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Monthly Archives: May 2013

  • Preview: Medium Voltage Cable Accessories - A Book By Derek Goulsbra Part 2

    Continuing on from our preview of the first chapter of Derek Goulsbra's new book, available here in part one, this post previews the start of chapter 2 'Medium Voltage Cable Accessories: A Theoretical and Practical Appraisal.

    Chapter 2 – Electrical breakdown of air and solid dielectrics

    In order to understand accessories and how they perform it is necessary to look at the fundamental electrical breakdown of air and solid dielectrics, which are, of necessity, mixed in dry cable accessories.

    2.1: Electrical Breakdown of Air

    Air at normal temperature and pressure, which means a temperature of 20 ° C and pressure of 760 mm Hg, has an intrinsic breakdown strength of 3 MV m-1 . This means that if a voltage is applied between two electrodes, which provide a uniform field, and the electrodes are spaced 10 mm apart, electrical breakdown of the air in the form of an arc will occur at 30 kV. This value is the same for a.c. (the peak value), d.c. and impulse. The electric strength is simply found by dividing the applied voltage at breakdown by the electrode separation. In the present example:

    Breakdown strength E = Voltage applied / Electrode separation
    = 30 kV per cm
    = 3 kV per mm
    = 3 MV per m

    A uniform field is shown in fig.1. By definition lines of equal voltage are spaced equally across the shortest distance of the electrodes.

    In practice, the intrinsic electric strength of air is of little use because conductors at high voltage are not conducive to producing a uniform field and the earth plane is often a flat sheet interspersed with bolt heads and nuts for various fixings. At the other end of the scale from a uniform field is the point-plane configuration shown in fig. 2.

    Figure 1: Uniform Field

    In this situation, it will be noted that the equipotential field lines are much closer together near the point electrode and spaced further apart as the earth electrode is approached. If the separation of the electrodes is again 10 mm, it will be found that the voltage required to break down the air will be as low as 10 kV, depending upon the sharpness of the point. The reason for this is that as the field lines are close together near the point, the breakdown strength of the air, i.e. 3 MV m-1 will be reached with only a few kV applied. At this point the air breaks down locally to create corona around the needle electrode. There will not be complete breakdown of the gap initially because the electric stress nearer the earth plane is very much lower. The corona produced does, however, ionise the air thus reducing its electric strength and if the voltage is increased further, total breakdown occurs at a much lower level than if the electrodes presented a uniform field.

    Corona occurs in many situations without causing complete failure of the system – e.g. on overhead power lines and around insulators on these lines, especially when there is rain or fog in the atmosphere.

    Whenever air is used as part of the insulation in electrical equipment, there is the possibility of corona occurring. It is not desirable for this to happen in an enclosed space and thus designers ensure that sharp profiles are eliminated and spacing is adequate to prevent corona at working voltage.

    It should be borne in mind that corona in an enclosed piece of switchgear or cable box can, if the humidity is high, create various acids, which will eventually corrode the metal work of the equipment and this must be avoided. It will be shown later that this effect can also be caused by badly installed cable termination.

  • Preview: Medium Voltage Cable Accessories - A Book By Derek Goulsbra

    Derek Goulsbra has been heavily involved in product development, failure analysis and engineering and jointer training for over thirty years. His new book, published by Nexans, is a detailed look at the Medium Voltage cable accessories which will be of value to jointers and engineers alike.

    The book contains theoretical and practical appraisals on the workings and failure modes of a range of cabling accessories, including separable connectors, terminations, joints and associated components, as well as a practical consideration of cable preparation using present day techniques and tools prior to installing the accessory. Derek rounds the book up with a set of examples of failures which are presented with explanations of how these could have been avoided.

    We have been lucky enough to get an early copy of the book and have featured the following teaser from chapters 1 and 2.

    Chapter 1 – Introduction

    The cables used for electrical distribution until the 1960’s almost exclusively used impregnated paper to insulate the conductors and were usually three core. A change to polymeric insulation then gradually gathered momentum and with it a change in the technology required for cable accessories, although, ironically some of the first dry terminations were developed for paper cable.

    Early so-called dry type accessories for polymeric cables were usually of the pushon variety produced from elastomers and used on single core cable. The sound basic technology employed in the early days ensured that such products continue to be supplied today.

    Heat shrinkable terminations were introduced in the late 1960’s and had the distinct advantage of being range taking and capable of use on both single and three core cables. Heat shrinkable joints were developed a few years later following the success of terminations and their suitability for both paper and polymeric cables resulted in a major revolution in cable accessory technology. This technology was a complete departure from the previous techniques, which required a cast iron box or lead sheath filled with oil or compound.

    Cold shrink products made an appearance a little later and are now proving popular within the industry. The debate as to which system is preferable continues. It is fair to say that the heat shrinkable system is the most versatile and well known, but on single core polymeric cables in particular cold shrink systems are seen to have certain advantages. It is not the intention of the author to express a preference for any one system.

    The use of screened elbows to terminate cables is now widely accepted and it will be shown later in the book that this system is theoretically superior to other systems for certain applications.

    The early development of dry cable terminations concentrated on three core paper cables in the United Kingdom and single core polymeric cables in the United States of America. The accessories for paper cable were originally developed for paper belted cable rated at 11 kV using heat shrink technology; whilst for polymeric cable elastomeric push on systems were developed principally for 8, 15 and 25 kV.

    As with many totally new concepts, it was shown that extensive laboratory development and testing is no substitute for field experience and many of the early terminations failed. The failures were generally not the result of poor installation, as each one was carefully monitored at this stage. The first heat shrinkable terminations developed for paper cable were pole top mounted and therefore exposed to the elements. The materials used were not themselves to blame and the formulation has changed little since that time. Several weak links in the chain were, however, present simple things such as inspection windows in cable lugs allowing water into the termination and the sealant used did not always exclude moisture. However, the major problem was the regular occurrence of breakdown between one core and the lead sheath.

    Gradually, as the problems were identified, the weaknesses were eliminated and the products became reliable. At the present time dry type accessories, if installed correctly, will give satisfactory operation for the life of the system. Many terminations installed thirty years ago are still in service today.

    As the practices of utilities have change and many of the jointers and engineers have retired, the need to make the installation of accessories less skill sensitive is the main driving force for the manufacturer. However, there are still some fundamental rules which should be obeyed to ensure satisfactory installation and hence trouble free service life.

    A constant problem within the industry is the shortage of fully trained jointers to install the ever increasing variety of products in the market place.

    The following chapters will examine how accessories work, why they can fail and good practices required ensuring trouble free service. It is hoped that there will be information of interest to engineers and jointers alike which will enable them to better understand the workings of cable accessories and hence reduce the potential for problems at a later stage.

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  • Considerations When Choosing Cable Glands

    There are a number of factors that need to be considered when choosing the correct type of cable glands for industrial applications.

    Unnecessary problems, such as time wasted on site can be caused if the glands ordered are the incorrect size or type. These problems can be reduced by paying attention to the following considerations when specifying what glands are required.

    Cable Gland Selection Guidelines

    For all cables:

    • Determine the type of cable being terminated
    • Identify the construction, size and material properties of the cable used

    For armoured cables:

    • Check the material used for cable armouring (Aluminium Wire Armoured and Steel Wire Armoured etc) and specify the gland is manufactured from the correct material. i.e. brass, aluminium, nickel-plating
    • Check the inside diameter of cable against the gland manufacturer specifications
    • Check the outside diameter of the cable
    • Identify the environment of gland installation and consider any corrosion protection requirements
    • Check the ingress protection (IP) rating required by the equipment and application
    • Consider if an entry thread seal is required for IP66 ratings and above
    • Check whether other cable gland accessories are required (locknuts, earth tags etc)
    • Check the gland meets necessary requirements if being installed within Hazardous Areas
    • Identify the equipment entry thread and size and consider if any thread adaptors or converters are required
    • Consider any unused cable entries and specify stopper plugs if needed

    It is recommended that extra time for correct and proper cable gland selection be included within a purchasing schedule, in order to reduce the likelihood of inconvenience and time wasting at the critical point of installation.

    ETS are happy to assist, at no cost, in carrying out gland selection and sizing in the event of a contractor having a cabling schedule. Please contact our Sales Team for more information about this service.

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  • Genuine LSF Gland Shrouds vs Non-Genuine LSF Shrouds

    In recent years there has been a massive influx of imported products, such as cable gland shrouds, which claim to have equal characteristics to their traditional counterparts. Our video outlines the difference in quality between genuine LSF gland shrouds and 'economy' LSF shrouds.


    Transcription:

    "The following video clip demonstrates the differences in so called LSF termination components. The first shroud in the video is an imported low smoke alternative, which is marketed by some companies as a cost effective or economy LSF range.

    As the demonstration shows, the shroud immediately catches alight emitting a thick black toxic smoke and does not exhibit any self-extinguishing characteristics. This is clearly not LSF. In addition, glands that these shrouds are generally supplied with are inferior in both design a brass content, causing badly fitting components and poor earthing capabilities. If you are asking for an improved LSF termination, is this the quality of material you would expect?

    Genuine LSF, low smoke and fume components, are designed to exhibit self extinguishing characteristics, and only emit white, semi-transparent, non-toxic smoke in the event of combustion taking place. This allows fire exit signs to still be visible, enabling trap persons a safe passage through smoke, which contains no poisonous toxins, namely halogens.

    This shroud is a genuine LSF Zero Halogen product, as manufactured by CMP Products. It is clearly self extinguishing and emits only white, non-toxic smoke. The material used in the manufacture of these shrouds has been independently tested by London Underground technical services and as such, as been certified for use on both LUL's network and many other mass transit systems, the world over.

    If you request a LSF product, this is the quality of material that you should expect."

    If you would like more information on what makes an LSF cable gland shroud high quality, please leave your comment below.

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  • Earthing Requirements of Cable Glands Used on SWA Cables

    When cable glands are used to terminate SWA (Steel Wire Armour) cables, the gland must be able to provide earth continuity from the termination of the armour through to the body of the equipment, either via the enclosure itself (if it is metallic), via a gland plate or through an external earth path with the use of an earth tag.

    Direct To Ground Earthing

    Typically when earthing an armoured cable, a number of direct-to-ground external earth link cables are used and connected to the cable gland through an earth tag. As a minimum requirement, the cable will be earthed at least one of its two ends, meaning in the event of a fault or short-circuit the most direct route to ground will be achieved.

    'Daisy Chain' Earthing

    If multiple cable entries are required in a non-metallic equipment enclosures, a 'daisy-chain' earthing method can be adopted. Wherein an external earth cable is connected to an earth tag within each cable gland, with at least one earth tag used to connect the earthing cable directly to ground, typically via an earth bar.

    Earth Tags or CIEL Glands?

    The above methods of earthing SWA cables via cable glands, can be achieved by adding the relevant aluminium or brass slip-on earth tags to the required gland assembly, this method would be suitable where lower levels of short-circuit protection are required. If higher levels of short-circuit protection are needed, in Medium Voltage installations for example, cable glands with a heavy duty Cast Integral Earth Lugs can be supplied. These CWCIEL glands are suitable for MV installations between 11kV – 33kV and are also available in both brass and aluminium variants.

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  • How engineering has changed on the London Underground in the past 150 years

    2013 marked the 150th year since the London Underground became the first underground rail system. On that day, around 40,000 people were carried on the service that ran between Farringdon Street and Paddington.

    Since the early days of gas lit platforms, steam powered trains and class division and now with around 3.5 million daily users, the tube system has come a long way since then. We look back on the history and practices of the victorian era and look at how they have evolved into the standards and codes of practice prevalent on the LU network today.

    The worlds first underground network

    When it was first opened, the London Underground was the first of its kind in the world, it was a pioneer of engineering and has driven many developments since then. One of the biggest differences compared to the current underground was the development of increased safety measures. In its early days the tube was remarkably different to the modern day system we know today. A key difference was the use of steam powered carriages.

    The problems of using steam trains in an underground environment were obvious from the very first day, with a porter being taken to hospital and several passengers having to leave the platforms due to the smoke and fumes. In efforts to improve the air quality within stations, even smoking was restricted, as to reduce its contribution to the poor air quality.

    Electric powered carriages

    Electric locomotives were first used on the London underground as early as 1890, with the district and circle lines switched fully from steam power to electricity in 1905. Although it wasn't until 1961 when the last steam-hauled passenger trains were replaced.

    With the introduction of a fully electrical underground rail system, old systems would have to be upgraded to accommodate the new power cables and supplied that lined the tunnels. Introducing the use of various cabling accessories such as cable hangers, cleats and flexible conduits.

    Introduction of LUL approval and LUL standard 1-085

    In 1987, a fire broke out at the Kings Cross underground station where 31 people lost their lives and 100 others were injured during the incident. It was revealed after the accident that the fire itself was only part of the reason for the deaths and injuries. Many of the products used on the London underground during the time contained harmful hydrocarbons that not only released harmful, toxic smoke which affected the breathing of the victims but also burnt with a thick black smoke, severely reducing their visibility and ability to find the exits.

    As a result of the tragedy, the Fire Precautions (Sub-Surface Railways Stations) Regulations were introduced in 1989. These regulations, also known as Section 12, require that all electrical equipment within the Underground Network must adhere to strict safety standards.

    All electrical cable and accessories installed within Section 12 locations must meet the requirements of LUL standard 1-085. This standard outlines the fire safety performance of the materials used, including smoke emission, flammability and flame spread.

    Demand for LSFZeroHalogen (LSZH) products

    Since the Kings Cross fire, the use of halogenated compounds within the LUL network has been restricted and more low-smoke, zero (or low) halogen products have been specified. When exposed to flame, these LSZH products not only burn a much less toxic smoke, but the smoke is white in colour, minimising its effect on visibility during a fire.

    Manufacturers are now able to supply products that have these LSF properties while maintaining the performance of their traditional counterparts.

    Limiting Oxygen Index (LOI)

    It should be understood that just because a product is classed as halogen free does not indicate fire resistance or self-extinguishing properties. The Limiting Oxygen Index (BSEN ISO 4589) determines the percentage of oxygen in the atmosphere that would need to be present for the material to combust. The oxygen percentage present in the air during normal conditions is 21%, meaning the higher the percentage, the greater flame retardency.

    Since the 1987 fire, the rail industry set standards that all materials used in underground passenger carriages demand an LOI of 34% of greater, compared to 28% LOI on overground rolling stock. This shows the extra precautions now being taken on the LUL network.

    Full LUL product range

    We are able to offer a wide range of LSFZeroHalogen and LUL approved products, suitable for use within the London Underground network and other mass-transit systems. Key LUL approved products include cable cleats, glands and trackside flexible conduits.

    Engineering the London Underground Video:

    Watch this short film made to inspire the next generation of engineers.

    The film takes you on a trip through the Tube’s history to the present day and celebrates 150 years of civil engineering on the Tube.

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  • The Importance of Correctly Cleating Cables

    The importance of correctly cleating cables in a trefoil formation within electrical installations is frequently underestimated, often due to a lack of understanding. Many people assume cleats are used purely to hold the power cables to the desired route, but for an installation of single core cabling system to be deemed safe, it is vital these cables are restrained in a manner which is able to withstand the forces generated in potentially lethal short-circuit situations.

    The below video shows an example of the extreme force that can be generated by a short-circuit:


    More awareness yet inferior products

    In recent decades, the demand for electrical power has risen dramatically and specifiers are becoming more aware of the issues involved with incorrectly cleating trefoil cable formations, but issues still occur with inferior quality products within the market-place.

    To address the issue of inferior products, in addition to the existing European Standard (EN 50368), there is now an international standard which provides global recognition of the need to provide suitable restraint for power cables; IEC 61914:2009.

    Up-to-date international accreditation

    Coupled with the International standard, it is equally important that any product specified is capable of demonstrating its suitability for the required task. It is no longer acceptable that in-house testing certificates are used as proof of a products pedigree and this is why current, third-party testing is vital.

    Here at ETS, we continue to believe that only the highest quality, UK-manufactured, EN and IEC standard conforming cable cleats should be used. Our expertise and extensive cleat range allows us to supply cleats designed specifically for the task and third-party accredited as such.

    We also provide a service that can recommend which type of cleat should be used and importantly, the spacing interval the cleat should be fixed for each project application. This is calculated from the short-circuit withstand installation requirement and the diameter and construction of the cable being installed.

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  • Heat Shrink vs Cold Shrink: Knowing When Each One Is More Suitable

    Cold Shrink and Heat Shrink tubing may appear similar from the outside, but their key characteristics are very different. Both types of tubing have different installation techniques, applications and physical properties.

    HeatShrink and ColdShrink tubing can be used for a variety of cabling applications including: terminating, splicing and providing environmental seals on LV and MV cables. More than price, ease of use and environmental conditions should be taken into consideration when choosing the most suitable product.

    The differences

    The most obvious differences between the two products are how each of them is applied. Cold Shrink comes stretched over a removable plastic core, allowing the tube to be slid over the application, the core removed and the tubing will contract to create a watertight-seal around the cable or connection due to the 'active memory' contained within the EPDM rubber or silicone material. Heat Shrink, also comes pre-stretched but as a sleeve rather than over a removable core. The sleeve requires a heat source for installation, usually from a gas torch, to heat the polyolefin tubing so that it shrinks to its original size, creating a seal over the cable or joint.

    Silicone-rubber Cold Shrink tubing has the greater UV-resistance of all the types of cold or heat-applied tubing and is therefore used in outdoor, exposed environments, for example: trackside terminations to rail power lines. EPDM rubber is also used in Cold Shrink tubings and is much more abrasion resistant than other cold or heat-applied products, being ideally suited for direct burial applications such as cable-to-cable jointing.

    Cold Shrink tubing has an “active memory” seal characteristic, meaning the tubing is always trying to return to its original size and able to maintain its sealing capability around cables as they expand and contract under large load-swings or temperature fluctuations.

    Another advantage of Cold Shrink tubing is that, because there is no direct flame required to install components, there is a reduced risk to the installer, especially in the presence of combustible gasses. Hot-work permits are not required and quality and reliability of the installation is guaranteed due to fewer critical installation techniques, with resultant time and cost savings.

    The polyolefin-based material used in Heat Shrink is resistant to most chemicals and also becomes very rigid once it has been heated, therefore making it a good choice for mechanical protection. However, the downside to this rigidity is the products inability to expand and contract with the cable, meaning an environmental seal cannot be maintained without the aid of hot-melt adhesives or mastic tapes. Heat Shrink materials are not suited for installation areas experiencing high heat or humidity, but are ideal for use in most industrial/commercial installations where operating temperatures are below 90oC and where chemical resistance is required.

    Video: ColdShrink Joint vs HeatShrink Joint

    Knowing which one to choose

    Every cabling installation has a number of different requirements when it comes to chemical, abrasion and moisture resistance. When adding space limitations and temperature considerations into the equation, choosing the correct product can become difficult and confusing. Ultimately, ETS can provide a vast selection of Heat Shrink and Cold Shrink products to suit a wide variety of cable accessory requirements from low-voltage straight joints, all the way up to 72kV distribution line terminations.

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  • Stainless Steel Cable Cleats and Galvanic Corrosion

    How the issue of galvanic corrosion affects installation of stainless steel trefoil cleats onto galvanised steel support structures in various environments.

    The term galvanic corrosion refers to corrosion induced when there is electrical contact between two dissimilar metals in the presence of an electrolyte, i.e. water or corrosive vapour. The severity of the corrosion is determined by whether the metal is deemed cathodic, e.g. 316 stainless steel in its passive state, or anodic, e.g. zinc (the predominant metal of galvanised coatings).

    In the majority of cable cleat installations, galvanic corrosion is not experienced due to the lack of an electrolyte in the atmosphere. Stainless steel has an inherent chrome oxide film layer which protects it from the environment, in what is deemed an “active” state, this layer can however be eroded in the presence of halogen salts in the form of fluorine, chlorine, bromine, iodine or astatine, many of which can be found in contaminated water or gas vapour. As the majority of ETS stainless steel cable cleats are installed within dry industrial or commercial environments, without the presence of any electrolyte and remaining in their “active” state, galvanic corrosion does not occur between the cleat and the surrounding galvanised steel support structure.

    Cause and effect of galvanic corrosion

    Galvanic corrosion is an issue which must be addressed in areas contaminated with high levels of halogen salts, namely exposed environments close to the sea or areas prone to contaminated rainfall, e.g. major conurbations and oil or gas fuelled power stations etc. In such areas the integrity of the stainless steel product is attacked by these halogen salts, breaking down the protective chrome oxide film, making the material deemed to be “passive” in its characteristics. Over a period of time and in the continual presence of an electrolyte, any zinc coated (galvanised) structure is adversely affected, dissolving the zinc and corroding faster than it would if not in contact with the passivated stainless steel, which conversely corrodes slower! In real terms, this means that the galvanised coated steel-work would corrode due to the loss of its zinc coating, although the stainless steel cleat suffers no adverse effects.

    Avoid galvanic corrosion with a 91-ST washer

    To overcome the issue of galvanic corrosion in such areas, we recommend the use of the 91-ST cleat separation washer. This provides an insulation layer between the dissimilar metals, increasing the distance between each metal and removing the electrical and physical contact points critical to the development of corrosion.

    The importance to this process of time and “wetness” (as well as the chemical characteristics of the “wetness”) should be stressed. There is no definite period of time which can be mentioned as there are so many variables to the process, consideration should be paid to the projected life-span of the installation and the potential corrosiveness of the environment. In general terms, we would advise the use of the cleat separation washer for all external installations.

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  • Thameslink Blackfriars Station Redevelopment

    Blackfriars station in London is at the end of a major redevelopment programme which started in 2008. As a major landmark station which is visited by 44000 passengers a day, this Thameslink project was initiated by Network Rail to increase the service capacity and frequency of trains from 12 to 24 trains an hour. As well as improving the interchange service between the National Rail and LUL services.

    Station Redevelopment

    The redeveloped station is the first in London to span the River Thames, allowing the platforms to accommodate 50% longer trains.

    ETS have provided relevant cable accessories for the duration of the project from the commencement of the enabling phase in 2008. Products have included Cembre terminals, Stainless steel cable ties, Ellis LUL approved cable cleats and CMP LUL approved Cable Glands.

    Cable products to meet demand

    Mick Searle – Project Buyer for Balfour Beatty Civil Engineering explains:

    “ETS provided a flexible service to meet the demands of the project which sometimes meant goods having to be delivered on a same-day basis. We knew that we could also count on ETS to provide products that met the correct criteria for both Network Rail and LUL installation standards”.

    Images courtesy of Network Rail

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