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Single-core power cables can be run in a number of formations, the most common include flat or trefoil formations. Each cable formation has its benefits and drawbacks, we’re going to look at the differences between each way for laying these cables.

Trefoil Phase Formation

One of the main reasons trefoil formations are used is that it places the phases the same distance apart, so the magnetic field and circulating currents are equivalent for each cable phase. Typically, trefoil phase formation is more commonly used for Low and Medium Voltage applications up to 132kV due to ease of installation and the reduction in space the formation has across the containment system used. However, installing single-core cables in trefoil formation can mean that the touching cables will exhibit worse heat-dissipation when compared to flat formation, therefore lowering the current-carrying capacity. Considerations which should be taken into account by specifying electrical engineers when designing such systems.

Flat Phase Formation

Rarely used at voltages below 275kV due to the mounting centres having to allow for sufficient heat-dissipation. Running in flat formation, the central phase of the three-phase set is adversely affected by the magnetic fields around the neighbouring phases, leading to a higher running temperature on the middle phase and subsequent voltage imbalance. Phase transposition can be used to counter-act these affects, but the necessary mounting space required for such installations usually precludes their use in most industrial/commercial installations and flat formation tends to only be used by major DNOs in their distribution networks.

Trefoil formations are chosen for applications where space is at a premium and flat formations also tend to be less cost effective due to the increase potential cable route space they take up. Our experience of such installations has generally been with cables above 132kV, with cables under 132kV being laid in a trefoil formation.

Triplex Formation

An adaptation of the trefoil formation can be found with the increasingly popular Triplex formation. Triplex cables are three conventional single-core cables supplied pre-wound in trefoil formation by the cable manufacturer/vendor, the three cores having been slowly twisted together during the manufacturing process and supplied on one drum, offering significant benefits in installation time.

Due to the constant phase rotation of the cores, using conventional trefoil cleats is not possible as the position of the mounting base will not be constant. For this reason our Triplex Cable Former provides a cleating solution, allowing traditional trefoil cleats to be used. Learn more about our offer of power cable lugs.

Cable cleats suitable for each cable formation:

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Flexible conduit systems can provide a proven safe and cost effective protection solution for cabling installed in explosive atmospheres. Better safe than sorry is a sensible approach, but make sure that you are not fooled into unnecessarily expensive options warns Tim Creedon, Sales and Marketing Director for Flexicon.

Explosive areas exist where a flammable mixture of gas and air or dust and air exist in large enough quantities and for long enough. If an ignition source exists then there is a real danger of an explosion.

Naturally we all think of the oil and gas industry in such situations, but there are a surprising number of other industries where explosive atmospheres could exist such as in building and construction, transport, marine and defence, food processing, water treatment and power generation to name but a few.

Wherever possible it is important to minimise the risk of explosive mixtures forming and/or prevent the risk of ignition. Where this is impossible or impractical then you need to consider providing protection.

Any electrical installation in such an environment is a potential source of ignition. The degree of protection any equipment requires depends to a large extent on the risk of an explosion occurring in a given area.

To understand the level or protection required, you must understand the nature of flammable mixtures and ignition sources and also how different zones are classified depending on the level of risk.

Gases are classified into 3 groups with group A being the least explosive and group C being the most. Equipment classification is from T1 to T6 according to the maximum allowed temperature resulting from the ignition temperatures of the gas/air mix. It is important to remember that certain fine dusts dispersed in the air can also be explosive.

Hazardous Zones

The above table shows the zone designations, which are divided first into the hazardous areas for gases, vapours and mists and secondly into the hazardous areas for dusts. It also shows their risk categories, i.e according to the probability of a risk being present.

Assuming that the electrical equipment is correctly specified, it is important not to overlook any cabling that connects into it.

Until recently those specifying cables for such areas had to select from the products offered by various cable manufacturers. In addition each individual cable needed a flameproof gland, which added to both the cost and the time needed for installation.

Such cables could also be difficult to terminate and, if a number need terminating in an enclosure, could necessitate the specification of a larger enclosure.

The development of flameproof ATEX and IECEx approved barrier glands for flexible conduit means that you can use liquid tight conduit systems in hazardous areas without compromising safety.

There are a number of ways of classifying protection techniques used to address hazardous zones. Most glands are classified as Ex d or Ex e.

An Ex d classified gland forms a flameproof or explosion proof barrier – they are strong enough to contain any explosion or fire that may occur. An Ex e classification is defined as ‘increased safety’.

If something is classified as Ex d then it can also be used for Ex e applications in Zone 1 and Zone 2 areas for gases and Zones 21 and 22 where explosive dust may be present. In most cases such glands do not add to the temperature of the enclosure into which it terminates, so it can be used with all temperature classes.

By using Ex d glands with liquid tight conduit, you can group several cables together into one system. The conduit provides protection for all of these cables and, if correctly selected, offers all the necessary mechanical protection for a given application.

So, for example, by using steel cored armoured metallic conduit you might be able to use standard cables instead of more expensive SWA specialist cables. In this example one braided conduit effectively takes the place of several more expensive braided cables. It also means that only one flameproof barrier gland is needed, rather than several.

This single termination, instead of the multiple terminations that would be needed with individual cables, limits the risk of the enclosure integrity being compromised since there is only one point of entry. Using a flexible conduit system also provides additional mechanical protection for the cables.

It is worth pointing out at this stage that it is the flameproof gland that is rated at Ex d so that if there were an explosion within the electrical equipment enclosure it would be contained. Some conduit manufacturers have in the past muddied the waters and inferred that the conduit is also classified as Ex d and been able to charge a premium.

The role of flexible conduit in any application is to protect cabling so you should take care in its specification. This is even more important in hazardous areas since you want the installation to remain safe throughout its lifetime, not just once the system is installed.

Fortunately when you specify the correct barrier gland, you can effectively use a liquid tight flexible conduit for the vast majority of applications.

There are a number of different types of liquid tight flexible conduit that are suitable for both indoor and outdoor applications.

With this in mind the specifier needs to conduct a thorough risk assessment of all of the hazards potentially faced by the conduit in addition to the explosive atmosphere.

From this risk assessment they can then accurately specify the most appropriate liquid tight conduit for the project. As an illustration Flexicon has eight different grades of liquid tight conduit which, between them, would suit most applications.

Other environmental factors

If conduit is left in an exposed situation, it could be crushed or there is a chance that something could be dropped onto it. For such applications you should specify conduit with an adequate compression and/or impact strength.

Other factors that you may need to consider when selecting the correct conduit might include: extremes of temperature, EMC screening requirements, moving equipment, abrasion, resistance to chemicals or corrosion and UV resistance for external installations.

Note this list is not comprehensive, hence the need for a full risk assessment. If you have any concerns then most manufacturers should be able to advise.

A great deal of attention has been paid to hazardous area equipment. Do not forget cable protection and electrical lugs.

For more information on our range of Flexicon flexible conduits, please contact our Sales Team.

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