Introduction to Safety Relays
Redundancy and safety are often seen together in industrial safety systems. Safety relays monitor emergency devices to switch contacts based on status - but what makes them different from normal relays?
Why Do We Need Safety?
With any automated machine, no matter how big or small, there will always be some kind of safety system installed. Machines do not know when people are close enough to get hurt so we need a foolproof way to stop a machine or process when people are close by. The devices need to not fail because if they do, people could get seriously hurt or even killed. This article will review safety relays, and to be clear, I’m referring to safety relays commonly encounter with emergency stop buttons or light curtains. Sometimes safety contactors are confused with safety relays but the two have very different uses.
Figure 1. Safety system with various relays and emergency devices. Image used courtesy of Pilz
Safety Relay Versus a Standard Relay
A safety relay’s mechanical actions is very similar to a standard relay. There will be an input signal that will trigger the output contacts to close, completing the circuit and allowing the machinery to move. An industrial electromechanical relay will have one signal (perhaps 24 or 120 volts) that energizes a coil of copper wire to trigger the output contact sets.
A safety relay has two input signal circuits, often solid-state for a quick response and to enable time-delay capture between the inputs. They need to be triggered at the same time, typically within fractions of a second of each other. These two input circuits will be completed by the side-by-side redundant contact sets of all E-Stop and other emergency safety control devices.
The safety relay will also typically require separate control voltage and ground in order to power the internal circuitry.
A standard relay can have from1-6 double-pole (NO and NC) or single-pole (NO or NC) contacts. With safety relays however, normally open output contacts are more common, but there are some on the market that have a few normally closed auxiliary contacts as well. When a standard relay fails, it may stick in its closed or energized state; this could be very hazardous if the relay is used to monitor a safety system such as an emergency stop pushbutton. If the relay fails to switch states, the machine won’t stop when the emergency stop button is pressed. With a safety relay, the internal features ensure that if the relay fails it will fail in the open state or not energized state.
Sometimes the normally closed contact is used as a monitor signal within the chain of safety devices to ensure the relay is off before allowing the system to energize, for example a signal enable for a safe “teach” mode which operates when safety doors are open. Typically safety relays require no setup or adjustment but there are some on the market that have a timer function that will delay energizing, these relays will need to be adjusted to meet the requirements of the system.
Unique Features of Safety Relays
There are many different kinds of safety relays with many different options and features but the defining feature is the monitoring chain. Typically, safety relays have two output voltage terminals and two input terminals for the coils, these are redundant monitor channels. The idea is to run both output terminals in daisy-chain fashion through all the emergency stop buttons, palm buttons, or any safety-rated device and then back to the input terminals for the coils.
The safety relay is looking for electrical continuity between those output and input terminals. If there is no continuity - either by failure or by pressing an E-stop - then the safety contacts will open and stop motion within the machine. The reason there are two outputs and two inputs is for redundancy, if even one contact of an emergency stop button doesn't open when pressed, the system will fault and the output contacts will open. The chance of an internal contact failing is low, but not impossible. The chance of two failing at the same time is far less likely.
Figure 2. Various sizes of safety relays. Image used courtesy of Banner Engineering
Safety Relay Varieties and Applications
Even with modern safety PLCs, there are still lots of different kinds of safety relays on the market. Any device that is safety rated needs to be used in conjunction with a safety control element (relay, etc) to maintain its safety rating. This means if you have an emergency stop pushbutton on a machine that has moving components or a process that could injure people, a safety relay will be required in order to maintain the safety rating of that component. You can still use an emergency pushbutton without a safety relay it just won’t be classified under the many machine safety standard protocols such as ANSI B11.19 or ISO 13.110.
Safety relays are sized according to the type and number of safety components they are monitoring, the number of contact outputs, and the safety level rating. Some relays have special features like safety muting or blanking which will temporarily mute the safety inputs under special circumstances. Other relays allow for additional contacts to be added, and some advanced safety relays allow for a small amount of programming using a graphical function block program.
Figure 3. IDEC safety relay modules, with the indicator of K1 and K2 displaying the redundant input coil channels. Image used courtesy of IDEC
Safety Relays and the Central PLC
With the safety relay already having a safety level rating, there is no need to use a safety-rated PLC or safety-rated inputs for the PLC. Some designers will use one of the normally open outputs to pass a signal into the PLC for monitoring the state of the safety system. If you would like to know which safety component is causing the system to not energize, a separate contact on each safety device should be used. It is not recommended to branch off of the two-channel monitoring chain to monitor the individual states of the safety devices, rather use an additional contact. Once the state of the safety system is known in the PLC, faults can be set when the system is deemed unsafe.
Troubleshooting Safety Relays
Most safety relays will have some indicator lights on the front of the relay. These lights indicate power to the relay, channel one and channel two status, and sometimes an error light. In order for the safety relay to energize both channel lights need to be on and if there is a reset chain it also needs to be satisfied. If there are no status lights on the relay, a technician can check for continuity between the output and input channels.
One common issue, during the initial wiring process, channel one’s output will be swapped with channel two’s input. The error light can indicate channels are out of sync or that the relay itself is defective. If there is continuity between the monitor channels and the reset channels and the relay still will not energize, check for proper voltage at the proper terminals. If the relay still will not energize you may have a defective relay and should contact the supplier or manufacturer.
Planning for Automation Safety
Safety relays are great at monitoring a few safety devices and ensuring safe stops of machinery when required. If your system has a lot of safety devices, then a more advanced safety PLC or safety controller might be a better option. Today most PLCs offer a safety-rated version that can communicate with remote or distributed safety input and output racks.
