Where the system peak fault current and the cable diameter are known, the following formula, taken from The international standard IEC 61914, can be used to calculate the forces between two conductors in the event of a three phase fault:

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 has been determined then the force for each potential cleat can be calculated.

Metric ladder typically has rungs at 300mm intervals, so cleat spacing is usually a multiple of this distance. So, Ft x 0.3 gives the force a cleat will see if spaced at 300mm, Ft x 0.6 for 600mm etc.

Ft x cleat spacing can then be compared to the maximum recommended mechanical loop strength of the cleat and then the cleat type and spacing can be selected.

Cleat type: | Loop Strength: |

Alpha | 15,000 N |

Vulcan+, Protect and Standard Duty Flexi-strap | 36,000 N |

Emperor, Colossus and Heavy Duty Flexi-strap | 63,000 N |

Centaur Saddle and Clamps | 85,000 N |

Whole job cost should always be considered as costs can often be reduced by using a stronger, more expensive cleat at a wider spacing than a cheaper option at more regular intervals.

The formula uses peak current, however this is often unavailable with a Root Mean Square (RMS) value given instead. To calculate the peak current from the RMS, IEC 61439-1 Low voltage switchgear and controlgear assemblies is commonly referred to, which uses the following multiples:

10-20kA =2

21-50kA =2.1

51kA =2.2

Peak fault: 110kA

Installation: Ladder

Cables in trefoil with an outside diameter of 38mm

Ft x CLEAT SPACING | REQUIRED LOOP STRENGTH |

0.3 for 300mm | 16,240 N per cleat |

0.6 for 600mm | 32,480 N per cleat |

0.9 for 900mm | 48,718 N per cleat |

1.2 for 1200mm | 64,958 N per cleat |

This force per distance can then be compared to different cleat loop strengths to ascertain the appropriate cleat and spacing requirements for specification

Additionally, in this example, the Ellis recommendation was for Vulcan+ cleats (LS: 36,000) spaced every 600mm, or Emperor cleats (LS: 63,000) every 900mm.

RMS fault: 30kA

Installation: Ladder

Cables in trefoil with an outside diameter of 33mm

Firstly multiply the RMS by 2.1 (or other system specific factor) to give 63kA peak. Then:

Ft x CLEAT SPACING | REQUIRED LOOP STRENGTH |

0.3 for 300mm | 6,134 N per cleat |

0.6 for 600mm | 12,268 N per cleat |

0.9 for 900mm | 18,401 N per cleat |

1.2 for 1200mm | 24,535 N per cleat |

As with example 1, force per distance can be compared to the cleat loop strengths and the appropriate cleat and spacing specified.

In this example, Alpha cleats (LS: 15,000) spaced every 600mm are the best option.

Before a cleat and spacing are finalised, two other factors should be considered irrespective of the short-circuit level.

First, that it is strongly recommended that a system employs a fault rated cleat or restraint at a maximum spacing of 1500mm.

Second, on bends and risers it is recommended that the maximum cleat spacing is 300mm.

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All our cable cleats are manufactured by Ellis Patents in the UK.