How to make an entrance
Chris Lloyd, Sales and Marketing Manager for Spelsberg UK, explains the strengths and weaknesses of different cable entry options and provides details on the latest developments.
When specifying an enclosure for electrical applications one of the key considerations has to be cable entry. The cable entry points of an enclosure can help to determine the overall level of ingress protection (IP) provided against the external environment as well as cable retention force and crucially, installation speed, all-the-while balancing utility with cost. Recent developments in materials, moulding technology and push-fit grommets have changed the options that are available providing a far more specialised solution for a variety of applications; time to reassess the options then.
Making the right choice of enclosure and entry method can make the difference between a profitable job and a loss, as well as guaranteeing the all important safety of installers and end users.
With new 'moulded-in' entry methods now on the market offering extremely fast entries, and traditional cable glands being replaced in some instances by lower cost fast-fit grommets, it is essential to know how to specify the right solution for any given application. For example, by far the most common cable entry method is traditional cable glands fitted to the body of the enclosure through knock-outs. While this option offers a safe choice; what a lot of people don't realise is that they are often over-specifying and that this can massively increase costs and installation times, as well as making maintenance more time consuming down-the-line.
Push fit cable entries have become increasingly popular as plastic moulding technology has advanced, enclosure manufacturers regularly use two types of material moulded into one box providing rigidity in the walls and soft, flexible membranes to allow cables to be pushed through, but still offer a good IP rating, protecting from moisture as well as physical intrusions. Cable retention force on offer however, is lower than with other alternatives.
If an enclosure is going to be used in a clean, dry and well insulated environment, only lower levels of IP are needed. In these situations both the enclosures and wires are likely to be well secured in their position so there is little need for high cable retention. If there are a number of enclosures to be wired then using cable glands will be extremely time consuming and unnecessary.
Many enclosures are available with a flexible membrane which provides a seal simply by pushing the end of the cable through the centre nipple. As the membrane stretches, the point of greatest extension opens and allows the cable through. The inverted seal grips the cable and provides the fastest entry seal so far possible. As IP and cable retention are not large concerns this method is ideal for reducing installation times and costs; in the event that re-wiring is required in the future the old wire can simply be pulled out and the new wire inserted.
There are a number of membrane plug/grommet products available that offer good sealing up to IP67 that are very fast to use. As a grommet they are a push fit design both to install and to feed the cable through. Recent grommet designs will allow for a variety of cable and conductor widths and are resistant to vibration.
There are fast-fit grommet seals available that clip into standard drilled and knockout holes that provide positive cable retention. The best of these, the Klikseal from TST, is moulded using two materials, a softer compound for the body and/or seal, and a stiffer inner material that applies positive clamping and cable retention.
The Klikseal provides most of the advantages of a cable gland while avoiding many of its disadvantages, including its high purchase cost. The main advantage being the fitting time for a Klikseal, this can be as low as ten seconds, compared with minutes for a cable gland. The reduction in fitting time can speed-up installation, and reduce costs on large installations and OEM assembly lines.
Despite the advantages of quick-fitting seals and grommets in many applications, there are still some situations where the most appropriate cable entry method is the clamped, dome-top, cable gland. These glands provide an aesthetically pleasing and professional look to any installation. A number of variations of this type are available in both nylon and metal but the salient features that ensure superior performance to the traditional stuffing gland are in its construction. The cable is fed through a sealing ring which is placed in an 'iris' that closes onto the sealing ring, clamping the cable and forming a high IP seal as the dome top is screwed down. This forms an IP68 seal around the wire, meaning that the enclosures can be fully submerged without fear of ingress, as well as creating very high cable retention, alleviating any risk of connections failing due to pressure on the wire.
In reality there are many different cable entry options available, all of which carry different benefits to the user. Choosing the correct solution can save money and time at the point of installation and during subsequent future maintenance. When specifying for a specific application it is a good idea to speak to an expert who can recommend the ideal product.
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