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Monthly Archives: August 2013

  • Bentonite vs Marconite – Earthing Compound Comparison

    Bentonite and Marconite have certain similarities, they are both ground enhancement materials used to lower the resistance to earth. However, there are a number of differences between the products and the applications they should be used for.

    Bentonite Moisture Retaining Clay

    Bentonite is often used to reduce the resistance between the soil and earth-electrode (earth rod or earth mat) by retaining moisture. The naturally occurring Bentonite compound consists mostly of montmorillonite, the sodium based clay swells to many times its size when mixed with water.

    Available in both powder and granular form, preference usually lies with granular form as it is often considered easier to handle and prepare.

    Marconite Conductive Aggregate

    Marconite is a synthetic material manufactured specifically for earthing purposes. Utilising specific raw materials and minerals, mixed in carefully controlled ratios. The Marconite compound then goes through a range of manufacturing processes and thermal treatments, designed to create a consistent, fit-for-purpose earthing compound.

    The resulting precisely measured, granular mixture is virtually dust free and has exceptional electrical properties.

    Bentonite vs Marconite

    Resistivity

    All earthing aggregates can be called conductive with a resistivity of 5 ohms metres or less. Bentonite has a typical resistivity level of around 3 ohms.m. Marconite with a resistance level of .001 ohm.m, has a far low resistance than its competitors, even when mixed with cement, the resulting resistance level is still only 0.19 ohms.m.

    Chemical balance

    As Bentonite is a naturally occurring material, there can be anything from 15-20% impurities which can be corrosive and will corrode any earth-electrode connections, resulting in earthing system failure, which can cause costly damage and lengthy repairs.

    Marconite is a chemically inert compound and as such is non-corrosive to steel or copper, it does not attack cement structures and has a pH level within the neutral range. Allowing Marconite to be used with all conventional types of cement, as well as most proprietary resin-based cements, adhesives and gypsum plasters.

    Versatility

    One of Bentonite's key features is also one of its weaknesses. The ability to absorb rain water once installed, helps increase the conductivity but also makes the material liable to drying out and therefore shrinking or even being washed away entirely. Therefore requiring maintenance every few years, such as adding additional water or salts to continue to achieve the desired earth values.

    Suitable for most ground conditions, Marconite becomes a permanent solid structure, especially when mixed with concrete and is not prone to these problems.

    High strength

    Due to its clay-like nature, Bentonite has limited strength levels, this added to the materials expansion when mixed with water, makes it unsuitable to be included within building structures themselves. Whereas Marconite can achieve strengths higher than Grade 25 concrete and therefore is suitable to be used as part of the building structure.

    Cost effectiveness 

    When it comes to cost, there is a clear difference between Bentonite and Marconite. Being considerably cheaper, Bentonite offers an earthing solution for cost conscious buyers, although the cost of maintenance every few years should be considered, it can still work out as an effective solution.

    Marconite on the other hand is a more expensive product, however once installed, the permanent earthing solution requires no ongoing maintenance so this should be factored in when considering the more costly material.

    Ease of use

    Both earthing compounds are simple to instal as a backfill for earth-electrodes. However as Marconite can be used to replace sand and aggregates within conventional concrete mixes, it should be carefully mixed in ratios of 3 parts Marconite and 1 part cement by weight, with the addition of 1 litre of water per 4kg total mix.

    Due to the slightly more complicated nature of using Marconite within a permanent concrete structure, it is available in a premixed bag. The 25kg premix sack contains pre-measured and pre-mixed ratios of Marconite and Portland Cement powder, requiring only 5 litres of water. Reducing on-site confusion and ratio errors. However the hydroscopic Portland cement used within the premix means it must be used within 6 weeks of purchase.

    Marconite and Bentonite each have their own advantages and disadvantages for different applications, with cost and longevity of installation often being the deciding choosing factors. For more information on which earthing compound is best for your needs, please contact our Sales Team for advice.

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  • Centaur Cable Saddles - A Short-Circuit Tested HV Cleating Solution

    The Centaur Cable Saddles are the only solution for cleating large diameter High Voltage power transmission cables, that has been put through, and passed, the most rigorous of short-circuit testing procedures. The cable saddles were designed by Ellis Patents to fill the gap in the market which presented serious safety risks.

    The Centaur product range typically handles 275kV to 400kV cables, from 100mm up to 162mm outside diameter, and is generally installed within tunnels and affixed to supporting steel-work.

    Why is a short-circuit tested High Voltage cable cleat so important?

    At present, neither the British or European standards take into account cleats on cables of this size. As a result, those specifying for such projects are very much left in the hands of the manufacturer, who in most cases, simply provide warranties for their products. The risk lies in the fact that none of the limited number of products available have been short-circuit tested, meaning warranties are based purely on calculations and mechanical tests.

    How the Centaur Cable Saddles are tested

    Using cables manufactured by ABB in Sweden, the centaur cleats were tested to 162kA peak (63kA RMS) for one second, in both three-phase and phase-to-phase fault installations. A copy of the full test report from KEMA is available upon request.

    The saddles have also been successfully tested for corrosion resistance having been subjected to an independent salt spray test, carried out in accordance with BS EN 9227:2006 corrosion tests in artificial atmospheres.

    High Voltage Cable Cleats Centaur by Ellis

    Centaur Saddle Design features

    The product consists of an extruded and pressed aluminium saddle and hinged aluminium over-strap retention which incorporates a LSFZeroHalogen (LSZH) nylon liner to cushion and protect the cable in the event of a short-circuit. The saddles are available in lengths of either: 400mm, 600mm and 800mm to allow for different mounting centres. The product range can also be supplemented by intermediate short-circuit straps as well as a roller system for easy pulling of cables.

    For more information on the Ellis Centaur saddle range, please contact our Sales Team.

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  • Galvanic Corrosion Protection and Prevention in Harsh Environments

    When specifying products for use within harsh environments, choosing the correct materials is vital. For example, in offshore oil and gas installations, equipment experiences exposure to high levels of seawater and salt-spray, which are highly corrosive due to the dissolved chlorides within. Offshore installations are not the only projects that experience harsh, corrosive environments, other corrosives that affect a wide range of industries include: chlorine, ammonia and hydrochloric acid.

    In May we reviewed the effects of galvanic-corrosion between two dissimilar metals, in that example we explored the use of stainless steel cleats with galvanised steel support structures. Corrosion can take many different forms and choosing the correct materials and finishes is key to minimising its effects. We have outlined common corrosion control methods, explaining how and where they should be implemented.

    Choosing materials based on their galvanic characteristics

    Different metals have different electrode potentials, therefore when two different metals are electrically connected in the presence of an electrolyte, such as seawater, the more active metal will become anodic leading to loss of electrons and increasing oxidisation in a process known as galvanic-corrosion.

    Metals and alloys are ranked in order of noble to active, in what is called the galvanic series. A more noble metal, such as stainless steel, will have much better corrosion-resistance than a more active metal such as cast iron.

    Galvanic Series MetalsGalvanic Series of Metals

    Galvanic corrosion can be controlled and minimized by selecting metals in similar positions within the galvanic series. A more active metal can also be used as a sacrificial anode, attracting corrosion in order to protect the more noble metal from corrosive attack. Alternatively, depending on the installation, a cleat separation washer can be installed between the dissimilar metals increasing the distance between each metal and removing the electrical and physical contact points, critical to causing corrosion.

    Passivation

    During manufacturing, certain metals form a layer of metal oxide on the surface in a process known as passivation. Although the process occurs naturally, it can be enhanced through chemical passivation treatments and anodising. The layer protects against corrosion as it is tightly bound to the surface, preventing further penetration of oxygen and corrosive molecules.

    The most common examples include aluminium and stainless steel. Usually, if these metals are damaged the layer reforms quickly, however this process is not without its faults. Pitting corrosion can occur in aluminium when chloride ions interfere with the reforming process and high chromium carbides can affect welded stainless steel.

    If the products are properly selected for the intended application, passivated metals can provide high levels of corrosion-resistance. This is why stainless steel cleats are the preferred product within offshore installations.

    Protective Finishes and Coatings

    Various protective coatings can be applied to metals to help shield metallic surfaces from the surrounding environment, including epoxy power coating, oven-baked enamel, hot-dipped galvanisation and PVC-type coatings. Care must be taken when installing products with these coatings as any damage during installation and fitting, drilling and cutting etc. must be re-protected to ensure the integrity of the rest of the coating.

    Hot-dipped galvanised products offer two forms of protection as the zinc coating protects the surface of the steel and becomes a sacrificial anode if damaged.

    Non-metallic components

    In highly corrosive environments, where metals are not suitable due to high levels of corrosion, non-metallic products are commonly being specified as an alternative. It is important to remember that plastics are not completely impervious to chemicals and often suffer from UV exposure, which should be taken into consideration when specifying project materials.

    The possibility of corrosion may not be at the forefront of material specifications and in times of economic hardship, may be over looked. However, with the World Corrosion Organisation estimating the annual cost of corrosion, throughout all industries, at $2.2 trillion, it truly is a global issue. We believe any short-term savings made from overlooking the extensive effects of corrosion will be more costly in the future and the figure from the WCO backs this up.

    If you are concerned about potentially corrosive environments affecting products you may be specifying, we are able to offer expert advice on the best product to suit your specific requirement.

    Have you had any experience with corrosion and would like to share your experiences then please leave a comment below.

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  • Repairing PVC Sheathed SWA and Armoured Cables

    cable sheath repair swaCables and cable jackets can be damaged by a number of causes and accidents, from digging near unmarked underground cables to damage during installation. There are different degrees of damage that can be inflicted upon an armoured cable, below we outline different techniques designed to repair damage to the cables primary (outer) sheath.

    If the sheath or jacket of an armoured cable is damaged it is important to repair the sheath as soon as possible to prevent further problems such as moisture ingress.

    Heat Shrink Repair Sleeve for SWA Cables

    Heat Shrink Repair Sleeves

    Repairing a cable with a heat shrinkable repair sleeve is suitable for power cables at any voltage. The SWRS sleeve is adhesive lined and features a stainless steel, self-locking channel, providing corrosion protection to the cables armouring and water tight seal around the sheath.

    Supplied as standard in 1 metre lengths, the sleeve and channel closure system can be cut as required to fit the application. A soft-flame gas torch is also required to heat the cable repair sleeve.

     

    Self Amalgamating Tapes

    Self amalgamating tapes can be used to repair smaller sections of damaged SWA cable. 3M Scotch Silicone rubber or 3M Scotch EPR self amalgamating tapes are available, depending the the cable type being repaired.

    self amalgamating tape for SWA cable repairTo use self amalgamating tape for sheath repair, first, the cable must be prepared, which includes wiping clean the area and making sure it is completely dry. The PVC sheath surrounding the damaged area should be roughened with a wire scratch brush or coarse emery cloth to reduce water ingress along the smooth surface of the undamaged sheath.

    Be sure to keep any tools or sheath abrasion brushes or cloths clean, as any oil or grease contamination on the outer cable jacket will prevent proper bonding and cause early sheath repair failure.

    Starting at least 30mm from the damage, wrap the tape around at an angle of about 30o and pull the tension on the tape to reduce the width by about 1/3rd as you wrap. The tape should be overlapped 50/50 as you go. Once you've covered one way continue 30mm past the damaged sheath area, then wrap the tape back the other way, alternating the angle of the application to oppose the first layer.

    3M Scotch Cable Jacket Repair Tape 2234



     

    For more information on SWA cable repair techniques and products, please contact us.

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