Exothermic welded connection (Cadweld) — permanent earthing connections

The guide on cable lugs and crimp connections covers the common, mechanically demountable connection method for conductors. This article covers an alternative technique used specifically for earthing connections where a permanent, non-demountable connection is desired: the exothermic welded connection, in practice often referred to by the brand name Cadweld (from manufacturer nVent ERICO), although other manufacturers offer comparable systems.

The operating principle: a thermite reaction

An exothermic welded connection does not use an external heat source such as a welding machine, but a chemical reaction between a metal oxide (copper oxide) and a reducing metal (aluminum) in powder form, placed in a graphite mold around the conductors to be joined. Upon ignition of the mixture — typically with a spark igniter — the reaction itself releases enough heat to let the mixture react completely: the aluminum reduces the copper oxide to molten copper, which then solidifies around and between the conductors being joined, forming a homogeneous, molecular bond — not a mechanical clamping connection as with a crimp connection, but an actually molten and re-solidified copper connection.

Why this differs from a crimp connection

A crimp connection (see the related guide) essentially remains a mechanical connection: the cable lug is physically deformed around the conductor, with a contact resistance that depends on contact pressure and area. An exothermic welded connection, by contrast, is a metallurgical connection: the conductors and the added copper melt together at the connection point into one continuous material, without a physical dividing surface between the original conductor and the connection material. This has two practical consequences: the connection has a lower and more stable contact resistance that does not depend on tightening torque or crimp pressure, and the connection cannot be dismantled without damaging the conductor — suitable for permanent applications, not for connections that may later need to be disconnected.

Typical applications

Exothermic welded connections are mainly applied for:

Earth electrodes and earth mats: connecting earthing conductors to a rod electrode or to the nodes of an earth mat in the ground, where the connection remains permanently buried and inaccessible for maintenance.
Structural steel and reinforcement: connecting an earthing conductor to structural steel or concrete reinforcement as a foundation earth.
Cable screen earthing on high-voltage cables: where a long-term reliable, low-resistance connection is essential and periodic dismantling is neither needed nor desired.

IEEE 837: the reference standard for qualification

IEEE 837 ("Standard for Qualifying Permanent Connections Used in Substation Grounding") is the internationally established reference standard for qualifying an exothermic-weld connection configuration (a combination of conductor diameters, materials, and mold) for use in earthing installations. The standard subjects a connection to a sequence of tests, including:

Freeze-thaw cycling, to check that repeated temperature variations do not weaken the connection.
Mechanical pull test, to confirm the mechanical strength of the connection.
Current-pulse testing, in which the connection is subjected to a series of short-circuit current pulses representative of the fault currents an earthing system must be able to carry in practice, followed by verification that the contact resistance has not increased significantly.

A connection that passes this sequence is qualified for the full duty of a substation earth grid — a qualification level considerably beyond what is required for a common crimp connection.

Note: the outcome of an exothermic welded connection strongly depends on correct execution — the right mold size and conductor diameter for the specific mold, a dry and clean mold (moisture can cause the reaction to spatter), and sufficient preheating of damp or cold conductors. An incorrectly executed exothermic weld can produce a porous, mechanically weak connection that nevertheless looks visually acceptable.

Practical relevance

When assessing an earthing installation with exothermic welded connections at buried or otherwise inaccessible points, it is important to verify that the connection shows a closed, evenly solidified copper surface without visible porosity or cracks, and that the correct mold and material quantity for the relevant conductor diameters has been used — a visual check at handover is especially important here, because the connection is no longer accessible for later inspection or dismantling once the trench has been backfilled.

Common mistakes

Using a damp or wet mold without sufficient pre-drying — this can cause molten material to spatter during the reaction and results in a porous, weaker connection.
Applying the wrong mold size for the conductor diameter — a mold that is too small or too large provides insufficient material around the conductor and thus a weakened connection.
Not performing a final visual check before backfilling an underground connection — once backfilled, the connection is no longer accessible for correction or repair.
Applying exothermic welded connections at points where future dismantling or modification is likely — the connection is permanent and cannot be undone without damaging the conductor, unlike a crimp or screw connection.