Safety circuit design is the engineering process of creating electrical circuits designed to safeguard people and equipment. These circuits use safety switches, relays, and other technology to detect dangerous situations – such as open safety guards or unexpected obstacles. When a hazard is detected, the circuit triggers protective actions like shutting down machinery or isolating electrical power to prevent accidents and injuries.

When it comes to designing these safety circuits, there are a few lesser-known aspects that deserve attention to create a truly robust safety system. Here are five tips you can use to optimize your safety circuit design.

1. Know Your Resets

When deciding between automatic and manual resets, you’ll need to consider a few factors. These include the severity of the hazard, likelihood of re-injury, and the complexity of the restart process. From there, you’ll be able to select the most suitable reset mechanism for each safety circuit, optimizing safety and production efficiency.

Automatic Reset

In an automatic reset, the safety circuit automatically resets itself once the unsafe condition is rectified. The benefit here is that it allows for a quicker restart of the machinery. This is best for low-risk scenarios where the hazard clears quickly and there is minimal risk of re-injury. For example, a safety gate on a conveyor belt might trigger an automatic reset once the gate is closed.

Automatic resets can also work for situations where a temporary stoppage is needed, such as clearing debris from a machine. Once the debris is removed, the circuit resets and operations continue.

Manual Reset

Manual resets require deliberate action, usually by pressing a button at the machine control panel to reset after a safety top. This reset type is good for machinery with high-risk potential because it allows a thorough investigation of the cause of the safety stop before restarting. For example, machines with pinch points or high-speed cutting operations.

Manual resets are also appropriate for processes requiring specific actions beyond simply closing a safety gate. An example here would be a manual reset on a large press might require resetting dies or clearing jams before restarting safely.

Monitored Manual Reset:

Monitored manual resets combine aspects of automatic and manual resets by requiring manual action to reset the circuit and incorporating a timer function. The reset must be pressed and released between 0.25 - 3 seconds. This stops operators from bypassing the safety system by keeping the reset button pressed.

This reset type is ideal for scenarios where a manual reset is preferred for investigative purposes, but you don’t need the machine to be inoperable for an extended period. For example, a machine with a complex restart procedure. The timer ensures the machine is restarted within a reasonable timeframe after the safety stop is addressed.

2. Mitigate the Domino Effect of Cascading Faults

Cascading faults occur in safety circuits when a malfunction in one component triggers a series of failures in other parts of the circuit. This domino effect can render safety measures inoperable and leave machinery in an unsafe state.

In short, one point of failure can compromise the entire safety system, leaving employees at risk of injury, possibly damaging your machinery, and impacting product quality.

Here’s an example scenario of a cascading fault:

A relay responsible for monitoring air pressure in the sealing mechanism malfunctions and sticks in the "on" position.
Since the relay thinks there's enough air pressure (when there isn't), the sealing arm descends to close on the box.
However, due to insufficient air pressure, the seal doesn't form properly. This creates a leak, causing a pressure sensor downstream to detect an anomaly.
The pressure sensor, designed to stop the machine if pressure drops below a safe limit, triggers a safety stop. However, due to the faulty relay still stuck "on," the shutoff valve for the air supply doesn't close.
With the air supply still on and the safety stop not functioning correctly, the situation worsens. Pressure continues to build in the malfunctioning sealing mechanism, potentially leading to a component rupture or creating a dangerous situation for an operator nearby.

How do you prevent this? Implementing redundancy and/or isolation techniques will help you design safety circuits that are more resistant to cascading faults, ultimately enhancing the overall safety and reliability of your machinery.

Redundancy

Redundancy involves incorporating duplicate critical components within the safety circuit – like having a backup system in place. If one component fails, the redundant one takes over, keeping the safety function operational. There are two main types of redundancy:

Hardware Redundancy: This involves duplicating safety switches, relays, or contactors within the circuit. For example, a safety gate might have two separate switches wired in series. If one switch fails, the other can still trigger a safety stop.
Logic Redundancy: This technique utilizes programmable logic controllers (PLCs) to implement voting systems. Multiple sensors feed data into the PLC, which analyzes the inputs and triggers a safety stop only if a pre-determined number of sensors (e.g., 2 out of 3) detect an unsafe condition. This helps to eliminate false positives caused by a single faulty sensor.

Isolation Techniques

Isolation focuses on electrically separating different parts of the safety circuit. This prevents a fault in one section from propagating to other parts and compromising the entire safety system. Here are two common isolation methods:

Physical Isolation: This involves physically separating safety circuits from control circuits using dedicated wiring channels, transformers, or optical couplers. This creates a physical barrier that prevents electrical faults from jumping between circuits.
Functional Isolation: This approach utilizes safety relays with built-in isolation features. These relays have separate power supplies and contact sets for the safety circuit and the control circuit. Even if a fault occurs in the control circuit, it won't affect the power supply or operation of the safety circuit.

Choosing the Right Technique

The selection of redundancy or isolation techniques depends on several factors:

Complexity of the safety circuit: More complex circuits might benefit from a combination of both techniques.
Cost-effectiveness: Redundancy can be more expensive due to the additional components required. Just remember that safety is an investment, and you can’t put a price on employee safety.
Criticality of the machinery: High-risk machinery warrants a more robust approach, potentially involving both redundancy and isolation.

By implementing techniques like redundancy or isolating different parts of the circuit, you can mitigate the risk of cascading faults and ensure a single point of failure doesn't compromise the entire safety system.

3. Keep Your Guard Up with Testing and Maintenance

Safety circuits aren’t a set it and forget it thing – they require ongoing review. Regularly test and maintain your circuits to make sure they’re functioning as they should. This includes following manufacturer recommendations for testing safety switches, relays, and other components at designated intervals. A proactive approach to maintenance will also help you identify and address potential issues before they become safety hazards.

4. Remember, Not All Components Are Created Equal

You want to avoid using generic electrical components for your safety circuits and prioritize safety-related components designed specifically for your systems. These components undergo rigorous testing to ensure they meet strict performance standards, boosting reliable operation in safety-critical situations.

5. Don’t Discount Human Error

Even the most meticulously designed safety circuit can be compromised by human error. To address this, consider incorporating features like tamper-proof switches that discourage bypassing safety measures. Additionally, clear indicator lights can enhance operator awareness of the safety circuit's status, promoting a culture of safety within your facility.

We Can Help

By understanding these often-overlooked aspects of safety circuit design, industrial safety professionals can create more dependable and effective safety systems. This translates to a safer work environment, reduced downtime due to accidents, and ultimately, a more productive and profitable operation. Start with a safety assessment to identify any safety gaps. Contact the experts at Rexel today to get started.

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5 Tips to Optimize Safety Circuit Design in Industrial Manufacturing

6/12/24 | Dave Cucerzan, Senior Industrial Automation Solutions Specialist