This part of IEC 60099 applies to non-linear metal-oxide resistor type surge arresters without spark gaps designed to limit voltage surges on a.c. power circuits.
BS EN 60099-4:2004+A2:2009 history
2014BS EN 60099-4:2014 Surge arresters. Metal-oxide surge arresters without gaps for a.c. systems
2004BS EN 60099-4:2004+A2:2009 Surge arresters — Part 4: Metal-oxide surge arresters without gaps for a.c. systems
2004BS EN 60099-4:2004 Surge arresters - Metal-oxide surge arresters without gaps for a.c. systems