- 24hr. Next Day UK delivery
- Nationwide distributor of power cable accessories
- Supplying the trade for over 30 years
As stockists of Marconite, we're able to supply the conductive concrete and pre-mix with short lead times and with no minimum order quantity, to celebrate this, we've decided to look at the history of Marconite Conductive Concrete, where it comes from and why it was developed.
Conductive clay such as bentonite has been used for electrical earthing for decades, however as technology and needs have increased, the demand for a more permanent earthing compound arose.
Formulating an earthing material from purposely manufactured synthetic material, that provided lower resistivity, higher strength and lower maintenance than traditional Bentonite clay, was required. The mixtures comes under many guises: conductive concrete, admix and pre-mix, but one of the resounding industry names is Marconite.
For an inside look, we've invited Dave Wilson from the James Durrans Group to talk about Marconite's history and how the Marconi Company developed this effective earthing product.
In the 1960’s the Marconi Company were at the forefront of the worlds’ communication and defence sectors. Designing highly sensitive electronic systems, their engineers increasingly encountered requirements beyond the technology of the time. Marconi realised that to overcome these issues that they would need an entirely new and highly conductive method of electrically earthing installations.
From inception, this new method needed to be permanent, required no maintenance, must be chemically inert, capable of forming strong structures and could be used anywhere, even in difficult ground conditions. In 1973 Marconi’s research, combined with process engineering skills from the James Durrans Group, culminated in the product named Marconite®.
From concept, Marconite® has been solely manufactured from unique raw materials and to a specific process, at their Scunthorpe works. The resulting product is a true semi conducting material that does not need water in order to conduct electricity. When used as an aggregate replacement and mixed with cement it forms a conductive concrete that can be used to encase earthing systems significantly magnifying their effect.
Aside from electrical earthing, resulting from its exceptional electrical properties, Marconite® has been widely used as an antic-static flooring system. If made into solid block, when assembled into buildings they act as a Faraday Cage, protecting the sensitive electronic equipment housed within from EMP radiation. Finally, if used in its dry granular state, it can be placed in trenches or beds to enhance existing ground conditions, a process known as soil enrichment.
Today, over 40 years since its inception, Marconite remains the industry default and standard to which all others are benchmarked. Its’ continued use around the world are testament to the vision of its original development and to the longevity of its abilities.
In the past year within the UK it forms an essential part in the latest high speed broadband networks, is extensively used in the latest power generation plants and provides rail engineers maintenance free earthing solutions. Whilst across the world it has been used in mass transit systems in Malaysia, solar power farms in the Middle East and as an essential component of national defence systems.
Diagram of Marconite installation:
As Marconite stockists and distributors, please contact our Sales Team with your earthing requirements.
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When identifying products, you may see terms such as “suitable for outdoor use” and “weatherproof”. It is important to be careful when selecting products that claim these properties, and to establish to what “weatherproof” actually means.
Bearing in mind that we live in temperate conditions and climates vary throughout the world, just look at Britain’s weather in the past 18 months; drought last winter, followed by the second wettest calendar year, sub-zero snow-storms bringing the country to a standstill and so far a summer which is only just rearing its head. With such unpredictable weather within the UK, you can understand why weatherproof can mean different things in different parts of the world, to different manufacturers.
Flexicon's technical director, Ian Gibson, warns that before specifying outdoor protection, make sure you clearly define exactly what it is you require. With such a wide scope of local climates, manufacturers should have a wide range of products to choose from. Flexicon, for instance, has 48 different flexible conduit systems with hundreds of variations when it comes to terminations.
There are a number of factors that should be considered when evaluating the true level of weather-proofing a product possesses. IP ratings are considered a primary factor but this should not be the only factor considered.
The first number on a product’s ingress protection rating describes its resistance to solids and dust, the second number represents the protection against water and moisture ingress.
It's important to remember the IP rating of a termination product, such as a cable or conduit gland, usually refers to the outer seal and an extra washer may be required to achieve the same IP rating on the equipment or enclosure entry. More information can be found here.
Ian recommends, when considering flexible conduits, the conduit should have a minimum of IP6x when it comes to dust ingress, regardless of the conduit’s application.
The different levels of ingress protection are outlined in the chart below.
1st Digit (Solid Objects) 2nd Digit (Water and Moisture) # Explanation # Explanation 0 Non-Protected 0 Non-Protected 1 Protected against solid foreign objects of 50mm diameter and greater 1 Protected against drops of water falling vertically 2 Protected against solid foreign objects of 12.5mm diameter and greater 2 Protected against drips of water falling up to 15o from the vertical 3 Protected against solid foreign objects of 2.5mm diameter and greater 3 Protected against spraying water at up to 60o from the vertical 4 Protected against solid foreign objects of 1.0mm diameter and greater 4 Protected against splashing water from all directions 5 Dust-protected 5 Protected against jet of water from all directions 6 Dust-tight 6 Protected against jet of water similar for to heavy seas 7 Protected against the effects of immersion 8 Protected against prolonged effects of immersion under pressure to a specified depth. 9K Steam clean, high pressure temp jet wash to DIN40050
When considering what moisture ingress protection is required, it is recommended to take a ‘belt and braces’ approach to prevent water ingress over longer periods of time. Therefore, a rating of IPX6 or IPX7 should be specified, unless the fitting is suitably sheltered, such as on the underside of an enclosure.
The amount of UV radiation that a product is exposed to will depend on location, altitude and climate. The ultraviolet rays will affect the performance of plastic materials by breaking down the long polymer chains, reducing the impact strength, flexibility and working life of the product.
Although other coloured plastics can be manufactured to be UV resistant, Flexicon predominately offer black plastics as the carbon black within them helps protect the polymer chains.
Corrosion becomes an issue if metal conduits and fittings are used. Coated steel is preferred, as long as its IP rating is sufficient to prevent water entering the conduit.
Nickel-plated brass fittings will discolour through oxidisation over time, especially in coastal areas. Grade 316 stainless steel is recommended for use in marine environments.
When considering temperature, generally the maximum temperature reached naturally by the weather will not present an issue with most products. Colder temperatures can cause more of a problem; you should consider the potential hazards to which a product might be exposed, to prevent failure during operation or installation.
Flexicon understand the need to have products that fit the harsh and sometimes unpredictable climates around the world. Their LTP flexible conduits, when used with stainless steel fittings, represents an all-round “weatherproof” product, with an IP rating up to IP69k and operating temperature ranging between -20oC to 105oC. The LTPHC range extends this temperature range to -60oC to 150oC.
Ian Gibson, technical director at Flexicon is also the chairman of both the IEC (Worldwide) and CENELEC (European) committees that prepare conduit standards.
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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.
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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