PLC/HMI What is the norm? Alarm controls.

I’m trying to perfect the HMI interface for alarm control. I’ve provided the user alarm options for all analog inputs, as follows.

• HH Boolean alarm, HH time delay
• H Boolean alarm, H time delay
• L Boolean alarm, L time delay
• LL Boolean alarm, LL time delay

I’m having an issues with the analog signal that reaches 0 or goes below (-3.8mA signal). So what would be the recommended normal approach to deal with 0 values?

• Add Boolean button to bypass L and LL if it’s not needed? Eg. Flow meter.
• Add a silly small number to everything so it’s never zero? N + 0.0001?
• Put ‘999’ as the time delay and use that number as a flag for alarm bypass?
• What would the user prefer?

I’m trying to keep the interface generic but hopefully still useable - to expedite deployment by developing a generic template.
Best regards,

IMHO, you should have an enable for each of these alarms. Not all situations require all alarms. For some values, they may want high high only, for other Low low only.
Then the Client, can enable or disable as needed. Plus, I would also add a reset value. If HH alarm is 100 and reset is 2, alarm is active until value is less than 98.

If you are not going to latch the alarms you will need a deadband for them to automatically reset.

I never use HiHi or LoLo, just Hi and Lo. If the operator is going to ignore the Hi alarm he will ignore the HiHi too. I do distinguish Hi and Lo information alarms from Hi and Lo alarms that will cause a shutdown or other process change and give them separate identifiers and limits.

You can have muting functions for alarms, but in principle the alarms should not be muted.
To discourage misuse of the muting functions we add a warning triangle on the HMI so that everyone can see that a muting function has been enabled. Also, the muting of alarms require a supervisor access level, and every change is logged.

That a sensor goes below its nominal range may have many causes. It can be that the designer didnt account for the correct measuring range, or it can be that the sensor is defective, or it can be a problem in the process.
If a measured value is critical, it may not be enough to consider the <4mA situation. Sometimes you make much more complicate discrepancy checks where the sensor value is checked against other sensors, or against certain steps of a process (i.e. if heating has been turned on but no temperature increase can be detected).

I'd add that along with the delay time you'd probably want a gate as well? If the pump feeding the flowmeter is turned off, the flow will be zero but it's not an alarm.

Do use two bits for this, one that specifically does this and another that selects whether the alarm is to be considered.