Safety Relays Explained: A Guide to How It Works

Estimated reading time: 9 minutes

Introduction

A safety relay is an electromechanical or electronic device designed to reduce risk and implement safety functions, particularly in industrial environments. Its primary goal is to shut down power and remove risk safely and reliably. It reduces the risk of hazards and decreases the chance of damage, injury, or death.

It’s important to note that a safety relay is not a stand-alone device. Still, we install it as part of a more extensive control and safety circuit inside a machine. It’s an interface between safety devices and the machine components.

Unlike regular relays, we build safety relays to meet stringent safety standards, incorporating fail-safe principles that guarantee operational safety in critical conditions. A safety relay helps to check for losses in safety functions and detect internal failures, including faults with their inputs and outputs.

Safety relays are essential in protecting workers from hazardous machinery or preventing system failures in automation. While installing a safety relay is not obligatory, it will simplify any safety circuit design, increase the machine’s safety level, and help detect faults.

History of the Safety Relay

The German automation manufacturer Pilz developed the first safety relay and patented the first emergency stop relay, the PNOZ brand, in 1987.

Various manufacturers now make safety relays with many different functions and features. For example, a two-hand safety relay is an example of a function-specific safety relay.

Risk Assessments

You should base the safety relay and overall safety system you choose on the results of a risk assessment for the machinery.

Although I won’t cover the details here, it’s essential to understand performance levels (PL), safety categories, and Safety Integrity Level (SIL) when selecting the appropriate safety system. Suppose you connect a low-PL component to a high-PL component. In that case, the entire system will be downgraded to the lowest PL.

Basic Construction of a Safety Relay

Depending on its type, a safety relay will have several safety inputs and outputs. These are terminals where you can connect your wires or cables.

So, if we start on the input side, we will connect some kind of safety device to the safety relay. An example of a device that would connect to the input of a safety relay would be a safety interlock switch, an emergency stop button, or even a light curtain. You could even connect a safety relay’s outputs to the inputs of another safety relay. (This is a way to “daisy-chain” safety relay modules.)

An important aspect of the safety inputs of many safety relays is that they have dual-channel redundancy features.

What does this mean?

Signal redundancy ensures that a single fault on a line will not lead to the loss of the safety function. We will discuss this in more detail later, but it is important to wire up the safety relay correctly to maintain this redundancy with devices. You will always have two wires for each safety input on a safety relay.

Now, this is not the case for all safety circuits. Depending on the risk assessment, a single-channel device could also connect to a specific safety relay that accepts single-channel inputs. However, a single-channel safety relay would not be able to detect all the faults a two-channel safety relay could detect.

Similarly, for the output side of a safety relay, these terminals will connect to another safety relay or a device such as a contactor. As with the inputs, the outputs are also redundant and are monitored periodically to check the status of the signals.

Usually, a compact DIN-rail-mounted unit contains all these input and output terminals. We install these units inside a machine’s control or electrical cabinet. The safety relay should also have a couple of indicator lights on its face to show its output status. Sometimes, you can diagnose a fault by seeing which indicator light or channel of a safety input is not resetting.

Finally, the safety relay’s “brain” or internal components are a function-specific microcontroller or other circuit. It verifies and continuously checks safety functions and itself. Therefore, the safety relay has redundant circuits with built-in self-monitoring. If a component inside the safety relay fails, it will remain operational.

An Example Safety Relay Wiring Diagram

Here’s a very basic example of a typical safety relay wiring diagram. Note that the wiring of the safety relay varies widely, so ensure that you read your safety relay’s instruction manual carefully.

There are several essential things to note about the electrical schematic above.

1. Power—The safety relay and contactors are powered by a 24VDC power supply connected directly to the A1 and A2 terminals.
2. The Emergency Stop Button (E-Stop)—Notice that the emergency stop button has two normally closed contacts that are mechanically connected. This is part of the redundancy-centric nature of safety systems. A pulsed output from terminal S11 is connected to both the e-stop contacts. When the E-Stop is depressed, both contacts are forced open, and a redundant signal is sent to terminals S12 and S14.
3. Safety Outputs—Terminals 14 and 24 are the input side of the safety outputs section of the safety relay. These two are connected directly to the power supply. When all safety conditions are met, the relay will close a set of force-guided and redundant relays. This will send power to terminals 13 and 14, powering the two contactors.
4. Contactors—The safety outputs provide a dual-channel, redundant signal that is connected to two contactors. These contactors will then be connected in series to control a motor.
5. Feedback Loop – The feedback loop monitors the contactors. In their normal state, a force-guided, normally closed contact will inform the relay of faults within the contactor, such as welded or stuck contacts. The input for these contacts comes from the safety relay as a pulsed signal. The two contacts are wired in series and returned to the safety relay on terminal S21.
6. Reset Button—If the safety relay is tripped, this button will reset the relay to normal operation once all safety conditions are met. In this case, the signal goes through the feedback loop of the contactors.

How Does a Safety Relay Work?

A safety relay monitors one specific safety function or a safety device, such as a light curtain. The safety relay will electronically evaluate inputs and control its outputs based on these evaluations.

Term: OSSD

OSSD Stands for Output Signal Switching Device. An electronic circuit sends out a certain pulse, recognized as a safety signal. OSSD signals prevent accidental or intentional bypassing of safety devices. The OSSD signal is not a steady 24VDC current but rather a coded signal transmitted over a pair of wires that are out of phase with each other.

Safety Relay Functions

On a basic level, the safety relay has one or more of the following functions:

1. A safety relay will still function if an internal fault is detected, allowing the system to stop or shut down due to the built-in redundancy and self-monitoring.
2. The safety relay will monitor input faults such as shorts between conductors, stuck contacts, and shorts to ground.
3. It has at least two inputs that take signals from an OSSD or dry contacts, such as those inside an emergency stop button.
4. A safety relay sends test pulses along its inputs/outputs to detect faults, such as shorts, to the power source.
5. A safety relay detects if dual-channel inputs change state simultaneously. This is to detect potentially failed contacts on an external safety device. Sometimes, a coded magnetic interlock closed too slowly will make the safety relay think there is a fault due to the two channels not closing simultaneously.
6. It has at least two safety outputs, and many safety relays have more than two. These safety outputs use positively guided contacts to ensure contact sets move as one unit.
7. It allows for monitoring of output devices such as contactors. This is called EDM or External Device Monitoring.
8. A safety relay must be power cycled or reset after a fault has occurred to restore it to normal operating condition.
9. A safety relay may have built-in time delay or other functions such as speed or frequency monitoring.
10. It provides electrical isolation between the safety devices and the power control circuit.

Proper Wiring is Important!

One crucial thing to consider is that for a safety relay to work as expected and reduce risk, we must wire it correctly. I have seen interlock switches wired to a safety relay’s A1/A2 power input terminals, effectively removing and deactivating power. Not only is this a wrong way of connecting a safety relay, but the safety relay also effectively becomes bypassed and useless in the circuit.

A typical use of a safety relay is the interconnection between a safety interlock switch mounted on a guard and the contactors that power a motor.

The safety relay will check and turn on the motor contactors when all safety conditions have been met.

General Purpose Relay

This general-purpose safety relay can be used with our EOS4 light curtains, E-Stop buttons, or interlock switches.

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Typical Applications & Usage Examples

We commonly find safety relays in industrial environments on machinery such as production lines, robots, and other machinery and devices where an increased safety level and risk reduction are required.

Here are examples of standard safety devices that we can connect to the input terminals of a safety relay:

• Light curtains
• Safety Mats
• Two-hand button control
• Safety guarding interlocks
• Emergency stop buttons

On the output side of a safety relay, a standard connection is contactors, which, in turn, connect back into the safety relay’s inputs for monitoring the contactor’s electrical contacts. In addition to contactors, we can connect a 2^nd relay or into special OSSD inputs of VFDs/drives.

Examples of machinery and equipment that make sure of safety relays:

• Elevators
• Cranes
• Robots
• Stamping Presses
• Cutting Machines
• Printing Machinery
• Production Machinery

Relevant Standards

OSHA 1910.211(d)(62) – Safety system means the integrated total system, including the pertinent elements of the press, the controls, the safeguarding and any required supplemental safeguarding, and their interfaces with the operator, and the environment, designed, constructed and arranged to operate together as a unit, such that a single failure or single operating error will not cause injury to personnel due to the point of operation hazards.

ANSI B11.19 & ANSI B11.20 – Control Reliability

Further Reading

https://www.ferndalesafety.com/connecting-safety-interlock-switches-in-series/

https://www.ferndalesafety.com/two-hand-control-for-machine-safeguarding/

Practical Machinery Safety By David Macdonald

Hero image: “Industrial Pipes” by André Robillard/ CC0 1.0