Cable Cleats and Their Operating Temperatures
Our standard range of cable cleats is designed to function in ambient temperatures from -50°C to +60°C and with cable conductor temperatures up to 90°C.
Cable Cleats and Eddy Currents
Ferro-magnetic materials that fully enclose single conductors in AC circuits can heat up due to eddy currents. Generally, eddy currents at mains frequencies require a complete electrical and ferro-magnetic circuit around each conductor. However, in installations where all three phases are within the same cable cleat, such as a trefoil cable cleat with three cables, the magnetic fields of the phases cancel out, negating the eddy currents and the associated heating effect.
Despite this, it is preferable to use cable cleats made from non-magnetic materials like aluminum, injection-molded polymers, or stainless steel, which have minimal magnetic properties.
Remember: When using single cable cleats made from ferro-magnetic materials, ensure not to form a complete iron loop around the cable.
Cable Cleats and Multi-core Cables
There is a common belief in the electrical industry that multi-core cables can protect themselves during a short-circuit, negating the need for fault-rated cable cleats. However, research shows that the forces between the conductors of a multi-core cable during a fault are similar to those between three separate single-core cables in a trefoil arrangement. Therefore, when specifying multi-core cables, contact the manufacturer to confirm the cable’s ability to withstand these forces.
Note that regardless of the withstand rating, an unrestrained multi-core cable will move in the event of a significant fault. Most wiring regulations clearly state that: “Every conductor or cable shall have adequate strength and be installed to withstand the electromagnetic forces caused by any current, including fault current.”
Cable Cleats and Fire
Currently, there are no European or IEC standards for fire-rated cable clamps, but other standards can be followed to avoid specifying unsuitable products. The international standard IEC 61914 requires non-metallic and composite cable cleats to resist flame propagation. UL94, the standard for the flammability of plastic materials, classifies plastics based on how they burn in various orientations and thicknesses. Specifiers should demand adherence to its V-0 rating for polymers.
The term LSF (low smoke and fume) is common but misleading as it doesn’t relate to any published standard and can be interpreted in many ways. To ensure performance in a fire, all Nasco plastic products have been tested at the Building Research Establishment (BRE) per the London Underground 1-085 specification regarding smoke emission, limited oxygen index, and fume toxicity. These products are listed in the London Underground Approved Products register.
Fire-rated (FP) cables typically operate in temperatures from 850°C to 950°C, making plastic cable cleats inappropriate. Even aluminum, with a melting point of 660˚C, would fail to support FP cables in a fire. Nasco manufactures the Phoenix range of clamps for use with FP cables, tested by Exova Warrington Fire and BRE, to ensure continuous operation in the event of fire.
Cable Cleats and UV Resistance
While metal cable cleats are impervious to UV attack, composite and polymer cable cleats can be at risk. Nasco composite cable cleats, such as Emperor, Vulcan, and Atlas, have polymer liners but are designed to be UV resistant because the polymer is shielded by either the cable cleat’s body or the installed cables. Polymer cable cleats exposed to UV should be made from materials containing carbon black or other UV-stabilized substances.
Remember: All cable cleats supplied by Nasco for UV applications are provided in UV-resistant materials.
Does Nasco Provide Advice on Cable Cleat System Designs?
As a cable cleat manufacturer, we do not offer design advice on the principles and choices between different types of cable installations. However, we provide expert advice on the suitability of particular cable cleats for any installation type.
Flexible and Rigid Cable Installations
In most projects, the constant movement of the cable due to the thermo-mechanical effect is a major consideration. Two principal installation designs accommodate this:
- Flexible Systems: Cables are “snaked” either vertically or horizontally, allowing for free expansion and contraction between fixing points.
- Rigid Systems: Cables are rigidly fixed, with the longitudinal thermo-mechanical force absorbed by the cable stiffness, the cable cleat reaction force, and the support structure’s rigidity.
Cable cleats are designed to withstand axial forces exerted by the cable, relevant to both flexible and rigid systems, and vertical installations.
Flat, Trefoil, and Quadrafoil Installations
Three-phase installations with single conductor cables are typically flat spaced, flat touching, or trefoil. The 17th Edition Wiring Regulations (BS7671) provides current ratings and voltage drop values for these arrangements and includes information on grouping factors and circuit spacing for thermal independence. IET Guidance Note No. 6 also offers valuable advice for installations with multiple cables per phase.
Another method for single-core cables is using quadrafoil cable cleats, where the neutral is bundled with the three-phase conductors. Although BS7671 does not provide advice for this, a report by ERA on behalf of Nasco offers the following guidelines:
- Current ratings for touching trefoil formation in BS7671 apply to quad bundles.
- Derating factors for touching trefoil formation in BS7671 apply to quad bundles.
- Voltage drops for quad formation circuits should use BS7671 values for flat touching formation.
- Guidance Note No.6’s advice for trefoil groups applies to quad bundles.
- The induced voltage in the neutral conductor of a quad group is minimal and can be ignored.
Preventing Thread Galling When Installing Cable Cleats
Stainless steel fasteners are prone to “pick-up” when threaded surfaces slide against each other, potentially welding together under sufficient speed and pressure—a phenomenon known as thread galling. To avoid this, reduce the speed and downward pressure when closing fasteners and use lubrication where appropriate.
Learn more about our cable cleats on our cable cleats page.