The standard ISO 3731 for trailer connectors was issued in 1980 as a supplement for the standard ISO 1185 from 5 years prior. The standards include a name affix to further define their area of application - ISO 3731 is Type S (supplemental) whereas ISO 1185 is Type N (normal). The purpose of the Type S connectors is to transfer signals and power for functionalities which cannot be served by the 7-pin Type N plugs and sockets (for example rear fog light or steady plus for a consumer on a trailer). The latest revision is from 2003. For a better recognizability, the housing of the supplementary system is white while the "normal" system features a black housing. Due to a different contact arrangement whithin the connectors, users can not accidentally plug a normal plug into a supplementary socket and vice versa.

Application of ISO 3731 Type S

When a trailer coupled to a truck necessitates more than just the basic lighting functions, the connector system according to ISO 1185 is not sufficient as the 7 pins cannot accommodate any further functionalities. The plugs and sockets according to ISO 3731 add 7 more lines to provide a steady power supply line for a consumers on the trailer (for example refrigeration trailers or semitrailers) or to control additional lighting functions (for example rear fog light or reversing light).

The supplemental socket is continually replaced by the 15-pin connector according to ISO 12098, which has been developed in the 1990s and which integrates the functionalities of the Type N and Type S 7-pin connector systems.

Scope of ISO 3731 Type S

The standard ISO 3731 specifies the dimensions of, and requirements for, 7-pole connectors of type 24 S and their contact allocation for the electrical connection of towing and towed vehicles with 24 V nominal supply voltage, thus ensuring interchangeability. A 24 S connector is intended to be used in addition to a 24 N connector according to ISO 1185 where more than 7 poles are required.