earthing philosophy reference standards legend: …

1

2

3

4

ECC

GPR

PEN

MV

LV

NOTES:

1)

2)

3)

4)

5)

6)

7)

8)

Medium Voltage

Low Voltage

LV system earth fault current is routed back to source transformer neutral through respective protective conductor (PEN / ECC & protective bonding conductor) as illustrated in Annexure-4. As per note-1 above, very negligible earth

fault current flows through general mass of earth and hence not indicated in the diagram.

Earth Continuity Conductor

Ground Potential Rise

Protective Earth & Neutral combined conductor

Earth Fault

LV earth fault current flow path back to source (Refer note-7)

MV earth fault current flow path back to source (Refer note-8)

Instrument earth conductor for ICA system (Y/G PVC insulated copper)

EARTHING PHILOSOPHY

IEC-61000-5-2

Protective conductor (bare copper for buried grid - size as per earthing design calculations & Y/G PVC insulated copper for

grid risers to equipment - size as per Table-C of this document)

Main protective bonding conductor (Y/G PVC insulated copper, size at least half of protective conductor as per clause 544.1

of IEC-60364-5-54)

Protective / supplementary bonding conductor (Y/G PVC insulated copper, size as per Table-A of this document)

ECC - Earth continuity conductor (Y/G PVC insulated copper, size as per Table-54.2 of IEC-60364-5-54, refer Table-B of this

document)

LEGEND:REFERENCE STANDARDS

IEEE-80

IEC-60364-5-54

BS-7430

Earth Dispatcher (Copper bar)

ABBREVIATIONSConcrete inspection earth pit with earth rod (copper, copper bond steel, etc.)

4-pole main incomer breaker (3 phase + PEN)

As per clause 15.8 of IEEE-80, for ground faults on distribution transformer secondary side or LV consumer side, the fault current circulates in the protective conductor (PEN / ECC & partly through protective bonding conductor) with

negligible leakage current into the earth and, thus, has no effect on the Ground Potential Rise (GPR). The transformer's PEN conductor and protective conductor of connected consumers are sized to carry full ground fault current

expected on the circuit and have very low impedance (resistance) compared to earth grid. This is the reason the earth grid is not involved in this fault loop and thus the low-voltage ground fault current is not a sizing parameter for grid.

The main earth grid within the plant needs to be designed only for carrying MV side ground fault current. Utility company, that supplies main power, will provide value of maximum MV ground fault current that it is capable of

supplying. This fault current forms the basis of designing earth grid at distribution substation. Refer note-8 below for further details.

As per Figure-54.1b of IEC-60364-5-54, PEN conductor shall be connected to neutral bar of LV MCC and then further connected with link to its PE bar. The PE bar is connected to earth network through main earth terminal as illustrated

in subsequent annexures. This scheme classifies as TN-C-S earthing scheme with TN-C up to main LV switchgear / MCC and TN-S thereafter on LV distribution side.

Lightning system must be connected to protective bonding loop for equipotential bonding as per Figure-6 of BS-7430 & Annex-B of IEC-60364-5-54

If instrument earthing system is kept isolated from plant earth, in case of lightning or power system ground fault, dangerous transient voltages can occur between isolated instrument earthing system and other parts of the installation

thus compromising personnel safety and EMC. In order to comply with EMC requirement & ensure personal safety, instrument earthing system must be bonded to plant earthing system to have common earthing reference in line with

clause 5.3.1 and figures 5 & 8 of IEC-61000-5-2.

Refer Figure-30 of IEEE-80-2013. MV earth fault current returns back to utility transformer star point; partially through associated incoming utility cable armor and / or separate protective conductor and balance through general earth

mass. Current flowing through earth mass is partly routed through main earth grid and balance through multiple earthed points throughout the plant. Refer Annexures-1 till 3 for illustration of MV ground fault current flow under

various scenarios. Current passing through multiple earthed points will return back to utility transformer star point through general mass of earth without passing through main earth grid. Fault current which flows through main earth

grid is used for sizing the grid and contributes to Ground Potential Rise (GPR) at distribution substation. The part of the current flowing through various earthed points across the plant will cause potential rise at those points & helps

minimizing ground potential difference (across various points of the plant) with respect to main earth grid. The split factor (for current division between main earth grid & other paths) should be referred from Figure-C.1, page-153 of

IEEE-80-2013. This split factor is used in earthing design calculations for designing earth grid. Usually the same is 0.65 (65%).

Since lightning system earth pits & instrument earth pits are finally connected to main earth grid as indicated in subsequent annexures, separate calculations for their earthing resistance are not required since overall earthing

resistance through such connection will be same as main earthing grid.

In case main earth grid requires larger area and is spread around the plant, it will also act as main protective bonding conductor loop within that area and separate bonding conductor loop is not required within that area.

of 6

Sr.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

Sr.

1

2

3

4

Annexure-2 Illustration of ground fault current flow path for fault at distribution transformer MV primary side

Annexure-3 Illustration of ground fault current flow path for fault at MV motor

Enclosure of ICA panel

Cable tray / ladder joints and LV cable gland earth tags

MV motor

Enclosure of LV Motors above 55 kW

Skids housing electrical equipment

Steel supports, steel structures, etc.

25

25 (maximum 50)

70

Annexure-4 Illustration of ground fault current flow path for fault at LV consumer

LIST OF ANNEXURES

Annexure-1 Illustration of ground fault current flow path for fault at MV switchgear / RMU

Instrument earth conductor for ICA system 35

Metallic tanks, fence, gate, etc.

6

6

16

MV cable armor / screen earth bond 35

TABLE-B

"S"

Cross sectional area of line

conductor, copper, sq.mm

Minimum cross sectional area of

ECC (Earth Continuity

Conductor), copper, sq.mm

S ≤ 16 S

16 < S ≤ 35 16

S > 35 S / 2

Half of protective conductor

Conductor Size, sq.mm

70

16

25 (maximum 50)

EARTHING PHILOSOPHY

TABLE-A

Main protective bonding loop around the plant

Local Control stations, Junction boxes & Instruments

Enclosure of LV electrical equipment having main power cable size up to & including 35 sq.mm

Enclosure of LV electrical equipment having main power cable size above 35 sq.mm

Enclosure of LV Motors up to & including 55 kW

Y/G PVC insulated copper protective bonding conductor sizes (minimum recommended)

Item Description

16

25

TABLE-C

Transformer body

Generator body

MV switchgear & MV VFD

Enclosure of LV panels connected directly to distribution transformer

Y/G PVC insulated copper earth grid riser conductor sizes

Same as buried grid conductor

Conductor Size, sq.mm




Item Description

of 6

DISTRIBUTION TRANSFORMER

GENERATOR

NEUTRAL BAR PE BAR

PE BAR

REMOTE I/O

OR SUB-ICA

PANELSIE BAR

NOTE-2IE BAR (ISOLATED FROM BODY)

REFER NOTE-4

LV MOTOR / DB /

PANELS

INSTRUMENT

EARTH PIT

INSTRUMENT

EARTH PITREFER NOTE-4

REFER NOTE-3

LIGHTINING

SYSTEM

EXTRANEOUS CONDUCTIVE

PARTSMultiple earth pits around

main protective

bonding loop IE BAR

MV MOTOR

Utility side HV/MV

Power Transformer

MV

Power

CableMV SWITCHGEAR / RMU

EARTHING PHILOSOPHY - ANNEXURE-1

MAIN ICA PANEL

PE BAR

Cable Armor and / or

separate PE conductorMV Fault current partial

flow through general

mass of earth

Main Earth grid (Note-1 & 6)

COMBINED PROTECTIVE EARTH & NEUTRAL (PEN) CONDUCTOR (Refer note-2)

Multiple Earth

pits as required

MAIN EARTH TERMINAL (Copper bar with twin disconnecting links)

LV SWITCHGEAR / MCC

ILLUSTRATION FOR GROUND FAULT CURRENT FLOW PATH FOR FAULT AT MV SWITCHGEAR / RMU

MV/LV

RYB

RYB

RYB

R Y B

R Y B

R Y BR Y B

Y

of 6


GENERATOR

NEUTRAL BAR PE BAR

PE BAR

REMOTE I/O

OR SUB-ICA

PANELSIE BAR


REFER NOTE-4

LV MOTOR / DB /

PANELS

INSTRUMENT

EARTH PIT

INSTRUMENT


REFER NOTE-3

LIGHTINING

SYSTEM



main protective

bonding loop IE BAR

MV MOTOR

Utility side HV/MV

Power Transformer

MV

Power



MAIN ICA PANEL

PE BAR




mass of earth



Multiple Earth

pits as required


LV SWITCHGEAR / MCC

ILLUSTRATION FOR GROUND FAULT CURRENT FLOW PATH FOR FAULT AT DISTRIBUTION TRANSFORMER MV PRIMARY SIDE

MV/LV

RYB

RYB

RYB

R Y B

R Y B

R Y BR Y B

Y

of 6


GENERATOR

NEUTRAL BAR PE BAR

PE BAR

REMOTE I/O

OR SUB-ICA

PANELSIE BAR


REFER NOTE-4

LV MOTOR / DB /

PANELS

INSTRUMENT

EARTH PIT

INSTRUMENT


REFER NOTE-3

LIGHTINING

SYSTEM



main protective

bonding loop IE BAR

MV MOTOR

Utility side HV/MV

Power Transformer

MV

Power



MAIN ICA PANEL

PE BAR




mass of earth



Multiple Earth

pits as required


LV SWITCHGEAR / MCC

ILLUSTRATION FOR GROUND FAULT CURRENT FLOW PATH FOR FAULT AT MV MOTOR

MV/LV

RYB

RYB

RYB

R Y B

R Y B

R Y BR Y B

Y

of 6


GENERATOR

NEUTRAL BAR PE BAR

PE BAR

LIGHTINING

SYSTEM



main protective

bonding loop IE BAR

MV MOTOR

REFER NOTE-3

REMOTE I/O

OR SUB-ICA

PANELSIE BAR

MAIN ICA PANEL


REFER NOTE-4

LV MOTOR / DB /

PANELS

INSTRUMENT

EARTH PIT

INSTRUMENT


Very small part of fault current



Multiple Earth

pits as required


LV SWITCHGEAR / MCC

PE BAR

MV/LV


ILLUSTRATION FOR GROUND FAULT CURRENT FLOW PATH FOR FAULT AT LV CONSUMER

Utility side HV/MV

Power Transformer

MV

Power



separate PE conductor

RYB

RYB

RYB

R Y B

R Y B

R Y BR Y B

Y

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earthing philosophy reference standards legend: …

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