Safety relays are devices that implement safety functions. In case their intervention is requested by the circumstances, the safety relay must act according to a specific safety routine to mitigate, reduce, or eliminate a hazardous condition.
The safety relay's response is to initiate a safety procedure to transition the application's condition into a safe stage. This response must be prompt, and the device responsible for it, in our case the safety relay, must be reliable. While a standard relay can perform a simple task of activating or deactivating a load, there are no guarantees that, under certain conditions, a standard relay can fulfill such an important duty. The short answer to our question "why do I need to use a safety relay?" can be summarized in one simple word: trustworthiness.
Prevention
Prevention is defined as one or more actions that aim to stop or prevent something from happening or arising. The terminology here is pertinent and perfectly summarizes the reason safety relays are used in a vast number of applications across several industries. Prevention, in an industrial process and machinery, is the fulcrum of critical application management. The role that safety relays play is critical within the prevention concept, as the device is responsible of averting dangerous conditions in case of hazardous events. Accidents, injuries, damages, or discontinuation of a production process are deleterious and detrimental, and not just for the company and its assets but, more importantly for personnel and the environment. What it is possible to see in plain site are usually actuators such as emergency stop buttons, safety switches, light curtains, safety mats, and so on, but behind the scenes, safety relays are the ones that not only drives these devices but also monitors their conditions. It is true that when a hazardous condition is detected, the safety relay must initiate a safety response to stop or control the machinery, but it is also true that a safety relay is responsible for ensuring that, in this exact moment, the final element can operate according to the safety logic.
What has been introduced in these few lines are basically two separate things. One is defined as a safety function, which ensures that a movement is stopped or initiated in a controlled and therefore safe manner. Turning this in practical examples means mentioning, among others, safety interlocking systems where the safety relay ensures that specific safety measures are met before allowing a machine to operate. For example, a safety relay may monitor a safety gate switch and ensure that the gate is securely closed before enabling the machine. Safety relays are commonly used to implement emergency stop systems. When an emergency stop button is pressed, the safety relay immediately cuts off power to the machine, bringing it to a safe status. The same applies to the opposite safety function, which is the activation of a load, such as a sounder or a beacon, to warn the personnel that a hazardous event is imminent or already occurring.
The second concept relates to the monitoring procedure that a safety relay can perform to guarantee the integrity of the loop and the final element itself. All dormant safety functions, which can be found, for example, in the process industry, are activated or deactivated only in case of perilous situations, and their functionalities need to be constantly monitored over time to ensure that, in case of emergency, they will be capable of performing their job. The safety relay can monitor the status of the loop, detecting short or open circuits as well as the health condition of the actuator itself. In some cases, safety relays can auto-diagnose their internal status and notifying the control room whether they are in a condition to operate or not.
Surveillance
If we take a Fire and Gas system as an example, we can state that under normal conditions, it is in stand-by mode, a state of readiness to intervene. In this case, a safety relay is undoubtedly capable of doing something that a standard relay cannot do: monitoring the health of the application. One of the reasons a safety relay is use is, in fact, its surveillance capability, which in technical terms is the ability to continuously monitor both its own vital functions and the health of the loop and the load, such as:
- Welded relay contacts diagnostic
- Relay coil integrity
- Filed power loss or fluctuation outside a pre-determined range
- Field short circuit
- Field open circuit
Filtering
In this paragraph of our article, we are referring to the ability of the safety relay to filter the pulse test performed by the majority of safety systems in, but not only, the process industry. It is worth noting that not all safety relays on the market have this characteristic, but many are pulse-test compatible, which is certainly a plus. By filtering the signal that the PLC sends though its terminals, the life expectancy of the safety relay increases. What is important to note is that the relay is not only capable of filtering such tests during normal operations, but it is also typically capable of disrupting them in case of a failure. If a fault occurs, the internal impedance of the relay changes, and this variation is reported through the coil terminal of the safety device. As such, the PLC card can detect the fault and take the necessary actions. In practical terms, this can also be seen as a money saving feature since it can spare additional digital input cards and wiring effort.
Safeness
The term "Functional Safety" is generally used in cases where the effectiveness of a protective measure depends solely and directly on the proper functional operation of an independent control system. When this circumstance occurs, it is the reason a safety relay must be used. A device such as a safety relay is used whenever a safety instrumented function, and consequently a safety instrumented system, is at the core of a plant's risk prevention policy. Along with sensors, logic units, and other final elements, a safety relay must ensure safety. By using safety relays, industries can achieve a higher level of safety for their employees, protect valuable equipment, and comply with safety regulations. These devices are a fundamental component of safety-critical systems, helping to prevent accidents, injuries, and damages in various industrial applications.
Flexibility
Safety relays come in various configurations to suit different safety functions and applications. They can be easily adapted or expanded to accommodate changes in the safety requirements of the system. Of course, this depends on the specific features of the relay, but generally, safety relays are well known for their pronounced flexibility and scalability. The advantage of using safety relays is also related to how easily they can be adapted and integrated into existing control systems, making them cost-effective and user-friendly. Safety relays often offer simplified wiring compared to complex safety PLC systems, which also means easy troubleshooting and maintenance. These features help identify faults and ensure that the safety relay is functioning correctly. Under the safety relays umbrella, there are also the so-called “smart relays” that can be programmed to diagnose specific events within predefined ranges. This is the case when a relay is capable of reading and interpreting live parameters from the field. The power of the load can be monitored, for example, to ensure that it fits the ideal operating conditions of the load. If the measured voltage is not within the specified range, a fault is issued so that proper actions can be taken. The same thing can be done by a smart relay with the load resistance or other crucial variables.
Reliability
The solidity of a safety relay is what makes it reliable, and this is another reason why a safety relay is used in safety-related and critical applications. Many safety relays have redundant channels, meaning they have multiple independent circuits. Redundancy improves reliability and safety by reducing the risk of a single point of failure. As explained in a series of articles regarding this piece of equipment, a safety relay is a complex device that incorporates complex electronic circuitry and include more than one standard relay. Regardless of the circuit structure that constitutes the internal workings of a safety relay, the concept of redundancy is always present. For example, if the function of a safety relay is to de-energize a load in case of an emergency, leading the application to a safe state, two or three relays are involved in the process. This ensures that even if two elements fail, there is still one capable of performing the safety function. The same concept applies in the opposite condition, where paralleled relays are used to ensure that a load can always be activated in case of an emergency. Like any other mechanical, electrical, or electronic device, a safety relay is not fault-free, but the probability of a dangerous failure drops drastically and can be considered close to zero. Safety Integrity Levels (SIL) or Performance Levels (PL) are the two main international rating systems used to define the reliability of a safety relay. Both classifications assign a grade to the devices and certify their abilities. The rating of a relay is determined by accredited certifying bodies through rigorous analysis and evaluations. The higher the level, the lower the probability of failure on demand (PFD).
One final requirement for ensuring the reliability of a safety relay, which falls to the end user, is maintenance and testing. SIL and PL rated relays often require periodic testing and maintenance to ensure that they continue to meet their performance requirements over time. The time period for performing such procedures is usually defined by the certifying bodies. What is known as t-proof or proof testing can be manual or automatic depending on the manufactures’ market policy, but it must be carried out according to specific procedures depending on the particular model installed in the end-users’ cabinets.
