STAINLESS STEEL CABLE CLEATS – PREVENTING GALVANIC CORROSION OF CABLE FIXINGS
bc26bc > 10-27-2021, 05:46 AM
In the following article we provide an evidence based specification case for stainless steel cable cleats where levels of atmospheric corrosion preclude use of other cable fixing materials.
One of the most important issues to consider when specifying cable cleats is the risk of material corrosion – not just as a result of the installation environment, but also from other metals which the cable cleat is in contact with.
Galvanic corrosion occurs when dissimilar metals are placed in contact with each other in the presence of an electrolyte. There are two factors that affect the rate of galvanic corrosion, the first is the distance between the two metals in the galvanic series.
The further apart the two metals are in the galvanic series, the greater the risk of galvanic corrosion – with the metal higher up the list (more anodic) being the one whose rate of corrosion is accelerated.
The second factor to consider is the relative surface areas of the different metals.
If the more anodic (higher up the list) metal has a smaller surface area than the metal it is in contact with, the difference in surface area causes the rate of corrosion of the anodic metal to increase.
Conversely, if the more anodic metal has a much larger surface area than the cathodic metal, it may be sufficient for the effects of galvanic corrosion to be discounted. In terms of cable cleat selection, the surface area of the cleat is generally significantly smaller than the structure it is mounted on.
Therefore, if it is made from a metal that is more anodic than its support structure it will be susceptible to galvanic corrosion.
Conversely, if the cable cleat is more cathodic than its support structure, there is little risk of galvanic corrosion.
Using this criteria, if galvanised ladder is the support structure, and there are no other significant factors, it is safe to use ball lock stainless steel cable tie. However, if the support structure is stainless steel, separation should be provided if aluminium or galvanised cleats are used.
Galvanic corrosion is not easily predictable and can be influenced by the type of electrolytes present such as salt water or fresh water containing impurities. In general terms when guarding against galvanic corrosion, the safest course of action is to separate dissimilar metals with polymer separation washers.
This separation should be carried out between the cable cleat and its mounting surface and the cleat’s mounting fixing.
All Ellis cable cleat products constructed from dissimilar metal are designed in a way that completely avoids bimetallic contact. As a result of this you can be confident that cable cleats will have a service life measured in decades.
All Ellis single stainless steel cable cleat are produced from 316L austenitic stainless steel.
In general, cable cleats are manufactured from austenitic stainless steel due to its non-magnetic and corrosion resistant properties – the former ensuring the cable cleat will not induce eddy currents or localised heating of the LV-HV cable.
Austenitic stainless steel does become a little magnetic as a result of work hardening when processed. This magnetism can barely be detected with a magnet and so is not significant from an eddy current perspective.
304 austenitic stainless steel, often referred to as A2, is one of the most commonly used stainless steels. It has excellent corrosion resistant properties in most circumstances, although is susceptible in atmospheres where chlorides are present, making it unsuitable for use in coastal or marine environments.
316 austenitic stainless steel, often referred to as A4, contains Molybdenum, which provides resistance against chlorides. 316 is often referred to as marine grade stainless steel due to its suitability for use in coastal and offshore applications.
If unsure a simple chemical test can determine whether Molybdenum is present and so differentiate between 304 and 316 types of stainless steel.
There are many different types of stainless steel, but there are two principal variants when it comes to cable cleats. 304 and 316 stainless steel are available in low carbon variants, namely 304L and 316L. These variants are immune to sensitisation (grain boundary carbide precipitation).
Any cable cleat which is manufactured from stainless steel and includes welding in the manufacturing process should be made in a low carbon (L) variant.
Should you require any further assistance in the selection or specification of stainless steel cable tie, MV Medium Voltage or HV High Voltage cables please do not hesitate to contact us.
The corrosion resistance properties of stainless steel are a result of chromium, which reacts with oxygen and forms a self-healing impervious layer of chromium oxide on the surface of the steel.
In most circumstances the chromium oxide layer is extremely durable and helps in resisting galvanic corrosion. However, in certain installation locations, such as railway tunnels, the oxide layer can be continuously penetrated.
This occurs due to trains frequently applying their brakes, which releases mild steel dust into the atmosphere that then settles on the stainless steel. If moisture is present, then corrosion occurs at an exaggerated rate. In such circumstances, if regular washing is not feasible, use of aluminium as an alternative to stainless steel products and/or coating processes are strongly recommended.
Ellis Patents offers special coatings to suit specific cable installation environments – e.g. our London Underground Approved electrostatic plastic coatings.
CABLE CLEAT FIXINGS
Closure fixings on cable cleats are fundamental to the loop strength of the cable cleat and its short-circuit withstand capability.
All Ellis Patents 316L stainless steel cleats use 316 fixings, which are manufactured to a precise and specific tensile strength. Fixings are sourced directly from approved manufacturers and any fixing on any cleat is directly traceable back to the batch quality records at that manufacturer.
GALVANISED STEEL
Contracts often require a guarantee regarding the life expectancy of a cleat. If the installation is designed correctly and all other corrosion issues have been considered this is a relatively simple exercise for wing lock stainless steel cable tie. With galvanized steel, life expectancy is determined by the thickness of the zinc coating.
The resistance of galvanizing to atmospheric corrosion depends on a protective film that forms on the surface of the zinc. When the newly coated steel is withdrawn from the galvanizing bath, the zinc has a clean, bright, shiny surface. With time a corrosion process occurs which produces a dull grey patina as the surface reacts with oxygen, water and carbon dioxide in the atmosphere.
This leads to the formation of a tough, stable, protective layer, which is tightly adherent to the zinc. As the corrosion process is continuous, the thickness of the zinc layer reduces over time and it is the speed of this reduction that is used to accurately predict the life span of the cable cleat.
Corrosion Rates for the UK
Permission to use the information relating to galvanising was granted by the Galvanizers Association for galvanised steel. If a galvanised steel cable cleat is specified for use in a zone 3 area then the corrosion rate is 1.5 microns (µm) per year.
If the contract for this specification states a required life expectancy of 40 years, then the initial galvanising thickness will need to be a minimum of 60 µm in order to meet the required longevity.
The international standard governing Cable Cleats used in electrical installations is IEC 61914:2015. In this standard Cable Cleats are defined as “devices designed to provide securing of cables when installed at intervals along the length of the cables”. Simply put, cable cleats are used to secure, fix and route electrical cables in the positions required in an electrical installation. They can consist of single or multiple parts, plastic or metal material and include some sort of provision for securing itself to a surface or structure. Mounting surfaces that may be specified include; ladder, tray, strut, rail, wire and beam. Examples of different types of cleats securing cable to cable ladder can be seen in the image below.
Cable cleats should be designed to ensure that cables are fixed, supported and routed in a manner that provides safe operation and reduces the risk of damage or injury in the event of an emergency or accident. Improper clamping of cables can result in loss through unnecessary downtime or even injury and death. They should at a minimum:
Be rated for the specified cable OD.
Provide a means of securely fixing the cable.
Have adequate strength to secure the cable.
Prevent excessive cable movement.
Avoid chafing and undue stress in the cable.
Cleat Selection and How to Specify Cable Cleats
If you’re here it’s likely you may have asked yourself ‘which cable cleat should I use for this job?’ Unfortunately the answer frequently is ‘well, it depends!’ More specifically you may be wondering ‘what size cable cleats for 2.5mm SWA?’ or ‘what size cable cleat for 6mm armoured cable?’. The latter questions are much easier to answer and if you scroll to the end of this article you’ll find selection tables for all of our cleats and common cable sizes.
Cable Arrangement
The cable arrangement/configuration will primarily dictate the type of cleat required. Cable arrangements for 3 phase installations utilising single conductor cables are typically flat spaced, flat touching or trefoil. A parallel or flat arrangement of single core cables can be completed with a range of single or two-part cleats. Whereas a trefoil or quad arrangement would require a trefoil or quad type cleat respectively.
Cable Type - The type of cable being used, Single or Multi-core, as well as its Voltage Levels and Construction Low Voltage (LV), Medium Voltage (MV) or High Voltage (HV)} should be considered.
Cable Diameter - Knowing the overall diameter of the cable (measurement across the entire cross-section) is essential in ensuring the correct size of cleat is selected. It is also required to calculate the short circuit forces that the cleat may be subjected to, this can be used to determine correct cleat spacings.
Performance - A range of factors will dictate the level of performance your installation will require. The size, weight and length of run of the cable also spacing will usually influence whether you require a polymer or metallic cleat. Things such as the support structure material and environmental conditions (corrosion) can also affect your decision of cleat material. Other factors such as project specification may require special provision for performance in the event of a fire. Low Smoke or Zero Halogen options are available along with cast iron in extreme cases.
In summary, to correctly select they type of cleat you require you should be looking to obtain the following information;
1. Calculate the system peak fault current.
2. Confirm cable type and arrangement, including the overall diameter and manufacturing tolerance.
3. Confirm the support structure type and material.
4. Consider any other environmental conditions and project specification requirements.
5. Consult with the Remora Sales team who, given your requirements, will advise the most cost effective solution.
In the UK the 17th Edition Wiring Regulations (BS7671) provides current ratings and voltage drop values for all these cable configurations. Information is also available on grouping factors and spacing between circuits to achieve thermal independence.
A KPMG International survey recently found that 78% of surveyed engineering and construction companies believe building project risks are increasing. Clients are challenging engineers to complete projects at an unprecedented pace to realize the return on investment, and the need for infrastructure development in remote and complex environments is increasing. A study by the G20-backed Global Infrastructure Hub and Oxford Economics estimates that $3.7 trillion will be invested in infrastructure every year to meet global demand. It’s no wonder engineering and construction firms are constantly pushing the competitive edge.
To maintain an advantage, it’s paramount to stay at the forefront of safety compliance and best practices. A fundamental move: Specify cable cleats because they assist with electrical system protection. To ensure long-term integrity of electrical infrastructure, follow cable cleat installation standards according to the International Electrotechnical Commission 61914:2015, the most robust standards established globally.
Panduit cable cleats ensure cables remain contained in the event of a short circuit fault, minimising disruption and damage to personnel and property. Uniquely engineered for ease of installation in a range of applications and harsh environments, Panduit has the right product to fit your needs while providing on the job productivity, reliability and safety.
Panduit’s cable cleat solution is the industry’s first solution that has streamlined the selection process, tested to IEC standards bringing the vision of creating an engineering specifiable products to the EPC and Contractor firms. Panduit’s cable cleats have simple and intuitive design that helps to improve productivity, have industry-unique mounting brackets and installation tools, and are compatible with a variety of ladder racks and cables
The rest of this article will explore IEC 61914:2015 standard at a high level outlining the methodology and testing criteria to successfully design and test a Panduit’s cable cleat to meet standard requirements as well as the appropriate methodology to correctly specify a cleat system in the protection against a short circuit fault.