Challenges of Series Wired Safety Devices
“Pros” and “Cons” of Series Wiring vs Other Options
This blog post will talk about safety controls and safety devices as related to automated industrial machinery. I’d like to shed a bit of light on working with wired safety controls and safety circuits. Specifically, I am going to investigate the challenges and opportunities of series wiring in a safety circuit.
Machine Safety Components
Let’s say you have a machine you are working on upgrading. Or maybe you are overseeing the purchase of a machine from an OEM, and they need guidance. The machine needs a safety system and you’re considering the best way to set up the safety system architecture. Here are the requirements for safety system input:
- Safety Interlocks
- Light Curtains (maybe)
E-Stops. Of course you need emergency stop devices, E-Stops for short. Beyond required standards and guidelines (like OSHA), e-stops are used in most manufacturing environments to protect personnel.
Safety Interlocks. Some of the safety interlocks need to be guard locked, to keep personnel from opening doors with a high inertial load behind them. Locked doors allow the hazard to come to a complete stop before opening is allowed. Some applications will just need non-contact switches, as that segment of the machine will stop quickly when the door is opened.
Light Curtains. Perhaps some light curtains may be needed as well. A light curtain is a functional safety device that allows access to a hazardous area, while still protecting your personnel from the hazard. Being a single-ended OSSD device these devices are not able to be wired in series. *OSSD= Output Signal Switching Device(s)
Series versus Individual Wiring? Which one is better?
Quick answer: it depends. Each represents challenges and opportunities (“Pros” and “Cons”) when working with wired safety controls. The traditional trade-offs of series wiring versus other options in a safety circuit are as follows.
Series. Wiring everything in series will cost far less in labor and complexity due to less connections.
You will lose some diagnostic information. Sure, the machine will stop, no matter the door or E-Stop that is triggered, but the operators may struggle to find the issue that caused the stop. Especially if you have many safety inputs. Any locking devices will have to share a common lock signal, so they are either ALL open or ALL closed. No mixing and matching is possible, and that may not be safe. You cannot series wire the OSSD devices, like the light curtains (OSSD= Output Signal Switching Device).
- Individually. Wiring to each device individually to obtain diagnostic info will take up a lot of expensive safety IO and more time/labor to wire.
- Hybrid. Hybrid of Series and Individually. It IS possible to wire the non-OSSD devices in series and have auxiliaries on each device. The auxiliaries will allow diagnostics, and don’t use up valuable safety IO, but they still are added IO points to connect none-the-less. Additionally, this doesn’t solve the locking switch dilemma. You want to be able to control those individually, not as a group.
And there is another consideration. If your risk assessment indicates a high level of risk, you cannot wire traditional safety devices in series AND reach the highest Safety Performance Level. This is due to the possibility of Fault Masking, covered in ISO 14119 (a subject for another day).
GuardLink Technology for Safety Control
Do you feel a headache coming on yet? Many users wonder “How do I handle all this?” We’ll need to design a functionally safe circuit, tailored to the application needs, yet optimize the cost within my budget.
“IF ONLY there was some sort of cost-effective safety network where I could run one line and T-off to each safety device. Where I could communicate to each device individually to find out status AND send signals to individual locking devices.”
You want safer, smarter operations and we can help you with that.
We will get rid of that headache with GuardLink. GuardLink™ technology is a safety-based communications protocol from Rockwell Automation / Allen-Bradley® that enhances safety and increases efficiency.
Applications wise, on your factory floor – What can it do?
Yes, you can run a single inexpensive trunk line of 4 conductor patch cables from the GuardLink safety relay
Yes, you can T-off to each device using a ‘Smart T’
- All connections are quick disconnects; M12, 4- or 8-pin
- The ‘Smart T’ will communicate device status back to your controls, if you desire
- The ‘Smart T’ also has visual LED status indications
- One indicating the system is OK
- One indicating the individual device is OK
Yes, you can send locking and unlocking signals to any individual device and confirm its status to your PLC
Yes, you can even incorporate OSSD type devices such as light curtains
Want to learn more? Read the virtual brochure for GuardLink here
How Will Safety Improve Your Efficiency?
GuardLink technology can be used with any Rockwell safety device. From electro-mechanical devices like a key style door interlock to the new OSSD style cable pull switch. Just choose the ‘Safety T’ to match the type of device.
Allen-Bradley® Guardmaster® smart safety devices that feature GuardLink technology deliver information, advanced functionality, and flexibility. This technology helps enhance safety and increase machine- and plant-wide efficiency.
If you need to stay within your budget and be confident your employees are safe, check out GuardLink technology. The solution is simple and elegant, yet extremely functional. There is an entire team of qualified safety specialists at Horizon Solutions to help make your next GuardLink safety project a success. Contact us today with your wired safety controls questions.
What machine safety wiring challenges do you face? Are you using individual wiring or series wiring? Let us know in the comments section.