eBook: Automating Manufacturing Systems; with PLCs



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.
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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