Down to the wire
January 1st 2006 The specifying of electrical conduit is often ignored, relegated to last minute, commodity, purchase, usually at the lowest price with choice based on whatever is available from stock. Martyn Turner of Kopex International explains the basics of conduit choice and the issues that need to be considered to stay cost-effective and, in some cases, legal.
Cheaper conduits such as those made of plastic may well be fit for purpose but even the type of plastic must be carefully checked for suitability. Most manufacturers provide a guide to the conduit's performance when in contact with abrasive or corrosive substances. For confined spaces or where there is delicate computer equipment, halogen-free material can be specified.
The IP Rating (Ingress Protection) of conduit systems should always be checked against the application. Often an IP68 rating is required and this should always state pressure and time, in order for the customer to determine suitability.
Smoke produced by conduit in a fire can vary from a slight wisp to billowing black clouds. Standard PVC is messy when it burns and more users are turning to Nylon materials, like PEEK, providing reduced smoke and dangerous gases.
These can be particularly important when used in confined spaces or public places where safety is highly important.
Just as cables are sized to suit currents, conduits are sized for the cables they carry and protect. Cables are usually pulled through a conduit in a bunch and there should be plenty of cross section to get all the cables through without jamming. A space factor of 40% is generally used, the spare space aids ventilation to keep cables cool and allows cables to be changed or added.
If a conduit is bent back on itself without crunching up the metal core or distorting the plastic form, then it is at its minimum bend radius; it can be reversed without permanent damage. It is not good practice to continuously work in this position because it affects the life expectancy of the tube.
Flexible conduits working below freezing grow stiff, if they can move about without damage they are said to "cold flex". As temperatures drop, they "cold bend".
Eventually the PVC cover will become brittle and crack open. Many plastics begin to soften above 100°C, and extruded coverings will wrinkle and begin to sag.
Long periods of high temperature can "age" plastics.
Even when assembled correctly, the weakest link in a conduit system is the joint between the conduit and connector.
In some applications this joint can be under tension, for example, when equipment is moved, dragging conduit and cables along the floor. Connector pull-off loads are quoted in technical literature to advise customers on the strength of this joint.
An "earthed" system has metal contact from the core through the connector and down to "earth". In pliable systems, the metal core is exposed to make the necessary contact inside the connector body. This is also the principle used for screening, where a combination of connector and fitting will protect sensitive computer cables from foreign signals by conducting them down to Earth.
Connectors for steel flexible conduits generally screw directly into the metal core, providing the earth path as well as giving a high pull-off load. Conduits themselves must not be used for Earthing.
In areas where a fire or explosion hazard is present, conduit systems must be able to resist explosions or prevent the spread of flame outside the immediate area. All Zone 1 equipment must be tested and approved by a recognised testing authority (BASEEFA, SIRA etc.).
Glands used must be approved against the ATEX directive (in Europe) and matched to the appropriate conduit.
The crush resistance of a conduit depends on such things as strip materials and thickness, depth of conduit wall, shape of corrugations and so on. Crush is usually graded by pushing a 50mm wide steel block into the conduit, with various degrees of "push" or load between "very light" to "extra heavy" as detailed in BS EN 50086 part 2. The conduit will be graded depending on what weight it will hold, without collapsing more than a quarter of its own diameter and returning to within 10% of its original shape, preventing damage to cables inside.
Specialist manufacturers of conduit systems, like Kopex International, have the experience and products to cater for a range of conditions, configurations and regulatory considerations.
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