Choosing cable conduit systems to match the application
20 October 2015
Incorrect conduit specification remains a primary cause of cable failure in heavy duty applications. Meirion Buck explains the key mechanical and environmental properties of conduit that can protect vital cables.
Conduit systems which comprise of conduits, seals, fittings and enclosures are primarily designed to protect cable systems. If the conduit system specified is incorrect or unsuitable for the application, then premature damage can be caused to the cable, which could lead to electrical failure and interruption to the system’s power supply.
Conduit selection is a serious matter, particularly when consideration is given to the number of potential mechanical failures that could occur in an application as a result of flexing, exposure to extreme temperatures and impact from climatic conditions. All-too-often, however, fire protection is the dominant specification criteria with the other potential failures relegated to an afterthought. The fact is that all conduit systems must comply with international conduit standard IEC 61386. This is a series of 12 tests that classify a conduit according to mechanical and physical properties detailing the minimum requirements that all conduits must meet.
There are three key properties that are tested. The first property is compression impact and tensile strength. These tests classify the maximum and minimum operating temperatures as well as IP rating for solids, water and corrosion resistance. A minimum flexing requirement for conduits of 5000 reversed bends at the minimum temperature and bend radius is specified for conduits to be classified as flexible.
Vibration can be extremely damaging to electrical equipment, particularly on heavy duty applications such as rail, marine or oil and gas. Conduit systems can significantly help to alleviate the damaging effects of vibration on delicate cables and connectors and the interface between them. The conduit system needs to be capable of withstanding the forces associated with any heavy duty application to ensure that valuable equipment is fully protected and remains operational.
The final key property is ingress protection or IP. Ingress Protection ratings are extremely important for protecting electrical equipment from damaging ingress of dust and liquids. ISO 60529 outlines an international classification system for the sealing effectiveness of products against the intrusion into electrical equipment of foreign bodies which can include everything from tools and moisture to dust, fingers and dirt.
This classification uses the letters IP which stands for ingress protection followed by two digits. The first digit of the IP code indicates the degree that persons are protected against solid foreign bodies intruding into an enclosure. The second digit indicates the level of protection against harmful entry of various forms of liquids, such as dripping, spray and submersion.
Additional protection may also be required against high pressure jet washers for cleaning in the form of IP69k. It is important to remember that IP ratings apply in dynamic as well as static applications and that sometimes a higher IP rating that is carried out in a static test may not offer the same level of protection in a continually dynamic situation. The highest IP68 ratings must be defined with a pressure and time of immersion, for example IP 68 4 bar 30 minutes.
The moral of the story is to get up to speed with the IP ratings and remember that tests of up to IPX6 are dynamic spray tests and tests for IPX7 and IPX8 are static immersion tests. The key test for product which has a ‘quoted' IPX8 rating is simply to ask the manufacturer or supplier at what pressure and for what duration of time the test was carried out.
This can often lead to a level of misunderstanding in the marketplace regarding IP ratings, because many buyers believe that ‘if the IP rating number is higher, it therefore follows that it has to be better than the lower number rating.' Technically, this is incorrect at the higher IP ratings.
Adaptaflex's new flexible conduit systems catalogue clearly shows the IP rating for each individual system, to assist in selection for suitability for a given application. The moral of the story is that the higher the IP number does not necessarily mean better performance, as clearly defined by the spray and immersion tests. Clear knowledge of IP ratings means you can apply the most economical solution, designed to meet the requirements of the installation and fit-for-purpose.
As we have seen, the correct specification of conduit depends very much on the application. My advice to end-users is that, if in doubt, ask the manufacturer for advice. The wrong conduit, specified in the wrong application can have adverse effects on a system’s power supply which can be easily avoided if a few simple rules are followed.
Meirion Buck is senior design and technical manager at Adaptaflex
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