Machine safety design: Relays versus a single safety controller

There comes a point in machine safety design when the designer needs to decide if it makes more sense to use multiple safety relays or to upgrade to a safety controller. This point typically occurs when the application requires three or more safety relays.

There are three key influencing factors that will impact this decision for the designer: basic hardware design, safety controller configuration software and cost implications.

In terms of functionality and safety I/O density, a safety controller solution is a step above a safety relay solution and a step below a safety PLC. Safety controllers must typically be configured using software. The software allows for simple-to-implement safety circuit flexibility and functionality.

Basic hardware design

One of the key influencing factors that will impact whether a designer selects multiple safety relays or a safety controller is hardware design. This includes:

• Load switching: Most safety relay designs have traditionally used safety relays constructed with internal electromechanical, force-guided relays. The key advantage here is voltage flexibility, as nearly any typical control voltage can be switched. As an example, the nominal current at 24 VDC is about six amps for a resistive-based load. Contrast this with a safety controller with typically solid-state outputs that is only designed to switch 24 VDC at a typical maximum of two amps.

• Modularity: Safety controllers are typically modular, so safety I/O can be added. If safety controller expansion modules are needed, these costs will be an important consideration.

• Dimensions: Width and space are also important factors if DIN-rail space is valued. Safety relay width is directly proportional to the number of safety output contacts and overall functionality.

• The number of safety I/O: Safety controllers carry a bigger number of available safety I/O in a relatively compact housing compared to a standalone safety relay. This contrast in size becomes very apparent when wiring for Safety Category 4 or Performance level “e”.

Safety controller configuration software

For a designer accustomed to using safety relays, a safety controller’s configuration software is one of the main intimidating unknowns. Designers may worry about the required programming expertise level and additional costs. In this writer’s opinion, the configuration software can be a positive aspect. It increases flexibility and offers advanced functionality that a multiple safety relay circuit cannot provide. The software includes:

• Drag-and-drop menus: Today’s safety controller software is designed with “drag-and-drop” functionality with “non-programmers” in mind to simplify the design process.

• Safety project simulation: Another powerful aspect of the safety controller is the availability of a “Safety Simulation Mode.” This mode is perfect for designing a safety project at your desk and going into simulation mode to validate your safety project before going onsite to download it to the safety controller itself. Simulation mode is a perfect way to begin your evaluation of a safety controller. It’s a no risk “test drive” of the fully functional software without the need of any hardware attached.

• Monitoring and diagnostics: Next, consider the diagnostics ability of the software. It will save time during startup and the troubleshooting of safety circuit failures. Troubleshooting a discrete safety relay circuit can be a daunting task due to all of the wiring and the variety of safety relays being used. With a safety controller, you can connect the software via the USB, upload the project and go online. At this point, the software will indicate the reason the circuit failed to activate.

• Documentation: Safety documentation can be a very important part of the machine delivery package. Software configuration packages have a predetermined offering where you can easily document key details pertaining to the safety program.

Cost implications

Let’s do a cost comparison exercise. For this cost comparison, we will use a baseline of safety relays with basic functionality. This writer assumes that a fair price for a safety relay with dual input channels and two normally open safety outputs is around $220. This writer will also say that a fairly priced safety controller lists for around $800.

Safety controller software

The next extremely important consideration is the potential cost of safety controller configuration software charges. Pricing for a safety controller configuration software package can range from a free download to around $1,800. This writer recommends a free downloadable configuration software package with no licensing fees and a simulation mode. Note: Since free options are available, we will not include configuration software in the pricing examples.

Pricing example A – Analysis

With three standard safety relays at $220 each, the total cost is $660. The safety controller is $800. The safety relay solution costs $140 less, and space is a trade-off. Since the price of safety relays is directly proportional to the number of safety contacts and to functionality, the safety relays would have to average more than $266 each before the designer saw a cost advantage. However, safety controller flexibility, the ability to stock only one part number and other benefits might be attractive enough to make the safety platform change.

Pricing example B – Analysis

With five standard safety relays at $220 each, the total cost is $1,100. The safety controller is $800. The safety relay solution is $300 (38 per cent) more expensive and uses 67 per cent more space on the DIN rail. If six safety outputs on a safety controller are enough, then it’s no question the safety controller is a viable option at this point.

Conclusion

When an application requires at least three medium-priced safety relays, it makes economic sense to consider moving to a safety controller platform. The more safety relays that are involved, the more this solution makes sense. If a platform change is decided, the next step is to evaluate the safety controller technology to determine if it fits the specific application needs. The designer should also consider functional flexibility, stocking a single part number and the monitoring/diagnostic power.

Kian Sanjari, P.Eng., is product marketing manager for I/O & Networking for Phoenix Contact. He can be reached at ksanjari@phoenixcontact.ca.

This column originally appeared in the November/December 2013 issue of Manufacturing AUTOMATION.