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

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

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

4. 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,

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

6. 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

7. 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

8. (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

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

10. 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

11. 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

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

13. 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

14. Safety Risks That occurs in MV LV Distribution Systems

15. 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

16. 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

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

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

19. 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

20. Equipment Damage

25. 5 x Iph

26. 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.

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

28. 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

29. 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

30. 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

31. 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

32. 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

33. Safe and Reliable