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Duffanator

Cutting Machine Power with Safeties

9 posts in this topic

Hey all, I recently rebuilt a rollpack packaging machine for one of our plants and one of the changes I made to it was to cut the power to the heating plates (there are 4 heating plates that are fed 480 volts to heat) whenever a safety is not made. I do this two ways, one is hardwired through contactors and the other is software, I stop firing the SSR's that feed the heating plates. All of our new machines do this so I'm assuming it's some kind of standard since you can access the heating plates once the safety covers are removed and there could possibly be 480 volts there. Well, they don't like it. They claim that they are loosing a lot of production time waiting for the heating heads to heat back up (why are they pulling safeties off that much and why for that long? I don't know, I already asked that question). They want me to change it back to the way it was but that would involve a major rewire of the machine controls as well as program changes. Not to mention that it's unsafe, potentally exposing maintenance and production to 480 volts. Are there written codes or anything for this? I could not find any but I'm also not sure where to look. I'd love to show them codes and say too bad, live with it since it's probably their fault they are loosing production time. Especially since I did the rebuild, I don't need someone getting lit up and then it coming back on me. Any information would be much appreciated! Thank you!

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OH NO you have agitated the Production God - you are actually making them wait on heaters heating up. You did get paid I hope Not sure what you mean by safeties OVERTEMP prevents burnout of heaters or destruction of machine components - may require manual reset OPERATIONAL HI LIMIT turns off heaters because desired temp is met will more than likely have auto reset OVERCURRENT trips heater control circuit because of malfunction - more than likely manual reset INTERLOCKS may be required as part of machine setup ie no heater til you have water in tank ACCESS interlocks may be required to keep people from zapping when they gotta get in there to tweak E STOPS darn good idea in general After 20 years in safety I am darn sick n tired of people having to defend good practice and good machine design with a bunch of codes. Let them find the codes that say the above are not needed. Until then tell em your design stands and if they tinker with it all warranties are null n void and they own all liability and dont call you. Dan Bentler A

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1. What is the risk from the 480V? Are the heaters removable and on plug/sockets? 2. The risk from the heat itself is there whether the heaters stay on or not. The heaters will start to cool but still be hot. You can cover for 1 with a label and instruction to not disconnect with power on, or use an interlocked plug/socket. The heat risk is not your problem. Additionally: A. SSR's might not meet the requirement for safe isolation/cut-off. B. If you put this one back they'll want the new ones changing as well! Ask the manufacturer why they do it this way. Finally: written codes....dunno!

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Yeah, I know. I don't trust them for that either. I feed the main power through a contactor to cut off the power to the heaters then from there I distribute it to fuses to protect each heater and then through the solid state relays. So both the contactor and the SSR's will shut off. Next thing you know they will want the machine to keep running if you hit an e-stop because they are loosing production!

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You did the right thing. Tell them you will not change it unless those exposed connections are modified so that they cannot cause a shock hazard. There are some situations in which following code to the letter can cause more safety issues than it solves by creating production jams etc, but in your case, you did the right thing. I am not a code expert but I am sure someone here will reply with specific code examples that you can use to support your decision. Paul

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Exposed live parts, you might dig around in NFPA 70e. You might also look at ANSI B11.19, This standard provides performance requirements for the design, construction, installation, operation and maintenance of the safeguarding (guards, safeguarding devices, awareness devices, safeguarding methods, safe work procedures). This standard does not provide the requirements for the selection of the safeguarding for a particular application, though there is guidance provided. Or even the OSHA website. Edited by robh

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I would. E-Stops are only required for presses (part of the OSHA regulations). For everything else, they are optional. If you put in an E-Stop, there are very tight and specific requirements about how to implement it, specifically in NFPA 79. Also the risk assessment codes (such as ANSI R15.06) determine whether an E-Stop can be implemented as a simple control relay or PLC logic, or has to be something more exotic involving safety relays. See, when you do a risk assessment, you go through the equipment, the operating procedures, maintenance procedures, and so on, and identify every conceivable scenario where an accident can happen. You look at the accident severity (how badly can someone get hurt), the avoidance (can someone get out of the way easily), and the frequency (how often could the scenario occur). These factors determine how risky the situation is. If the risk is great enough to require some sort of engineered solution, the safety procedure determines how to implement a safety system to prevent something from happening. An E-Stop is almost never useful in reality as a safety device. In non-stress situations, humans are reliable about 90% of the time. So if a safety system relies on a human being, in a non-stress situation, 90% of the time they will do the right thing. Outside of those parameters, your success odds rapidly drop to nothing. Since an E-Stop by nature is only designed to be used in an emergency (stressful) situation, by nature it is next to useless. It is also human nature that the natural human response to a catastrophic emergency is to either flee or freeze, which is not conducive to hitting an E-stop button. On a very generous day, I might grant you a 10% chance of success. I have spent far too many meetings discussing why an operator was being entirely too human in a bad situation and didn't press an E-Stop, and whether they should be disciplined for it. Thus, risk assessments usually kick the E-Stops out because they are completely useless in any scenario with any reasonable amount of risk involved. I automatically don't put them in because they are also a source of all kinds of operator shenanigans (malicious or not), the associated circuitry is notorious for down time, and they usually get used for lots of things that are not emergency stopping scenarios. E-Stops are usually the result of failing to do a proper risk assessment in the first place..."well, for anything we haven't thought of, we'll put in an E-Stop!" In reality, if you can't determine what the impending doom factor is, the proper device would be a control reliable system that would rapidly detect impending, unforseeable doom and eliminate anyone in the area from getting into harm's way. I suggest that the only device capable of achieving that is a box of donuts strategically placed in the break room!

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Well written post Paul as always. But I must strenuously disagree with one statment. STDs {Strategically Placed Doughnuts} have proved to be an ineffective safety device in our facility based on past data. STPs {Strategically Placed Pizzas} and STHWs {Strategically Placed Hot Wings} work far better.

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There are some points by handling the cutting machine

  • Always wear eye protection.
  • Always be sure that the pilot drill extends beyond the cutting edge of the saw by at least 1/8″.
  • Be sure to secure the material to be cut to keep it from spinning or slipping.

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