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Electrical apparatus in industry for use in the presence of explosive gases must be specially designed to prevent the apparatus from igniting the gas. In the case of flameproof enclosures, any holes and gaps in the enclosure wall must be sufficiently long and narrow to prevent hot combustion products expelled by a gas explosion inside the enclosure from igniting an external explosive cloud. There are tight requirements as to the maximum permissible gap surface roughness. This article describes an experimental study of the influence of severe mechanical and corrosive damage of flame gap surfaces on gap performance for IIB and IIC gases in air, using ethylene and hydrogen as test gases. In agreement with previously published findings for propane (IIA gas), it appeared that even with ethylene and hydrogen gap surfaces can suffer considerable damage without this causing any reduction of gap performance. © 2013 American Institute of Chemical Engineers Process Saf Prog 33: 49–55, 2014