Calculating Forces Between Conductors During a Three-Phase Fault
When the system peak fault current and cable diameter are known, you can use the formula from the International standard (IEC 61914) to calculate the forces between two conductors during a three-phase fault:
Ft=0.17×Sip2
Where:
- Ft = force in Newton/metre (N/m)
- ip = peak short-circuit current in kiloamp (kA)
- S = distance between the centrelines of the conductors in metres (m)
Once Ft in N/m is determined, you can calculate the force for each potential cable cleat.
Applying the Formula
Metric ladder typically has rungs at 300mm intervals, so cable cleat spacing is usually a multiple of this distance. To find the force a cable cleat will experience:
- For 300mm spacing: Ft × 0.3
- For 600mm spacing: Ft × 0.6
You can then compare the calculated force to the maximum recommended mechanical loop strength of the cable cleat to select the appropriate type and spacing of the cable cleat.
Considerations for Cost Efficiency
Whole job cost should always be considered. Costs can often be reduced by using a stronger, more expensive cable cleat at wider spacing rather than a cheaper option at more regular intervals.
Additional Resources
For more examples and information on calculating cable cleat spacing and selecting the appropriate cable cleat type, refer to the Nasco Patents Black Book.
Learn more about our cable cleats on our cable cleats page