The One Where Everyone Wins:
A Mutually Beneficial Contracting Method
As machine tools and process operations become more complex, the shortcomings of traditional safety relays become more obvious.
Shrinking budgets and growing to-do lists are all too common in the world of construction. Owners struggle to meet budgets and timelines while satisfying transparency goals. Traditional procurement models, such as price-based contracts; design, bid, and build; time and materials; and construction manager at-risk, typically don’t emphasize healthy relationships and partnerships between owners and contractors. Job Order Contracting is a great alternative that fosters mutually beneficial relationships while addressing common project challenges.
Traditional safety-control systems.
Single-Function Safety Relays
Single-function safety relays ensure proper working of the safety function, keeping both people and equipment well protected. They interface between the control and the actuating device. When you look only at the cost of safety relays, they can be a low-cost solution in limited applications, typically controlling three or fewer safety functions/devices. They are simple to operate and have a clear, predefined structure with fixed functionality.
Typical Applications for Single-Function Safety Relays
- Stopping Movement in a Controlled and Safe Manner
- Monitoring the Position of Movable Guards
- Interrupting a Closing Movement During Access
- Emergency Off/Stop
Typical Shortcomings of Single-Function Safety Relays
Difficult troubleshooting due to complex wiring between each device.
Requires significant engineering for system modifications or upgrades.
Inability to operate in mixed modes (safety and standard).
Tedious functionality changes because of each relay’s defined purpose.
Additional safety functionality requires additional safety relays.
The Logic is Done by Wiring
Despite their limitations, safety relays are widely used, are commonly built into new equipment, and could be the right choice in simple applications. This would be based on the risk assessment for the machine or application. The relays are typically connected to a standard PLC to control the safety application. It’s worth noting that the need for a safety relay is eliminated entirely when safety PLCs are used instead of safety relays.
Learn about the benefits and advantages of switching to safety PLCs.
Multi-Function Safety Relays
This solution represents a configurable device with a few more inputs and outputs. As a rule of thumb, a programmable safety relay can control between two or three safety functions, representing an incremental flexibility improvement over a single-function safety relay. Additionally, they have a smaller footprint that enables more-compact control panels or cabinets and will reduce and simplify wiring. That simplicity, though, is dependent on the safety functionality required. Multi-function safety relays are available in a number of styles and configurations.
Typical Applications for Multi-Function Safety Relays
- A simple base unit can handle a variety of inputs.
- Most require no software for configuration or operation. Typical setup and maintenance is simple (but with no flexibility).
- Multi-function safety relays are often a choice when single-function relays can’t meet the safety functionality.
- Suitable for systems with more-complex requirements such as machine tools or robots with many I/O points or a high number of safety-related tasks.