A cable cleat spends its whole life holding cables quietly in place — until a short circuit, when for a few tens of milliseconds it has to restrain a force that can reach several thousand newtons per metre. Get the selection wrong and the cables tear free of the structure. The selection itself is a short calculation chain: fault current → peak current → peak force → cleat class at a spacing. This walks through it.
During a fault, adjacent power cables repel each other with a force proportional to the square of the peak short-circuit current and inversely proportional to the spacing between cables. Because it is a force per unit length, the load on each cleat is that force times the cleat spacing. Selection means matching an IEC 61914 short-circuit class (a tested peak current at a stated cleat spacing and cable centre distance) to the project's prospective fault current. If the fault level exceeds the rating, reduce the cleat spacing rather than assume a bigger cleat.
- Drives the force
- Peak current squared · cable centre-to-centre spacing
- Drives the load per cleat
- Force per metre × cleat spacing along the run
- The rating
- IEC 61914 short-circuit withstand: peak kA at a stated cleat spacing
- First lever if under-rated
- Reduce cleat spacing (halving spacing ≈ halves cleat load)
§ 01 What Happens in a Fault
Two parallel conductors carrying current in opposite directions push apart; carrying current the same way, they pull together. In a three-phase fault the currents are large and the phase relationships make the net force on the cables violently repulsive for the first peak. The cables try to fly apart, and the only thing holding them to the structure is the cleats.
The current that matters is not the steady running current — it is the peak asymmetric short-circuit current, the very first current peak after the fault strikes, before it decays. That peak is what the cleats must survive. For the difference between cleats and ordinary clamps in this duty, see cable cleats vs cable clamps.
§ 02 The Peak Force Calculation
For cables in a trefoil (three-cable clover) formation, the peak force per unit length between cables during a three-phase fault is approximated by:
| Term | Expression / value | Note |
|---|---|---|
| Peak force per metre | F ≈ 0.17 × ip² / s | F in N/m, ip in kA, s in m |
| ip — peak current | ip = κ · √2 · Isc | κ ≈ 1.8–2.0 by X/R ratio |
| Isc | RMS symmetrical fault current | From the project short-circuit study |
| s | Cable centre-to-centre spacing | Set by cable OD in trefoil |
| 0.17 coefficient | μ₀/2π × trefoil factor | 0.2 for two conductors × ≈0.866 |
The two things to take from the formula: the force scales with the square of peak current — doubling the fault level quadruples the force — and it scales inversely with cable spacing, which in trefoil is fixed by the cable OD and bundle size. You do not get to choose cable spacing freely; you choose the cleat and its spacing along the run.
§ 03 From Fault Current to Cleat Class
IEC 61914 defines a short-circuit withstand test: a cleat is subjected to a declared peak current at a stated cleat spacing and cable configuration, and passes if the cables do not become detached and the cleat does not fracture. The manufacturer publishes the result as a short-circuit rating — a peak kA figure tied to a specific spacing.
- Get the prospective fault current (Isc) and X/R ratio from the electrical design or short-circuit study
- Convert to peak current ip with the κ factor — offshore and near-generator positions tend to higher κ
- Match a cleat class whose IEC 61914 tested peak current at your intended spacing meets or exceeds ip, with margin
- Check the test configuration matches — a rating for trefoil does not transfer to flat formation, and cable OD must be within the tested range
The independent evidence for the rating is the test report; what to ask for is covered in IEC 61914 cable cleat test reports.
§ 04 How Spacing Changes the Load
Because F is a force per metre, the load an individual cleat must restrain is F × cleat spacing. This is the main design lever:
- Closer cleats, lower load each — halving the cleat spacing roughly halves the force each cleat carries, so a given cleat can hold a higher fault current when installed closer together
- This is why ratings quote a maximum spacing — a cleat rated for a peak current at 600 mm spacing is not rated for the same current at 1200 mm
- First move when under-rated — if the fault level exceeds the cleat's rating at the planned spacing, reduce the spacing before assuming a heavier cleat; often a spacing change closes the gap
- Coordinate with support spacing — the mechanical support spacing from cable cleat spacing on the tower and the short-circuit spacing must both be satisfied; take the smaller
§ 05 Selection & RFQ Inputs
A cleat enquiry that a supplier can answer for fault duty contains:
- Prospective short-circuit current (Isc, kA RMS) and X/R ratio or κ factor
- Cable formation — trefoil or flat — and cable OD, which sets the centre spacing
- Intended cleat spacing along the run, or a request for the maximum spacing at the fault level
- Material and corrosion category — SS316 vs aluminium for the environment; offshore stays A4/316
- Standard reference — IEC 61914 short-circuit class and the test report requirement
For the full parameter set and a ready structure, adapt the trefoil cable cleat specification guide.
For floating offshore routes, pair this checklist with dynamic cable-cleat RFQ and inspection data before approving the replacement or new-build package.
Evidence and decision boundary
- Direct evidence
- IEC 61914:2021 covers requirements and tests for cable cleats and intermediate restraints, including declared resistance to electromechanical forces. IEC 60865-1 provides calculation procedures for mechanical and thermal short-circuit effects, while IEC 60909-0 provides the upstream short-circuit-current calculation framework.
- Engineering inference
- A cable-cleat release decision requires comparison of the project fault study with the tested cable formation, cable diameter, support spacing, mounting surface, cleat components and report acceptance criteria. A standard reference by itself is not proof that a particular product/configuration passed.
- Typical or indicative value
- Formulas, spacing adjustments and selection margins on this page are preliminary engineering aids. Final acceptance must use the project short-circuit study and a traceable IEC 61914 report for the offered configuration, or a documented engineering assessment of every difference.
Primary sources checked
- IEC 61914:2021 - requirements and tests for cable cleats
- IEC 60865-1:2011 - calculation of mechanical and thermal short-circuit effects
- IEC 60909-0:2016 - calculation of three-phase AC short-circuit currents
Related commercial route: Compare the relevant clamp systems and project inputs.