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Technology | January 2014

Safe practices for power cables

Power cable systems have been one of the cutting edges of any process industry. Sanjay Jha shares on how we could achieve better reliability by adopting components more suited to the practical site conditions.

The design and selection of the materials used in cable joints and terminations is extremely important for safe power supply to fire prone industries such as oil and gas. Power cable systems are a critical part of the power distribution systems in any process industry. Any unforeseen failure of cables or its joints and terminations could be a very serious affair with respect to huge production losses and associated safety hazards. This is all the more true when it comes to oil and gas industry as the environment could possibly be contaminated with explosive gas and air mixtures. Besides, contamination in air, the underground buried cable and cable joints may also be subjected to ingress of hydrocarbons present in the soil leading to deterioration of the materials used in the joint. This degradation could lead to ultimate failure over a period of time. Hence the design and selection of the materials used in cable joints and terminations is extremely important. It is equally important to ensure that the installation of such joints and terminations are done by skilled and trained installers using appropriate tools as required. It is estimated that approximately 30 per cent of the network failures are caused by splicing issues and most of which are caused by installation or workmanship issues. Any failure explosions, sparks in the connections, surface discharges in terminations, etc. could be a possible fire hazard in an operational plant.

Taped type joints are used depending on the experience and preference of the user. Heat shrinkable type cable joints and terminations are currently most commonly used in the industry. Both the heat shrink and taped type joints and terminations require high level of skill and craftsmanship. Installation of heat shrink type joints and terminations requires use of open flames, which could pose fire hazards in the oil and gas environment. Hence, elaborate safety precautions like having a fire tender, water curtain, fire extinguishers, etc. need to be arranged for undertaking the installation of such joints and terminations in an operating plant. Further, less than perfect quality of installation by contracting companies during the construction phase could lead to failures during the operation later.

The reliability of electrical power cable joints and terminations depend on many factors such as jointing kits design and technology; quality of jointing kit materials (manufacturing defects); quality of installation; simplicity of installation procedure; operating conditions and environment (hydrocarbons); and other influence like load pattern, surges, etc.

Some of the examples of cable accessory defects and failures are shown below:
Failures such as the above, could be overcome by choosing the right products and investing in training and the right tools. To achieve best results, it may be necessary to customise the design and use specially suited materials taking into account special operating conditions such as hydrocarbon contaminated air or soil. The scarcity of skilled and trained installers makes the situation even more challenging. On the other hand, somewhat limited internal technical expertise in this highly specialised field is also a worrying factor. In this situation, the practice of entrusting the cabling and splicing work to contracting companies, as widely practised in the construction phases, involves much risk. In the case of operational oil and gas plants, the increased safety regulations and urgency to reinstate the power supply poses additional challenge. In older plants, an ageing cable distribution network could sometimes be subjected to increasing load current demands subjecting the old cable and its joints to increased risk of failure.

Let us now look more closely at the main functions performed by medium voltage power cable terminations and joints and how we could achieve better reliability by adopting components more suited to the practical site conditions.

The main functions of a medium voltage cable termination are conductor terminal, electrical stress controle, external insulation, providing flashover protection, track resistance property, sustain contamination (including hydrocarbons) in environment, protection against ultraviolet rays and environment sealing.

Different types of joints and terminations like taped, heat shrinkable, cast resin, push-on and cold shrink type accessories all perform the same functions as mentioned above. However, one type scores over others on certain parameters while the reverse could also be true on a different parameter. For instance, taped type is very universal in terms of cable sizes but suffers from dependence on skill and workmanship of installer. On the other hand, push-on type minimises dependence on skill but has very limited range taking ability. Likewise, cast resin type have limited shelf life but is excellent in mechanical and water sealing properties. Specially formulated resins can have very good hydrocarbon and chemical resistance features. Based on the required functions, some of the desirable features of good termination and joints are fail safe design and technology; near-to-workmanship independent quality; materials suited to oil and gas environment; easy and effective training; fast, easy and safe installation; direct energising after installation; minimal number of installation components; wide application ranges (low inventory); and no open flame for installation.

Present day cold shrink type silicone rubber cable terminations and hydrocarbon resistant resin encapsulated cold shrink joints combine all the above desirable features and attempts to address this pain area of oil and gas industries. The hydrocarbon resistant resin, having chemical and hydrocarbon resistance properties, used for encapsulating the joints prevents degradation under the influence of hydrocarbon contaminated soil. The cold shrink component is basically a one piece molded or extruded component which is expanded and loaded on a removable core. The term ´Cold Shrink´ simply indicates that the shrinking of the part is done without the need to apply heat or flame. This means, it is lot safer as compared to the heat shrinkable accessories as it does not involve use of flames for installation. The continuous attempt of the cold shrink component to shrink further and compress on the cable surface provides the living seal and high interface pressure. The one piece design uses the ´made in factory´ concept as against the´made at site´ concept to minimise the dependence on skill and workmanship. The construction of a typical cold shrink termination body is shown in the figure below:

The silicone rubber used in most cold shrink terminations and joints offer very good track resistance, UV resistance and hydrophobic properties. The one piece concept adopted in cold shrink type joints and terminations helps simplify the installation steps drastically as can be seen from the example comparison of installation steps of cold shrink joint with heat shrink type joint shown below.

The performance of joints and terminations equally depends on the performance of the connectors and lugs. Typically, crimping method is used for the conductor connections in India. However, the crimping method relies on the suitability and condition on the crimping tool, crimping die sets and their compatibility with the connectors and lugs. Sometimes, mismatching of some of these components leads to poor connections causing hot spots and eventual failures. Today, shear head type lugs and connectors are available which do not depend on the crimping tools and dies for installation and reliable performance. These connectors are suitable for a range of conductor sizes and can generally accommodate both aluminum and copper conductor cables. Installation is very simple and less time taking as compared to crimping solutions.

The importance of good tools, effective training and site cleanliness cannot be under estimated. There is a shortage of skilled, trained and experienced cable jointers today and many times, less skilled jointers are employed for installation in construction sites due to scarcity of skilled and experienced cable jointers. The risks associated with this, need to be mitigated by investing more on rigorous training and special tools. The best of the products can fail if not installed properly. The cold shrink type joints, terminations and shear head bolt type lugs and connectors are easy to train for. Cable preparation is also much simple if the right tools are used for stripping the semiconducting screen layer of the cable. Examples of some good practices which can be adopted are shown below.

In addition to the above, several other practices such as fire barriers, fire wraps and fire coatings could be adopted to reduce the propagation of fires in cables and joint areas. Several oil and gas companies use cables with lead sheath to prevent degradation of cables under the influence of hydrocarbons. Like cables and cable accessories, these fire barrier products also should be compatible with the oil and gas environment. Examples of such products are shown below:

To conclude, cold shrink type cable joints and terminations provide a simple and reliable solution for splicing and termination requirements in oil and gas industry. The installation procedure is safe, easy, does not require open flame and is easy to provide training. The silicone rubber provides excellent track resistance, hydrophobic and UV resistant properties. The use of hydrocarbon resistant resin for encapsulating underground joints provides a reliable protection against degradation due to chemical attack. The shear head bolt type connectors and lugs provide an easy and simple alternative to crimping connections. Use of fire stops and fire barriers provides protection against propagation of fires and provides enhanced safety. Last but not the least, investment in training and tools would go a long way to enhance reliability of power cable distribution systems.

The author is Sr. Technical Manager, Electrical Markets Division-Energy Products at 3M India

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