Andy Booth's Guide To Cleat Specification
- Published: Saturday, 19 December 2015 06:56
When it comes to electrical installations no product causes as much confusion as the cable cleat, which when you consider it has its own International and European standards does beg the question “why?”
Andy Booth, technical director of Ellis, talks to Electrical Digest about how to ensure correct cleat specification.
“Cleats may be small, but their importance is mighty. Without them the dangers posed by a short-circuit are plentiful; costly damage to cables and cable management systems, plus the risk to life posed by incorrectly installed live cables.
The International Standard, that’s IEC 61914:2009, states: A cable cleat is a device designed to provide securing of cables when installed at intervals along the length of the cables.
While, the British Standard (BS7671:2008) gives us: 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.
Put in layman’s terms, for an electrical installation to be deemed safe cables need to be restrained in a manner that can withstand the forces they generate, including those generated during a short-circuit.
And that, in a nutshell is the job of the cable cleat.
It all seems so simple, so why the confusion? The issue is that cleats have long been underestimated, so instead of being treated as a vital element of any cable management installation they are frequently lumped in with the electrical sundries and seen as fair game for cost cutting.
So what exactly should people be aware of to ensure correct cleat specification?
Firstly, knowing the mechanical strength of a cleat prior to specification is imperative. It is though vital to check what test it’s undergone, as in our experience a cleat that has passed a mechanical tensile test at a given force will not necessarily survive a short-circuit test at the same force.
A short-circuit test is the only reliable way of proving that a cable cleat is capable of withstanding a specific set of fault conditions.
We recommend that any claims of cleat strength be supported by a short-circuit test certificate, which shows the test has been carried out in an independent and accredited laboratory. A full test report should also be supplied, including before and after photographs, and a table of results and conclusions.
F = 0.17 x ip²
F = force in Newton metre (N/m)
ip = prospective peak short circuit current in Kiloampere (kA)
S = spacing between the conductors in m
Correct cleat spacing is vital to ensure optimal performance. Once the system peak fault current and cable diameter are known the formula (see FIG 1*) can be used to work out the forces between two conductors in the event of a three phase fault (F). This can then be multiplied by different cleat spacings and compared to the maximum recommended mechanical loop strength of the cleat, meaning both the correct cleat and spacing can be selected.
For example, metric cable ladder typically has rungs every 300mm, so cleat spacing is usually a multiple of this distance. F x 0.3 gives the force a cleat will see if spaced at 300mm, F x 0.6 for 600mm etc.
One of the most important issues to consider when specifying cleats is the risk of material corrosion – caused by either the installation environment or other metals.
Galvanic corrosion occurs when dissimilar metals are placed in contact with each other in the presence of an electrolyte. There are two factors that affect its rate. The first being the distance between the two metals in the galvanic series, the second, the surface areas of the different metals.
Unfortunately, this isn’t easily predictable and can be influenced by the type of electrolytes present in the air – e.g. those coming from salt water or fresh water containing impurities –. Therefore, the safest course of action is to separate dissimilar metals with polymer separation washers.
Stainless steel is the material of choice for the vast majority of cleats and fixings due to its non-magnetic and corrosion resistant properties.
These properties are a result of chromium, which reacts with oxygen and forms a self-healing impervious layer of chromium oxide on the surface of the steel. This layer is extremely durable, but in certain locations, such as railway tunnels, the oxide layer can be penetrated as a result of mild steel dust in the atmosphere, which reacts with moisture to exaggerate corrosion. In such circumstances Aluminium products or electrostatic plastic coatings should be specified.
The life expectancy of a cleat is often needed and is relatively simple to calculate – at least if the installation is designed correctly and all other corrosion issues have been considered. Complications arise if the cleat or its fixings are manufactured from galvanised mild steel. In such situations, the Galvanizers Association map should be referred to in order to ascertain the required thickness of the galvanised / zinc coat.
Other elements worth considering include:
• Fire – there are no current standards for fire rated cable clamps, but common sense dictates the cleat being used should have the same fire retardancy as the cable
• Operating temperatures – most cleats are designed for use in ambient temperatures ranging from –50°C to +60°C and with cable conductor temperatures up to 90°C
• UV resistance – metal cleats are impervious to UV attack. Composite and polymer products aren’t. If they’re likely to be exposed they should be supplied in materials containing carbon black or other UV stabilised additives.
Correctly cleated cables are a must for safe electrical installations. This brief specification checklist has been taken from our very first technical guide to cleats, which aims to help eradicate the specification issues that have dogged the electrical industry for years. Hopefully both this article and the guide will play their parts in eradicating the cleat specification issues that have dogged the electrical industry for years.”
Ellis is the world’s leading cable cleat manufacturer. For more information on its products and services, visit www.ellispatents.co.uk or call +44(0)1944 758395.
Balfour Beatty - Power Transmission and Distribution on the London Power Tunnels Project.
Description: Photograph showing a section of the tunnel between St Johns Wood and Kensal Green Substations, part of the London Power Tunnels 400kV project. The cable was installed onto Ellis Patents Centaur saddles and secured using Ellis Patents short circuit straps supplied to Sudkabel directly. Balfour Beatty carried out the cable installation using its bespoke cable installation machine.