Earthing of MV and LV Distribution Systems: A multi-faceted problem. Hendri Geldenhuys Gareth Stanford Industry Association Resource Centre (IARC) Eskom

Considerations related to: Public , Customer (end user?) And Line’s man safety considerations Merged with System “protection” and performance considerations

MV Line LV Feeder Distribution transformer LV Earth MV Earth Service Box Service connection Communication line Artwork: A Dickson

Design Philosophy Consider the Bonding and Earthing of the structure: There are no single solution that fits all applications and environmental conditions All solutions has risk, some less than others,

Design Philosophy Wood, LV Feeder MV Feeder Telephone Wood, Concrete or Steel MV Feeder Wood, Concrete LV Feeder or Steel Telephone

Design Philosophy BIL wires on all shared structures (no gap no insulation of BIL wire.) BIL wires only on suspension structures with no stays (no gap no insulation of BIL wire.) No BIL wire BIL wire on all shared structures- insulate bottom 2m of down wire No BIL wire- Double arresters on transformers on MV side BIL wires on all – Move gap down below LV BIL wires on all – Split air gap above & below LV to prevent LV Faults

BIL wire on all shared structures (no gap no insulation of BIL wire.) LV Feeder MV Feeder Telephone Wood, Concrete or Steel MV Feeder LV Feeder Telephone

(no gap no insulation of BIL wire.) BIL wires only on suspension structures with no stays (no gap no insulation of BIL wire.) LV Feeder MV Feeder Telephone Wood, Concrete or Steel MV Feeder LV Feeder Telephone

No BIL wire LV Feeder MV Feeder Telephone Wood, Concrete or Steel

BIL wire on all shared structures- insulate bottom 2m of down wire LV Feeder MV Feeder Telephone Wood, Concrete or Steel MV Feeder LV Feeder Telephone

Double arresters on transformers on MV side No BIL wire- Double arresters on transformers on MV side LV Feeder MV Feeder Telephone Wood, Concrete or Steel MV Feeder LV Feeder Telephone

BIL wires on all – Move gap down below LV LV Feeder MV Feeder Telephone Wood, Concrete or Steel MV Feeder LV Feeder Telephone

BIL wires on all – Split air gap above & below LV to prevent LV Faults LV Feeder MV Feeder Telephone Wood, Concrete or Steel MV Feeder LV Feeder Telephone

Safety Risks That occurs in MV LV Distribution Systems

Separate MV and LV Earths FEEDER LINE ( 3Ph , Ph-Ph or SWER ) TANK fuses SURGE ARRESTOR + 242 V - 242 V N HV TO LV NEUTRAL ARRESTOR L N E MV EARTH LV EARTH

MV Voltage Transferred to LV Earth FEEDER LINE ( 3Ph , Ph-Ph or SWER ) TANK fuses SURGE ARRESTOR + 242 V - 242 V N > 5 kV HV TO LV NEUTRAL ARRESTOR > 5 kV L N E MV EARTH LV EARTH

MV-LV Supply System L N E N L N E

MV-LV Supply System L N E N 2.5 kW 10A 20A -100V 100V LV Neutral Break

LV MV 161 V [ =230 x (70/100) ] I = 2.3 A 69 V [ =230 x (30/100) ] Neutral SA ☻The ratio between the MV and LV earth electrode resistance determine the voltage on the LV neutral- earth 70 Ω 30 Ω Step potential

Equipment Damage

5 x Iph

Earthing and Bonding in different Environments Pollution Low Pollution High Lighting Low No BIL down wire or bonding of hardware Bonding of hardware (no BIL down wire) Lighting High BIL wire and co-ordinated gap Bonding of total structure, no gap, high insulator BIL.

MV Line LV Feeder Distribution transformer LV Earth MV Earth Service Box Service connection Communication line Artwork: A Dickson

AC power system related risk Design Philosophy MV Conductor drop onto LV system MV conductor contact to BIL down wire only LV contact to BIL down wire BIL wires on all shared structures (no gap no insulation of BIL wire.) Fast clearing of MV fault (1sec). Auto Reclose repeat to lock out. High GPR on LV Protective Earth. Exposure to all LV installations and BIL wire locations Med risk Slow clearing of fault (10 sec) even a small risk of not clearing fault. Very High GPR on BIL wire. Fault not cleared BIL wire stay live High risk BIL wires only on suspension structures with no stays (no gap no insulation of BIL wire.) Fewer BIL wires stay live No BIL wire Exposure to all LV installations No risk BIL wire on all shared structures- insulate bottom 2m of down wire BIL wire not accessible No BIL wire- Double arresters on transformers on MV side BIL wires on all – Move gap down below LV BIL wire will remain live until it flashes to LV protective earth. If Fault to LV occurs-clearing of the fault is the same as column to the left. Low risk BIL wires on all – Split air gap above & below LV to prevent LV Faults

Public&Consumer lightning risk Design Philosophy Effective earth Public&Consumer lightning risk Equipment Damage BIL wires on all shared structures (no gap no insulation of BIL wire.) MV earth + LV earth +BIL wire earths. Best practice (not totally safe) Best Practice BIL wires only on suspension structures with no stays (no gap no insulation of BIL wire.) MV earth + LV earth +fewer BIL wires No BIL wire MV earth+ LV earth only High risk BIL wire on all shared structures- insulate bottom 2m of down wire MV earth + LV earth only No BIL wire- Double arresters on transformers on MV side MV earth + LV earth +BIL earths Medium risk BIL wires on all – Move gap down below LV MV earth + LV earth + BIL earths BIL wires on all – Split air gap above & below LV to prevent LV Faults

Overall Risk 2 MED 3 MED 1 MED AC power risk Lightning Risk Design Philosophy MV Conductor drop onto LV system MV conductor contact to BIL down wire only LV contact to BIL down wire BIL wires on all shared structures (no gap no insulation of BIL wire.) MED HIGH LOW BIL wires only on suspension structures with no stays (no gap no insulation of BIL wire.) No BIL wire BIL wire on all shared structures- insulate bottom 2m of down wire 2 MED No BIL wire- Double arresters on transformers on MV side BIL wires on all – Move gap down below LV 3 MED BIL wires on all – Split air gap above & below LV to prevent LV Faults 1 MED

LV Feeder MV Feeder Telephone MV Feeder LV Feeder Telephone 50 mm LV gap 500 mm BIL wood MV Feeder 500 mm BIL wood gap LV Feeder 50 mm LV gap Telephone

Designs can be optimised for specific areas and applications A (simple) Wood Pole Structure has complex safety and performance considerations Designs can be optimised for specific areas and applications No Power System is ever 100% safe

Safe and Reliable