Cable cleats should be regarded as a vital element of any cabling installation, says Richard Shaw.
Laying electricity cables is time consuming and costly, and ensuring systems are properly installed is vital not just economically, but in terms of health and safety. Therefore, it would seem extremely remiss, especially in an offshore environment, if one significant installation aspect was frequently neglected – cable cleats.
The importance of cable cleats is frequently underestimated. What this means in practice is that instead of being treated as a vital element of any cabling installation, they are lumped in with the electrical sundries and seen as fair game for cost-cutting.
But for an 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 this is exactly what cable cleats are designed to do.
Without cleats, the dangers are obvious – costly damage to cables and cable management systems, plus a risk to life posed by incorrectly or poorly restrained live cables. All of which is in stark contrast to the stringent safety requirements associated with offshore work.
Unfortunately, it’s not just a question of installing any old cleat. It has to be correctly specified for the project in hand. If not, the cables might as well be secured with plastic cable ties. The reason being that different cable cleats are designed to withstand specific forces, meaning the only thing underspecified cleats will do in a short circuit situation is add to the shrapnel.
One reason for this worrying level of confusion is that the market is very much manufacturer driven. Therefore, the choice of product tends to be reliant on third party certification – in the form of a short circuit testing certificate – but unfortunately this can be misleading.
For example, it is not uncommon for manufacturers to claim a given short circuit withstand at a given cleat spacing and legitimately provide third party certification to support this. However, the overlooked fact is that the quoted short circuit withstand is only valid for a cable diameter equal to or greater than the diameter of the cable used in the test. If the project in question uses smaller cables (and the fault level and spacing is the same) then the force between the cables is proportionally greater and the certificate is inappropriate.
Plainly and simply you cannot say that a specific cable cleat has a short-circuit withstand without qualifying the statement. So instead of claiming a withstand of 150kA you would need to say that the cleat has a short-circuit withstand of 150kA when securing 43mm cable in trefoil at 300mm centres.
To me the only way of rectifying the issue is through the adoption of cable cleats as short circuit protection devices. The reason for this is simple. By giving cable cleats the same degree of importance as fuses or circuit breakers nobody would be left in any doubt about ensuring their correct specification.
The reasoning behind this is simple. In the event of a fault, the forces between cables reach their peak in the first quarter cycle, which is the point that cable cleats earn their crust. In contrast, circuit breakers typically interrupt the fault after three or even five cycles by which time, if the cleats are underspecified, the cables will be long gone, meaning the expensively assembled circuit breaking system will never been given the opportunity to be tested.
This change is not going to take place over night, and so specifiers and engineers need to take steps to aid the correct specification of cable cleats. Perhaps key is being aware that levels of cable protection can be enhanced by selecting only products that are classified in section 6.4.4 of the International Standard. What this means, is that the cable is guaranteed to still be intact and operable after a short circuit, as opposed to just the cleat.
Unfortunately, eradicating the problem isn’t that simple. Most specifiers and engineers are diligent when it comes to system design. Where the problems really begin is when the specification reaches the buyer. At which level there isn’t the technical understanding of cleats and instead the focus is far more on cost.
Should a buyer see a specification for 4,000 of our Emperor cleats it is natural that they may look elsewhere to find a better per unit price. The issue though is that the specification is particular to the project and the product, having been set according to the strength of the cleats and their spacing along the cable. As such any change in the type and strength of cleat will result in a change in the numbers required, which in turn will have a knock-on effect on price. Therefore, if a buyer is intent on changing the specification they really need to look at overall as opposed to per unit cost.
Looking at things from a budgeting point of view, it is fair to stay that certain initial costs would be increased by adopting practices that ensure the correct specification of cable cleats. But if you consider the kind of sums involved, in terms of time, materials and manpower, in replacing an entire cable management system due to a short circuit occurring and causing irreparable damage it’s easy to see the point of the additional expense.
Ellis is the only leading manufacturer in the electrical industry that focuses solely on cable cleats. Its products are used extensively in the power industries. Recent successes include the UK's National Grid’s London Power Tunnels project, the Zagorskaya Pumped Storage Plant (PSP) in Russia’s Moscow region and the new Ledvice Power Plant in the Czech Republic.
Richard Shaw is magazine director of Ellis, Malton, North Yorkshire, UK. www.ellispatents.co.uk
