|Cleats, cleats, cleats – a selection of cable cleats|
The key issue surrounding cable cleats is that their importance is frequently underestimated.
This means 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 when companies are seeking to keep within tight budgets – a practice that is exceptionally dangerous and causes a wholly unnecessary risk to life.
To me, the only way of rectifying the issue is by educating all those involved in the process of specifying, buying and installing electrical cables as to the sheer importance not just of cable cleats, but their correct usage. And this is something that can only be done by going back to basics.
In a nutshell, 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 them, the dangers are obvious – costly damage to cables and cable management systems, plus the risk to life posed by incorrectly or poorly restrained live cables.
Take for example an electrical cable installation fitted with underspecified cleats. Some may think this would still provide a certain level of short circuit protection, but it does not. In fact, 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.
Educating all parties
One of the major contributing factors behind this complicated picture is that although cable cleats are recognised by industry regulators, having their own International Standard (IEC61914), this is simply an advisory guideline rather than an obligatory regulation.
|High-voltage Centaur cable saddle undergoing short-circuit testing in Holland|
As a result, the market is very much manufacturer driven, and so the choice of which product to use is very much reliant on third-party certification – in the form of a short circuit testing certificate – but unfortunately this can be very 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 or greater than the diameter of the cable used in the test. 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 and the certificate is inappropriate.
|National Grid’s ambitious London Power tunnels project|
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 150 kA you would need to say that the cleat has a short-circuit withstand of 150 kA when securing 43 mm cable in trefoil at 300 mm centres.
Unfortunately, there is a major problem with easily misinterpreted reports. A recent report that came across my desk proved exactly this point. At first glance it suggested the product could withstand a peak short circuit of 138 kA, but on full reading it became clear that the test rig was set up with four trefoil circuits in parallel. What this means is that while the overall fault level was 138 kA, as per the headline figure, each of the four trefoil groups only saw a quarter of the fault, equivalent to just 34.5 kA.
In the long term, the whole situation needs to be resolved through a process of education and agreement involving manufacturers, regulators, specifiers, contractors and installers. And to me the only way of achieving this is to take the somewhat radical approach of cable cleats being adopted 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.
When considered in more detail this argument is not as outlandish as it first sounds. Do you know that in the event of a fault the forces between cables reach their peak in the first quarter cycle? This is the point where cable cleats earn their crust, while, 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.
New vs. old installations
Anyway, that is looking quite some way down the track. What of the short term? What should those responsible for electrical installations be doing in order to ensure corners are not cut and safety sacrificed when it comes to cable cleats? To answer this, installations need to be split into two categories – new and old.
In the case of new installations, the process should begin during the design stages. Ideally, this would commence with the forces between cables being calculated so as to ascertain the type and strength of the cleat required. A number of other factors then need to be taken into account, including physical performance, mounting surface and the environment in which the installation will be situated. Only when this has all been tested and measured can the designer be confident of specifying the correct cleat for the installation.
|The Centaur cable saddle in production in the Hurco machining centre|
Old installations meanwhile pose a completely different set of problems. Many will have been installed before the introduction of any related standards, while those that came later may still not be suitable. Therefore, it is advisable to review all cabling in such installations to ensure it is safely restrained.
However, understanding the need for cleats and the processes involved in their correct specification is only half the story. Cleats themselves come in a variety of sizes and types and it is important to understand this variation in order to ensure they are correctly chosen.
Of course, the simplest way of doing this is to ensure the product is third-party certified and can withstand the forces it claims to, but this process, as previously outlined, cannot always be relied upon. Therefore, in order to ensure peace of mind, the current best course of action is to ask the manufacturer to conduct project specific testing.
Take for example a job like National Grid’s London Power Tunnels project. Over 30 km of tunnels are being built to carrying high-voltage (HV) cables between substations in Wimbledon, Hackney, Willesden and St John’s Wood.
|Close-up of the Centaur cable saddle in production|
The cost of the project is enormous, and the implications of under specified cleats failing to prevent a short-circuit fault are too numerous to consider in one of the most densely populated cities on the planet. But until very recently the methods for supporting large HV cables at fixed points were designed on a project-by-project basis, but there were no tests or related publications to determine how these HV cable fixings should perform in the event of a fault.
We secured the cable cleat specification for the London Power Tunnels project, the value of which topped £1.5 million ($2.3 million), thanks to a product specifically designed for exactly this type of project.
A number of years ago we made the decision to move away from bespoke products designed on a project-by-project basis and develop a specific range designed to secure large HV XLPE cables in response to what we saw as a major gap in the market – especially at a time when so many HV cable tunnel projects were coming on line. Our product, Centaur, underwent full-scale short-circuit testing before coming anywhere near the market and is sold using a highly scientific approach.
In fact, this research and testing led type of approach has served us well in a host of recent specification successes in the power sector. These include the Ledvice power station in the Czech Republic, the London Array offshore wind farm, Pembroke power station in Wales, the new Astute nuclear powered submarine and an ever increasing number of offshore projects around the world.
Importance of the specificers
While it does seem that a lot of my short-term solution argument is putting the emphasis firmly on the manufacturer, there are steps that can be taken by specifiers and engineers to ensure they play an active part in 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.
Of course, these things are far easier said than done. The problem we face is not simply getting people to acknowledge the issue, but actually getting them to actively do something about changing practices that are so deeply ingrained within the specification and installation process that many are not even aware they are doing anything wrong.
Yes, most specifiers and engineers are very diligent when it comes to system design. System fault levels will normally be calculated so that anticipated forces between cables will be known. And, in most instances, the type of cleat and appropriate spacing will be given careful consideration. This often goes as far as a product defined by part number being included in the specification, but this is usually balanced with the words “or equivalent”.
Thereafter the specification is usually passed to a number of contractors who bid competitively for the contract. Where the problems really begin is when the successful contractor passes the specification on to a buyer. At this level there is not the technical understanding of cleats and instead the focus is far more on cost. For example, if a buyer sees a specification for 4000 of our Emperor cleats they may look elsewhere to find a better per unit price. The issue though is that the specification is particular to both the project and the product, and will have 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 the cleat will result in a change in the number of cleats 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 the overall cost as opposed to the per unit cost.
In simple terms the customer has diligently designed a system to comply with all current legislation and guidelines, but the system finally installed (and probably warranted) by the contractor may not meet the same standards. All of which means the onus is on the end customer to ensure what they specify and what is being installed tallies up.
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 is easy to see the point of the additional expense.
What also cannot be forgotten is that electricity is as dangerous as any substance or liquid mankind works with. It can and does kill and maim. Cable cleats are designed specifically to restrain cables in potentially lethal short-circuit situations. Therefore, there is no doubt that any short cuts that increase risk, whether to the installer, the end user or the passer-by, need to be stamped out quickly.”