An arc flash incident occurred at a coal mine when a 3.3kV back to back coupler failed under load. The investigation found a high resistance joint caused a phase to phase fault. While electrical protections operated, the closest protection relay did not operate fast enough to minimize incident energy. Modeling found actual clearing times were longer than settings. Protection settings were optimized to halve the incident energy by clearing faults up to 4 cycles faster through reduced time delays. Future considerations include challenging protection designs and settings to be as low risk as reasonably practicable.

1. ULAN WEST
UNDERGROUND
GLENCORE
3.3kV back to back failure

2. Incident Summary
ULAN WEST
UNDERGROUND
GLENCORE

3. On the 3rd November 2017, During the installation and commissioning of LW04 face equipment, A shearer
supply 3.3kV flameproof back to back coupler and plug combination, located along the monorail,
catastrophically failed under load during start-up of the shearer drums.
The investigation concluded there was a high resistance joint between the thimbles and sockets resulting
in a catastrophic phase to phase fault within the restrained plug and back to back. The plug was ejected
from the back to back causing damage to the locking mechanism and the arc flash caused severe burning
of the back to back phase pins and melting of the phase pin insulation material.
A review of the electrical protection which failed to protect the 3.3kV restrained back to back coupler and
plug combination during the internal arcing fault was carried out, focusing on the protection design,
protection settings, fault levels, and clearing times.
3

4. Physical Layout
ULAN WEST
UNDERGROUND
GLENCORE

5. Monorail Layout Overview:
5
11kV DCB
Outbye (To Longwall Substation)410M
Inbye (To Longwall Shearer)
447M

6. Monorail Layout Overview:
6
3.3kV DCB11kV DCB
Outbye (To Longwall Substation)410M
Inbye (To Longwall Shearer)
447M

7. Monorail Layout Overview:
7
3.3kV DCB11kV DCB 3.3kV Back to Back
Outbye (To Longwall Substation)
49M
410M
Inbye (To Longwall Shearer)
447M

8. Monorail Layout Overview:
8
3.3kV DCB11kV DCB
Outbye (To Longwall Substation)
3.3kV Back to Back
3.3kV Back to Back
49M 41M
410M
Inbye (To Longwall Shearer)
447M

9. Monorail Layout Overview:
9
3.3kV DCB11kV DCB
3.3kV Back to Back
Outbye (To Longwall Substation)
Inbye (To Longwall Shearer)
3.3kV Back to Back
3.3kV Back to Back
49M 41M
41M
410M
447M

10. Monorail Layout Overview:
10
3.3kV DCB11kV DCB
INCIDENT LOCATION
Outbye (To Longwall Substation)
3.3kV Back to Back3.3kV Back to Back
3.3kV Back to Back
49M 41M
Inbye (To Longwall Shearer)
447M

11. Incident Scene – As Found
ULAN WEST
UNDERGROUND
GLENCORE

12. 12
Looking Outbye

13. 13
3.3kV Back to Back

14. 14
Ejected Cable

15. 15
Ejected Cable

16. 16
Charing
Sprayed
Copper Slag

17. 17
Sprayed
Copper Slag

18. 18
Sprayed
Copper Slag

19. 19

20. 20

21. 21
Missing Phase Pin

22. 22
Locking Mechanism
Damaged But Remained
Locked In Place

23. 23
Missing Phase Pin Found
Below Back to Back

24. Electrical Protection Layout
ULAN WEST
UNDERGROUND
GLENCORE

25. Electrical Protection Layout SLD
25
• There are four layers of
overcurrent protection inline
from the supply substation
(furtherst), to the 3.3kV supply
outlet (Closest)

26. Understanding The Fault - Electrical Protection Review
ULAN WEST
UNDERGROUND
GLENCORE

27. What Protection Operated?
Two overcurrent protection relays operated during the fault and one Earth Leakage Relay.
1. The LW 11/3.3kV substation 11kV CB Sepam OCR, tripped on Instantaneous overcurrent
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28. What Protection Operated?
Two overcurrent protection relays operated during the fault and one Earth Leakage Relay.
1. The LW 11/3.3kV substation 11kV CB Sepam OCR, tripped on Instantaneous overcurrent
2. The LW 11/3.3kV Txf BUELR. This relay senses a 3.3kV earth fault and trips the 11kV CB.
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29. What Protection Operated?
Two overcurrent protection relays operated during the fault and one Earth Leakage Relay.
1. The LW 11/3.3kV substation 11kV CB Sepam OCR, tripped on Instantaneous overcurrent
2. The LW 11/3.3kV Txf BUELR. This relay senses a 3.3kV earth fault and trips the 11kV CB.
3. The 3.3kV BSL DCB main bus protection OCR
29

30. Understanding The Fault –
Layers Of Protection Review
ULAN WEST
UNDERGROUND
GLENCORE

31. Layers of Protection Analysis – Incident Location
31
• Following the principles of EES005 – Protection Safety Factors:• Calculated Fault Level at incident location was 5.092kA
• All Protection relays were confirmed as set correctly as per the current protection design
• Following the principles of EES005 – Protection Safety Factors:

32. Layers of Protection Analysis – Incident Location
32
Operated Protection
• Calculated Fault Level at incident location was 5.092kA
• All Protection relays were confirmed as set correctly as per the current protection design
• Following the principles of EES005 – Protection Safety Factors:

33. Understanding The Fault –
Why didn't the closest protection relay operate?
ULAN WEST
UNDERGROUND
GLENCORE

34. What happened to the layers of Protection?
• Why didn’t the closest protection relay operate?
• The settings were all correct as per the current design
• The grading appeared sufficient
34

35. What happened to the layers of Protection?
35
• Why didn’t the closest protection relay operate?
Closest

36. Clearing Time Settings
36
Closest

37. Clearing Time Settings
37
Should have cleared
fault
Closest

38. What about the ACTUAL clearing time?
• A review was then conducted of protection relay operation and associated clearing times.
• The intent was to understand actual clearing times of faults based on the different hard wired circuit
arrangements in use, in order to better understand the incident energy that was clearly evident.
38

39. 11kV Protection Relay - OPERATED
1.1 SS901 11kV side Sepam
Intentional Delay: 100mS
Sepam internal delay: 20mS
UV and S/T coils
Evolis CB 60mS
Total clearing time: 180mS
39
Closest

40. 3.3kV BSL DCB outlets (dual feed) – DID NOT OPERATE
40
1.1 SS901 3.3kV BSL DCB outlets (dual feed)
Intentional Delay: 100mS
IPD Internal delay: 20mS
Finder relay (via MCR contact): 10mS
UV and S/T coils
Evolis CB 60mS
Total clearing time: 190mS
Closest

41. 3.3kV BSL DCB Bus IPD protection - OPERATED
41
1.1 BSL DCB Bus IPD protection
Intentional Delay: 20mS
IPD Internal delay: 20mS
Finder relay: 10mS
Safety Relay: 60mS
UV and S/T coils
Evolis CB 60mS
Total clearing time: 170mSClosest

42. 3.3kV Shearer Outlet – DID NOT OPERATE
42
1.1 Shearer Outlet
Intentional Delay: 100mS
IPD Internal delay: 20mS
Finder relay: 10mS
Safety Relay: 60mS
UV and S/T coils
Evolis CB 60mS
Total clearing time: 250mS
Closest

43. Set Versus Actuals
43
Closest

44. Set Versus Actuals
44
Closest

45. • Ulan West then engaged the original equipment manufacturer and an independent consultant to review
the current protection design, protection settings, and model the load flow to gain a better
understanding of:
• the prospective fault current,
• the arcing fault hazard based on these tripping times,
• and review the safety factors applied to protection settings
• Models were run on the current and proposed design changes.
• We focused on optimisation of the protection settings to minimising the protective circuitry time delays.
45

46. Conclusion – Protection Settings Optimisation
ULAN WEST
UNDERGROUND
GLENCORE

47. Protection Settings Optimisation
The protection as it was set for the system, operated correctly.
• The immediate improvement opportunity focused on was to reduce the incident energy by clearing the
fault quickly.
• All protective elements were able to gain improvements in clearing times of between 2.4 to 4 cycles and
this is reflected in the arc flash energy calculations.
• The results of the arc flash study show that at the cable plug fault point using a conservative working
distance of 300mm, and utilising the original settings, presented arcing energy of 27.87J/cm2, (Category
2 arcing fault).
• A final model was then used with the revised settings to demonstrate a reduction in the incident energy
to 14.06J/cm2 (Almost halving the incident energy), and the requirements drop to a Category 1 arcing
fault. (Standard U/G PPE)
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48. Conclusion – Future Consideration
ULAN WEST
UNDERGROUND
GLENCORE

49. Our Future Consideration
• Challenge Protection Settings:
• Are they Optimised to make use of the latest protection relay software and functionality
• Are they actually ALARP
• Challenge Protection Circuit Design:
• Its easy to unintentionally accumulate in excess of
100mS of mechanical lag into an otherwise
“instantaneous” trip circuits through the addition
of indication, safety, or interposing relays.
• Do We Need More Layers of Protection
• The inclusion of Fault Limiting Fuses
• The Ultra Fast Earth Switch
49
OC Relay

50. Thankyou,
Peter Every-Burns
Special Thanks:
Jacob Brown, Tom Tyler, Daniel O’Keeffe, Roger Devery
ULAN WEST
UNDERGROUND
GLENCORE