Who would have thought that something as simple looking as a cable cleat could cause so much debate, but in recent years it has been one of the hottest topics in the electrical industry - not least because the recently introduced International standard has elevated its prominence to a whole new level. Unfortunately, the problem with this prominence is that the importance of a cleats role in any electrical installation is still not fully appreciated. Therefore, we've decided to try and rectify things by talking cleats with Richard Shaw, managing director of leading cleat manufacturer, Ellis Patents.

ER: First things, first, why do we need cleats?

RS: 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 this is job that cable cleats are specifically designed to do. 

Take them away and the dangers posed by a short circuit are obvious - costly damage to cables and cable management systems, plus the risk to life posed by incorrectly or poorly restrained live cables.

And it's important to bear in mind that it's not just the use of a cleat that is vital, but the use of a correctly specified cleat. Because all an underspecified product would do in a short circuit situation is add to the shrapnel.
 
ER: Well that seems fairly straightforward, where's the problem?

RS: The key issue surrounding cable cleats is that their importance has been, and still is, severely underestimated. Therefore, instead of being treated as a vital element of any cable management installation they are simply lumped in with the electrical sundries. 

What this means in practice is that even if suitable products are specified, they are still seen as fair game for cost-cutting when it comes to companies seeking to keep within tight budgets. This is a potentially dangerous practice that, if allowed to continue, could lead to the wholly unnecessary loss of a life.

ER: Have the International (IEC61914 - 2009) and European (EN50368) standards not helped deliver this level of awareness and education?

RS: Yes, the introduction of the two standards was a huge boost for everyone associated with cable cleats. And yes, they have helped to provide global recognition of the need for secure cleating in electrical installations, which when you consider that as recently as 2003 there wasn't even a European standard for cleats demonstrates just how far we've come in the journey towards the widespread adoption of safe cleating practice. 

But, they still fall some way short of ensuring the cleat is universally understood and used correctly. The main reason being that the standards are advisory rather than regulatory, meaning that the onus is on the manufacturer to self certify their products - a situation that has led to a market awash with a mish-mash of products of differing quality, which in turn means further confusion for specifiers and installers.

ER: What needs to be done then?

RS: Compulsory third party certification really should clear up this confusion, but the problem is that the quoted short circuit withstand, which is seen as the indicator of a cleats suitability for a project, is only valid for a cable diameter equal to or greater than the diameter of the cable used in the test. 

So if the project in question is using smaller cables than those referred to in the test (and the fault level and spacing is the same) then the force between the cables is proportionally greater, meaning the certificate is inappropriate and the cleats will not provide the protection they are installed to give.

What all of this means is that at present the only tried and tested way to ensure correct cleating is through project specific testing - a process that we currently offer customers that means they can install our cleats with complete confidence.

ER: You've talked a lot about short circuits, can you explain what happens to cables during a short circuit situation?

RS: We do a lot of short circuit tests and a good way of explaining what happens to the cables is to look at the difference between those that are correctly restrained and those that aren't. 

In recent tests we did with our American distributor, kVA Strategies, we performed three short circuit tests on 3 x 1/C-777kcmil, 2kV marine cables at 59kARMS in trefoil formation. One test was conducted on cables tied with 1/2" wide stainless steel cable ties, while the other was conducted on cables restrained by our Emperor trefoil cable cleats. During the short circuit the mechanical forces between the cables exceeded 4,500 lbs/ft.

After one short circuit, the cables restrained with the metal cable ties were damaged beyond repair - suffering multiple tears in the cable jackets and insulation, as well as evidence of electrical arcing. In fact, 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 also severely damaged the cable tray.

In contrast, the correctly restrained cables were subjected to not one, but two successive short circuits and after careful inspection no damage was found. In fact, the testing lab team stated that the cables still passed the required IEC voltage withstand test and so could continue to be used at full-load.

ER: Aren't electrical cables meant to be fully protected by circuit breakers?

RS: That's a common misconception, but in the event of a fault the forces between cables reach their peak in the first quarter cycle, while circuit breakers typically interrupt the fault after three or even five cycles. And by this stage, if the cleats are underspecified, the cables will be long gone.

ER: What's the best cleat to use?

RS: How long is a piece of string? There are a large variety of cleats available and all of them are designed for different installations. For example, our Emperor cleats are recommended for the highest short circuit fault duty applications. Meanwhile, our Centaur cleats are designed specifically to restrain high voltage cables up to 400kV with a diameter range of 100 to 160mm.

ER: So, is there a rule of thumb for picking the appropriate cleat for an application?

RS: In order to ensure the correct cleat the best idea is to go to a manufacturer with information concerning the installation environment, mounting structure, cable configuration, peak short circuit fault level and cable diameter and they should be able to advise on the most suitable cleat and the spacing at which it should be installed.

ER: And what about a recommended spacing between cable cleats?

RS: Again there's no hard and fast rule to suits all installations. The optimum spacing needs to be 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.

ER: Finally, the use of multi-core cables, which we are told don't need to be restrained, is growing enormously - what's your view on this?

RS: This is a question we are being asked with increasing regularity and so in order to be able to provide meaningful advice we have carried out some preliminary research and carried out a series of short circuit tests. 

At present we aren't aware of any published data that indicates any preferred particular fixing method, but custom and practice suggests that most users seem to working under the assumption that any forces on the conductors that arise in the event of a short circuit will be restrained within the cable jacket, meaning cable cleats aren't required. 

The tests we carried out were on armoured and unarmoured 3 core, copper conductor, multi-core cables from various cable manufacturers. These cables were tested across a variety of conductor sizes but because of the number of manufacturers, the variety of cable types and the different methods of construction available, it wasn't feasible to carry out exhaustive tests. 

That said the results of the tests, although varied, were certainly interesting. They showed that it is unsafe to presume that the forces between the conductors will always be restrained within the jacket of the cable, whether or not the cable is armoured or tightly helically wound. 

Therefore, our conclusion is that unless the relevant cable manufacturer can give assurances regarding the performance of their specific multi-core cable at the anticipated fault level, then fault rated cable cleats provide the safest option for securing multi-core cables.

For further information about Ellis Patents visit www.ellispatents.co.uk  or call 01944 758589.