Braid for earthing and grounding

What an earth braid has to do

An earth braid provides a low impedance path from a metal part to the earthing system. In normal service it carries no current, or a small leakage current. During a fault it carries a large current for the time it takes the protective device to clear, and it has to do so without melting, arcing at a joint or setting nearby material alight.

It also has to survive the environment. A braid is often used where a rigid conductor would be inconvenient: across a door hinge, between a removable panel and the frame, around a vibration isolation mount or between two parts that move as the equipment warms. The braid has to stay flexible and stay conductive for the life of the installation.

Fault current and duration

The first input is the prospective fault current at the point where the braid is connected. That is not the supply fault level in general; it is the current that would flow through this path, which depends on the impedance of the fault loop and on how the current divides between parallel paths.

The second input is the clearing time of the protective device. A braid that carries 10 kA for 0.1 s is a different proposition from one that carries the same current for 3 s. The energy rises with the square of the current and linearly with time, so a longer clearing time drives the section up quickly.

The third input is the initial temperature. A braid that starts at 30 °C has more thermal headroom than one that starts at 70 °C inside a warm enclosure. The permitted final temperature depends on the material, the insulation around it and the risk of damage to adjacent parts.

Sizing the braid

A first estimate uses the adiabatic equation: the conductor section in square millimetres equals the square root of the current squared times the duration, divided by a factor k. The factor comes from the standard for the conductor material and the permitted temperature rise. For illustration, with k of 143 for a copper conductor starting at 90 °C and limited to 250 °C, a fault of 10 kA for 0.5 s gives about 49 mm².

That calculation assumes a solid conductor with uniform heating. A braid is not solid. The strands heat individually, the current sharing between carriers is uneven, and the joints add resistance and heat. The adiabatic result is therefore a starting point, not a rating. A braid for a high fault duty needs data from the maker or a test on the actual construction.

There is also a mechanical minimum. A braid that is adequate for a small fault current can still be too fragile for the installation, especially at a door hinge or a removable panel. The section has to survive handling, vibration and repeated movement, not just the fault.

Corrosion and dissimilar metals

Copper is cathodic to aluminium and to steel. In the presence of moisture, a copper braid bolted to an aluminium bar forms a galvanic cell, and the aluminium corrodes. The corrosion products are resistive, so the joint resistance rises and the earth path degrades without any visible fault.

The usual defences are a bimetallic transition, a plated interface, a sealed joint or a joint compound. Tin plating on the copper and on the aluminium contact face reduces the potential difference and slows the corrosion. A sealed boot keeps moisture out but has to survive the temperature and the movement.

The environment sets the severity. A dry indoor cabinet is mild. A coastal site, a wash-down area or an outdoor enclosure is severe, and the joint needs more protection. The braid itself is usually tinned copper in these locations, because bare copper oxidises and the oxide is resistive.

Termination and surface preparation

The earth joint is often the one that gets the least attention. Paint, powder coat, anodising and oxide are all insulators. The contact area has to be bare metal, and the paint has to be removed around the stud, not just under the washer, so the clamp load is spread over a clean face.

Use a dedicated earth stud where possible. A stud welded or brazed to the frame gives a stable contact and a known torque. A self-tapping screw into thin sheet is a poor earth connection, because the thread can loosen and the contact area is small.

A serrated washer can bite through a thin oxide layer, but it also cuts the plating and creates a path for corrosion. A flat washer and a clean, plated face is usually better, with a sealed cover if the environment is wet. The torque has to be stated and checked, because a loose earth joint is a hidden fault.

Continuity and resistance testing

The first test is continuity from the part to the earthing bar. A low-resistance ohmmeter or a millivolt drop test at a stated current gives a number that can be compared with a limit. A simple buzzer proves almost nothing, because it will sound through a joint that is far too resistive.

The second test is the joint resistance across each termination. Measure from the braid to the bar, not from one end of the braid to the other, so the reading isolates the joint. Record the current, the reading and the temperature, as you would for any millivolt drop test.

The third check is visual. Look for corrosion, a loose fastener, a braid that has been twisted or crushed, and a sleeve that has slid away from the pad. On a door or a moving part, open and close it through the full travel and check that the braid is not pulled tight or trapped.

Writing the specification

State the fault current, the clearing time and the start temperature. Add the conductor and the construction, the termination and the environment. Then add the test: the continuity check, the joint resistance limit and the visual criteria. A braid specified only by width and length leaves the fault duty undefined.

For a moving earth, add the movement duty. A door braid that flexes every shift needs a bend life check, even though its fault duty is short. The bend life and the fault capacity are separate requirements, and both have to be met by the same part.

Finally, state the installation. The braid should reach both pads without tension, with a loop that stays in plane and clear of sharp edges. The fastener torque and the surface preparation belong in the assembly instruction, because the best braid in the catalogue can be defeated by a painted frame and a loose bolt.

• Fault current, clearing time and start temperature
• Conductor, plating and braid construction
• Pad size, stud or bolt and torque
• Environment and corrosion protection
• Movement duty and bend life
• Continuity and joint resistance limits
• Installation route and loop rule