What is alarm sealing?

Hi Friends
Please explain the concept of alarm sealing? Why is this important? I tried looking on the web, but unfortunately couldnt find a relevant description.

Thanks

If i understand what you are getting at.

Alarm sealing in my opinion is latching the alarm bit.

The trigger for the alarm is no longer active, but the alarm stays latched on (bit is high) until an acknowledge bit is triggered.

Hi Rob
Thanks for your prompt response. Yes, your description does match the logic which states that the alarm seal becomes active when the trip interlock is unhealthy making sure operator sees the alarm and acknowledges before the valve operates again.

Like so

Depending on circumstances, the PLC may detect an alarm condition and take an action which then causes the alarm condition to no longer be true. If this happens quickly, the HMI, which is monitoring the alarm, may never see that there was a problem, and thus the operator will not understand why the action occurred.

By "sealing" in the alarm (making the alarm itself a condition for setting the alarm, as Rob showed), the alarm persists long enough for the HMI to "see" it.

Always bear in mind that most HMIs typically only read a PLC value once per second or longer. One second is an eternity to a PLC. So some events must be captured and held, in order to build a usable interface.

But if held, the question then becomes, "what releases the hold?". While 'acknowledgement' is a common method, it is not necessarily the best. And thus is the can opened, and the worms start pouring out....

there are some ideas shown in the PDF attached to the post below which might be helpful to you ...

http://www.plctalk.net/qanda/showthread.php?p=538924&postcount=5

Check this tutorial about Fault Capturing Example in Ladder Logic RSLogix 500 Allen Bradle:
https://solisplc.com/plc-alarm-prog...le-in-ladder-logic-rslogix-500-allen-bradley/

tarik1978 said:

Check this tutorial about Fault Capturing Example in Ladder Logic RSLogix 500 Allen Bradle:
https://solisplc.com/plc-alarm-prog...le-in-ladder-logic-rslogix-500-allen-bradley/

Click to expand...

Wow! Thank you for linking to my site Tarik, really appreciate it.

Great question OP. Alarm sealing is very basic in practice, the goal is to latch in the alarm which was triggered. You're accomplishing 2 things here:

1. The alarm which was triggered can be monitored and seen by the operator even after the issue has been resolved. For example, a motor which lost position may recover by itself, but with the alarm in place, it will be obvious that the event occurred. If you don't latch it in, it's nothing but a mystery stop.

2. An explicit reset of the system is required to unlatch the alarm. I've seen systems which would auto-reset and in my personal experience, this is can be a serious risk from a safety and equipment standpoint. Requiring an explicit reset means that an operator understood the alarms presented to him, acknowledged that they have been fixed and reset the machine.

Cheers,
Vlad

Thanks a lot for the valuable feedback. The concept is very clear now. Really appreciate for all the explanations.