Earthing in Mines and Quarries Slide Presentation Phillip Wall

Earthing in Mines & QuarriesExpectations for Power System & Lightning performance

Phil WallSenior Electrical Engineer

21st Electrical Engineering Safety Seminar - Nov 2011

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

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

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?

Power System Performance

Touch Voltage limits AS3007.2-2004

Impedance

to Ground

Fault

Current

Earth Grid

Voltage =

Earth Potential Rise

(EPR)

V Touch Voltage

V Step Voltage

Voltage =

Earth Potential Rise

(EPR)

V Touch Voltage

V Step Voltage

Touch Voltage

from

hand to both feet V

Step Voltage

from

foot to footV

Voltage Limits

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

Calculation of Unique Voltage Limit Curves

Deterministic approach in AS2067 follows IEC 61936-1 closely

AS60479.1 Voltage Limit Calculations

Determine Body Path Impedance

Determine Current through the heart for a

given time

Determine Voltage limit

Body Path Impedances from AS60479.1

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

Body Path Impedances from AS60479.1

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

Touch Voltages

Determine Body impedance

from tables 1-9 of AS60479.1

Touch Voltages

Add lumped resistance

such as boots

Touch Voltages

Add lumped resistance

such as gravel

Touch Voltages

Add lumped resistance

such as gloves

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

AS60479.1 Voltage Limit Calculations

Determine Body Path Impedance

Determine Current through the heart for a

given time

Determine Voltage limit

Current through the Heart (AS60479.1)

Current through the Heart (AS60479.1)

Current through the Heart (AS60479.1)

Choose c1 curve

Scale for calculated impedance of body = unique touch curve

Custom Voltage Limit curve

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

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

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’

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.

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

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.

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.

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.

Mine Earth

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)

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

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.

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.

Why would we separate?

Why would we separate?

Why would we separate?

But what to do with

the cable screen earth?

Cable screens

bared back

Why would we separate?

Cable screens

bared back

Why would we separate?

Why would we separate?

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: contact@powerearth.com.au

Website: www.powerearth.com.au