How not to is a series dedicated to learning from the mistakes of others. While learning from your own mistakes is generally more memorable, learning from others is (1) safer, (2) a better career move and (3) more pleasant.
We are going to cover a crucial topic: what should be in your local controls? Basically, you want anything that is always required to be in your system. Things you never want bypassed.
Example 1: A co-worker of mine told me about a system he dealt with recently. He went to turn off the gear for preventative maintenance (a 'PM'). As per that PM, you test the E-stop by hitting it while running. It did not come off. He immediately cut power and started poking around. Indeed, a single wire came loose. The PLC never picked it up, as the E-stop circuit was never engaged. That was easily a near miss.
Emergency Stops (E-stop) should always be in your system. Whether you are in auto or manual, you do not want to be able to bypass them. Any sort of E-stop should be wired fail-safe, so any wire coming loose will shut the device down. Typically, this means you could have each as a normally closed contact that is engaged by opening it. The status coming to your PLC is far less important than your gear deactivating and protecting your team!
Example 2: I was called in to look at some PLC code for a pump station using positive displacement pumps. When I went online, I saw each and every single high discharge pressure alarm was forced off, bypassing each in the PLC. They were also physically disconnected, leading to a fairly unpleasant discussion between the superintendent and the lead maintenance manager.
Positive displacement (PD) pumps are excellent opportunities for a novice designer (or programmer) to cause issues. PD pumps work by deforming the cavity a liquid is held in. Often, this is done via a piston.
An issue with PD pumps comes in when a clog occurs. You can quickly have the outlet pressure exceed the design specifications of the piping down the line. If you have a PD pump, make sure you have a pressure sensor in the outlet line and that it is hard-wired into the local motor controls. I say hard-wired to the local controls so that your system will not be bypassed by a programmer who decides to force that input "because we really need the pumps right now."
If someone goes through the effort of manually bypassing a high-pressure switch, you can still have the issue... but you should make sure the site managers are aware of this immediately. This is an easy way to damage a lot of equipment or injure someone.
Learn from every mistake you can. Be safe!
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