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A question of cleats

In recent years the cable cleat has been one of the hottest topics in the electrical industry. The reason, as Richard Shaw, managing director of Ellis Patents explained to EWM, is that the cleat’s importance is still not fully appreciated.

A question of cleats

EWM: An easy question first – why do we need cleats?

RS:
For any 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, or use underspecified products, 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.

EWM: That seems simple enough, what’s the problem?

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

EWM: Surely last year’s International standard (IEC61914 – 2009) has eradicated this problem?

RS:
The introduction of the International standard, and its European equivalent (EN50368 – 2003), was a boost for everyone. And they have certainly helped to provide greater recognition of the need for correct cleating practice, but 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 the market becoming awash with a mish-mash of products of differing quality.

EWM: What needs to be done then?

RS:
People need to be made fully aware of what exactly happens when cables aren’t correctly restrained. We do a lot of short circuit tests, which are an ideal way of demonstrating exactly what happens and how cleats prevent this.

We recently 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.

EWM: Right, now that you’ve got across just how important a cleat is how would you go about ensuring they are correctly sold and specified?

RS:
In an ideal world compulsory third party certification would clear up all the problems. Unfortunately, this is open to misinterpretation and possible abuse because 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.
Therefore, if the project in question uses 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.

As a result, this means that at present the only tried and tested way to ensure correct cleating is through project specific testing.


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