Managing earthing hazards around pole top transformers at mines and quarries James Derricott

1. Managing Earthing Hazards around pole
top transformers at mines and quarries
– common vs separated earthing
James Derricott
BE(Hons I), MIEAust, CPeng

2. Pole top transformers in mines
• Pole top transformer earthing in mining and
quarry context
• Often the highest earthing related hazard on
site
• Key inputs (fault level, auxiliary paths) very
different for remote pole top

3. Why earthing exists
• To allow protection to operate and clear faults
(abnormal power system operation)
• Provide a voltage reference
• Lightning and transient energy dissipation
• Provide path for out of balance current (or
load in the case of SWER lines)
• Minimise voltage difference and protect
people during fault events

4. What are earth faults
• Line to ground (single or double)
• Current doesn’t flow unless there is a circuit
• Current needs to return to the source
• Creates voltage rise (EPR) at each end for the
circuit (earth grid)

5. Earth Fault Circuit
• V = I x R
• EPR = Grid Current x Grid Resistance
• EPR = Fault Current x System Impedance

6. Voltage Hazards
• Voltage across body
• Hand-hand, foot-foot, hand-
foot
• Right hand to left foot is worst
• Heart may go into ventricular
fibrillation (unnatural rhythm)
• May easily be mistaken for
heart attack (unlikely to be burn
marks)
• Probability of a fault occurring
• Probability that someone will
be there
• Probability that the fault
duration will line up with the
heart beat and enter VF

7. EPR Transfer

8. Common Earthing
• HV and LV earthing bonded together
• Lower system impedance
• Lower EPR / Vt & Vs
• Transfer of EPR

9. Separate Earthing
• HV and LV earthing separated
• Higher system impedance Zsys = Rg
• Higher EPR / Vt & Vs (lower If)
• Minimise EPR transfer
• Corruptible
• Undetected LV fault

10. Safety Standards
• Safety standards have been changing
• Now voltage based targets that include
likelihood and risk based assessment
• AS3007, AS2067, EG0

11. Key Inputs
• Soil resistivity and layering
• Fault level
• Clearing time
• Nearby assets – LV earthing, nearby metallic
items, distance away, contact scenarios and
frequency

12. Example – Bore pump
• 11kV/440V Pole top supplying a bore pump
• No overhead earth wire
• Surface soil resistivity of 200 Ωm top, 20 Ωm
underlying
• Fault level of 2kA
• Drop out fuse clearing time – 0.2s
• 1 fault / 10 years
• Bore pump 20m away

13. Bore Pump
Calculate Safety Criteria
• AS3007 – Argon tool, Mining/MEN contact scenario (2000
contacts, 4s each)
• Backyard (HV standalone) scenario (416 contacts, 4s each)
• 1 fault per 10 years, 200Ωm surface soil
• Allowable touch
voltage = 654 V
(Mining/MEN)
• Allowable HV =
1,021 V
(HV/backyard)

14. Bore Pump – Common Earthing
• Combined Zsys = 1.5 Ω
• EPR ≈ 3,000 V

15. Bore Pump Common Earthing
• Touch voltage to pole ≈ 1,750 V
• Touch Voltage to bore/pump ≈ 1,500 V
– LV is 229% of allowable limit

16. Bore pump – common earthing
Mitigation Options
• Lower the system impedance further (any
OHEWs?)
• Reduce fault level
• Reduce clearing time
• Separate the HV and LV to reduce voltage
transfer

17. Bore Pump – Critical Zsys
• Allowable touch voltage of 654 V
• Touch voltage at LV is 50% of EPR
• Limit EPR to < 2x 654 = 1,308 V
• Zsys max = EPR/Ifault = 1,308/2,000 = 0.65Ω

18. Bore Pump – Separate Earthing
• 5m separation between HV and LV electrode
(far enough to limit voltage picked up on soil)
• Rg = 4.8 Ω
• EPR = 9,697 V (> VLG for 11kV?)
• EPR = 6,350 V

19. Bore Pump – Separate Earthing
• Vt LV ≈ 430V, 66% of allowable limit
• Vt HV ≈ 4,300V, 421% of allowable limit

20. Bore Pump – Separate Earthing
• HV touch voltage above allowable
– Grading ring/operator mat
– Insulate pole/downconductor
– Crushed rock/asphalt surface layer
• Downsides
– LV tank fault may not clear
– Need to maintain the separation
– Ensure no inadvertent connections made later

21. Bore Pump – Summary
Common Earthed
• Zsys = 1.5Ω
• EPR = 2kA x 1.5Ω = 3,000V
• Vt LV ≈ 1,500 V (229% of allowable limit)
• Vt HV ≈ 1,750 V
Separately Earthed
• HV Rg = Zsys = 4.8Ω
• EPR = 6,350V
• Vt LV ≈ 430V, 66% of allowable limit
• Vt HV ≈ 4,300V, 421% of allowable limit
• Hazards at HV earth only and easier to mitigate

22. When to use what
Common earthing if you can easily satisfy the following:
• Metallic return path all the way back to the main
substation and solidly bonded both ends (OHEW, cable
screen, LV neutral)
• Highly built up areas such as process plant with many
LV earth points connected by continuous neutral
Separated if the following is true:
• No metallic return path
• Sparse or small and isolated local LV earthing