This may come from the misconception that cold shrink insulation requires cooling or freezing to activate it, in the way that heat shrink components need the application of heat.
This is not so. Cold shrink insulation moulds round a joint under its own elasticity – a feature that is effective in temperatures down to -20°C. It is very simple to use and needs no special tools or techniques.
The insulating material in cold shrink components does not shrink in the true meaning of the word but returns to its original shape from a stretched form.
During manufacture, the insulation is stretched to two or three times its diameter over a coiled former. This is the key to the classic simplicity of the concept. On site, once the coil is pulled out, the insulation contracts down to its original diameter and grips onto the cable.
Cold shrink replaces the traditional methods of wiping and sweating, plumbing tape and hot poured bitumen. This means less training time for the workforce, reduced installation times, no possibility of damage by overheating and assured quality and reliability of joints and terminations.
It also eliminates the problems associated with heat shrink technology such as having to carry gas cylinders up transmission towers; working round the back of terminations in confined cable boxes without setting light to other cabling; and heating a joint or termination evenly all the way round in less-than-perfect working conditions.
In conjunction with today's medium voltage polymeric cables, the time saved by using cold shrink components can mean an extra joint or termination being carried out each day.
Advanced materials
The advanced materials used in cold shrink products mean that the insulation retains its flexibility for a very long time. After fitting, it will settle around a joint and grip the cable throughout its life.
This feature of cold shrink technology is vitally important at medium to high voltages. Because the insulating material settles round the joints, there are no voids to encourage corona discharges inside the cold shrink component. It does not shrink or creep under prolonged loads and resists water ingress up to 3 bar pressure, even when cable dimensions alter under extreme changes of temperature.
In contrast, the insulating material of heat shrink components is set by the application of heat and loses its elasticity. So heat shrink components need an internal coating of adhesive to hold them in place on the cable.
The heat can also degrade polymeric cable and there is the disadvantage that permission has to be obtained to use an open flame in hazardous areas such as underground and petrochemical plants.
For cold shrink, silicone rubber is the preferred material. It is hydrophobic, or averse to water – important in outdoor terminations. Water forms drops on the surface of silicone rather than spreading into an even film that can increase the chances of tracking or insulation breakdown. Other materials such as porcelain are hydrophilic, or attractive to water. This encourages water to spread evenly over their surface, reducing the insulating properties.
Silicone is mostly inorganic, so leakage currents or arcing on the surface do not create a conducting carbon path. As an additional safeguard, track-resistant filler is used. It is uv-stable without the need for inhibitors and its smooth surface provides minimal grip for contaminants while rain will wash them away.
It is also inherently fungus resistant and, being a cross-linked polymer, it contains mechanical damage such as a cut so it will not run into a longer, axial split.
Peter White, company cable engineer for Western Power Distribution, recently oversaw the installation of 50 cold shrink outdoor terminations to electrify remote farms in Exmoor. He is a firm advocate of the benefits of cold shrink: "Not only are cold shrink terminations better suited to polymeric cables because the material spends its life trying to return to its original size, it's also extremely easy to use and requires little additional training for jointers."
The products
The cold shrink technology used by 3M Electrical Products is based on the properties of the special silicone elastomers which the company has developed over the last three decades and which exert permanent radial pressure throughout their life.
The spiral former inside these cold shrink products is made from a specially formulated polypropylene-type material, lightly spot-welded every few turns. This makes it possible to apply one cold shrink component on top of another such as on a multi-core cable where individual cores are insulated and the whole then covered with a larger cold shrink component.
But before the cold shrink accessory is fitted, the jointed cable still has to be built up to ensure it resembles the original cable. Insulation and shielding have to be replaced so the lines of electromagnetic flux have a smooth path over the connection. Any sharp deviation from the original profile can cause the flux to break out under the excessive electrical stress and the joint to blow up.
To boost take-up of cold shrink technology, 3M Electrical Products has taken several initiatives. Firstly, the recent nationwide launch of its modular Cold Shrink System. This consists of the QS1000 cold shrink jointing system and QTIII third generation terminating system. These are suitable for 11 kV standard single and three core cables in polymeric, paper and lead.
The cold shrink components contain a number of different features designed to offer unprecedented protection. Each component contains semiconducting layers, earth fault protection and mechanical protection. The overall principle being to create a Faraday cage around the joint and contain electrical stress within the cable.
The silicone rubber insulation is injection moulded rather than extruded so each cold shrink component is produced individually. This enables individual components to be tested.
It also enables such features as rain sheds on outside terminations to be moulded into the main body of the component rather than being a slide-on extra. A further advantage of moulding over extruding is that a rounded end can be designed on to the end of the embedded semiconducting layer. This enables refractive stress control and keeps more stress lines within the cable profile.
The QTIII termination design includes a built-in ring of sealing compound to save the taping of top seals on outdoor terminations, and a ring of stress control compound. These features reduce the skill required to terminate a cable, which means less training time for the workforce while ensuring consistent quality every time.
Modular kits
Individual components have been assembled into modular kits that can be put together to suit all jointing and terminating needs at 11 kV. This system makes it easier to select the right product for the job and provides the end user with a full range of products for all popular cable configurations.
Customers just have to purchase the appropriate kit to ensure they have all the parts needed for any connection. This includes modules containing connectors, a copper stocking to go over the joint and connect the cable routers on either side, and a choice of encapsulation methods.
Around 84 modules can be combined into 698 kits covering over 1500 cable configurations from 25 to 300 mm². This includes transition joints between the older, belted paper cables still found underground and the polymeric versions used in new installations.
The initiative has also been backed up by training in the methodology behind the kits for several 3M accredited cold shrink distributors: Cable Services, Smith Electrical and Thorne & Derrick.
Distributors have been chosen for their experience in cable jointing and 3M has worked closely with each to ensure a comprehensive selection and quality control procedure.
Sarah King, 3M's technical manager, comments: "We know from customer feedback that the product selection process for jointing and terminating cables can be very complex. This is why we have invested a great deal of time and effort into simplifying the process through this modular kit system."
Given its consistent and reliable installation quality, cold shrink insulation is a technology that you should not overlook. And with new intiatives like this one set to kickstart the market, perhaps it's time to get that shrinking feeling.
Source
Electrical and Mechanical Contractor
