Effect of rusting and mechanical damage of gap surfaces on efficiency of flame gaps in flameproof electrical apparatus

Potentially incendiary electrical apparatus for use in the presence of explosive gas atmospheres have to be specially designed to prevent the apparatus from igniting the gas. Flameproof design is one of several options, and one requirement is then that any holes and gaps in the enclosure wall be designed to prevent a gas explosion inside the enclosure from being transmitted to an explosive gas cloud outside it. Current standards (IEC) require that flame gap surfaces have a surface roughness of <6.3 μm. Any damaged flame gap surface has to be restored to this quality. The present investigation has demonstrated that flame gap surfaces in flameproof electrical apparatuses can suffer considerable corrosive and mechanical damage without any reduction of gap performance. In some cases very significant mechanical surface damage in fact improves gap performance. Possible physical reasons for this are discussed. These findings indicate that current high costs of repairing and replacing flameproof electrical apparatus in process plants offshore and onshore can be significantly reduced without any increase of explosion risks.