Cable cleats and… Operating Temperatures
Our standard ranges of cable cleats are designed for use in ambient temperatures ranging from -50°C to +60°C and with cable conductor temperatures up to 90°C.
Cable cleats and… Eddy Currents
Ferro-magnetic materials that completely surround single conductors in AC circuits are susceptible to heating from eddy currents.
Generally, eddy current generation at mains frequencies requires a complete electrical and ferro-magnetic circuit around each conductor. But in installations where all three phases are contained within the same cable cleat e.g. three cables in a trefoil cable cleat, the magnetic fields of the phases cancel each other out, which in turn negates the eddy currents and the heating effect.
Despite this, it is preferable to use cable cleats manufactured from non-magnetic materials such as aluminium, injection moulded polymers or stainless steel, which has only very slight magnetic properties.
ALWAYS REMEMBER: When using single cable cleats manufactured from ferro-magnetic materials care should be taken to avoid forming a complete iron loop around the cable.
Cable cleats and… Multi-core Cables
There is a commonly held belief within the electrical industry that multi-core cables will protect themselves in the event of a short-circuit, meaning their installation does not require fault rated cable cleats.
However, research shows that the forces between the conductors of a multi-core cable in the event of a fault are similar to those between three separate single core cables laid in a trefoil arrangement.
Therefore, when specifying multi-core cables it is advisable that cable manufacturer should be contacted to ascertain the ability of its specific cable to withstand these forces.
It is worth noting that whatever the withstand quoted, in the event of a significant fault an unrestrained multi-core cable will move.
Furthermore, the requirements of most wiring regulations are clear and typically state that:
“Every conductor or cable shall have adequate strength, and be so installed as to withstand the electromagnetic forces that may be caused by any current, including fault current.”
Cable cleats and… Fire
There are currently no European or IEC standards for fire rated cable clamps, although there are requirements within other standards that can be followed to prevent unsuitable products being specified.
The international standard IEC 61914 requires non-metallic and composite cable cleats to have adequate resistance to flame propagation.
UL94, the standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances, is a plastics flammability standard that classifies plastics according to how they burn in various orientations and thicknesses. Adherence to its V-0 rating for polymers should be demanded by specifiers.
The use of the description LSF (low smoke and fume) is common terminology with regard to polymers, but is misleading as it doesn’t relate to any published standard and so can be interpreted in a wide variety of ways.
To ensure complete assurance of performance in a fire, all Ellis plastic products have undergone testing at the Building Research Establishment (BRE) in line with the London Underground 1-085 specification with regard to:
Smoke emission
Limited oxygen index
Toxicity of fumes
The appropriate products are listed in the London Underground Approved Products register. Identification numbers are 360, 361,362, 363, 364, 365 and 1661.
A great deal of focus is placed on fire rated (FP) cables and their performance in fire, but very little attention is given to the cable fixings used to secure these cables. Given that FP cable is typically rated for operation in temperatures ranging from 850°C to 950°C then the use of plastic cable cleats or clamps is clearly inappropriate.
Even aluminium only has a melting point of 660˚C, which means it would fail to support FP cables in a fire.
To counteract this shortcoming, Ellis manufactures the Phoenix range of clamps for use with FP cables. Independently tested by Exova Warrington fire and BRE, all products in the range are proven to perform to the same level as the FP cables ensuring continuous operation in the event of fire.
Cable cleats and… UV Resistance
While wholly metal cable cleats are impervious to UV attack, composite and polymer cable cleats can be at risk.
Ellis composite cable cleats such as Emperor, Vulcan and Atlas all have polymer liners, but are designed to be impervious to UV attack because the polymer is shielded by either the cable cleat’s body or the installed cables.
Polymer cable cleats that are likely to be exposed to UV should be supplied in materials containing carbon black or other UV stabilised material.
ALWAYS REMEMBER: All cable cleats supplied by Ellis for applications involving UV are provided in UV resistant materials.
Does Ellis provide any advice on cable cleat system designs?
As a cable cleat manufacturer, we do not offer advice on the design principles and choices between different types of cable installation. We will however provide expert advice on the suitability of particular cable cleats within any type of installation.
Flexible and Rigid Cable Installations
On most projects a major consideration is the constant movement of the cable due to thermo-mechanical effect. To accommodate this two principal types of installation design exist:
FLEXIBLE SYSTEMSwhere the cables are “snaked” either vertically or horizontally. The cable can expand and contract freely between fixing points.
RIGID SYSTEMSwhere the cables are rigidly fixed. The longitudinal thermo-mechanical force is withstood by the combination of the stiffness of the cable, the cable cleat reaction force and the rigidity of the support structure.
Cable cleats are designed to withstand the forces exerted by the cable in the ‘axial’ direction, this is relevant to both flexible and rigid systems. It is also important when the cables are installed vertically.
Flat, Trefoil and Quadrafoil Installations
Cable arrangements for three phase installations utilising 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 all these arrangements. It also contains information on grouping factors and spacing between circuits to achieve thermal independence.
Additionally, IET Guidance Note No. 6 delivers valuable advice on installation arrangements where there are multiple cables per phase.
An additional method for installing single-core cables is to use quadrafoil cable cleats where the neutral is bundled with the three phase conductors. In this arrangement, there is no advice in BS7671 but a report produced by ERA on behalf of Ellis delivered the following guidelines:
Current ratings, given in BS7671, for cables in touching trefoil formation are appropriate for cables in quad bundles
Derating factors, given in BS7671, for cables in touching trefoil formation are appropriate for cables in quad bundles
Voltage drops for circuits in quad formation should be calculated using the values tabulated in BS7671 for cables in flat touching formation
When considering multiple cables per phase, the advice given in Guidance Note No.6 for trefoil groups is applicable to quad bundles
The induced voltage in the neutral conductor of a quad group is minimal and can be ignored.
How do I prevent thread galling when installing cable cleats?
Stainless steel fasteners have a propensity to “pick-up” when the two threaded surfaces slide against each other. If sufficient speed and pressure is applied to the sliding surfaces then they can weld themselves together – a phenomenon known as thread galling.
All stainless steel fixings will thread gall if there is sufficient friction.
To avoid thread galling, reduce the speed and downward pressure when closing fasteners and use lubrication where appropriate.
Learn more about our cable cleats on our downloads page – https://www.ellispatents.co.uk/downloads/.
