Earthing in Mines and Quarries Slide Presentation Phillip Wall
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Transcript of Earthing in Mines and Quarries Slide Presentation Phillip Wall
![Page 1: Earthing in Mines and Quarries Slide Presentation Phillip Wall](https://reader030.fdocuments.us/reader030/viewer/2022033023/563dd47a55034635058b5b98/html5/thumbnails/1.jpg)
Earthing in Mines & QuarriesExpectations for Power System & Lightning performance
Phil WallSenior Electrical Engineer
21st Electrical Engineering Safety Seminar - Nov 2011
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What are we trying to achieve?
• Ensuring personnel protection throughsafe step and touch voltages
• Means for appropriate circuit protection operation
• Overvoltage protection
• Voltage stabilisation
• Minimise energy transfer to hazardousareas (ignition sources)
Electrical Safety through Earthing
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Typical Earthing Systems
Two Broad Catergories
• Power Systems– Main Incoming Substation
(Network Earth)
– Earthing systems associated with the Mine or Quarry (Mine Earth)
• Lightning Protection Systems– Surface Operations & Quarries
– Underground Mines
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Assessment Criteria
• Power Systems– Touch Voltages to comply with AS3007.2
– Step Voltages to comply with requirementsof AS2067
• Lightning Protection Systems– Systems to comply with AS/NZS1768
– CMHSR 2006 requirementsClause 19 part (i) subsection (ii)
– Is there any Touch or Step requirements?
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Power System Performance
Touch Voltage limits AS3007.2-2004
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Impedance
to Ground
Fault
Current
Earth Grid
Voltage =
Earth Potential Rise
(EPR)
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V Touch Voltage
V Step Voltage
Voltage =
Earth Potential Rise
(EPR)
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V Touch Voltage
V Step Voltage
Touch Voltage
from
hand to both feet V
Step Voltage
from
foot to footV
Voltage Limits
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Touch Voltages
AS3007.2 curves are useful where substation EPRs are small with no impact to surrounding infrastructure. If EPRs are not small and impact to surrounding infrastructure is unavoidable:we need a more detailed approach!
• How to apply wet areas?
• Is a 132kV system applied the same as a 3.3kV system?
• How does gravel in a switchyard help?
Use AS2067 with data from AS/NZS 60479.1
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Calculation of Unique Voltage Limit Curves
Deterministic approach in AS2067 follows IEC 61936-1 closely
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AS60479.1 Voltage Limit Calculations
Determine Body Path Impedance
Determine Current through the heart for a
given time
Determine Voltage limit
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Body Path Impedances from AS60479.1
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Contact surface area: Large,
medium or small
Pressure exerted
Dry, wet or saltwater wet
Tables 1 – 9 of AS60479.1
for a given ‘expected’ voltage across the body
Current Path through the body
Temperature
% of population (variance)
Calculation of Body Impedance from AS60479.1
Hand
to hand
a.c.
50/60 Hz
Frequency of supply
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Body Path Impedances from AS60479.1
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Contact surface area: Large,
medium or small
Pressure exerted
Dry, wet or saltwater wet
Tables 1 – 9 of AS60479.1
For a given ‘expected’ voltage across the body
Current Path through the body
Temperature
% of population (spectrum)
Calculation of Body Impedance from AS60479.1
Frequency of supply
Add lumped resistances such as
Gloves, Boots or Gravel
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Touch Voltages
Determine Body impedance
from tables 1-9 of AS60479.1
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Touch Voltages
Add lumped resistance
such as boots
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Touch Voltages
Add lumped resistance
such as gravel
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Touch Voltages
Add lumped resistance
such as gloves
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Touch Voltages
Back to the earlier questions:
• How to apply wet areas?
→ Apply a wet body impedance from tables in AS60479.1
• Is a 132kV system applied the same as a 3.3kV system?
→ Apply a voltage dependent body impedance from tables in AS60479.1
• How does gravel in a switchyard help?
→ Gravel acts as a lumped impedance in the shock circuit
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AS60479.1 Voltage Limit Calculations
Determine Body Path Impedance
Determine Current through the heart for a
given time
Determine Voltage limit
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Current through the Heart (AS60479.1)
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Current through the Heart (AS60479.1)
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Current through the Heart (AS60479.1)
Choose c1 curve
Scale for calculated impedance of body = unique touch curve
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Custom Voltage Limit curve
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Lightning Protection System Performance
For Power Systems, design and assessment isclearly defined: AS3007, AS2067 etc.
How do we assess personnel safety limitsfor lightning?
Sourced from AS/NZS 7000
Typical lightning <150µsec
Energy and Charge absorbed by the body
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Assessable Criteria
No single defined assessment criteria can be identified, however considerable focus shall be required for the following:
– Ensure all ‘at risk’ infrastructure is provided with a suitable path to discharge lightning energy
– Maximise personnel safety where possible through low ohm grids (step and touch) and equipotential bonding (hand to hand)
– Minimise side-flash
– Remove all potential ignition sources from hazardous zones
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Coal Mines Health and Safety Requirements
• NSW DPI Technical Reference EES005 states that earth electrodes provided for (lightning) must be separated from the mine earth electrodes .
• Required separations:– At least 3 metres in air.– At least 15 metres in the ground.
• For underground operations : ‘Prevention of the effects of lightningbeing transferred to the underground parts of coal operation’
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Historic Design Requirements
• NSW DPI Technical Reference EES005 states “In some locations in NSW the 15m distance may be inadequate.”
• No separation distance can be computed for buried earthing systems as no assessment criteriais defined.
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Lightning Protection System Performance
• For open cut and quarries , protection principles in accordance with AS1768
• For underground mines, bonding principles in underground in accordance with AS1768
What about U/G Coal Mines?
• Additional recommendations for U/G ‘preventeffects of lightning being transferredinto underground parts of coal operation’
• Minimisation of ignition sources in hazardousenvironments - by reducing transferred energy
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Potential Ignition Sources
• Under the right conditions a spark may occur between items of metalwork that are bonded to different earthing systems (or not effectively earthed at all).
• Equipotential bonding dramatically reduces potential ignition sources.
• It is recommended that equipotentialbonding techniques are implemented in the underground environment.
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Network Earth
… is the Earthing system associated with the incoming high voltage supply.
• Irrespective of earth connection to the upstream Network substation, the Earth Fault belongs to Network substation.
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Mine Earth
• A need to remove the confusion around Mine Earths.
• ‘Mine Earth’ is a designated earthing system for the electrical supply feeding surface or U/G mobile machinery.
• Mobile machines rely on cable screens in the supply cable and have very little local earthing. Systems are in place to control step and touch hazards from power system faults.
• Sometimes combined to other earthing systems especially when on the surface.
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Mine Earth
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Mine Earth
• For U/G coal mines where additional hazards exist with ignition of gas, the Mine Earth and Lightning systems have a complex relationship . Separated systems are recommended.
• Suggested definitions:
Mine Surface Earth (MSE)Mine Underground Earth (MUE)
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Lightning Earth
What is a Lightning Earth?An earth termination intended to discharge lightning currents into the general mass of the earth.
Unless suitably protected, all surface structure can beassumed to be incorporated into a lightning earth.
Examples: Product bins, Winder towers, stackouts, conveyor inclines
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Lightning and Surface Earthing systems
• Often difficult to separate.
• For underground mines, all surface infrastructure consisting of electrical circuits can be considered apart of a surface earthing system.
• The challenge is where this infrastructure passes into the underground workings.
• Buffer zones required to break conductive plant and services.
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Separate or Common Earthing Systems?
Why would we separate?
• Hazards to personnel and infrastructure from too many volts (EPR) on the Earthing System.
• Separation requirements can benecessary for lightning or power system.
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Why would we separate?
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Why would we separate?
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Why would we separate?
But what to do with
the cable screen earth?
Cable screens
bared back
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Why would we separate?
Cable screens
bared back
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Why would we separate?
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Why would we separate?
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Earthing, Protection and Power Engineers
Newcastle Office:
Unit 4 / 11 Glenwood Drive Thornton NSW 2322
Telephone: +61 2 4964 2077 / Fax: +61 2 4964 2088
Sydney Office:
Level 5 / 162 Goulburn St Darlinghurst NSW 2010
Telephone: +61 2 9265 7677
Email: [email protected]
Website: www.powerearth.com.au