Cables are electrical conductors; they have a metal core covered in PVC insulation.  The core is generally made up of a single thin solid length of copper wire or even thinner strands of fine copper twisted together to make them more flexible.

The size of cable used depends on the amount of electricity it has to carry.  Small currents (measured in AMPS) mean small cables.  Lots of current will require lots of copper area to prevent overheating.

Just as cables are sized by the customer to suit his currents, so conduits can be 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 "hole" to get all the cables through without jamming up.  To make this possible a space factor of only 40% is generally used, or in layman's terms 40%, just under ½, of the conduit is filled with cables. The spare space (i.e. 60% minimum) will help ventilation to keep cables cool, and allow cables to be changed or added later.

Crush strength

The crush resistance of a conduit is the measurement of how well the conduit resists being squashed under foot.  In depends on such things as strip materials and thickness, depth of conduit wall, shape of corrugations, etc.

Crush is usually graded by pushing a 50mm wide steel block into the conduit.  There are 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.  It must also return to within 10% of its original shape.  This might seem a small amount but by not allowing too much crushing, the cables inside the conduit will not be damaged.

Halogens

Halogens is the name given to a group of elements occurring in nature (they are astatine, bromine, chlorine, fluorine and iodine). The important note is that some of these halogens are contained in certain plastics such as PVC's.  When there is a fire the burning plastic will release its halogen in the fumes.  These are poisonous as well as being corrosive.  This is why many more customers are requiring halogen-free materials like nylon or type "T", particularly in confined spaces or where there is delicate computer equipment.

IP Rating (Leakability)
If a conduit or connector has a hole in the side, it will not protect the cables or equipment on the inside.  Obviously the larger the hole the bigger the object which could get in and damage the electrics.

A series of tests have been drawn up to show the amount of protection a system will provide.

Commonly known as IP Rating (Ingress Protection) it is a simple measure of how much dust and water the conduit and connectors will keep out.

The IP Rating is specified in BS EN 60529 by two individual numbers, each referring to a scale given in the standard.  The first is the size of the solid object the conduit system will keep out varying between a golf ball and talcum power.  The second figure shows how waterproof it is, from a drip to complete submersion.  Generally the higher these numbers, the greater the protection, although the IP Rating should always be checked against the application for suitability.

Often an IP68 is required, and this should always state pressure and time, in order for the customer to determine suitability.

Minimum bend radius
If a conduit is bent back on itself as tightly as possible 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 radii, it is not good practice to continuously work in this position.  Particularly on flexible applications it means that the whole construction is being pushed to the limit and could affect the life expectancy of the tube.

Pull off load
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.

Temperature rating
The temperature range of a conduit system depends on the materials used to make the connector or conduit.  The temperature range quoted in literature is the lowest and highest temperatures where the conduit will still function and protect the cables inside.

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, termed cold bend.  Eventually the PVC cover will become brittle and crack open.

High temperatures have an opposite effect on conduit behaviours.   Most of our plastics (with the exception of PEEK) will begin to soften above 100°, and extruded coverings will wrinkle and begin to sag.

Long periods of high temperature can "age" plastics turning them hard, brittle and eventually cracking them. Alternatively, uncovered pliables will "stew" happily up to 150° c.  Above this temperature any paper lining begins to smoulder and smell.

Lead coatings on steel begin to melt at 200°c and when cool the core becomes soldered together and is not longer pliable.

UV Resistance
Ultra violet rays occur in natural sunlight somewhere between visible light (the blue end of the rainbow) and x-rays. UV can make whites seem whiter as well as damaging living tissue.

Strong sunlight particularly in overseas applications will cause a breakdown of plastic materials over a long period of sunbathing. Plastics can be given additives to increase their UV resistance, black being better than coloured materials.  It is not easy to give definite advise, but generally black conduits will survive a minimum of ten years without deterioration.