CENTRAL ELECTRICITY AUTHORITY (MEASURES RELATING TO SAFETY AND ELECTRIC SUPPLY) REGULATIONS, 2010, AND EARTHING PRACTICES IN MINES. AJAY SINGH, DIRECTOR OF MINES SAFETY(ELECTRICAL), CENTRAL ZONE, DGMS, DHANBAD
REG 100: PROTECTIVE EQUIPMENT.- (1) appropriate equipment shall be suitably placed in the mines for automatically disconnecting supply to any part of the system, where a fault, including an earth fault, occurs and fault current shall not be more than 750 milliampere in installations of voltage exceeding 250 V and up to 1100 V for below ground mines and oil fields and 50 ampere in installations of voltage exceeding 1100 V and up to 11 kV in open cast installations of voltage exceeding 1100 V and up to 11 kV in open cast mines and the magnitude of the earth fault current shall be limited to these specified values by employing suitably designed, RESTRICTED NEUTRAL SYSTEM OF POWER SUPPLY.
� Body resistance= 1000 ohm SHOCK VOLTAGE = 317V � Flow of current through body ≃ ≃ ≃ ≃ 317mA � � Death most likely. � Body resistance= 1000 ohm � Extra resistance introduced in the path of earth fault current = 423Ω SHOCK VOLTAGE = 30.15V � Flow of current through body ≃ ≃ 30mA � ≃ ≃ � CHANCES OF SURVIVAL.
EFFECT OF RESISTANCE OF EARTH RETURN PATH ON SHOCK VOLTAGE/CURRENT � For 45 OHM plus NGR, SHOCK VOLTAGE = 30.15V / 30mA � For 100 OHM plus NGR, SHOCK VOLTAGE = 60.61V / 60mA � For 10 OHM plus NGR, SHOCK VOLTAGE = 7.32V / 7mA
Concept of touch potential
EQUIVALENT CIRCUIT:
� For a 550V system, per phase voltage is 317V and the value of NGR to restrict earth fault current within 750mA will be 423 Ω. � The magnitude of shock voltage ( or touch potential) will be: = (550/√3) x 45 / (423 + 45) = 30.53 V which is well within safe limits. � Now, the characteristics between touch potential and time shall be referred so as to appropriately set the timing of earth leakage relay (source: AS-4871) , failing which the protective scheme won’t be fool- proof and may not offer desired protection. proof and may not offer desired protection.
CONVENTIONAL TOUCH POTENTIAL ANALYSIS
1. Touch potentials resulting from ‘compliant’ fault limits and loop impedances are not necessarily ‘safe’. 2. AS/NZS4871.1:2012 relates the maximum duration of human exposure to prospective touch voltages that do not usually result in touch voltages that do not usually result in harmful physiological effects on any person subjected to that touch voltage (i.e. ‘safe’).
MAXIMUM DURATION OF 50Hz TOUCH VOLTAGE AS PER AS4871;
DURATION OF 50Hz TOUCH VOLTAGE
� At 162V, total protection clearance time (relay setting plus contactor actuation time) must be ~100msec. � If the operating time of our protection exceeds 100msec we must increase the NER impedance or reduce the return earth continuity limit accordingly. 1. The presence of an NER does not make the system safe. 2. Safety is reliant on active earth leakage protection. 3. If our active protection fails to operate we must restrict the continuous touch potential to less than 25V to remain safe.
� 5 Amp earth fault current limit, 35+40 = 75 Ohms earth return impedance. � Prospective touch voltage is 75/(115+75)*577 = 228V. � Clearance time: earth leakage relay 50 msec interposing relay delay 20 msec circuit breaker delay 130 msec. � the total clearance time is around 200 msec.
200msec clearance time for the Lp curve requires 110V touch potential limit. Can adjust the NER, E/C limit or both
Resultant touch voltage is 134V •Clearance time 400msec (E/L relay 250msec, interposing relay 20msec, CB delay 130msec). � At 400msec clearance time, touch voltage req’d ~57V Back calculate current limitation for 35 Ohm return impedance as 1.8A �
PROPOSED AMENDMENT OF REG 100(1): In the interest of safety, the earth fault current shall not be more than 750 milliampere in installations of voltage exceeding 250V and up to 1100V for below ground mines and oil fields, and 10 A in installations of voltage exceeding 1100V and up to 11KV for belowground mines, open cast mines and oil mines or oil fields , and the magnitude of the earth fault current shall be limited to these specified values by employing suitably designed, restricted neutral system of power supply including neutral monitoring protection. Appropriate equipment shall be suitably placed for automatically disconnecting supply to any part of the system in the event of earth fault or earth leakage. Provided that the settings of protective devices thereof shall not exceed 100 mA for individual apparatus of voltage Provided that the settings of protective devices thereof shall not exceed 100 mA for individual apparatus of voltage up to 1100V. Provided further that in installations where magnitude of aggregate capacitive current is more than 10A, the owner of installations or user shall limit the earth fault current to appropriate value , with the approval of Electrical Inspector. Appropriate PPE shall be used by the personnel for handling such apparatus to protect against electric shock due to touch potential.
UNGROUNDED SYSTEM: I A = I B = I C = 1A UNFAULTED I A = 0A I B = I C = √3*1 = 1.73A N I F = - (I B + I C ) = 3A PHASE-A FAULTED
RESTRICTED NEUTRAL SYSTEM 1. Touch potential for a typical 3.3KV network with solidly neutral system will be around 1905.25V . 2. Touch potential for a typical 3.3KV network with 50A restricted neutral system will be around 1905.25÷(38.10+45)Ax 45Ω= 1031.72V . 3. Touch potential for a typical 3.3KV network with 5A restricted neutral system will be around 1905.25÷(381.05+45)Ax 45Ω= 201.23V .
In case when the proportion of capacitive current is lesser than resistive current: desairable V s > V R In case when the proportion of capacitive current is more than resistive current: not desairable V R > V S
What about 10:1 tripping ratio? Under 4871:2012 test current for E/L relay is 120% or 1.2:1 Typical tripping ratios in UK are 3:1, US are 2.5:1 •Tripping ratio should be maintained high as possible, but at 350mA trip (5:1) system would be safe •Increased trip current avoids sympathetic trips on unfaulted outlets and relays still proven/tested to trip
CASE STUDIES: 2. 1. JAMADOBA REGIONAL SUB-STATION, M/S JEETPUR COLLIEY, M/S SAIL: TATA: 10MVA, 33/11KV TRANSFORMER 2MVA, 11/3.3KV TRANSFORMER
3. MONIDIH PROJECT SUB-STATION, M/S BCCL: 10MVA, 33/6.6KV TRANSFORMER Magnitude of capacitive currents for various schemes: 1. With 4 nos. of feeders in circuit = 5.67A 2. With 3 nos. of feeders in circuit = 4.32A 3. With 1 nos. of feeders in circuit = 1.07A 3. With 1 nos. of feeders in circuit = 1.07A
√(6 2 +15 2 ) = 16.15A √(6 2 +10 2 ) = 11.66A √(4.5 2 +15 2 ) = 15.66A √(4.5 2 +10 2 ) = 10.96A
TOUCH POTENTIAL or current through human body of netwok SHOCK VOLTAGE 1000Ω resistance I capacitive NGR 15 NGR5 NGR10 voltage VNGR10 V NGR15 I NGR15 VNGR5 INGR5 INGR10 11KV 6A 423Ω 610V 610mA 1270Ω 635Ω 217V 420V 217mA 420mA 6.6KV 6A 254Ω 573V 573mA 762Ω 380Ω 212V 402V 212mA 402mA 3.3KV 6A 127Ω 498V 498mA 381Ω 190Ω 201V 364V 201mA 364mA
���������������������������������������� Capacitance between ��������������������������� A and B in the delta ������������������������ C 1 system = C c + 0.5C C = 1.5 C c = 0.5C 1 , C 1 = 3 C c 1.5C c . 1.5C c . C 0 = C 1 + C S = C S + 3C C = A
C 0 = C 1 + C S = C S + 3C C C b ��� C S C A-BC ��� C C + C S
Thank you all. For any doubt feel free to contact me at 9771499979, 9434070734 ajaysingh.ddms@gmail.com
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