Using E-Stops Over Longer Spans
We all know emergency stops or e-stops on automated machinery are a requirement. But what is the best way to apply and position them, especially for long spans?
ISO 13850 Standard
The ISO 13850 standard is a great resource in understanding the correct application of E-Stop devices. However, spacing between e-stop buttons on a piece of equipment is not defined. It’s up to your design team to determine the risks and the position of the buttons. For a longer piece of equipment how far is appropriate?
The related passage from the standard below:
What to do about e-stops on a long piece of equipment?
We know you must do a risk assessment. This is a VERY important step that I’ve discussed in other posts. How do you know what to do if you don’t have a full understanding of the risks, right? So, take the time and do it correctly.
Let’s assume you have done a risk assessment and are dealing with a long conveyor. Because employees are loading and unloading material along the whole length, every point along one edge of that conveyor is a potential risk in your application. You cannot guard that edge.
Cable Pull Option
The best E-Stop device, in this case, is a pull cable attached to a safety switch. It’s likely you have seen these if you have spent much time in industrial facilities. If there is a safety demand, a person can yank on the cable that will deflect the safety switch it is connected to and stop the machine. They are a solution that is offered in section 4.5 of the ISO 13850 standard.
Cable Pull Switch Challenges
In my experience, however, traditional cable pull e-stops are not always popular. Why?
- They can be challenging to set up.
- If the environmental temperature changes, causing a tension change, they can nuisance trip.
- If they are accidentally bumped while loading and unloading the conveyor they may trip also.
As far as setup, traditional devices are simple dry contact safety switches that require a couple of people to set the spring tension just right. If the tension is too strong or slack, it will trip. And with a longer line, you’ll need two people a fair distance apart; one person at the tensioner and one person at the switch, trying to communicate effectively.
Any time you have a safety device that causes heartache for maintenance personnel or operators, there is a higher probability it’s going to be defeated, so you want to choose the right device.
Fear NOT, there is a solution in the latest generation of cable pull switches. The newer Allen-Bradley® Lifeline™ 5 series from Rockwell Automation® solves the above problems and adds information gathering capability
- No more nuisance trips.
- It’s an intelligent solid-state device, using a load switch to determine tension level.
- It only trips with fast changes in the tension created by a yank on the cable
- It will not trip for slow tension change (e.g., if leaning into the cable or a temperature change induced tension change).
- One person can set it up.
- It has bright LEDs that can be seen from a long distance. One person can tension the line from a distance and watch the LEDs flash to indicate when it’s set correctly.
- Versions for corrosive areas.
- A stainless-steel body option is available.
- Know it’s going the trip BEFORE it shuts the machine down.
- Tension status can be seen on the LED and collected over the wire to a PLC.
- Trip status can also be seen on an LED or over the wire.
We Can Help!
E-stops, switches, and everything in between, we’re here to help. Our machine safety and automation specialists can help you address challenges and offer expert advice. Contact us today!