What is a cable cleat?
A cable cleat is a device which, when installed along the length of cables, secures and restrains them. Cable cleats are typically bolted to a mounting surface such as a cable tray rung or channel strut, but do not rely on that surface for the retention of the cables. Cable cleats are a superior choice for securing cables in long vertical runs. A properly specified and installed cable cleat is capable of protecting cables by containing the electromechanical forces between the conductors in the event of a short circuit. Cable cleats also protect cables from damage during seismic, vibration and shock conditions, as well as from chafing due to differential movement between cable fixings (a severe problem plaguing marine and offshore installations). Cable cleats provide retention and support to both single conductor and multi-conductor cables.
Why are cable cleats required?
Adequately restrained cables can eliminate costly cable repairs, replacements and unplanned downtime. A correctly rated cable cleat, properly specified and installed, will allow you to meet the most stringent cable installation standards. For example, NEC Article 392.8 (D) specifies that parallel connected single-conductor cables are to be “SECURELY BOUND in circuit groups to prevent excessive movement due to fault-current magnetic forces”. If the cable cleat is declared to be "resistant to electromechanical forces", it must successfully withstand short circuit testing as defined by IEC 61914:2009 clause 6.4.3 or 6.4.4, while simultaneously protecting the cables.
What happens to cables during a short circuit?
kVA Strategies and Ellis Patents performed 3Ø short circuit tests on 3 x 1/C-777kcmil, 2kV marine cables at 59kARMS in trefoil formation at an independent third party testing laboratory. One test was conducted on cables tied with 1/2" wide stainless steel cable ties. The other test was conducted on cables restrained by Emperor trefoil cable cleats. During the short circuit, the mechanical forces between the cables exceeded 4,500 lbs/ft. The Short Circuit Videos documented and graphically demonstrate the necessity for adequate cable restraint and the ability of fault rated cable cleats to prevent excessive movement.
After one short circuit, the cables tied with the metal cable ties were damaged (multiple tears in the cable jackets and insulation, as well as evidence of electrical arcing). The metal cable ties catastrophically failed before the first quarter cycle current waveform peak, ejecting the ball bearings from the cable tie buckles with sufficient velocity to lodge deeply into the plywood test bay walls. The subsequent cable thrashing severely damaged the cable tray.
The cables restrained by the cable cleats were subjected to two successive short circuits. After the second short circuit, careful inspection by the testing lab revealed no cable damage. The cables passed the required IEC 61914:2009 voltage withstand test (aka hi-pot test), proving the cables were undamaged and available for continued operation at full-load.
How can the forces between conductors during short circuit conditions be calculated?
The maximum mechanical force between conductors during a short circuit (equation recognized by IEC 61914:2009) is:
Ft= [(0.17 x IPEAK2) / S]
Where:
Ft = Maximum Force on Conductor (Newtons/ meter)
IPEAK = 1st Cycle Peak Asymmetrical Short Circuit Current Magnitude (kA)
S = Spacing between conductor centers (meters)Example:
A 3Ø short circuit current of 65kA RMS-Symmetrical (143kA 1st cycle peak) flows through three single conductor #4/0 cables (OD=0.8386” nominal +/- 5%) in close trefoil orientation.
Substituting IPEAK = 143kA and S = [(0.95 * 0.8386 * 25.4)/1000] = 0.020235m
Therefore, Ft = [0.17 x (1432)] / 0.020235 = 171,798 N/m = 11,544 lbs/ft
Aren't electrical cables fully protected by circuit breakers?
Even for circuit breakers calibrated for instantaneous protection, irreversible cable damage can occur in inadequately restrained cables within the first 1/4 cycle of the fault initiation (i.e. before the circuit breaker can open to interrupt the fault). While current limiting fuses may limit the amount of current flowing through the cables, the restraint device must be certified for use at the maximum let-through current.
What are the relevant standards for cable cleats?
Cable cleats should be installed according to the relevant wiring standard (eg NFPA-70, The National Electrical Code or C22.1, The Canadian Electrical Code). The internationally recognized peer reviewed performance standard for cable cleats is the International Standard IEC 61914:2009, Cable Cleats for Electrical Installations. The Founder and President of kVA Strategies, Charles A Darnell, PE CFEI is the US Technical Expert on IEC Subcommittee 23A that is responsible for preparing and maintaining the international cable cleat standard.
What information is required to specify cable cleats?
Customers should provide information on the spacing between conductor centers, maximum available short circuit fault duty and length of cable run to enable kVA Strategies to calculate the forces between cables and recommend the appropriate cable cleat(s). For cables operating over 34.5kV, customers should also provide cable data sheets. For HV and EHV transmission cables, customers should call kVA Strategies to discuss the installation. The comprehensive Cable Cleat Engineering Specification will assist engineers, designers, electricians and end users by providing the necessary information to design and install cable cleat systems that protect cables and maximize circuit availability and reliability.
Which cable cleat is best?
kVA Strategies offers several models of cable cleats. Generally, the cable cleat choice is dictated by the available short circuit fault duty. Emperor cable cleats are recommended for high short circuit fault duty applications and for cable systems operating over 15kV. For cable systems operating at 15kV and below that are in moderate or low short circuit fault duty applications, the choice is typically based on user preference. For guidance on cable cleat selection, please refer to the Cable Cleat Selection Guide.
What is the recommended spacing between cable cleats?
Cable cleat lineal spacing is determined by engineering calculation to ensure the cable cleats are suitable for the electromechanical forces encountered during the maximum available fault duty of the system. For guidance on lineal spacing for Emperor Cable Cleats, please refer to the Emperor Spacing Recommendation. For lineal spacing guidance on other cable restraint products, Contact kVA Strategies. Horizontal spacing between adjacent cable cleats (e.g. parallel cables) is based on the width of cable cleat when fully opened. Since every installation may be different, kVA Strategies will be pleased to provide further information on request.