In field accident investigation reports of shear bolt connectors, one type of root cause of failure has been systematically ignored for a long time.
The report will record the breakdown of the EPDM rubber insulation layer and describe the carbonization trace path on the joint surface, but it will almost never trace back to the real starting point of the accident – the bolt that failed to break cleanly and the 1–3mm metal burr (Burr) it left on the inside of the rubber insulation sleeve.
That burr was the origin of failure. It just took two years for it to become visible.
How are burrs formed?
When the shear bolt fails to achieve a one-time flush fracture under the design torque, metal protrusions – that is, burrs – will remain on the surface of the connector. Causes include: non-compliance of bolt material, geometric size deviation of the shear groove, repeated force application during installation, or force application direction off-axis.
Burrs are nearly invisible on visual inspection, especially if EPDM heat shrink or cold shrink tubing has covered the connector.
But what happens next inside the casing is far from an “invisible small flaw.”
Complete expansion of the failure chain:
A point contact stress concentration area is formed between the burr and the EPDM rubber sleeve, resulting in a tiny air gap. The dielectric strength of the air gap (approximately 3 kV/mm) is much lower than that of the EPDM body (typically over 20 kV/mm). Under normal operating voltage stress, the air gap reaches the breakdown threshold first, and partial discharge (Partial Discharge, PD) is initiated.
Partial discharges continue to erode the EPDM surface: micro-pits are formed, surface roughening intensifies, and the rubber gradually becomes hydrophilic. The rough surface absorbs moisture and pollution particles, forming a local conductive layer. Once the surface conductive channel is established, the electrical tracking process starts: the carbonized conductive path continues to extend on the insulating surface until flashover or body breakdown is triggered.
The entire process complies with the typical failure mechanism described in the IEC 60587 tracking and erosion test standard – slow, self-reinforcing, and undetectable in the early stages by conventional thermal imaging inspections.
What does “Clean Fracture Design” mean at the manufacturing level?
Qualified shear bolts must achieve flush fracture at one time under the preset torque: no residual protrusions, no burrs, and the cross section is completely flush with the surface of the connecting body. This cannot be verified through visual inspection of the finished product and can only be guaranteed by material specification control and shear groove geometric accuracy during the manufacturing process.
The design of the stepless bolt hole (Stepless Bore) is also critical: any steps or edges in the inner hole will create a similar air gap between the conductor and the insulating sleeve, forming the same starting point of partial discharge. Together, these two design flaws pose the greatest potential threat to the long-term reliability of EPDM insulation systems.
I believe: “Burrs are not accidental quality defects, but systemic signals of bolt material, shear groove geometry, or manufacturing process control out of control—its cost is borne by the on-site operation and maintenance team years later, and is often not traceable to the connector itself.”
Being able to continue to provide customers with high-quality termination products is something that our SUNJ team considers very important, and it is also the guarantee that every EPC engineer responsible for O&M (Operation and Maintenance) and QA/QC (Quality Assurance and Control) really hopes to get.
During field inspections or failure investigations of installed cable connectors, have your teams ever traced the root cause of insulation failure to the connector manufacturing process? Welcome to share in the comment area. I hope to have in-depth exchanges on this topic with more industry professionals.
If you are responsible for connector selection, acceptance criteria development or operation and maintenance procedures for 1kV–36kV systems, please follow me and chat with me about your specific scenarios. I will be very happy to give you some useful suggestions from the manufacturer’s perspective!
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