In brief
A safety relay is a switching device that evaluates safety functions such as emergency stop or guard door monitoring redundantly and safely shuts down hazards.
Contents
What is Safety relay?
A safety relay monitors signals from safety devices, such as emergency stop buttons, guard door switches or light curtains, and shuts down the hazardous movement when required. Unlike a standard relay, it has an internally redundant design, usually with force-guided contacts, and detects its own faults such as welded contacts or cross-circuits in the wiring.
Typical ratings are the number of enabling contacts, single- or dual-channel connection, the start mode (automatic or monitored start), feedback circuit, supply voltage and switching capacity. In addition to versions with instantaneous opening contacts, there are devices with off-delay for stop category 1 and variants for two-hand control or standstill monitoring. The achievable level of safety is stated as a performance level to EN ISO 13849-1 or as a SIL claim limit to IEC 62061.
Safety relays are safety-relevant components: when replacing one, the new device must achieve at least the same safety function, category and performance level. A different model can affect the validation of the machine. Because the devices often remain installed for decades, product discontinuations and the search for functionally equivalent alternatives play an important role. Replacement devices should therefore be stocked in good time.
In practice: maintenance and procurement
For a spare parts enquiry, the manufacturer part number, supply voltage, contact configuration, start mode and required performance level are needed; a photo of the nameplate and terminal assignment helps. Replacement with a different model should always be agreed with the person responsible for machine safety and documented.
Synonyms and abbreviations: safety relay module · emergency stop relay · E-stop relay · safety switching device
Frequently asked questions
What is the difference between a safety relay and a standard relay?
A standard relay simply switches a contact without monitoring itself. A safety relay works with two channels, uses force-guided contacts and checks in every switching cycle whether all contacts have opened correctly. If it detects a fault, it prevents a restart. This is why only a safety relay may evaluate a safety function such as an emergency stop.
What information is needed to replace a safety relay?
Important are the manufacturer part number from the nameplate, the supply voltage, the number of enabling and signalling contacts, single- or dual-channel wiring, automatic or monitored start and any delay times. It should also be known which performance level or SIL is required for the safety function, so that an alternative device can be assessed correctly.
When is a safety controller better than a safety relay?
Safety relays are suitable for a few simple safety functions, such as an emergency stop circuit or a guard door. As soon as many sensors, several zones or logic operations are added, wiring with individual relays becomes confusing. A programmable safety controller is then usually more economical and easier to extend and diagnose.