The earth strip is the cheapest part of the earthing system and the one people guess at most. I have watched a 25 by 3 mm GI strip get specified for a fault duty that needed at least 50 by 6, chosen for no better reason than that the last job used 25 by 3. A protective conductor has one job in its worst moment: carry the full fault current for as long as the breaker takes to clear it, without melting and without opening a joint. Size it for that moment, not for what happens to fit the trench.
What the strip has to survive
When a fault dumps current into the earthing system, the conductor heats almost instantly, because there is no time for the heat to escape into the soil. The standards call this adiabatic heating. The cross-section you need comes from three things: the prospective fault current, how long the protection takes to clear it, and the material constant of the conductor. IS 3043 gives the relationship. In plain terms, the conductor area in square millimetres is the fault current in amps times the square root of the clearing time in seconds, divided by a material constant k. For copper k is about 159 and for steel about 80, and that single difference is why steel sections have to be so much larger than copper for the same duty.
A worked feel for the numbers
Take a 25 kA fault cleared in one second. For copper, 25000 times the square root of 1, divided by 159, is about 157 sq mm, so a 25 by 6 mm copper flat at 150 sq mm is right on the edge and a 31 by 6 is comfortable. For steel, with k around 80, the same duty needs about 312 sq mm, which is why you step up to a 50 by 6 mm GI flat at 300 sq mm, or larger. Halve the clearing time and the area drops by only about 30 percent, because it follows the square root, not a straight line. This is the reason a relay setting and a conductor size are one conversation, not two.
GI, copper or copper-bonded
- GI (galvanized steel) flat: lowest cost, fine for above-ground grid risers and structure bonding in dry or moderate soil. It needs a bigger cross-section for the same duty, and the zinc is its only corrosion defence below ground.
- Copper flat or tape: highest conductivity, smallest section for the duty, the right choice for main earth grids, substation mats and anywhere long life and good joints matter. It costs more and can be a theft target above ground.
- Copper-bonded steel: a steel core with a molecularly bonded copper layer, giving most of copper's underground life at a lower price, common for buried grid and electrode tails.
- A common split is GI above ground and copper or copper-bonded below, but never run bare copper and bare steel together in the same wet soil without thought, because the copper drives galvanic corrosion of the steel.
Corrosion allowance, because soil eats steel
The adiabatic size is only the electrical minimum. Below ground you add a corrosion allowance on top of it, because a strip that is electrically fine on day one can be half gone in fifteen years if the soil is aggressive. For buried GI, common practice is to add sacrificial thickness and never drop below 6 mm on a buried flat, since thin strip perforates and breaks the grid where you cannot see it. Copper needs little allowance but still wants its joints protected. Where the soil is corrosive, copper-bonded or solid copper earns its price by simply still being there at the next audit.
Joints and burial, where good strip still fails
A correctly sized run is only as good as its joints and its depth. Bolted laps should overlap at least the width of the strip, use two bolts, and be tinned or greased against corrosion. Better still for buried grid is exothermic (thermite) welding, which gives a joint that will not loosen or corrode the way a bolted lap does. Bury the main grid below 500 mm so it stays in moist, stable soil and clear of cultivation and frost. Bring test points up to accessible links. And bond the strip to structural steel, fencing and cable tray so the whole installation sits at one potential when a fault hits.
Size the earth strip from the fault current and the breaker time, then add metal for the soil to eat. The strip that passed the calculation but skipped the corrosion allowance is the one sitting open-circuit at the fifteen-year inspection.
Need earthing strip and tape sized to your fault study? We supply GI, copper and copper-bonded flats and tapes cut and drilled to drawing, with MTC stating grade and cross-section, plus exothermic joint kits for buried grid.