Cable Fault Bracing Calculator

Check the cleat strength needed to restrain cables during a short-circuit event, then compare the result against a selected cleat rating or the project Cable Schedule.

Method & Assumptions

During a short-circuit fault, the peak current flowing through adjacent conductors generates repulsive electromagnetic forces. Cable cleats or securing ties must resist these forces across their tributary span. This calculator quantifies the required rated tensile strength of the securing component per IEC 61914:2015.

1. Peak factor κ (IEC 60909-0:2016 §4.3.1.1):

κ = 1.02 + 0.98 × e^(−3R/X)

Range: 1.02 (purely resistive) to ≈ 2.0 (purely inductive). Typical values: LV switchboard X/R ≈ 5 → κ ≈ 1.36; MV cable fault X/R ≈ 15 → κ ≈ 1.73.

2. Peak fault current:

i_peak = κ × √2 × I_sc

3. Electromagnetic force per unit length (Biot–Savart / Ampère's force law):

Single-phase:   F = 2×10⁻⁷ × i_peak² / d   [N/m]
Three-phase:    F = √3×10⁻⁷ × i_peak² / d  [N/m]

where d is the centre-to-centre cable spacing in metres. The three-phase formula applies to both trefoil and flat (single-layer) arrangements and represents the analytically derived maximum force on the worst-case conductor over one fault cycle.

4. Cleat tensile load:

T = F × L_cleat_spacing   [N]

5. Required rated cleat strength (IEC 61914 default safety factor = 2.5):

T_req = T × SF   [kN]

Select a cleat whose IEC 61914 rated strength ≥ T_req. Verify the cleat OD range covers the cable outer diameter.

Citations: IEC 60909-0:2016, IEC 61914:2015 — Cable cleats for electrical installations.

See also: Seismic Bracing Calculator, Support Span Calculator, Short Circuit Study.

Fault System Parameters

Switch spacing and cleat span entry labels between imperial and metric units.

Obtain from a short-circuit study at the fault location. Typical values: LV (≤1 kV) 10–25 kA; MV (1–36 kV) 16–50 kA; HV (>36 kV) up to 85 kA. Use the prospective (maximum) fault current for a conservative cleat selection.

Common starting points:

Determines the DC offset (asymmetry) of the fault current. Typical: LV distribution board 2–10; MV cable system 10–25; MV/HV transformer terminals 15–40. A higher X/R gives a higher peak current (more conservative).

Three-phase: uses √3×10⁻⁷ coefficient (worst-case conductor). Single-phase or DC 2-conductor: uses 2×10⁻⁷ coefficient.

Both trefoil and flat arrangements produce the same peak force on the worst-case conductor for a balanced three-phase fault. The arrangement affects spacing: for touching trefoil/flat cables, spacing equals the cable OD.

Cable Geometry

Distance between the centres of adjacent conductors. For touching cables this equals the cable OD. For separated cables use the actual centre-to-centre distance. Smaller spacing increases the force (more conservative).

Cleat / Securing Device Parameters

Distance between adjacent cleats along the cable run. Each cleat must resist the cumulative force over this span. Shorter spacing reduces the load per cleat. Typical range: 300–1500 mm.

IEC 61914:2015 requires cleats to be proof-tested to 2.5× their rated load. The calculated cleat load is multiplied by this factor to give the minimum rated strength to specify.

If you have already selected a cleat, enter its IEC 61914 rated tensile strength to see whether it is adequate. Leave blank to skip the pass/fail check.

Evaluation Mode

Calculation Setup

Single Cable
Peak Multiplier
-
Spacing Source
-
Cleat Interval
-
Safety Factor
-

Review the inputs above before calculating.