Network Communications News (NCN) September 2016 - Page 31

cable management & labelling Intermetallic compound formation – the diffusions between the plating and substrate material will form intermetallic compounds, thus generating compressive stress within the plating. Diffusion – atomic substrate species may have high diffusivity when they diffuse into the plating film. External stress – or applied stress, which can be introduced by turning a nut or screw, bending or stretching of the surface etc. Coefficient of thermal expansion mismatches – between the plating materials and substrate. Surface oxide – it is possible to form zinc oxide and tin oxide on the tin surface and cause stress due to specific volume change. Although the fundamental mechanism of whisker growth is still not fully understood, the consensus of opinion is that whisker growth will proceed either via a dislocation mechanism or through a grain growth/recrystallisation mechanism. Furthermore, many of the theories agree that stress provides the driving force for whisker formation. Eight out of 10 data centre operators don’t prefer them. So how big is the risk? Unitrunk commissioned a report on zinc whiskers from Dr Colin Gagg, a leading expert on the topic with a PhD in Forensic Engineering, who has studied contamination phenomenon in electronic assemblies. In general, Gagg reported that zinc whiskers will not be a problem until they break off and begin circulating freely in the air. This is exactly why handling older electro-plate galvanised fixtures, such as cable trays and baskets; computer floor tiles and supports, etc, will raise the potential for problems. Even the lightest touch can dislodge zinc whiskers off their growing surfaces, allowing them to be circulated by air conditioning systems – and thus drawn into sensitive computer equipment. Even if the electronic equipment is equipped with dust filters, zinc whiskers can still filter through – only High Efficiency Particulate Air (HEPA) filters are fine enough to stop them being drawn into computer equipment. Once inside any electronic or electrical equipment, electrically conductive zinc whiskers can settle onto exposed circuit boards, where they can and do cause short circuits. Symptoms of zinc whisker related failure range from minor data corruption or anomalies to catastrophic hardware failures. A high incidence of power supply failure is a commonly reported problem. Furthermore, equipment malfunctions can be ongoing and intermittent, so diagnosis is often difficult. How should I prevent zinc whiskers? Unfortunately, there is a lot of misinformation in the marketplace about how best to tackle zinc whiskers. Unscrupulous suppliers may well offer hot-dipped galvanised products as a solution. Although hot-dipped galvanised products may not form zinc whiskers in such a short time as electro-galvanised systems, the evidence suggests that it will only be a matter of time before whiskers initiate and grow. Therefore in terms of zinc whisker elimination, hot dipped galvanising may, at best, only be a delaying tactic. S P E C I A L F E AT U R E The presence of whiskers can cause short circuits and arcing in electrical and electronic equipment. Powder coating is another option put forward. The reality in a site environment is that once powder coated solutions are cut or formed, the open end is unlikely to be treated to provide complete integrity of the coating. Another issue of powder coating is in achieving continuity throughout the system. This is very often only achieved by scraping away the coating from an area and applying an earth lead across joints. As our report expressed, if you want to eliminate the risk you have to specify stainless steel cable management systems and apply the same rigour to other parts of the installation. The reality is that this is not happening. Very few, if any, data centres are opting for this approach to risk. The message is this: specify stainless steel or accept there is a level of risk in the installation. What you can’t do is opt for anything less than stainless steel and believe that you are fully protected, as some suppliers may well argue. These whiskers belong to something with a nasty bite. With more than 20 years’ experience within the cable management sector, Tim Brown joined Unitrunk as specification manager in 2011 and was made national sales manager in 2014. Originally trained as an electrical apprentice, Tim began his career in the electrical contracting sector, later training as an electrical engineer before moving into estimating and surveying roles. He left contracting in 1993 to take on a sales role and has specialised in cable management ever since. 31 30-31 Cable Management – Unitrunk.indd 31 26/08/2016 15:03