We lost power to the whole plant on Tuesday, about 20 minutes. When it came back the sump pump in the pit under press 2 ran on its own. No start from the HMI and nobody was down in the pit.
Controller's a 1769 L30ER with a PF525 on the pump. Start is an OTL on a run bit from the HMI start button, stop is an OTU from the stop button and from the high current alarm.
I checked the HMI log first and there's no start press anywhere after the restart, so then I pulled the drive fault log and that's empty as well, although the drive reckons it got a run command from the PLC.
Has the L30 got a battery in it that could be flat, or is the HMI firing a start when it reconnects? I've got to tell the safety guy on Monday why a pump started on its own, so any ideas which of the two it is?
Was the pump running when the power went off? If it was, the run bit was on, and an OTL bit keeps its value through a power cycle on Logix. No battery in a 1769 L30ER, the program and the tag values come back exactly as they were.
It was running, yes. It had just been started to empty the pit before the outage. So the bit sat there through 20 minutes of no power and the drive got a run command the second it had power back. We spent an hour looking at the HMI for a start that never happened.
That pit has a ladder down into it. If a person can be down there, a pump restarting on power return isn't a programming quirk, it's a hazard, and the risk assessment for that pit should say so. Whatever you do in the PLC, the pump has to need a fresh start after power comes back, and I'd want that in the contactor circuit too, start button and a hold-in contact, not just a bit in a processor.
Ron's right about the pit. The tag values are all still there after an outage, the difference is what the instruction does with them on the way back in. Logix prescans before the first real scan and every OTE gets its destination cleared. OTL is left alone, which is the whole point of it. So a seal-in does what you want: Start OR Run in parallel, Stop in series, OTE on Run. Prescan clears Run, first scan has no Start, the pump waits for a person. Keep the latches if you'd rather and clear them on S:FS in a rung of OTUs. Same in TIA with S and R.
I'd take the OTL out. There's a writeup with the rung in it: https://plctr.com/introduction-to-plc-programming-for-beginners/
Changed it to the seal-in yesterday, Start OR Run, Stop and the current alarm in series, OTE on Run. Killed the panel power with the pump running, brought it back, pump stayed off. Three times.
The OTL held the run bit right through the outage, so the drive had a run command waiting the moment the L30 came back. The seal-in rung with an OTE is what stopped it. Our controls guy is adding the hold-in contact on the contactor like Ron said, and the pit's going on the risk assessment.
One thing we turned up on the way: the PF525 had its own restart after power loss parameter switched on as well. Not sure whether that alone would have started it with the bit off. Thanks jcasey and ronh55.
Marking solved. Pit access belongs in Safety PLC if it goes further.
plctr.com team