Think twice about bolt-on, safety control system additions

Rich Gibson
Certified Machinery Safety Expert (TÜV NORD)
ABB Jokab Safety Products

When faced with a new or changed machine-safety risk, bolt-on solutions may seem like the easiest, cheapest approach. In most cases, it makes sense to consider a safety PLC, which can provide a far superior, lower-cost solution.

Back in 70s, young guys often “customized” their cars with a variety of bolt-on accessories. These hot- rodder wannabees screwed mud flaps, hood scoops, curb feelers, and more onto their cars. They may have looked cool for a while; but in the long run, most of these bolt-on additions did more harm than good.The same is true for control panels. “Bolt-on solutions,” where people meet a new system requirement by attaching an additional piece of hardware or patch the software, usually deliver mixed results. Bolt-ons are typically fast and easy to install and usually meet the immediate need. But they often don’t offer the best long-term outcome. They seldom integrate fully with the existing systems fully, limiting functionality and effectiveness. And while they almost always cost less in the short run, actual lifecycle costs are usually higher.

“A great example is the recent call I got from a distributor who wanted to add a light-curtain muting controller on a production conveyor,” says Rich Gibson, marketing manager for ABB’s Jokab safety products. “This control allows product to pass through the curtain, while an arm or tool would create a trip. My response to him was, ‘Why would you want to add another control device when you already have the capability in your existing control panel?’ Bolting

on the necessary additional devices would have solved his problem, but I recommended a far better solution: the central, intelligent controller.”

The bad, old days of hardware-based safety systems

In pre-digital days, every point of protection started with a sensor, whether a light beam, door switch, e-stop, or whatever. Between that sensor and the panel, was a dedicated controller mated to that sensor. The controller for a door stop was different than the controller for light beam. You also needed one or more relays. All these devices had to be hardwired together.

It was a classic case of the law of diminishing returns. As they addressed new risks by adding more hardware, they reduced overall system reliability. Each device and every connection created new, additional, potential points of failure.


It’s hard to overstate the benefits people are realizing as they transition to digital safety PLCs for machine- safety applications. The controllers are largely device agnostic, making it possible to directly connect most safety devices and sensors. Both for panel builders and owners, that simplifies systems and reduces the parts inventory.

All these advantages come with a price tag that’s typically smaller – significantly smaller – than add-on solutions. The cost of a programmable controller in any multi-safety-function panel is more than offset by the savings from eliminating dedicated controllers and relays. In applications with a simple door stop and estop, a digital controller might be overkill. But, as the number of safety functions grows, the cost benefits of digital technology quickly make it the better choice. Savings in the vicinity of 40% are common.

All-in-one creates concerns

Some people have reasonable concerns about replac- ing multiple components with a single controller. It’s true you eliminate many potential points of failure, but instead you have all your safety-control-system eggs in one metaphorical basket. But offsetting that concern is the ease of troubleshooting and replace- ment when there’s a controller issue.

Rather than the frustration of running down problems in individual devices, diagnosing issues of all sizes is typically facilitated by the built-in diagnostics of a programmable controller. It’s possible to plug an HMI into a safety PLC and access a trove of system infor- mation. If there is a failure, swapping out the control- ler will probably be faster than the diagnosis and re- pair of relay-logic components. Controller programming is readily transferred to the replace- ment part.

Get the bigger picture

“Rather than looking at safety control systems as a collection of individual safety components, take a more holistic approach,” Gibson suggests. “Start with a risk assessment of the machine or process to iden- tify all the tasks and hazards. This should include in- put from everyone who interacts with the machine. In- experienced people making assessments may do a tremendous job of identifying all the potential risks faced by an operator, but fail to consider how techni- cians perform maintenance or do changeovers.”

You can typically rely on your safety-controller provid- ers for a no-cost risk-reduction evaluation. Before in- viting them in to do an assessment, though, clarify what the deliverable will be. You don’t simply want a list of risks; you want a detailed set of solutions as well.

It’s reasonable to ask for the required wiring dia- grams to implement those solutions, but that may take them beyond what they are willing to provide for free. They may also offer installation services, providing a turnkey package. That level of service will definitely incur additional costs.

To ensure the quality of their assessment, it may be worthwhile to require they be credentialed in these systems. Your local pipe and wire distribu- tor can probably provide all of the necessary com- ponents, but it’s not likely they will be able to as- sess the machine-safety issues. It will be even less likely that they will be able to guide your installa- tion and maintenance. Certified Machine Safety Expert is one designation that indicates the indi- vidual has demonstrated capabilities in this area.

Turning over safety-control-system design and in- stallation to an expert may be the easiest ap- proach, but it’s probably not the best. Keeping your experts involved in the process will help them develop their skills and knowledge. That will give them skills and knowledge to update and fur- ther evolve the system over time. The initial tran- sition from analog to digital is a fairly large step, making modifications and maintenance on the system in the future much less intimidating.

For simple safety-control systems that require the addition of a basic function like a new door switch or safety mat, bolting on the components to pro- vide that new capability may be the best way to go. In most cases, though, a programmable safety controller will deliver superior results in terms of safety, functionality, and integration with both lo- cal and plant systems. It also lays the foundation of faster, easier modifications to safety systems, and provides all these benefits at a significantly lower cost.