A safety relay is the component that turns a safety device an emergency stop, a guard switch, a magnetic safety switch into a reliable stop command that removes power from a machine. Choosing the right one means matching it to the safety function you need to monitor, the Performance Level your risk assessment demands, and the way the machine is wired and reset. This guide explains how safety relays work, the specifications that matter, and how they pair with the safety contactors that break the motor power.
It is written for machine builders, panel builders and maintenance engineers specifying or replacing a unit. If you already know the part you need, you can go straight to our safety relays and contactors range.
What does a safety relay actually do?
A safety relay continuously monitors its input circuit rather than simply switching it. It sends test pulses down the wiring to a device such as an E-stop or interlock switch and watches both channels of a redundant (dual-channel) circuit. If a button is pressed, a guard opens, a wire is cut or a contact welds, the relay detects the change or fault and de-energises its force-guided output contacts, removing power in a controlled way. Because the outputs are force-guided, a welded contact is detected and the relay will not reset that self-checking behaviour is what makes it a safety relay and not an ordinary control relay.
Key things to get right when choosing a safety relay
1. The safety function you need to monitor
Start from the device. Safety relays are often function-specific one type for emergency stops and guard switches, another for light curtains, two-hand controls, or standstill and speed monitoring. Match the relay's rated inputs to your device, and check whether you need a single function or a unit that can handle several. For emergency stops and interlock switches, a general-purpose monitoring relay such as the Mechan SRL-1 safety relay (24V AC/DC) or the Allen Bradley Guardmaster MSR127TP covers the majority of machines.
2. Performance Level and safety category
Your machine's risk assessment sets a required Performance Level (PL a–e) under EN ISO 13849-1, or a Safety Integrity Level (SIL 1–3) under IEC 62061. That target dictates the architecture: a single-channel Category 1 circuit for low-risk stops, or a dual-channel, monitored Category 3 or 4 circuit typically PL d or PL e for higher-risk machinery. Choose a relay rated to reach the PL/SIL you need in the wiring category you will use.
Tip: the Performance Level is a property of the whole safety function device, relay and contactors together not the relay alone. Two contactors in series with feedback monitoring are usually what lifts a circuit to PL e.
3. Channels, contacts and reset
- Channels: dual-channel (redundant) wiring is required for most Category 3/4 functions; single-channel suits only low-risk applications.
- Safety and auxiliary contacts: count the outputs you need more contacts let one relay drive more contactors or signal to the PLC. The Guardmaster MSR138DP gives 2 safety contacts plus an auxiliary, the MSR127TP gives 3, and if a single relay runs out of outputs an extender such as the Mechan FX2 extender module adds more.
- Reset: decide between automatic reset and a monitored manual reset. Manual, monitored reset is the norm where an operator must confirm the area is clear before restart.
4. Supply voltage and mounting
Confirm the coil/supply voltage 24V DC is standard, though many units (the Mechan SRL-1 included) accept 24V AC/DC, with mains-voltage options such as the 110V AC Mechan FM1 safety control unit also available. Check the DIN-rail width and terminal type so the unit drops into your panel cleanly.
Safety relay vs safety PLC which do you need?
A dedicated safety relay is the right choice for machines with a small number of straightforward functions: it is lower cost, needs no programming, and is quick to wire and validate. When you have several guarded zones or need to expand a relay-based system, a modular safety control unit with extender modules (the Mechan F-series works this way) keeps things simple without programming. Once you reach many functions, muting, diagnostics and network integration, a configurable safety controller or safety PLC becomes more economical and far easier to modify. All three can reach SIL 3 / PL e; the choice is about scale and flexibility, not safety rating.
Where the safety contactor comes in
A safety relay does not usually switch the motor current itself it drives one or two safety contactors that do. Safety (mirror-contact) contactors have mechanically linked auxiliary contacts, so the relay can monitor whether the main contacts have actually opened. Size the contactor by the load current and voltage of the motor or circuit it isolates our stocked Allen Bradley 100S-E and 100S-D safety contactors run from roughly 100A to 420A. For redundancy at higher Performance Levels use two contactors in series with feedback wired back to the relay. You'll find relays and contactors together in our safety relays and contactors category.
Searching for a Pilz, Siemens or ABB safety relay?
The Pilz PNOZ family is the most widely recognised safety relay range, alongside Siemens SIRIUS, ABB and others and they are what most people search for. The important point when replacing one is that the circuit does not care about the brand on the housing: match the safety function, number of channels, contact count, PL/SIL rating, reset type and supply voltage and an equivalent unit will drop in. Pneutrol Spares stocks both Allen Bradley Guardmaster safety relays and 100S contactors and Mechan Controls safety relays, safety control units and MagnaSafe safety switches that cover these functions; send us the part number you are replacing and we'll match it to a Guardmaster or Mechan equivalent.
What else you'll need
A safety relay is one part of the interlock system. Specify the input devices and downstream parts alongside it:
- The MagnaSafe non-contact safety switches and emergency stops that trigger the relay.
- Trapped-key interlocks and access control for guarded zones.
- Safety contactors sized to the load, from the safety relays and contactors category.
Frequently asked questions
What does a safety relay do?
A safety relay monitors safety devices such as emergency stops, guard switches and light curtains, and removes power in a controlled way when one is triggered or a fault is detected. It cross-checks its wiring for faults like a welded contact and uses force-guided contacts, so it will not reset if the circuit is unsafe.
How do I choose the right safety relay?
Match it to the safety function you are monitoring, then to the Performance Level (EN ISO 13849-1) or SIL your risk assessment requires. Confirm the number of channels, the count of safety and auxiliary contacts, the reset type (automatic or monitored manual) and the supply voltage before ordering.
Do I need a safety relay or a safety PLC?
Use a safety relay for machines with a small number of simple functions: it is cheaper, needs no programming and is quick to validate. Choose a configurable safety control unit or safety PLC when you have many functions, several zones, or need diagnostics and network integration. Both can reach SIL 3 / PL e.
What is the difference between a safety relay and a safety contactor?
A safety relay is the monitoring and decision device; a safety contactor is the switching device it commands. The relay watches the safety circuit and, on a stop, de-energises one or two safety contactors, which physically break the load current. Safety contactors have mirror (mechanically linked) contacts so the relay can confirm they have opened.
Can I replace a Pilz PNOZ or other safety relay with a Mechan Controls unit?
Yes, provided the replacement matches the original on safety function, number of channels, contact count, PL/SIL rating, reset type and supply voltage. The brand itself does not matter to the circuit, so send us the part number you are replacing and we'll match it to a Mechan safety relay or safety control unit that meets the same specification.
Browse the range
Ready to specify? Browse our safety relays and contactors → including Allen Bradley Guardmaster relays and 100S contactors, the Mechan SRL-1 safety relay, FM-series safety control units and MagnaSafe safety switches. Replacing a Pilz PNOZ, Siemens or ABB unit? Contact our team with the part number and we'll match it.