IEC 61992-5 Ed. 1.0 b:2006

Covers particular requirements for surge arresters for specific use in fixed installations of d.c. traction systems. They consist of one or more non-linear resistors which may be in series with single or multiple spark gaps. Covers also low-voltage limiters, that is protective devices mainly used in fixed installations of d.c. traction systems to connect certain portions of the circuit.”