Exothermic welding vs mechanical clamps for earthing joints
Why buried earthing joints fail, how exothermic welds compare with bolted and compression connectors, and how to plan moulds, powder and joint types for a site.
Ashima Electrotech technical team · 3 min read
Short answer: Use exothermic welds for buried and concealed joints: electrode to strip, strip to strip, and cable to rod. They fuse the conductors into one piece, with no contact surface to corrode and no bolt to loosen. Keep bolted clamps for accessible test links, where you need to disconnect the electrode for testing.
Why earthing joints fail
Most earthing systems that "go bad" do not fail at the electrode. They fail at a joint. A mechanical joint relies on pressure between two metal surfaces. Underground, moisture and soil chemistry get between those surfaces and corrode them. Thermal cycling and vibration slowly relax the bolt tension. Dissimilar metals, such as copper against GI, set up galvanic corrosion. Resistance at the joint rises, sometimes until the conductor is effectively disconnected, while the electrode itself is fine.
How exothermic welding works
- The conductors are cleaned, dried and clamped into a graphite mould shaped for that joint and those conductor sizes.
- A metered charge of weld powder is placed in the mould and ignited. The powder is a mixture of copper oxide and aluminium.
- The reaction produces molten copper, which flows around and fuses the conductor ends.
- After cooling, the mould is opened and reused. The joint is a solid cast connection with a cross-section at least equal to the conductors.
Comparison
| Exothermic weld | Bolted / mechanical clamp | Compression connector | |
|---|---|---|---|
| Joint type | Fused, one piece | Contact pressure | Cold-formed contact |
| Long-term resistance | Stable | Can rise with corrosion and loosening | Stable if correctly crimped |
| Fault-current capacity | At least equal to the conductor | Limited by the clamp | Depends on the connector |
| Inspection needed | Visual at installation | Periodic re-tightening | Visual |
| Tools | Mould, powder, igniter, cleaning tools | Spanner | Hydraulic crimp tool and dies |
| Best for | Buried and concealed joints | Accessible test links | Where hot work is not allowed |
Planning a site
A mould is made for one joint geometry and one pair of conductor sizes. Before ordering, list for each joint type:
- the geometry: tee, cross, straight splice, cable to rod, or strip to rod,
- the size and material of each conductor, for example 25 × 3 mm GI strip, 32 × 6 mm copper strip, or a 17.2 mm rod,
- how many joints of each type there are. One mould handles many shots, but the powder is used per joint.
Add the accessories:
- handle clamps,
- a flint igniter,
- a wire brush and file card for cleaning the mould,
- sealing compound to close gaps around conductors,
- a slag scraper,
- gloves and eye protection.
Good practice
- Conductors and the mould must be clean and completely dry. Moisture causes porous welds and spatter. Pre-heat a cold or damp mould first.
- Use the correct powder charge for the mould. Too little leaves an incomplete joint.
- Inspect each weld: it should be fully formed, without large voids, and should cover the conductors.
- Follow hot-work safety rules: keep combustibles clear, wear PPE, and keep an extinguisher at hand.
From Ashima
We supply exothermic welding systems:
- graphite moulds for tee, cross, cable-to-rod, strip-to-rod and strip-to-strip joints,
- weld powder,
- the full set of accessories.
Send us your joint list and we will quote the moulds and powder quantity. See exothermic weld accessories.
Need a quotation?
Send the product, quantity, site details and delivery city. We will suggest the right item and quote.
Products mentioned
General technical guidance, not a design for a specific site. Follow your project specification, the current editions of the applicable standards and the requirements of the inspecting authority.