TRANSFORMERS, SUBSTATIONS + CABLES
information for maintenance and repair to meet the requirements of IEC 60079-17 and IEC 60079‑19 respectively. “…documentation relating to the suita- bility of the equipment for the area and environment to which it will be exposed, e.g. temperature ratings, type of protection, IP rating, corrosion resistance;” Clause 5.9 of the standard says: “Electrical equipment shall be selected and/or installed so that it is protected against external influences which could adversely affect the explosion protection.” And lists corrosion, chemicals, condensation and moisture as some of the influences to consider. The specifier/installer must consider the effects of hazardous gases or vapours on the integrity of the installation with regard to the performance of the flame path, the IP seals, and the terminals themselves. (This is evidenced in the photos, shown below left, of a similar failed junction box that was installed in a Zone 1 area.) Flameproof junction boxes made of cast iron are highly susceptible to corrosion, especially if the flame paths are unprotected. On the other hand, good quality Ex d flameproof junction boxes would have their critical flame paths treated with a metallurgical process that resists corrosion. In addition, they would have IP seals made from hydrocarbon resistant polymers that seal on the outer rim of the flame path which give additional protection to the flame path from corrosion. They would also have captive high tensile stainless steel screws which would not get lost at the time of installation. Unlike zinc coated steel fasteners, high tensile stainless steel fasteners would not weaken over time due to corrosion. This is critical as the fasteners need to secure the lid against explosive pressure for the lifetime of the installation. Such good quality junction boxes would also have been independently tested for resistance to environmental corrosion to ASTM B117-03 and ISO 6988 standards. Another important consideration is the fact that an installation in a Zone 0 environment could suffer from the risk of an explosive gas mixture (which is always present
Left: Corrosion of a flame path on a cast iron junction box after testing to ASTM B117-03 and ISO 6988 standards. Right: A good quality flameproof junction box which has been specially treated with a corrosion-resistant metallurgical process after testing to ASTM B117-03 and ISO 6988 standards.
Dr Geof Mood is the Technical Director for CCG Cable Terminations (Pty) Ltd. He holds a doctorate in mechanical engineering and sits as an independent expert on a number of IEC Standard committees, including the Maintenance Team for IEC 60079-14. In summary, the installer or specifier of Ex equipment should not be taking dangerous, subjective advice from a manufacturer but should objectively read the certificates and the installation standards and should certainly not believe that putting an Ex e terminal inside an Ex d enclosure will give the level of protection needed for Zone 0 applications. □ or present for long periods) or liquid migrating through the interstices of a cable to an area with a lower level of Ex protection, leading to an explosion. (Sceptics may think that this cannot happen, but the infamous Torrens Island explosion shows it does happen, even when only in Zone 1 in which an explosive mixture is present only for a relatively short period, compared to Zone 0 where it is always present). SANS/IEC 60079-14 clauses 9.3.2 and 10.6.2 are clear on how to address this risk.
In line with professional publishing practice Electricity + Control offered the author of the original article right of reply to the comment from Dr Geof Mood of CCG Cable Terminations. The invitation was declined.
MARCH 2022 Electricity + Control
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