Environment

 

3.1 Selecting the right material and finish

 

3.1.1 Preventing corrosion

In planning any cable ladder or cable tray installation the choice of an appropriate corrosion resistant material and finish is always a key issue at the specification stage. The correct choice has long term implications and is crucial for ensuring the longevity and the aesthetics of the complete installation.

Maintenance against corrosion of cable ladder and cable tray installations is generally impractical. It is vital at the specification stage that the selected finish for the equipment is capable of providing lifetime protection from corrosion within the intended environment, ideally with some margin of safety. Therefore it is important to establish the corrosive properties of an environment to ensure the right material and finish is chosen.

The following sub-sections give information on how corrosion occurs and contain supporting technical data on the standard construction materials and surface finishes available. Consult the manufacturer for further information.
 

3.1.2 Chemical corrosion

Few metals will suffer corrosion damage in a dry, unpolluted atmosphere at a normal ambient temperature. Unfortunately such environments are exceptional and atmospheric pollutants as well as moisture is likely to be present to some degree in most situations, thus some chemical corrosion may be expected in almost all situations.

Any support installation situated in an area where higher concentrations of chemicals exist must be subject to more detailed consideration in order to select an appropriate finish which provides the best combination of initial cost and expected life.
 

3.1.3 Electrochemical corrosion

When two dissimilar metals are in contact and become damp it is possible for corrosion to be induced in one of the metals. Such corrosion may progress rapidly and cause considerable damage so it is important to consider and, if necessary, take steps to eliminate this process.

Electrochemical (alternatively referred to as electrolytic or bimetallic) corrosion takes place because the two different metals each behave as electrodes and the moisture acts as the electrolyte as in a simple battery; as with any battery the resulting flow of current will cause corrosion of the anode.

The likely effects of this reaction can be predicted using the Galvanic Series.

3.1.4 Galvanic series

The rate of corrosion depends upon the differences in electrical potential of the metals as defined by the Galvanic Series (Figure 26). The strength of the electrolyte, the period for which the electrolyte is present, and the geometry of the connection between the dissimilar metals are all influencing factors. When corrosion occurs it is the anodic metal (which is higher in the galvanic series) which will corrode in preference to the cathodic metal (which is lower in the galvanic series).

The best way to prevent electro-chemical corrosion is to ensure that all system components have the same finish e.g. all components HDG or all components stainless steel. Where this is not possible then components with a low potential difference, as shown in Table 3, should be used.

Even when two dissimilar metals are in moist contact, electrochemical corrosion need not necessarily take place. Its likelihood depends upon the potential difference between the two metals; this can be obtained by taking their respective values from the Galvanic Series chart shown in Figure 26 and subtracting one from the other. When the potential difference is less than the values given in Table 3, corrosion is unlikely to occur.

If from consideration of the Galvanic Series excessive corrosion does appear likely then the risk can be largely eliminated by insulating the dissimilar metals from one another, breaking the electrical path between them. A layer of paint or grease on either surface is sometimes used but is not recommended because it only offers a short term solution. A better solution is to electrically isolate them by using an insulating material such as polypropylene, nylon or other non-conductive material, usually in the form of pads or washers.

In addition to the contact between dissimilar metals the relative surface areas between them also has an effect. If the anodic metal has a small surface area in relation to its counterpart it will be corroded very aggressively and any sacrificial protection it provides may be short lived. If on the other hand it has a large surface area in comparison to its less reactive counterpart, some minor corrosion may take place at points of contact but the process is likely to reach equilibrium rapidly so that any further reaction is insignificant as in the following example.

Consider the example of a tray or ladder with a thick protective zinc coating over a large area connected together using stainless steel fixings each having a small surface area. The stainless steel, in contact with the galvanizing, causes only minor corrosion of the zinc because of the small area of the stainless steel fixing in comparison with the much larger surface area of the zinc coating.

For further details on electrochemical corrosion see PD 6484 ‘Commentary on corrosion at bimetallic contacts and its alleviation.’

If copper is laid directly onto a galvanised surface the zinc will rapidly corrode. Thus cables should always have an insulating sheath if they are to be installed on galvanized cable ladder or tray.

The galvanic series illustrates the potential difference between a section of metal and a calomel electrode when both are immersed in sea water at 25 ºC.
 

3.1.5 The merits of zinc

The Galvanic Series Chart clearly indicates why zinc is such a useful corrosion resistant coating for mild steel.

Firstly it forms an impervious zinc barrier around the steel, coating it with a metal whose own rate of chemical corrosion is both low and predictable in most situations.

Secondly, if the coating is damaged at any point (e.g. at a cut edge) the zinc surrounding the damaged area becomes the anode of the electrolytic cell and is sacrificially corroded away very slowly in preference to the underlying steel. Corrosion products from the zinc may also be deposited onto the steel, effectively re-sealing the surface and maintaining the integrity of the barrier. This ensures the strength of the steel structure remains unaffected.

Because zinc appears near the top of the Galvanic Series it will act as a sacrificial anode in relation to most other metals; thus its relatively low cost and the ease with which it can be applied as a galvanized coating on steel means that it continues to be the most commonly specified protective finish for support systems.

Steel cable ladder or cable tray systems can usually be assigned to one of the following corrosion classes as shown in Table 4 and a suitable zinc coating system selected from Table 5 to achieve the required life expectancy of the coating.

3.2 Finishes

 

3.2.1 Hot Dip Galvanising (HDG)

Hot dip galvanizing after manufacture is an excellent, economical protective finish used on support systems in many industrial and commercial applications.

The galvanised coating is applied as a final manufacturing process by immersing a steel component (after various pre-treatments) in a large bath of molten zinc; the zinc forms an alloy with this the steel substrate and protects the steel from corrosion as above.

The life of a zinc coating is directly proportional to its thickness but in different environments this life does vary. However, because hot dip galvanizing has been used for many years its life in diverse environments has been well established. The most comprehensive guide to the design life of zinc coated systems in different environments is contained in BS EN ISO 14713-1 Zinc coatings: General principles of design and corrosion resistance (see Tables 4 and 5).

In the presence of certain atmospheric pollutants (such as sulphur dioxide in industrial areas) or when installed in an aggressive coastal or marine environment the rate of dissipation of the zinc will be accelerated; however in most situations hot dip galvanizing remains an extremely effective and economical corrosion resistant finish.

BS EN ISO 1461 provides the specification for a hot dip galvanized coating. Heavier gauges of steel will usually take up a thicker coating of zinc than lighter gauges so the standard defines the coating for different steel gauges. The coating thicknesses given in the standard is shown in Table 6.

3.2.2 Deep Galvanizing

A Deep Galvanized finish has all of the characteristics of hot dip galvanizing (HDG) but with a much thicker coating of zinc. This can give up to 3 times the life of the standard hot dip galvanized (BS EN ISO 1461) finish in certain environments.

Although the appropriate British Standard for Deep Galvanizing is BS EN ISO 1461 (the same as for hot dip galvanizing after manufacture) the process requires the use of steel containing a slightly higher proportion of silicon. When galvanizing normal mild steel the process effectively ceases after a short immersion time in the galvanizing bath which gives, depending on the gauge of the steel, the coating thicknesses laid down within BS EN ISO 1461. However, with silicon bearing steels the chemistry of the galvanizing process changes, resulting in the zinc coating continuing to increase in thickness as long as the steel remains immersed in the zinc.

Coatings of up to three times as thick as the minimum requirements of BS EN ISO 1461 are both possible and practical to achieve. However, in practice the most cost effective coating thickness is usually twice the thickness required by BS EN ISO 1461.

3.2.3 Pre-galvanised (PG)

A zinc coating can be economically applied to steel sheet immediately after its manufacture; the result, pre-galvanised steel (to BS EN 10346) can be an attractive, bright material which is suitable for non-arduous environments.

Pre-galvanised (or mill galvanized) steel is produced by unwinding steel coil and passing it continuously through a bath of molten zinc and then past air jets to remove excess zinc from the surface. The process is closely controlled to produce a thin, even and ripple free zinc coating with very few imperfections. Because this pre-galvanized steel coil must then be cut to shape during subsequent manufacture of support equipment, the edges of the finished components will have no zinc coating. This aspect, together with the relatively light zinc coating provided by the process, make pre-galvanized service supports suitable for indoor, low-corrosive environments (particularly where an aesthetically attractive appearance is important) but unsuitable for humid indoor or outdoor applications.

3.2.4 Electroplating with zinc

This coating process is often referred to as bright zinc plating (BZP).

Electroplating with zinc may be used when a smooth bright decorative finish is required. Parts can be coloured or colourless depending on the type of passivation process used. It is generally used for internal applications where a low degree of corrosion resistance is acceptable.

Electroplating involves connecting the metal substrate to a negative terminal of a direct current source and another piece of metal to a positive pole, and immersing both metals in a solution containing ions of the metal to be deposited, in this case zinc.

3.2.5 Zinc Whiskers

The phenomenon of zinc whiskers (Figure 27 and Table 7) has been a known issue for more than 60 years and was initially associated with access floor tiles that have a metal zinc coated base, used in the electronics and communications industries. Although the existence of zinc whiskers is widely acknowledged, there have been no reported instances of equipment failure attributed to zinc whiskers on cable management systems. Zinc whiskers are conductive crystalline structures that sometimes unpredictably grow outward from a zinc coated surface.

Over periods that may take many months or even years, zinc-coated surfaces may begin to exhibit hair-like filaments from the surface which grow by adding zinc atoms at the root of these metal crystals. The lengths, thicknesses, rates of growth, and population densities of zinc whiskers can be highly variable from sample to sample.

The process of zinc whisker growth is not fully understood, however, available information would suggest that compressive stresses within the coating are a key factor in their formation. It is believed that compressive stresses within a hot dipped galvanized coating after manufacture are inherently lower than in pre-galvanized and/or zinc plated coatings.

Whilst certain ‘organic’ coatings which can be applied over the zinc surface may delay whisker growth, there is no evidence that the coating will prevent the formation of whiskers.

Some typical attributes of zinc whiskers are as follows:

• Length: Often up to a few millimetres but rarely in excess of 1 centimetre,
• Thickness: Typically a few microns, but spanning a range from less than 1 micron to >30 microns. For comparison, zinc whiskers may be < 1/100th the thickness of a human hair,
• Rate of growth: Up to 1 millimetre in length per year,
• Incubation: Recorded from a matter of months to many years,
• Density of growth (number of whiskers per area) spans a very wide range:
• sparse growths approach 1 whisker per square centimetre
• very dense growths may exceed 1000 whiskers per square centimetre

Experience suggests that it is extremely rare that zinc whiskers will form on a hot dipped galvanized coating applied after manufacture. However, if the risk of zinc whiskers on a new installation is to be absolutely avoided then the following alternative materials may be specified:

• Stainless steel,
• mild steel with a protective organic coating,
• non-metallic.

Due consideration must also be given to the supports, brackets, fixings & fasteners.

Should there be a concern over zinc whiskers on an existing installation, contact the manufacturer whose product is installed. In some instances it may be necessary to instigate a periodic inspection and audit of the installation in order to determine any corrective actions.
 

3.2.6 Zinc flakes

These are associated with the process of hot dip galvanizing after manufacture. They are small zinc films usually formed in perforations, however, during the final finishing process, storage and transportation most zinc flakes become detached from the product, (see Table 7). Unlike zinc whiskers, due to their size and mass, zinc flakes do not readily become airborne and are therefore unlikely to enter and cause damage to electrical equipment. There are no known reported instances of zinc flakes causing failure of electrical equipment.

3.2.7 Other applied finishes

Powder coating may be applied as a protective finish but more generally it is requested as a decorative layer applied to systems already protected by a zinc coating.
 

3.2.8 Stainless Steel

For most practical purposes stainless steel can be regarded as maintenance free and suffering no corrosion. Inevitably there is a relatively high price to pay for these attractive properties but, in aggressive environments or where the cost or inconvenience of gaining subsequent maintenance access is prohibitive, this initial cost premium may well be justified.

Stainless steel contains a high proportion of chromium (usually at least 11%) and the steel’s remarkable immunity to corrosive attack is conferred by the chromium-rich oxide film which occurs naturally on its surface. This invisible film is not only inert and tightly bonded to the surface; it also re-forms quickly if the surface is damaged in any way. The fire resistance of stainless steel is particularly noteworthy; tests have demonstrated that stainless steel cable supports can be expected to maintain their integrity for considerable periods even when exposed to direct flame temperatures exceeding 1,000°C. This may be an important consideration where the electrical circuits being supported provide for emergency power or control systems.

Stainless steel is also used where hygiene is a major consideration. Its advantages in such applications are again its excellent resistance to the various chemicals and washes which are frequently used for cleaning purposes and the smoothness of surface (depending on the finish specified) which minimises the soiling or contamination that can take place.

Many grades of stainless steel are available but the one generally used in aggressive marine environments is BS EN 10088 Grade 1-4404 (equivalent to 316L31, BS 1449: Part 2). This grade has improved corrosion resistance (particularly in the presence of chlorides) and high temperature strength. It is often used in the chloride-laden marine conditions which exist on offshore installations and in coastal regions.

For less aggressive environments BS EN 10088 Grade 1-4301 (equivalent to 304, BS 1449: Part 2) is the normal grade. This grade may be used for aesthetic purposes and is commonly used in the dairy and food industries where cleanliness is of great importance. Final finishes with mechanical brushing or polishing are used to provide a good looking and robust surface finish.

A stainless steel surface will have excellent corrosion resistance due to the chromium oxide layer on the surface of the product. With some stainless steels however, the surface areas can become subject to corrosion due to the depletion of chromium during welding. To overcome this problem welded stainless steel products are often pickled and passivated after welding.
 

3.2.8.1 Pickling & Passivation

The pickling and passivation process gives optimum corrosion resistance and is carried out under a carefully controlled operation aimed at minimising risk to both the environment and individuals carrying out the process.
 

3.2.8.2 Pickling

The pickling process on the surface of stainless steel is carried out to remove a thin layer of metal from the surface of the component. Mixtures of nitric and hydrofluoric acid are usually used for this process. Pickling is also used to remove weld heat tinted layers from the surface of stainless steel where the steel’s surface chromium level may have been reduced. Finally pickling can be used to remove carbon steel contamination which occurs on the component during the manufacture process and to reduce small areas around a weld which may be deprived of oxygen allowing localised forms of crevice or pitting attack to form corrosion.
 

3.2.8.3 Passivation

A passive chromium rich oxide film naturally forms on the surface of stainless steel. Additional passivation adds a thick oxidising passive layer that is accelerated and forms a thickened protective layer. Unlike pickling no metal is removed from the surface and the passivation always occurs after the pickling has been completed. This passivation treatment reduces the corrosion risk on stainless steel and leaves a Matt grey smooth finish.
 

3.3 Non-Metallic systems

 

3.3.1 uPVC (Unplasticised Polyvinyl Chloride)

uPVC cable trays offer a light weight corrosion resistant alternative to steel systems. uPVC typically contains corrosion resistant additives. This makes uPVC cable tray resistant to chemical and aggressive agents such as hydrogen, benzene, liquid propane and methanol. In addition, cut or damaged edges do not corrode in adverse atmospheric conditions. However, the product’s resistance to some chemicals can vary depending on the working temperature, so it is advisable to check the manufacturer’s guidelines.

Whilst PVC cable trays are generally suitable for use at temperatures between -20ºC and + 60ºC, the products are subject to thermal expansion and contraction. Any holes drilled in the tray for screw or bolt fixings should be oversized to allow for movement due to temperature fluctuations and it is advisable that nylon washers are used under screw or bolt heads. Where required expansion gaps should be left at adequate intervals between lengths as recommended by the manufacturer.
 

3.3.2 GRP (Glass Reinforced Polymer)

Constructed from glass reinforced thermoset resins, GRP Cable Support Systems can be designed and manufactured to combine light weight properties with a structural integrity comparable with metallic systems.

GRP Cable Support Systems can be made to resist many corrosive environments and have non-conductive properties.

GRP Products can be produced by means of the pultrusion process or by moulding. The pultrusion process uses a combination of uni-directional and cross strand glass rovings and matting which is resin impregnated and pulled through a heated die to produce a very solid and structurally sound profile that is generally stronger than moulding. The pultrusion process is the one normally chosen to produce cable ladder and cable tray systems.

The resin that is used gives the final product different properties, the most common resins used are Polyester, Acrylic and Vinylester.

Polyester
This is the most common resin used, it offers good all-round protection against corrosion, has excellent mechanical strength and a good resistance to fire.

Acrylic
Acrylic resin is generally used where a high degree of protection is required against the effects of fire such as low smoke, fire propagation and flame spread.

Vinylester
Vinlyester is generally used in applications when additional protection is required against the effects of certain corrosive chemicals.
 

3.4 Loadings

In order to select and design the most appropriate cable ladder or cable tray system for an installation it is important to consider the necessary loads which will need to be supported and the distance between the supports, otherwise known as the span. The type of loads imposed on cable ladder or tray installations can be classed as distributed or point loads, dead loads or imposed loads. A cautious design approach should be taken when planning a cable support system.
 

3.4.1 Dead loads

These loads include the weight of any cable, pipes and secondary equipment carried on or installed on the cable ladder or tray plus the actual weight of the cable ladder or tray and any component of the system such as covers or accessories.

When designing an installation it is usual to consider whether future changes in the pattern of demands for building services will impose increased loading requirements on the support system. It is good design practice to allow both the physical space and sufficient load carrying capacity for the future addition of approximately 25% more cables or other equipment.

Weight data for cables is readily available from the cable manufacturer or cable supplier and is usually quoted in terms of kilograms per metre (kg/m).

On some occasions it may be necessary to select a cable ladder or tray design in the absence of accurate information on the likely cable load. To help with a potential situation such as this, and to safe guard the installation, a recommended approach would be to choose a size of cable ladder or tray and to estimate the maximum cable weight which is capable of being contained within the cable ladder or cable tray. The following formula will assist in the estimation of the cable weight.

This calculated maximum loading can then be used to select a suitable support span for the cable ladder or cable tray using the manufacturers published loading data.

The maximum cable laying capacity can be calculated by using the theoretical maximum value of 2800 for the density of the cable. In practice however, the value 2800 may be replaced by 1700 (kgm-3).

Weight data for secondary equipment should also be readily available from the equipment manufacturer or supplier and is usually quoted in terms of kilograms (kg). The unit weight for the secondary equipment can be converted into an equivalent weight per metre by using the following formula:
 
For Example a secondary item of equipment with a weight of 12kg has an equivalent weight per metre Wm of 8 Kgm-1 for a span of 3 m. This figure should be added to the sum of the individual cable weights. When determining the location of secondary items of equipment care should be taken to either mount the item centrally across the cable ladder or tray or fix the items adjacent to or directly onto the side members and as close to a support as the installation will allow.
 

3.4.2 Imposed loads

Imposed loads can include wind, ice and snow. The effects of imposed loads will vary from one installation to another and further advice relating to the specific influences of each should be sought at the design stage of the installation. Appropriate design data for U.K. weather conditions is given in British Standard BS EN 1991 : 2005.The following information on imposed loads is given as a general guide.
 

3.4.2.1 Snow

The magnitude of the additional load imposed by snow will be influenced by a number of factors including density of the snow, the degree of drifting which will alter the profile of the snow accumulating on the cable ladder or tray and the nature of the installation (i.e. covers fitted or percentage of cable loading area occupied by cables). The density of snow can also vary depending on the level of wetness and compactness. Further details can be found from BS EN 1991-1-3:2003 Eurocode 1.Actions on structures. General actions. Snow loads.
 

3.4.2.2 Ice

An allowance should be made for those locations where ice formation is likely so that the total load supported by the cable ladder or tray installation can be determined.

The most common form of ice build-up is glaze ice as a result of rain or drizzle freezing on impact with an exposed object. Generally only the top surface and /or the windward side of a cable ladder or tray system is significantly coated in ice. Where cable ladder or tray is installed in areas of low temperatures where ice is likely to form, the load imposed by the ice should be calculated and added to the maximum design load.
 

3.4.2.3 Wind

Wind loads exert sideways and vertical forces on cable ladder or cable tray installations. The force is a function of the wind speed and may be determined from BS EN 1991-1- 4:2005 + Amendment1:2010 (Eurocode 1. Actions on structures. General actions. Wind actions.)

Wind speed will vary relative to the height above the ground and the degree of exposure.

When covers are installed on outdoor cable ladder or cable tray, another factor to be considered is the aerodynamic effect which can produce a lift strong enough to separate a cover from an installation. Wind moving across a covered system creates a positive pressure inside the cable ladder or cable tray and a negative pressure above the cover (Bernoulli effect). This pressure difference can result in the cover being lifted off which can result in damage to the installation and possible injury to personnel or to the public.

It is recommended that closed cover types or covers with heavy duty cover clamps are used when an installation requiring covers is likely to be susceptible to strong winds.
 

3.5 Temperature

 

3.5.1 Effect of Thermal Expansion on Cable Tray and Cable Ladder

It is important that thermal expansion and contraction are considered when designing and installing a cable ladder or tray installation. Even in relatively moderate climates there will be sufficient seasonal thermal movement which could easily place undue stresses on the installation and the supporting structure.

To incorporate thermal displacement in the design of a cable ladder or cable tray installation expansion couplers should be used. For this reason it is important to establish the maximum temperature differential which is likely to be encountered at the site of the installation.

The temperature differential is based on the maximum and minimum seasonal temperatures. This temperature differential will determine the maximum spacing between expansion couplers within a cable ladder or cable tray installation.

See section 2.5.3 which gives details on expansion coupler installation.

Consult the manufacturer for more detailed information.
 

3.5.2 Effect of Thermal Expansion on Cables

The effect of cable expansion and contraction should also be considered and it is therefore advisable to ensure that some excess cable length, such as a loop or partial bend, is left at the position of the expansion joints.

A cable can be assumed to be an elastic body, and therefore under conditions of temperature change can expand or contract. In reality the expansion or contraction is dependent upon the material, shape and construction of the conductor and with small temperature changes it is linear until, with bigger temperature changes, it reaches a limiting value. Stresses of up to 50 N/mm2 can be expected and under the influence of such stress deformation takes place.

Whether the temperature rise of a conductor produces a longitudinal expansion force or a radial expansion force largely depends on the type of conductor, the adhesion of the insulation to the conductor, the type of cable and the method of cable cleating.

In multi-core cables the radial expansion of the conductors is hindered and therefore high longitudinal forces are developed in the conductors. In single core cables longitudinal expansion occurs when the deflection of the cable is hampered due to the design of the cable fixings.

Cables must be installed and secured in such a way that longitudinal expansion is equally divided over the full length of the cable and does not occur only at a few points. This is of particular importance when installing cables of large cross sectional area which in normal operation are heavily loaded with large cyclical currents.

Single core cables must be installed in long straight runs in a wavy line. Cables must be fixed to supports at sufficiently large distances to permit deflection. During the installation of cables the minimum bending radii must be strictly observed so as to avoid the development of excessive radial stresses in the bends and hence the possibility of damage to the insulation and outer sheath. Single core cables must be installed in such a way that damage e.g. pressure points caused by thermal expansion, are avoided. This can be achieved by installing the cables in an approximate sine-wave form and fixing at the ‘peaks’ of each of these waves. Sufficient space must be provided on the cable tray and cable ladder to accommodate the maximum deflection of the cable under normal operation.

Further advice should be given by the cable manufacturer.

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