Basic guide to specifying conduit systems

Martyn Turner of Kopex International explains the basics of conduit choice and the subjects that need to be considered to stay cost-effective and, in some cases, legal.

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 for immediate delivery.

This can be a recipe for disaster, especially if the application demands certain levels of specification and performance.

Often the original product specifier has acted properly but pressures from others in the chain have come to bear - and shortcuts taken - leading to recriminations back through the supply chain.

The first concern when choosing the right conduit for the job is material.

Cheaper conduits made of plastic may well be fit for purpose but even the type of plastic must be carefully checked for suitability.

Plastic coatings on metallic flexible conduits are usually soft and yielding but they are still able to resist attack from many substances in solid, liquid or gaseous forms.

Most manufacturers will have available a guide to the conduit's performance when in contact with these substances.

For confined spaces or where there is delicate computer equipment, it is possible to specify halogen-free material like nylon.

The IP rating (ingress protection) of conduit systems should always be checked against the application for suitability.

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, providing reduced smoke or dangerous gases.

These include special material mixes, treated nylons and materials like Peek, all of which can provide varying improvements over normal PVC.

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.

The main concern is that there is plenty of spare cross section to get all the cables through without jamming.

A space factor of only 40% is generally used, the spare space aids ventilation to keep cables cool, allowing cables to be changed or added later.

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.

At this point the conduit can be reversed and bent back again without permanent damage.

Although conduits are happy to sit at their minimum bend radius, it is not good practice to continuously work in this position because it affects the life expectancy of the tube.

Pliable conduits tend to have a wider temperature range because they only have to "sit" at the extreme temperature.

Parts containing soft materials such as PVC tend to have a smaller temperature range than all metal versions.

Flexible conduits working below freezing grow stiff, if they can move about without damage they are said to "cold flex".

As temperatures drop and they could continue to be bent, they "cold bend".

Eventually the PVC cover will become brittle and crack open.

Many plastics begin to soften above 100C, and extruded coverings will wrinkle and begin to sag.

However, plastic conduits are available to withstand higher temperatures.

Long periods of high temperature can "age" plastics turning them hard, brittle and eventually cracking them.

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".

When a fault occurs, stray current is directed away for safety.

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 and interference 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, and a separate earth wire must be run inside or outside of the conduits.

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 these must be used with the matching, appropriate conduit, such as liquid tight or stainless steel equivalent types.

An epoxy putty used inside the gland stops the spread of flame through the conduit system during an explosion and so prevents further fires.

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 EN50086 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 and configurations.

It is always best practice to consult with experts, particularly in view of the increasing legislative and regulatory minefield that affects almost every installation to some degree.

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