTS.

RULES/GUIDELINES: • INDIAN ELECTRICITY RULES 1956. • IS: 3043/1966, 1987 : CODE OF PRACTICE FOR EARTHING. • BS : CP 1013: 1965 • CBI & P RECOMMENDATIONS • CIGRE APPLICATION GUIDE • ANSI / IEEE STD. 80/ 1961, 1967, 1986.

TYPES OF EARTHING: • SYSTEM EARTHING. • PROTECTIVE EARTHING.

SYSTEM EARTHING EFFECTIVELY EARTHED SYSTEM • • (Xo / X 1) < 3 (Ro / X 1) < 1 Xo= ZERO SEQUENCE REACTANCE. X 1= POSITIVE SEQUENCE REACTANCE. Ro= ZERO SEQUENCE RESISTANCE. UNDER THE FAULT CONDITIONS THE VOLTAGES OF THE OTHER TWO HEALTHY PHASES WILL NEVER BE MORE THAN 80% OF THE LINE TO LINE VOLTAGE OF THE SYSTEM. THIS IS ALSO CALLED CO-EFFICIENT OF EARTHING.

OBJECTIVE OF SOIL RESISTIVITY MEASUREMENTS: • Estimating the ground resistance of a proposed sub-station or transmission tower. • Estimating potential gradients including step and touch voltages. • Computing the inductive coupling between neighbouring power and communication circuits. • Designing cathodic protection systems. • Geographical surveying.

FACTORS AFFECTING THE SOIL RESISTIVITY: • • Type of the soil. Moisture. Dissolved salt in water. Temperature. Grain size and its distribution. Seasonal variation. Artificial treatment.

EQUIPMENT USED FOR MEASUREMENT OF SOIL RESISTIVITY: • EARTH RESISTANCE METER(EARTH TESTER) Confirming to IS 9223: 1979 (Frequency shall be 60 to 90 Hz and voltage 30 to 250 V). • METROVATT WEST GERMANY. • EVERSHED & VIGNOL [EMI THORN OR MEGGER] UK.

EQUIPMENT USED FOR MEASUREMENT OF SOIL RESISTIVITY:

METHOD OF MEASURING SOIL RESISTIVITY • • Geological information and soil samples Variation of depth method Two point method Four point method - Equally spaced Wenner arrangement - Unequally spaced or Schlumberger – Palmer arrangement

WENNER FOUR ELECTRODE METHOD:

ρ= 4πa. R /[1+(2 a/√a 2+4 b 2)-(a/√a 2+b 2)] • • ρ= Soil resistivity in ohm-meter. a= spacing between adjacent electrode-meter. R= earth tester reading-ohms. B= depth of burial of electrode-meters. When ‘a’ is large compared to ‘b’ the above formula be reduced to ρ = 2πa. R. ALTERNATIVE ELECTRODE CONNECTION: ELECTRODE RESISTIVITY ARRANGEMENT FORMULA • CPPC ρ = 2πa. R 1 PCCP • CCPP ρ = 6πa. R 2 PPCC • CPCP ρ = 3πa. R 3 PCPC

The soil is said to be homogenous when the average value of ‘ρ’ lies within 30 percent • CASE STUDY: SL. NO a Meters 1 2 2 3 3 4 4 5 5 6 6 7 7 8 R Ω 4. 33 2. 60 2. 35 1. 986 1. 652 1. 462 1. 305 ρ Ohm-Meters 54. 412 49. 009 min 59. 062 62. 392 62. 279 64. 301 65. 593 max ρ Average 59. 579 130% of ρ Ave = 1. 3 x 59. 579 = 77. 453 70% of ρ Ave = 0. 7 x 59. 579 = 41. 705 Minimum & Maximum value lies within 41. 705 and 77. 453 The Soil is Homogeneous.

SURVEY OF SOIL RESISTIVITY MEASUREMENT: CASE STUDY: • SOIL RESISTIVITY MEASUREMENT: • Site of 120 MW Diesel Power Plant at Yelahanka, Bangalore. Location Yelahanka Site Size 92. 12 m x 73. 15 m Date of Measurement 23. 01. 91 Type of soil Agriculture Land Condition of Ground Dry Earth tester used ET 3/2 A 1 D 1 B 1 D 2 C 1 B 2 C 2 A 2

Inner- Electrode Spacing Megger Reading in ohms (Soil Resistivity = 2πa. R in ohm-m)* 2. 5 m 5 m 7. 5 10 m 15 m Location A 1 -A 2 2. 0 Ω (31. 42) 1. 03 Ω (32. 36) 0. 94 Ω (44. 30) 0. 72 Ω (45. 32) 0. 52 Ω (49. 02) Location B 1 -B 2 1. 74 Ω (27. 33) 0. 94 Ω (29. 53) 0. 83 Ω (39. 11) 0. 62 Ω (38. 96) 0. 52 Ω (49. 02) Location C 1 -C 2 1. 74 Ω (27. 33) 1. 03 Ω (32. 36) 0. 61 Ω (28. 74) 0. 61 Ω (38. 33) 0. 51 Ω (48. 07) Location D 1 -D 2 2. 45 Ω (38. 48) 1. 06 Ω (33. 30) 0. 83 Ω (39. 11) 0. 63 Ω (39. 58) 0. 51 Ω (48. 07) Average of the above Minimum Resistivity Encountered Maximum Resistivity Encountered 37. 99 ohm-meters 27. 33 ohm-meters 49. 02 ohm-meters Conclusion: since the maximum and minimum resistivity lies within 30% of the average value the soil is a homogeneous one

LIMITATIONS OF WENNERS METHOD: In the case of homogeneous soil for 8 times the spacing, the megger reading shall be one-eighth of R (megger reading), which may be beyond the lower limits of the megger reading. In order to overcome the above Schlumberger- palmer method is used.

UNEQUALLED SPACED OR SCHLUMBERGER – PALMER ARRANGEMENT:

ρ = [πc (c + d) x R] / d SL NO 1 2 c Meter 2 4 d Meter 20 16 R ρ Ω Ohm-Meters 7. 76 53. 633 4. 20 65. 973 ρAvg 59. 803 When c=d, The above formula will reduced to 2πa. R

OBJECTIVE OF GROUND RESISTANCE MEASUREMENTS: • Verify the adequacy of new grounding system. • Detect changes in an existing grounding system. • Determine hazardous step and touch voltages. • Determine ground potential rise (GPR) in order to design protection for power and communication circuits.

METHODS OF EARTH RESISTANCE MEASUREMENT: • • Fall-of-potential method. E. B. Curdts 61. 8% method. Slope method. IS: 3043 alternative method.

FALL-OF-POTENTIAL METHOD: Thumb Rule: Remote current electrode shall be at a minimum distance of 10 times the depth of the burial of the earth electrode being tested or 10 times the diagonal distance in case of earth mat.

Case Study of Fall of Potential Method REMOTE ELECTRODE(C 2)DISTANCE=90 M P 2 • Megger P 2 Megger Meters Reading Ohms 05 14. 2 45 23. 5 10 16. 7 50 23. 9 15 19. 0 55 24. 1 20 20. 2 60 24. 8 25 21. 0 65 26. 2 30 22. 0 70 29. 8 35 22. 7 75 38. 6 40 23. 1 80 60. 7 Maintenance purpose annually one reading may be taken with remote electrode at 90 m & potential electrode 45 m from the station ground to verify the value of 23. 5Ω (Previous year result).

E. B. CURDTS 61. 8% METHOD This method is applicable to single ground electrode. Potential electrode P 2 is placed at 61. 8% of remote current electrode C 2 from the station ground.

CASE STUDY: C 2 Metres 40 m 60 m 80 m 120 m P 2 Rg Metres Ω 24. 72 18. 5 37. 08 18. 7 Max 49. 44 18. 6 74. 16 18. 3 Min Average Rg 18. 525 105% of Rg average = 19. 45Ω, 95% of Rg average = 17. 60Ω. All values lies within ± 5% of Rg Average. This method is acceptable.

THE SLOPE METHOD OF TEST Was established by Dr. G. F. Tagg. The following is the summary of the paper published in IEE 1970. (Vol. No. 177, No. 11) This technique shall be used when testing earth electrode systems which covers a large area. This method is useful when the position of the centre of the earthing system is either unknown or inaccessible(e. g. if the system is beneath the floor of a building). This method yields results of greater accuracy than those detailed above. The procedure is as follows: a) The terminals C 1 & P 1 on the instruments are connected to the earth electrode. b) Connect terminal C 2 to a current electrode inserted in the ground 50 m more or away. The distance from the earth electrode to the current electrode is EC. c) The potential electrode connected to terminal P 2, is inserted at several positions between the earth and current electrodes, starting from near the earth electrode. (The electrodes must be in a straight line). At each position the resistance is measured and the earth resistance curve is plotted from the results e. g. , (as shown in fig) atleast 6 readings are needed. Drawing the curve will show up any incorrect points which may be either rechecked or ignored.

the curve the equivalent reading to potential electrode position 0. 2 EC, 0. 4 EC & 0. 6 EC can be found. These becomes R 1, R 2 & R 3 respectively. e) Calculate the slope co-efficient µ. d ) From Where µ=R 3 -R 2 R 2 -R 1 Which is the measure of the change of slope of the earth resistance curve. From the table shown in the next page, obtain the value of PT /EC for this value of µ. PT is the distance to the potential electrode at the position where the true resistance would be measured. Multiply the value of PT /EC by EC to obtain the distance P 2. From the curve, again read off the value of resistance that correspond to this value of PT. The value obtained is earth system resistance. It is important to note that: a) If the value of µ obtained is not covered in the table, then the current electrode will have to be moved further away from the earthing system. b) If it is required, further sets of test results can be obtained with different values of EC, or different directions of the line of EC. From the results obtained of resistance for various values of the distance EC a curve may be plotted.

This shows how the resistance is decreasing asymptotically as the distance chosen for EC is increased. The curve indicated that the distances chosen for EC in tests(1) and (2) were not large enough; and that those chosen in tests(3) and (4) were preferable because they would give the more correct value of the earth resistance. (c) It is unreasonable to expect an accuracy of readings of more than 5%, 10% is often adequate bearing in mind that this sort of variation could easily occur with varying soil moisture conditions or non-homogeneous soils. Chart for use with slope method is in Annexure II.

ANNEXURE –II Chart for use with slope method 3 4 5 6 µ 0 1 2 7 8 9 0. 40 0. 41 0. 42 0. 43 0. 44 0. 45 0. 46 0. 47 0. 48 0. 49 0. 6432 0. 6418 0. 6403 0. 6389 0. 6374 0. 6360 0. 6346 0. 6331 0. 6317 0. 6302 6431 6416 6402 6387 6373 6359 6344 6330 6315 6301 6429 6415 6400 6386 6372 6357 6343 6328 6314 6300 6428 6413 6399 6384 6370 6358 6342 6327 6312 6298 6426 6412 6397 6383 6369 6354 6340 6325 6311 6297 6425 6410 6396 6382 6367 6353 6338 6324 6310 6295 6423 6409 6395 6380 6366 6351 6337 6323 6308 6294 6422 6408 6393 6379 6364 6350 6336 6321 6307 6292 6420 6406 6392 6377 6363 6348 6334 6320 6305 6291 6419 6405 6390 6376 6361 6347 6333 6318 6304 6289 0. 50 0. 51 0. 52 0. 53 0. 54 0. 55 0. 56 0. 57 0. 58 0. 59 0. 6288 0. 6273 0. 6258 0. 6242 0. 6227 0. 6212 0. 6197 0. 6182 0. 6166 0. 6151 6286 6271 6256 6241 6226 6210 6195 6180 6165 6150 6258 6210 6255 6239 6224 6209 6194 6179 6163 6148 6283 6268 6253 6238 6223 6207 6192 6177 6162 6147 6282 6267 6252 6236 6221 6208 6191 6176 6160 6145 6280 6265 6252 6235 6220 6204 6189 6174 6159 6144 6279 6264 6248 6233 6218 6203 6188 6172 6157 6142 6277 6262 6247 6232 6217 6201 6186 6171 6156 6141 6276 6261 6245 6230 6215 6200 6185 6169 6154 6139 6274 6259 6244 6229 6214 6198 6183 6168 6153 6138 0. 60 0. 61 0. 62 0. 63 0. 64 0. 65 0. 66 0. 67 0. 68 0. 69 0. 70 0. 71 0. 6136 0. 6120 0. 6104 0. 6087 0. 6071 0. 6055 0. 6039 0. 6023 0. 6006 0. 5990 0. 5974 0. 5957 6134 6118 6102 6086 6070 6053 6037 6021 6005 5989 5973 5955 6133 6117 6100 6084 6068 6052 6036 6019 6003 5987 5971 5953 6131 6115 6099 6083 6066 6050 6034 6018 6002 5985 5969 5952 6130 6113 6097 6081 6065 6049 6032 6016 6000 5984 5967 5950 6128 6112 6096 6079 6063 6047 6031 6015 5998 5982 5964 5948 6126 6110 6094 6076 6061 6045 6029 6013 5997 5980 5964 5947 6125 6108 6092 6076 6060 6044 6027 6011 5995 5979 5962 5945 6123 6107 6091 6074 6058 6042 6026 6010 5993 5977 5960 5943 6121 6105 6089 6073 6057 6040 6024 6008 5992 5976 5959 5942

µ 0 1 2 3 4 5 6 7 8 9 0. 73 0. 74 0. 75 0. 76 0. 77 0. 78 0. 79 0. 5923 0. 5906 0. 5889 0. 5871 0. 5854 0. 5837 0. 5820 5921 5904 5887 5870 5853 5835 5818 5920 5902 5885 5868 5851 5834 5817 5918 5900 5883 5866 5849 5832 5815 5916 5899 5882 5865 5847 5830 5813 5914 5897 5880 5863 5846 5829 5812 5912 5895 5878 5861 5844 5827 5810 5911 5894 5877 5859 5842 5825 5808 5909 5892 5875 5858 5841 5824 5806 5907 5890 5873 5858 5839 5822 5805 0. 80 0. 81 0. 82 0. 83 0. 84 0. 85 0. 86 0. 87 0. 88 0. 89 0. 5803 0. 5785 0. 5766 0. 5748 0. 5729 0. 5711 0. 5692 0. 5674 0. 5655 0. 5637 5801 5783 5764 5746 5727 5709 5690 5672 5653 5635 5799 5781 5762 5744 5725 5707 5688 5670 5851 5633 5797 5779 5760 5742 5723 5705 5686 5668 5650 5631 5796 5777 5759 5740 5722 5703 5685 5666 5648 5629 5794 5775 5757 5738 5720 5701 5683 5664 5646 5627 5792 5773 5755 5736 5718 5699 5681 5662 5644 5625 5790 5772 5753 5735 5716 5698 5679 5661 5642 5624 5788 5770 5751 5733 5714 5696 5677 5659 5640 5622 5786 5768 5749 5731 5712 5694 5675 5657 5638 5820 0. 91 0. 92 0. 93 0. 94 0. 95 0. 96 0. 97 0. 98 0. 99 0. 5618 0. 5598 0. 5578 0. 5557 0. 5537 0. 5517 0. 5497 0. 5477 0. 5456 0. 5436 5616 5596 5578 5555 5535 5515 5495 5475 5454 5434 5614 5594 5574 5553 5533 5513 5493 5473 5452 5432 5612 5592 5572 5551 5531 5511 5491 5471 5450 5430 5610 5590 5570 5549 5529 5509 5489 5469 5448 5428 5608 5588 5547 5527 5507 5487 5467 5446 5426 5606 5586 5565 5545 5525 5505 5485 5464 5444 5424 5604 5584 5563 5543 5523 5503 5483 5462 5442 5422 5602 5582 5561 5541 5521 5501 5481 5460 5440 5420 5600 5580 5559 5539 5519 5499 5479 5458 5438 5418

µ 0 1 2 3 4 5 6 7 8 9 1. 30 1. 31 1. 32 1. 33 1. 34 1. 35 1. 36 1. 37 1. 38 1. 39 0. 4652 0. 4618 0. 4583 0. 4549 0. 4515 0. 4481 0. 4446 0. 4412 0. 4378 0. 4343 4649 4614 4580 4546 4511 4477 4443 4408 4374 4340 4645 4611 4577 4542 4508 4474 4439 4405 4371 4336 4642 4607 4573 4539 4505 4470 4436 4402 4367 4333 4638 4604 4570 4535 4501 4467 4432 4398 4364 4330 4635 4601 4566 4532 4498 4463 4429 4395 4360 4326 4631 4597 4563 4529 4494 4460 4426 4391 4357 4323 4628 4594 4559 4525 4491 4457 4422 4388 4354 4319 4625 4590 4556 4522 4487 4453 4419 4384 4350 4316 4621 4586 4553 4518 4484 4450 4415 4381 4347 4312 1. 40 1. 41 1. 42 1. 43 1. 44 1. 45 1. 46 1. 47 1. 48 1. 49 0. 4309 0. 4267 0. 4225 0. 4183 0. 4141 0. 4099 0. 4056 0. 4014 0. 3972 0. 3930 4305 4263 4221 4178 4136 4094 4052 4010 3968 3926 4301 4258 4216 4174 4132 4090 4048 4005 3964 3921 4296 4254 4212 4170 4128 4086 4044 4001 3959 3917 4292 4250 4208 4166 4124 4082 4040 3997 3955 3913 4288 4246 4204 4162 4120 4077 4035 3993 3951 3909 4284 4242 4200 4157 4115 4073 4031 3989 3947 3905 4280 4237 4195 4153 4111 4069 4027 3985 3943 3900 4275 4233 4191 4149 4107 4065 4023 3980 3938 3896 4271 4229 4187 4145 4103 4061 4018 3976 3934 3892 1. 50 1. 51 1. 52 1. 53 1. 54 1. 55 1. 56 1. 57 1. 58 1. 59 0. 3888 0. 3840 0. 3791 0. 3740 0. 3688 0. 3635 0. 3580 0. 3523 0. 3465 0. 3405 3883 3835 3786 3735 3683 3630 3574 3517 3459 3399 3878 3830 3781 3730 3677 3624 3569 3511 3453 3393 3874 3825 3778 3724 3672 3619 3563 3506 3447 3386 3869 3820 3771 3719 3667 3613 3557 3500 3441 3380 3864 3818 3766 3714 3662 3608 3552 3494 3435 3374 3859 3811 3760 3709 3656 3602 3546 3488 3429 3368 3854 3808 3755 3704 3651 3597 3540 3482 3423 3362 3850 3801 3750 3698 3646 3591 3534 3477 3417 3355 3845 3796 3745 3693 3640 3586 3528 3471 3411 3349

µ 0 1 2 3 4 5 6 7 8 9 1. 00 1. 01 1. 02 1. 03 1. 04 1. 05 1. 06 1. 07 1. 08 1. 09 0. 5416 0. 5394 0. 5371 0. 5349 0. 5327 0. 5305 0. 5282 0. 5260 0. 5238 0. 5215 5414 5391 5369 5347 5325 5303 5280 5258 5235 5213 5412 5389 5367 5345 5322 5300 5278 5255 5233 5211 5409 5387 5365 5344 5320 5298 5276 5253 5231 5209 5407 5385 5362 5340 5318 5296 5273 5251 5229 5206 5405 5383 5360 5338 5316 5293 5271 5249 5229 5204 5403 5380 5358 5336 5313 5291 5269 5247 5224 5202 5400 5378 5356 5333 5311 5289 5267 5244 5222 5200 5398 5376 5354 5331 5309 5287 5264 5242 5219 5297 5396 5374 5351 5329 5307 5284 5262 5240 5217 5295 1. 10 1. 11 1. 12 1. 13 1. 14 1. 15 1. 16 1. 17 1. 18 1. 19 0. 5193 0. 5168 0. 5143 0. 5118 0. 5093 0. 5068 0. 5042 0. 5017 0. 4992 0. 4967 5190 5165 5140 5115 5090 5065 5040 5015 4990 4965 5188 5163 5137 5113 5088 5062 5037 5012 4987 4962 5185 5160 5135 5110 5085 5060 5035 5010 4985 4960 5183 5158 5132 5108 5083 5057 5032 5007 4982 4957 5180 5155 5130 5105 5080 5055 5030 5005 4980 4955 5178 5153 5127 5103 5078 5052 5027 5002 4977 4952 5175 5150 5125 5100 5075 5050 5025 5000 4975 4950 5173 5148 5122 5098 5073 5047 5022 4997 4972 4947 5170 5145 5120 5095 5070 5045 5020 4995 4970 4945 1. 20 1. 21 1. 22 1. 23 1. 24 1. 25 1. 26 1. 27 1. 28 1. 29 0. 4942 0. 4913 0. 4884 0. 4855 0. 4826 0. 4797 0. 4768 0. 4739 0. 4710 0. 4681 4939 4910 4881 4852 4823 4794 4765 4736 4707 4678 4936 4907 4878 4849 4820 4791 4762 4733 4704 4675 4933 4904 4875 4846 4817 4788 4759 4730 4701 4672 4930 4901 4872 4843 4814 4785 4756 4727 4698 4669 4928 4899 4870 4841 4812 4783 4754 4725 4696 4667 4925 4896 4867 4838 4809 4780 4751 4722 4693 4664 4922 4893 4864 4835 4808 4777 4748 4719 4690 4661 4919 4890 4861 4832 4803 4774 4745 4718 4687 4658 4916 4887 4858 4829 4800 4771 4742 4713 4684 4655

Earth resistance curve Resistance Arbitrary Position of E electrode 0. 2 EC 0. 4 EC 0. 6 EC Position of C electrode Position of P electrode measured from E.

Field Measurement Area of the 11 k. V S/S is 6. 25 Mx 17. 5 M and the diagonal distance is 18. 58 M. • Measurement No. 1 • Current Electrode at a distance of 49 M from the Earth electrode (CE) • • • Megger reading: 0. 2 CE(09. 8 M) = 0. 402Ω, R 1 0. 4 CE(19. 6 M) = 0. 534Ω, R 2 0. 6 CE(29. 4 M) = 0. 884Ω, R 3 • µ = R 3 - R 2 = • R 2 - R 1 0. 884 -0. 534 =2. 65 0. 534 -0. 402 • From the chart (Annexure 1) µ obtained (2. 65) is not covered (max 1. 69)in the table, the current electrode will have to be moved further away from the earthing system.

• Measurement No. 2 • Current Electrode at a distance of 80 M from the Earth electrode (CE) • Megger reading: 0. 2 CE(16. 0 M)=0. 273Ω, R 1 • 0. 4 CE(32. 0 M)= 0. 354Ω, R 2 • 0. 6 CE(48. 0 M)=0. 541Ω, R 3 • µ = R 3 -R 2 = 0. 541 -0. 354 • R 2 - R 1 0. 354 -0. 273 =2. 31 • From the chart (Annexure 1) µ obtained (2. 31) is not covered (max 1. 69)in the table, the current electrode will have to be moved further away from the earthing system.

IS : 3043 ALTERNATIVE METHOD:

CASE STUDY: • Two suitable direction at 90 degree apart at one corner of the fence are first selected. The potential electrode and current electrode are placed in these direction 250 to 300 metres away from the fence at the same distance. A reading is taken under these conditions. The current electrode is then moved in 30 m. Steps until the same readings are obtained for three consecutive locations. This procedure is termed as locating the remote current electrode distance. SL Spacing in Metres Megger Reading NO P 2 C 2 Ω 1 270 0. 026 2 270 300 0. 039 3 270 330 0. 039 4 270 360 0. 039

• The current electrode is then left in the last foregoing position and the potential electrode is moved out in 30 m. Step until three consecutive reading are obtained without a change in value. The last reading then corresponds to the true value of earth resistance. SL NO 1 2 3 4 Spacing in Metres P 2 C 2 270 360 300 360 330 360 360 Megger Reading Ω 0. 039 0. 040 Resistance of the Grounding System = 0. 040Ω i. e. 40 milliohms.

GROUND RESISTANCE ROD AND PIPE ELECTRODE: • Depth of Buriel as per IS: 3043 1966 & 1987 is minimum 2. 75 meters • Rg = (100ρ / 2πL) loge(4 L / d) OHMS WHERE, Rg = GROUND RESISTANCE ρ = SOIL RESISTIVITY IN OHM-M L = LENGTH IN CM d = DIAMETER OF ROD OR PIPE. Ex: - ρ = 100Ω-m d = 4 cm L = 250 cm Then, Rg = 35. 15 ohms When d is 2. 5 times Rg = 29. 31 ohms.

GROUND RESISTANCE OF PLATE ELECTRODE: • • • Rg = (ρ/A) x (√ π/A) ρ= Soil resistivity in ohm-meter. A= Area of both sides of Plate in metre Sq. Size of plate shall be min. 600 mm x 600 mm, max. 1200 mmx 1200 mm depth min. 1. 5 m as per IS: 3043 1966 & 1987 The minimum thickness shall be: a) Cast iron 12 mm b) Galvanized iron 6. 3 mm c) Copper 3. 15 mm

GROUND RESISTANCE OF GRID (EARTH MAT) : • Depth of burial 0. 5 to 1 meter • LAURIENT AND NIEMANN • Rg = ρ/4 r + ρ/L = ρ/4 (√ π/A)+ ρ/L WHERE, Rg = GROUND RESISTANCE ρ = Average ground Resistivity in OHM-M r = Radius of a circle having the same area as that occupied by the Grid L = Total buried length of buried conductors (Both horizontal and Vertical) in meters A = Area of Grid (Earth Mat) • The fault current carrying capacity of the conductor is designed for one minute since the primary breakers are fast acting and trips in 0. 25 secs and the back up protection trips in 0. 5 secs.

GROUND RESISTANCE VALUE • Large stations : 0. 5 Ohm • Major stations : 1. 0 Ohm • Other stations : 2. 0 Ohm CORROSION • The average loss in weight of specimen 150 mmx 125 mmx 3 mm buried for 12 years in no case exceeds : • Copper : 0. 2 % per year • GI : 0. 5 % per year • MS : 2. 2 % per year

REQUIREMENT OF AN EMBEDDING MATERIAL • It should have high electrical conductivity which should be constant, unaffected by changes in temperature & moisture; • It should permanently remain once embedded and should not be either dissolved in or swept away by water; • It should have high swelling property to absorb water and retain the same over long periods of time; • It should not cause or accelerate the corrosion of the ground electrode material, such as steel; • It should be easily applicable; • It should not cost much in relation to the total cost of grounding installation.

BENTONITE • One of the most suitable substances for chemical treatment of soils which fulfills most of the above requirements is a clay known as bentonite. Bentonite Calcium based Sodium based

BENTONITE Contd. . • Bentonite contains Na o (Soda), K o (Potash), 2 2 Cao (Lime), Mgo (Magnesia) & other mineral salt that ionize forming a strong electrolyte with : a) p. H : 8 -10 b) ρ : 2. 5 Ohm-m at 300 % moisture c) Swell index by volume : ≥ 8 d) Quantity required for Pipe electrode as per IS: 3043 -1987 (2. 75 m long 100 mm Id 13 mm thick) : 45 Kg

HAZARDS DEPEND ON: • Frequency • Magnitude • Duration Frequency: • Humans vulnerable to the effects of ac at 5060 Hz • Slightly larger currents at low frequencies and DC • Higher currents at high freq: (3000 -10, 000 Hz) • Hundreds of amperes for lightning surges Contd. ,

Magnitude: Most common physiological effects in the order of increasing magnitude: • • • Perception (1 m. A) Let-go currents (1 -6 m. A) Muscular contraction (9 -25 m. A) Ventricular fibrillation (100 m. A) Burning

SAFE BODY CURRENTS: • Dalziel’s & LEE and recommended as per AIEE 80/1963 I = 0. 165/√ t Amps k IEEE -80/1976 I k = 0. 116/√ t Amps IEEE -80/1986 I = 0. 116/√ t Amps k 50 I = 0. 157/√ t Amps k 70 CBI&P (India): Ik= 0. 155/√ t Amps Where, t=Duration of the shock 8 ms to 3 secs

POTENTIAL GRADIENTS STEP VOLTAGE • The potential difference between two points on the earths surface, separated by a distance of one pace, that will be assumed to be one meter, in the direction of maximum potential gradient. TOUCH VOLTAGE • The potential difference between a grounded metallic structure and a point on the earths surface separated by a distance equal to the normal maximum horizontal reach, approximately one meter.

G. P. R FOR EHV/UHV STATIONS IN INDIA Limits of G. P. R is not prescribed in India. Following recommendations made by CIGRE: The G. P. R should be limited to A value such that • Touch voltages at places outside the fence and frequently visited by public should not be more than 650 volts • The transferred voltage between sheaths and the conductor of cables should be limited to 60% of 1 minute power frequency test voltage of the cable. • Transferred voltage between the overhead ground wire and a conductor of an overhead line should be 60% of 1 minute power frequency test voltage of line arrestors. Contd. ,

TRANSFER POTENTIAL SYSTEM VOLTAGE FROM 110 TO 400 k. V High Reliability Station Tripping time 0. 2 Sec and Never exceeds 0. 5 Sec 650 V Other Stations 430 V A survey conducted by IEEE indicates that the maximum G. P. R permitted by various Electrical utilities varies from 1 to 20 k. V

MAINTENANCE • Maintenance of earth-electrodes at power stations and substations • Records should be kept of the initial resistance of power station and substation earth-electrode systems and of subsequent tests carried out. Such tests should be made periodically to ensure that no serious increase in resistance occurs. Normally an annual test should suffice, but local circumstances may justify an increase or reduction in the intervals between tests in the light of experience. Where possible earth-electrodes should be arranged in groups so that they can be disconnected in turn from the general earthing system for test while leaving sufficient electrodes in service to provide an effective connection with earth.

MAINTENANCE Contd. . • In urban substations it is frequently impossible to make any effective check on the resistance to general mass of earth of the electrode system because of the connection to the metallic sheaths of numerous high voltage and low voltage cables. In such cases, the earthing connections within the substations should permit periodical and visual inspection. • In rural substations, particularly those connected to overhead high voltage and low voltage lines, greater reliance needs to be placed on the electrode system, and it is important that there should be facilities for testing the resistance of the electrode to general mass of earth. Normally annual measurements shall be carried out but local circumstances in the light of experience may justify increase or decrease in this interval but it should not be less than once in two years.

PRECAUTIONS TO BE TAKEN DURING RESISTANCE MEASUREMENTS • There is a possibility of lethal potential existing between a station ground a remote ground if a system fault involving the station ground occurs while ground resistance is being measured. The use of rubber gloves is advisable while making connections to the test electrodes. Under no circumstances should the two hands or other parts of the body of the testing personal should be allowed to complete the circuit between the points of possible high potential difference. • An isolated lightning arrester ground should never be tested with the arrestor in service because of the possible high potential gradient around the ground connection. • Since the resistivity of the upper soil layers is greatly influenced by the weather, a day test should be chosen which is free from extreme weather conditions.

SUPPLIERS OF EARTH TESTERS M/s Ashida Electronics Ram Rajya, Kopri Thane – 400 603, Maharastra M/s Agarwal Electronics 201, Shivashakthi Indus. Estate, Near Shreyas Talkies, LBS Marg Ghatkopar (w), Bombay – 400 086 M/s Cambride Instruments and Engg. company 23/2, Maulana Abdul kalam azad Road, Howrah – 711 001, West Bengal M/s Conin Prakriti Instrumentation REGD. Off & Facty. 16, Ragendra Nagar Industrial Estate, P. O. Mohan nagar, Ghaziabad 201 007 M/s Continental Electronics Naroda 177/4, Industrial Estate, Naroda -382 380, Dist. Ahmedabad Gujarat M/s Erricson Engineers Unigue House, 11 th Floor, 25, Parsee Bazaar street Bombay-400 023 Contd. ,

M/s Indotech Devices Pvt Ltd. , H. O & works, A-75 Sector V P. B. 19, Noida 201 301 Dist. Ghaziabad, U. P Instrument Techniques Pvt Ltd. , Regd. Off & works B-2 Co-operative Industrial Estate, Balanagar, Hyderabad – 500 037 Nippen Electric Instruments Company Sett & of 12 -A, Marol Maroshi Road, Andheri (E), Bombay – 400 059 M/s Sharmasons sakova Instruments Pvt Ltd. , P. B. No. 6251, 2, OLF Industrial Area, Najafgarh Road, New Delhi 110 015 M/s W. J. Alcock & Co. Pvt Ltd. , 7, K. S. Roy Road, Calcutta – 700 001 M/s Yokins Instruments Yokin Trade Lines Admn. Office B-3/40 Ashok Vihar Phase II, Delhi 110 052

BENTONITE SUPPLIERS M/s Amar minerals Pvt Ltd. , Netivali Baug, Kalyan Dist. Thane, Maharastra M/s Amarjyot Industries No. 45, Dr. Meisheri Road, Near Sandhurst road, Railway Stn. Bombay Works: A-108, MIDC, Dombivili, Dist. Thana M/s Industrial Minerals & Chemicals Co. Pvt Ltd. , No. 125, Narayan Dhauru street, Nagaevi, Bombay – 400 003 Works: Kurla Road, chakala Andheri Bombay-58 Works: Chinchpokli cross lane, Bomaby 400 027 M/s International Minerals & Chemicals Co. No. 54 -D, Govt. Industrial Estate, Bombay-400 067 M/s Ambika Minechem Industries No. 129/131, Kazi Sayed street 4 th Floor, Bombay – 400 003 Contd. ,

M/s Mysore Agencies 23/1, 3 rd Cross, Lalbaugh Road, Bangalore – 560 027 M/s Baroda Mineral Grinding Industries National highway, Naroda Ahmedabad M/s J. D. Jones & Co. Pvt Ltd. , C/5, Gillander House, No. 8, Netaji Subhash Road, Calcutta 700001 M/s Metamine Industries Rathnagarmata Road, Kapadvanj, Dist. Kaira Gujarat M/s Somanath Minerals & Chemicals No. 72, Chitra Industrial Estate, Bhavnagar 364 004 (Gujarath) Contd. ,

M/s Sunders India Corporation Near Indian Airlines Corporation Diwanpara Road, Bhavnagar – 364 001 M/s Manjunatha Pulverisers Sabuvani Buildings N. R. Road, Bangalore – 560 002 M/s Mysore City & Mineral Industries No. 1032, IV Block, Rajaji nagar, Bangalore 560 010 M/s Mine Chemical Industries, Peenya Industrial Estate Phase III, Bangalore M/s Binto Plast Pvt Ltd. , Industrial Estate, Jodhpur – 342 001 Contd. ,

M/s United Engineering Corporation APSRTC Complex, Vishakapatnam M/s Ashapura Group of Industries “ VIBHA” 55/3 13 th H Main HAL 2 nd Stage Indira nagar, Bangalore – 560 008 H. O. 81/82, Mittal Court C Nariman point, Bombay – 400 021 M/s OSWAL MINERALS No. 6, 2 nd Main Road, Rama chandrapuram Bangalore – 560 021

Remote electrode at: • • 60 M 02. 5 05. 0 07. 5 10. 0 0. 164 0. 174 0. 183 0. 190 • • • • • 12. 0 12. 5 15. 0 17. 5 20. 0 22. 5 24. 0 25. 0 27. 5 30. 0 32. 5 35. 0 36. 0 37. 5 40. 0 42. 5 45. 0 47. 5 50. 0 0. 205 0. 206 0. 210 0. 221 0. 225 0. 236 0. 240 0. 246 0. 261 0. 274 0. 296 0. 316 0. 329 0. 343 0. 346 0. 376 0. 406 0. 438 0. 493 0. 563 R 1 R 2 R 3 µ = R 3 -R 2/R 2 -R 1 = 0. 329 -0. 240/0. 240 -0. 205 = 2. 5 05 10 15 16 80 M 0. 190 0. 204 0. 222 0. 218 20 25 30 32 35 40 45 48 50 55 60 65 70 75 0. 227 0. 239 0. 242 0. 247 0. 258 0. 274 0. 290 0. 306 0. 307 0. 320 0. 330 0. 345 0. 434 0. 859 R 1 R 2 R 3 0. 306 -0. 247/0. 247 -0. 218 = 2. 03

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