Railway signalling circuits require relays whose contact state is unambiguous even after a fault: an NO and an NC contact of the same relay can never be closed at the same time, so welding of one of them is detected by the opposite contact. The OA 5603/39 PCB relay has eight such contacts, crown contacts for small signal currents and a design to EN 50578 (UIC 736, type C) declared in the datasheet.
For the boards of safety modules and interfaces DOLD offers the smaller OA 5621 and OA 5622 relays with forcibly guided contacts in a wash-proof housing, and for DIN-rail cabinets the HL 3094N interface module, which multiplies a controller signal onto forcibly guided contacts with monitoring by the NC contact and has a design to EN 50155 in its datasheet.
Selection of a PCB relay covers the coil voltage, the contact arrangement and the contact material: AgSnO₂ for inductive loads, gold-plated AgNi for low-current signal circuits. In safety circuits the validation of the complete system is carried out according to EN ISO 13849-2.
States that must survive a loss of supply are stored by the UG 8851 latching relay with forcibly guided contacts, switched by short pulses.
