SAFETY RULES SUMMARY
A set of safety rules was developed by Jim Rowell (http://www.mrplc.com, "Industrial Control Safety; or How to Scare the Bejesus Out of Me"). These are summarized below.
Grounding and Fuses
• Always ground power supplies and transformers.
• Ground all metal enclosures, casings, etc.
• All ground connections should be made with dedicated wires that are exposed so that their presence is obvious.
• Use fuses for all AC power lines, but not on the neutrals or grounds.
• If ground fault interrupts are used they should respond faster than the control system.
Hot vs. Neutral Wiring
• Use PNP wiring schemes for systems, especially for inputs that can initiate actions.
• Loads should be wired so that the ground/neutral is always connected, and the power is switched.
• Sourcing and sinking are often confused, so check the diagrams or look for PNP/NPN markings.
AC / DC
• Use lower voltages when possible, preferably below 50V.
• For distant switches and sensors use DC.
• Use properly rated isolation transformers and power supplies for control systems. Beware autotransformers.
• Use Positive or Force-Guided Relays and contacts can fail safely and prevent operation in the event of a failure.
• Some ’relay replacement’ devices do not adequately isolate the inputs and output and should not be used in safety critical applications.
• Use NO buttons and wiring for inputs that start processes.
• Select palm-buttons, and other startup hardware carefully to ensure that they are safety rated and will ensure that an operator is clear of the machine.
• When two-hand start buttons are used, use both the NO and NC outputs for each button. The ladder logic can then watch both for a completed actuation.
• E-stop buttons should completely halt all parts of a machine that are not needed for safety.
• E-stops should be hard-wired to kill power to electrically actuated systems.
• Use many red mushroom head E-stop buttons that are easy to reach.
• Use red non-mushroom head buttons for regular stops.
• A restart sequence should be required after a stop button is released.
• E-stop buttons should release pressure in machines to allow easy ’escape’.
• An ’extraction procedure’ should be developed so that trapped workers can be freed.
• If there are any power storage devices (such as a capacitor bank) make sure they are disabled by the E-stops.
• Use NC buttons and wiring for inputs that stop processes.
• Use guards that prevent operation when unsafe, such as door open detection.
• If the failure of a stop input could cause a catastrophic failure, add a backup.
• Wire so that the power enters at the top of a device.
• Take special care to review regulations when working with machines that are like presses or brakes.
• Check breaker ratings for overload cases and supplemental protection.
• A power disconnect should be located on or in a control cabinet.
• Wires should be grouped by the power/voltage ratings. Run separate conduits or raceways for different voltages.
• Wire insulation should be rated for the highest voltage in the cabinet.
• Use colored lights to indicate operational states. Green indicates in operation safely, red indicates problems.
• Construct cabinets to avoid contamination from materials such as oils.
• Conduits should be sealed with removable compounds if they lead to spaces at different temperatures and humidity levels.
• Position terminal strips and other components above 18" for ergonomic reasons.
• Cabinets should be protected with suitably rated fuses.
• Finger sized objects should not be able to reach any live voltages in a finished cabinet, however DMM probes should be able to measure voltages.
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