SLC Power Loss Question

Morning everyone,

I had a question regarding power loss in a SLC. I have a SLC 5/05 attached to a UPS, so during power loss we can run on backup power. However, in the case of the power outage being longer than expected, the UPS may not be sufficient to keep the SLC running.

My question is, what happens to the process? In terms of outputs that is. I am using Output Relay Cards (1746-OX8) and 1746-NOI8's.

1. Do the outputs drop out for the relay cards?
2. I assume there is no way to keep these energized during a power loss if this is the case.

Thanks!

Sounds like you need a bigger UPS or a generator... If it's absolutely critical that those items remain energized during a power failure, you need to find a way to prevent the power failure.

I just did an application where I did use a UPS to keep the SLC running, I also have to keep one of the input cards on...

Your outputs are not going to have power unless you wired up the UPS to them??

I did not need to run anything at a power loss... just keep the cycle running, so when power was resumed I would restart the cycle where we left off, this worked great also was better from a safety stand point as now we know how the machine will restart

Depending on the application you could have a small generator to keep you outputs on??

See here for my application http://www.plctalk.net/qanda/showthread.php?t=43235

Mark

No power no outputs will remain energized.
The only thing you can do is to keep memory of the outputs when the UPS kicks in and set the outputs high so that when power is restored your outputs will turn back on. That is about the best you can do in this situation. I guess why do you want output contacts to remain on if there is no power to your system?

Its more of a process that is running, but since we lost power, all the contacts dropped out which caused the process devices to stop (blowers/pumps/etc) so I mean its more of a case to attempt to re-start the process when we get power back as opposed to keeping the contacts on.

How would I go about storing the outputs in memory?

There are lots of ways. One way is to take a snapshot of your output card file to internal storage registers while running on UPS power. Then when you re-power up (after the UPS has run out of power) use the First Scan bit and UPS on (was running) to re-energize the state of the outputs that were turned on.

That is a good idea. I appreciate your help! Thanks.

You can also change your output instructions to OTLs/OTUs from OTEs. An OTL in conjunction with an OTU will maintain their state through a power cycle. OTEs in a seal-in-circuit will be reset upon a power cycle.

Seal-in-circuit =

|                            |

|-[Start]---|---------(Run)--|

|           |                |

|--[Run]----|                |

|                            |

I would approach this very cautiously - you really need to think about the safety aspects of what you want to do. In most systems, dropping out power puts the system in a idle state.

You need to consider the e-stop system of what you have. Most e-stops drop out power to the master relay. In you way, when you reset, will everything just start running like it never happened? What happens when someone locks out a machine, on unlock will everything just start back up in auto like it was never locked out?

I would really look at the risk assessment and ramifications if you keep everything running when power is restored. This isn't something that should be attempted without covering all bases and understanding what is going on. If someone gets hurt because the way you programmed everything to run on a power restore, you could be liable. It might sound great that the process just starts back running like the power never went off, but I think the operators should re-initialize the process.