Can we make mining operations immune from lightning induced explosions darren woodhouse

27th Electrical Engineering Safety Seminar
Can we make mining operations immune from
lightning induced explosions? Copyright © Safearth Consulting 2017
Sydney 2017
1/12
Electrical Engineering Safety Seminar 2017 - Sydney
Can we make mining operations immune from lightning induced explosions?
Dr Darren Woodhouse
ELECTRICAL ENGINEERING SAFETY SEMINAR 2017
November 2017
lightning induced explosions?
Dr Darren Woodhouse
Dr Darren Woodhouse
Bold Statement #1
Lightning is pretty cool …
… until it hits you!

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Dr Darren Woodhouse
Isokeuranic Charts
Dr Darren Woodhouse
Ground Flash Density

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Dr Darren Woodhouse
During one storm which lasted just
over two hours 5295 strokes!
Roughly 1 strike every 2 seconds!
Lightning Stroke Density
Dr Darren Woodhouse
• UQ MISHC  Minerals Industry Safety and Health Centre, Uni of Queensland
• Makes 16 “considered important” conclusions
• Makes overwhelming use to Capitalisation of the First letter of a word Randomly
Within a Sentence
• No.1 Conclusion
“… in most cases lightning voltages generated at the bottom of vertical metallic
pipes … are small enough for normal thicknesses of dielectric such as PVC ...”
ACARP Research Project C22003
Lightning in UG Coal Mines – UQ MISHC

Sydney 2017
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Dr Darren Woodhouse
10. Attenuation of the lightning current and voltage on buried vertical pipes increases as
soil resistivity decreases and mine depth increases.
12. The soil resistivity profile (number of layers and layer thickness) has a strong influence
upon the scalar potentials that can appear underground. However, when the top layer
thickness is relatively small compared to the mine depth, the overriding factor is still the
“infinite” soil resistivity below the top layer(s).
13. For a direct strike to the origin of the sloping pipe (above ground), the voltages seen
underground are considerably smaller than those for a vertical pipe (since there is a longer
distance over which attenuation occurs).
… and on
Attenuation
Dr Darren Woodhouse
What changes the voltage on the structure?
• The strike
• The soil resistivity
• The borehole length and material
• Connection (earthed) to the borehole
11
When lightning hits a buried conductive structure … ?

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Dr Darren Woodhouse
• Direct Injection of Current
• Electric Conduction
• Magnetic Induction
• … and combinations thereof!
Mechanisms for Transfer
Dr Darren Woodhouse
• Doesn’t have to be vertical!
Models Considered

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Dr Darren Woodhouse
Lightning strike characteristic
microseconds
Amps
Dr Darren Woodhouse
Soil - homogeneous
Pipe - homogeneous
Video – Uniform Borehole

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Dr Darren Woodhouse
Transients don’t behave like power frequency
Surge or characteristic impedance of a path
vrs
Terminating impedance
• Don’t make fast/severe changes to path type
• Divide and Conquer
Travelling waves
Dr Darren Woodhouse
If lightning doesn’t like something it gets upset!
Remember, by definition, any increase in voltage induced by lightning leads to a
flash.
Petulant Child

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Dr Darren Woodhouse
The old vanguard of earthing, to separate or not!
And what does it mean?
Beware that segregation, aka no interaction between systems, is almost impossible!
Separation or Bonded, not Segregated!
Dr Darren Woodhouse
The Ideal Solution …
Now what could it be?

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Dr Darren Woodhouse
• Low resistivity ‘soil’ or cover.
• Protection for the ‘lip’ of the entrance.
• Flat unattractive surface.
• Attractive elements can be located elsewhere.
Ideal solution
Dr Darren Woodhouse
• High resistivity ‘soil’.
• No protection for the work area.
• Irregular surface.
• Attractive elements located in close proximity.
Non-ideal Circumstances

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Dr Darren Woodhouse
• Could solve the borehole transfer problem, but is it
practical?
• Doesn’t solve the magnetic coupling scenario.
Insulated Boreholes?
Dr Darren Woodhouse 29
Video – Well earthed borehole at depth

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Dr Darren Woodhouse
1. Isolation is not always the answer –
• It can fail!
• Can be hard to implement and maintain!
2. Bonding is a possible answer in some circumstances.
3. Lightning can’t be controlled but it can be cajoled, hindered & encouraged!
4. Choose wisely!
30
Conclusions
Dr Darren Woodhouse
The rules are:
1. Don’t be attractive to lightning!
2. Make, or encourage, something else to be more attractive!
3. Assume that you are attractive … aka your asset will get hit!
4. Make sure it can take the attention … aka current/energy.
5. Make sure it can jettison the attention … aka current/energy.
6. “Lightning doesn’t read standards!” – Prof. M. Darveniza
The ‘Vogue’ rules of Lightning Protection

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Dr Darren Woodhouse
• Further research required into …
• … a standard protocol for measuring soil resistivity, …
• More analysis could be undertaken to better quantity quantify the effect of the
gaps in the roof mesh, the size required and the spacing required.
• … computer model calculations should be validated against low level in situ
measurements …
• The key currents and voltages necessary to ignite methane need to be better
characterised …
ACARP Research Project C22003
Lightning in UG Coal Mines – UQ MISHC
Dr Darren Woodhouse
1. Identifying better rules for separation and bonding.
2. Looking at ways to attenuate voltage in metallic services before reaching
workings.
3. Quantifying response of services in workings.
4. Increasing the understanding of transients, in particular lightning!
5. Understanding how to limit power transfer in the first place.
6. How to test/verify mitigation is effective.
7. …
Alternative Suggestions for Further Work

Can we make mining operations immune from lightning induced explosions darren woodhouse

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