Product category:
Electrical systems
News Release from: SPECIAL REPORT by the Editor | Subject: Conduit systems
Edited by the Manufacturingtalk Editorial
Team on 08 August 2007
Conduit systems safer when the rules are
followed
When electrical systems and even mechanical systems are designed and built, the specification of adequate or suitable conduit and conduit components is often ignored, writes Jon Hobden.
Usually the effect of this is that standard or cheapest parts are used, specified by a buyer rather than an engineer, and neither the application nor the environment is adequately considered Sometimes cost pressures post-design mean that unsuitable conduit is used
While plastic conduit systems are usually the cheapest, they can also be the right products for the purpose.
However, the type of plastic can be critical; although pliable, plastic can burn or melt easily so is not suitable for flameproof applications - though some plastics can be highly resistant to chemical attack.
Where flexibility is needed, such as for small spaces, a halogen free material is often the best solution; nylon systems are very popular.
Where an application calls for an ingess protection (IP) rating, there are no short cuts.
Always consult manufacturers' specification tables for the most suitable product.
For applications where smoke inhibition is required, materials like PVC are no-go.
Noxious gases and smoke are reduced when using nylon or PEEK, often used in spaces occupied by people or in confined areas.
Because of the way cables are routed - usually pulled through a conduit in a cluster - spare space is needed.
The space also helps to dissipate heat from the currents being carried by the cables.
The usual amount of cross section taken by cables is no more than 40% of the internal cross section of the conduit, allowing for smooth movement through the system and additional cables to be added.
Another consideration is the radius of any bends in the conduit.
A conduit system has a 'minimum bend radius' and this must not be exceeded if damage or cable overheating is to be avoided.
The life of a conduit system van be drastically shortened if excessive bending is evident.
The working temperature of a conduit system is another issue when specifying materials.
PVC, for example, has a relatively small operating range with premature aging and cracking symptoms of the range being exceeded, either by heat or cold.
Cold flexing then cold bending occur in freezing conditions, leading to the material turing brittle and eventually cracking.
Connectors, or the joint between the conduit and a connector, represents the weakest part of a conduit system.
Manufacturers generally publish loads for pull-off values and these must be adhered to.
Crush values should also be considered and allowed for.
Where the conduit is to be used as part of the electrical system itself, the full system must be earthed according to manufacturer's instructions and prevailing regulations.
Earthing can be effected from a metal core to connectors or, in steel conduit, the connector is usually screwed into the core.
This adds to the strength of the system as well as its electrical integrity.
Where necessary, a separate earth wire is run, either inside or outside of the conduit as the conduit itself should not be used as the earthing medium.
Glands are used to protect systems from ingress and to protect the surrounding environment from explosion.
Hazardous areas represent an additional challenge for conduit manufacturers and few have the relevant approvals for their products.
Always look for product that carries the necessary approvals.
Conduit systems must be capable of preventing flame spreading out of the system, with all components carrying the same capability.
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