Earthing systems in mines - Phillip Wall

Phillip Wall - MIEAust Phil.Wall@powerearth.com.au

Earthing Systems in Mining Operations – managing transfer voltage hazards

25th Electrical Engineering Safety Seminar - Nov 2015

Segregate and contain? OR

Bond all systems?

Earthing Systems in Mines

A high Earth Potential Rise (EPR) needs to be adequately managed

Should we:

Slide 1

Earthing Systems in Mines Managing transfer

voltage hazards

Fault Current

Impedance to Ground

Buried Earth Grid

Voltage = Earth Potential Rise

(EPR)

Voltage = Earth Potential

Rise (EPR)

V Touch Voltage

Transfer Hazard V

Step Voltage V

Mandatory commonly bonded systems:

Power Systems AS/NZS 3007 AS 2067 Single combined earthing systems Unless:

• Underground mining • Voltage limits cannot be achieved

Mandatory commonly bonded systems:

Lightning Protection Systems

AS/NZS 1768 & AS/NZS 3007 Single combined earthing systems Unless:

• Underground mining

• Reduce: impedance of the earthing system, earth fault current or fault clearing time • Surface insulating layers, grading rings or bonding concrete reinforcement • Separation of HV and LV earthing systems • Combined HV and LV earthing systems • Isolation • Protective barriers or signs • Remove non compliant infrastructure (eg telco pit)

Risk Management Treatment Methods

Mitigation methods for Lightning effects

- Minimizing the lightning collection area

- Supplementary bonding underground in accordance to the principles of AS/NZS1768

- Insulation between separate earthing systems

- Reducing earth connection resistances

Lightning Protection Systems in Mines

• Surface installations, protection - AS/NZS 1768

• For underground mines, bonding principles in underground - AS/NZS 1768

Transfer of lightning to other parts of the mine?

• Minimised through separated earthing systems.

Boreholes:

• Voltages can be controlled with PVC insulation. (Dielectric Isolation)

• Stopping the casing short (say 10m above roof mesh) is a similar outcome to PVC for current.

ACARP PROJECT FINDINGS Project C22003 published 1/6/2015

‘Investigation of the Potential Lightning Impacts on Underground Coal Mines’

Source: ‘Investigation of the Potential Lightning Impacts on Underground Coal Mines’ by Prof. David Cliff

ACARP PROJECT FINDINGS Boreholes (cont):

• Attenuation ↑ lower soil resistivity

deeper the mine

• Impulsive volts ~ tens of kV on roof mesh

• Gaps in roof mesh - reduced currents

Project C22003 published 1/6/2015

Direct strike to surface transformers feeding U/G parts

ACARP PROJECT FINDINGS Project C22003 published 1/6/2015

• Large Voltages (~MV) transferred by cable screens to U/G parts

• Potential difference in U/G

soil → cable screen armour was found to be very high.

• Voltages relatively independent of mine depth, frequency and length of power cable

• Smaller than direct strike BUT still significant

• The potentials in U/G Parts↑significantly

for increasing cable lengths for higher freq.

ACARP PROJECT FINDINGS Project C22003 published 1/6/2015

Indirect strike to a cable due to a horizontal lightning channel

Why would we separate earthing systems?

• Majority of cases safer to commonly bond.

– Improves overall system impedance

– Lower EPRs and touch hazards

– Simplest and easiest configuration to maintain

– Less damage to equipment

Separate or Common Earthing Systems?

Why would we separate?

A note on separations – two scenarios

1. Separation between power systems EPR from a power system fault:

Main Substation

mine infrastructure (or Mine Surface Earth)

What is the source of electrical energy?

A note on separations

2. Separation for lightning transfer

surface

underground

Separated Power Earthing Systems

When EPR cannot be managed through Common Bonding. Sparse networks

Separation comes with a few difficulties:

• Confusion from multiple earthing systems

• Hazards within yard due to different potentials on earths

• Lightning Flashover – correct insulation levels

• Earth switches/switchboard earths maintenance

• Single point bonding / cable damage

Common Bonded vs Separated

Q: Do the scales tilt at any point?

A: Yes, however the optimum arrangement is not always apparent;

Lower touch voltages with more exposure

vs

Higher touch voltages with less exposure

Each system differs and requires detailed assessment.

Independent

Earthing System

Impedance Earthed - IT systems:

The earthable point of the power system is either isolated from earth or earthed through an impedance

Provide for the safe management of voltages during earth faults

Impedance Earthed systems

According to AS/NZS 3007:2013

“There is potential benefit for electrical supplies entering underground being impedance earthed systems”

Readily controls touch and transfer potential

Typical Earthing systems … and how they go together:

Network Earth

Mine Surface Earth (MSE)

Mine Underground Earth (MUE)

Lightning Earth

Network Earth … is the Earthing system associated with the incoming supply.

Irrespective of earth connection to the upstream Network

substation, the earth fault belongs to network.

Mine Earth

Q: What is a Lightning Earth?

A: An earth termination intended to discharge lightning currents into the general mass of the earth.

Lightning Earth and transfer effects

What happens when the structure is used as a downconductor?

Can there be Lightning and Power System separation?

Lightning Earth and transfer effects

Lightning Earth and Mine Earths

Unless suitably protected, all surface structures can be assumed to be incorporated into a lightning earth.

Power Systems Separation

Neutral Earthing Resistor

Power Systems Separation

What is this earth?

Where to earth?

Power Systems Separation

Insulated and isolated earth

Power Systems Separation

Two earthing systems within the one yard

Power Systems Separation

Insulated and Isolated

Earth Bar

Power Systems Separation

Power Systems Separation

NER connected to Network Earth

How to connect these?

Power Systems Separation

Screens bared back and insulated Single point

bonded at Mine Earth

Power Systems Separation

Controlled Area

Single point bonded at Mine Earth

Power Systems Separation

Single point bonded at

Network Earth

Feeder Maintenance

Breaker 1 open Earth Switch 1 closed

Breaker 2 open Earth Switch 2 closed

Lightning transfer effects and U/G mines

According to AS/NZS 3007:

No likelihood of transfer:

AS/NZS 1768.

If likely transfer effects:

No direct connection

Lightning earth Mine Undergroud Earth (MUE)

Where do we provide the separation?

Mine Earths – Surface and Underground?

Screens bared back

Mine Underground Earth

• Transfer mechanisms

- Transition from Surface to UG

- Through different districts in UG areas

• Areas of interest:

- sealed areas

- working faces

- return airways

U/G Bonding Practices

Borehole Mitigations

• Depends on the construction type of borehole and what services use the borehole

U/G Bonding Practices

- High Voltage feeders

- Gas Drainage / Dewatering / Submersible pumps

- Ballast / Concrete / Stone dust drop holes

- Steel lined air shafts

- Piezometers and Extensometers

- Tube bundle (caternaries) and communications

Before sealing up longwalls

• Breaks in mesh in gate roads

• Removal or breaking of pipe lengths and cables

• Removing or treatment of mesh at seals

• Careful attention to pipe penetrations through seals

U/G Bonding Practices

Mines are being asked to implement controls to minimise lightning effects. Effective earthing systems such as separated systems are shown to reduce energy transfer.

Summary