Make the right specification choices

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Tim Brown from cable management specialist, Unitrunk, shares insights from the company’s CIBSE-accredited CPD outlining the different materials used to fabricate cable management systems, the performance capabilities associated with each and the considerations for correct specification of the most appropriate solution for varied environments

While many still consider cable management to be a commodity item that is incidental to the cabling installation, the reality is that systems vary considerably and the correct choice of cable management can have a significant influence on a wide range of factors.  Not only does system selection affect speed and ease of installation, it can also aid or inhibit installation design and layout due to fixings, loadings and bend radii, for example. Post installation, correct specification of the cable management system not only affects future-proofing and scalable capacity, it can also determine whether the infrastructure is fit for purpose, robust enough to withstand normal levels of vibration for the installation environment or able to deliver the intended service life.

While system type and design are critical across all these considerations, it is the material type that will determine the longevity of a correctly installed system of the right type and capacity. Under-specification can result in corrosion and a shortened service life.  Meanwhile, over-specification can lead to increased material costs and installation times without delivering any additional value in terms of durability or service life.

Unitrunk’s CIBSE-accredited CPD has been developed to help overcome the confusion surrounding the optimum material choice for varying environments and this general guidance should be supported by project-specific technical support from the supply chain for individual schemes.

Tailored to Requirements

Specifying the correct cable management for different environments can be a minefield that even experienced electrical consultants, engineers and contractors struggle to navigate effectively. Not only are basic decisions about whether cable ladder, cable tray, cable basket or cable trunking offers the most appropriate solution for each element of the build required, but the right size (width and depth), the right weight (heavy, medium or standard duty) and the right material or finish are also essential decisions.  

Indeed, the number of variables is so extensive that there should never be a one-size-fits-all or copy-and-paste approach to cable management specification.

The starting point for selecting the right cable management materials must be consideration of the environmental factors affecting each element of the project. For example, is the cable management required indoors, where it will be protected from the elements, or outdoors where it will be exposed? If it’s going to be indoors, will the atmosphere be clean or polluted? And if it’s going to be outdoors, will it be inland or in a coastal or marine location where a saline atmosphere is a factor?

The specification considerations should also include the expectations of the client or the end user. While the standard service life of a building is considered to be 60 years, the client or end user may have different expectations from this norm. They may also have specific parameters in mind for the maintenance period required for the installation and require accountability for the environmental impact of the materials chosen.

Corrosion risk

As standard cable management systems are usually pre-galvanised or hot-dip galvanised steel, it’s important to understand corrosion risk categories for zinc. There are six categories in all, ranging from C1 (very low), which would typically be inside an insulated building with relative humidity <60 per cent where zinc loss in the first year would be measured at <0.1µm. The scale rises to C5I (very high industrial) where the system is installed in an external environment with considerable humidity and an aggressive atmosphere or an interior location with almost constant water condensation and high levels of pollution or C5M (very high maritime) for coastal areas with high salinity and anticipated 1st year zinc loss of 4.2 - 8.4 µm.

For indoor environments it should be simple to understand the conditions to which the cable management will routinely be subjected. For outdoor locations, it is advisable to consult the British Galvanisers ‘Corrosion Map of the UK’ which highlights the locations most at risk of accelerated corrosion. www.galvanizing.org.uk

Galvanisation is used to protect the base metal from rust but it’s important to understand that there are other corrosion risks associated with galvanised cable management.

‘White rust’ is a wet storage stain that occurs as a white, powdery and porous substance when a fresh zinc surface is stored in a wet environment with limited oxygen and carbon dioxide. The white rust is around 100 times greater in volume than the zinc consumed so, for most cable management products, it will not be detrimental to the integrity of the item.  However, for electro-zinc or pre-galvanised products where the zinc coating is lighter, some damage may occur. Deposits should be removed when they occur and it’s advisable to be mindful of the storage conditions for all galvanised materials to prevent white rust risk.

Galvanic or bimetallic corrosion occurs when two dissimilar metals are immersed in a conductive solution and are electrically connected. One metal (the cathode) is protected, whilst the other (the anode) is corroded. 

‘Zinc Whiskers’ are metallic filaments that can grow on any galvanised surface and detach into the atmosphere. Considered a particular risk to business critical equipment in data centres, they are a form of corrosion that should be considered in the specification for any environment where there could be a risk to sensitive equipment.  Where this risk needs to be avoided, a powder coated or stainless steel alternative to galvanised cable management solutions should be specified.

Understanding Galvanised Products

One of the biggest hurdles to selection of a cable management material that provides the best fit solution for a particular installation is the level of ambiguity surrounding different materials and how they are formed. Over-specification can be costly and the cost increases exponentially with the size of the project.  However, under-specification can be equally costly for the client if the installation is not sufficiently robust.

Galvanising steel is a cost-effective way of protecting the material, which works in three ways; firstly, the zinc coating weathers at a very slow rate giving a long and predictable life. Secondly, the zinc coating corrodes preferentially to provide sacrificial protection to any small areas of exposed steel and thirdly, on larger exposed areas, the surrounding zinc prevents ‘creepage’.

However, there is often confusion surrounding the terms ‘pre-galvanised’ and ‘hot dipped galvanised’. Pre-galvanised refers to sheet steel that has been through a hot dipped galvanising process at the point of manufacture, with a typical galvanising application of 8-20µm. Hot dipped galvanised products, meanwhile are completely immersed in a bath of molten zinc at a temperature of around 450 degrees until the temperature of the item is the same as that of the zinc. During this process the molten zinc reacts with the surface of the steel to form a series of zinc/iron alloys. When the item is removed from the zinc bath, it has a 55-85µm coating of zinc and the surface solidifies, with excess zinc removed to ensure a smooth finish. This increased covering of zinc during the hot dip process makes hot-dipped cable management solutions much more robust and corrosion resistant.

Making the Right Choices

With a better understanding of galvanisation terminology it’s easy to see why pre-galvanised cable management should only be utilised for dry, interior environments and this smooth finish is ideal for exposed cable management installations where the services are on show. Similarly, Electro-zinc finishes, which provide low-cost and attractive protection with the ability to achieve colours including gold, black or olive by post-treatment, also offer a suitable indoor solution for non-corrosive environments.

Meanwhile, hot-dipped galvanised cable management may be more expensive but offers enhanced protection (provided any exposed steel after cutting is coated with a galvanised paint), and can be used for external installations where there is no additional corrosion risk, such as chemical or marine environments. It should be noted, also, that standard cable trunking cannot be hot-dip galvanised as the thickness of the material deforms the item during the process so the specifier must consider whether to downgrade the trunking to pre-galvanised or upgrade to stainless steel and should seek advice from the system supplier in this regard.

For more demanding environments, stainless steel is advisable. Stainless steel 304L offers good weldability and is ideal for brewery, dairy, food and pharmaceutical production environments. 316L stainless steel provides even greater corrosion resistance thanks to the addition of molybdenum, and is sometimes referred to as ‘marine grade’.

For installations where an extended service life is required, Corten A steel may provide a suitable solution. This material is galvanised after manufacture, with a thicker steel and higher silicon content, which attracts more zinc in the galvanising bath.

Cost Vs Value

Clearly cost and performance have to be considered in tandem, with pre-galvanised and electro-zinc providing the most attractive price point, and hot dipped galvanised cable management typically costing 35-40 per cent more. Stainless steel and Corten silicon rich cable management will cost around 40 per cent more again than hot dipped galvanised. 

However, all specification decisions should factor in whole life and installation costs rather than looking at material costs in isolation, taking account of replacement intervals and operational efficiency as part of the material selection strategy.

 

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