Range of SIL4 safety critical relay (vital)

Safety RS4 relays are certified and their functions are similar as NS1 signaling relays (which are SIL4). It is used a lot by SNCF, french railway operator, in railway applications; NS1 signaling relays are in particular used in relay-based interlocking.

Safety systems control is often done via safety relays. There is often confusion in the terms used for the notion of safety relay:

Relay forcibly guided according to en 61810-3standard

In reality a lot of safety relays are simply forcibly guided relays according to EN 61810-3standard, guaranteeing that it is impossible that a NO (Normally Opened) contact and a NC (normally closed) contact are established at the same time.

Relays guaranteed to be fail-safe and forcibly guided

In reality a lot of safety relays are simply forcibly guided relays according to EN 50205 standard, guaranteeing that it is impossible that a NO (Normally Opened) contact and a NC (normally closed) contact are established at the same time.

Relays like the NS1 relays which meet NF F 70 030 standard, are also guaranteed to be fail-safe (not all the NS1 relays – the RU are not – it is also the same analysis for Q-relay).

The fail-safe guarantee is obtained by the fact that the moving part of the relay is orientated in relation to earth’s attraction. This way, via the effect of gravity”, the “working” contacts open if the relays coil is not powered.

This implies that these relays may not be used in-board. These relays are mainly used in rail signaling facilities and are considered to be intrinsic safety relays with a SIL4 safety level.The standard emphasize on the fact that these safety characteristics are only valid if the terms of use for these relays are respected (maximum supply tension at 28.8V, maximum current in the limited contacts, etc.).

Why has CLEARSY developed the RS4 relays?

We have developed and set up systems like COPPILOT, DOF1 and DIL which are intended to control the platform screen doors of various underground rail systems. The vital function on these systems is SIL3.

The opening control is achieved through existing relay contact in working position. The security of the safety function on CLEARSY’s systems is based on the fact that we safely detect that the relay has remained “stuck” in the condition in which working contact is established. This principle was used to produce the RS2 relay: this relay is made up of 2 forcibly guided relays and allows, thanks to an external check of the contacts (by a security automaton for example) to guarantee a SIL3 safety level.

For security applications which have no checking system, we have designed an automatically checked relay named RS4. The RS4 is functionally comparable to the traditional vital relays because the RS4 relay is an intrinsic safety relay with a SIL4 safety level. The RS4 is “forcibly guided “AND ” fail-safe “, including design based on component failure according to EN50129 standard. Like the NS1 relay, certain conditions of use must be respected to guarantee the SIL4 safety integrity.