Over the years, I have had many conversations about how to best handle critical alarms. Most recently, a customer from a company in West Texas experienced an oil spill where an alarm notification failed to let him know that a tank was too full and it ran over. The root cause was identified and the reason a notification wasn’t sent was due to communications failure between the HMI and PLC. In this case, the register list in the HMI was being polled by a host system. To mitigate the risk of it happening again, we implemented a heartbeat status setpoint between the HMI and PLC to notify personnel in the event of a communications failure.
Generally speaking, events are usually processed through either a SCADA host or standalone systems. SCADA hosts use existing infrastructure in conjunction with higher polling frequencies to check register statuses for changes to setpoints. Although SCADA polling is very effective, it tends to tax DC power systems used on remote EFM and PLC’s and could potentially have several points of failure in the communications architecture. Once the events are processed, the SCADA system uses another piece of software to notify personnel. SCADA hosts usually require that the user login to the server using a Citrix or VPN connection to acknowledge alarms remotely.
Critical event monitoring through standalone systems utilize cellular or satellite infrastructure. These standalone devices usually have 4-6 discrete inputs that operate independently of one another, are tied to switches that toggle the inputs, and powered by lithium batteries or a solar panel and gel cell battery. Once an input has been triggered, the device sends a message through the carrier to an awaiting data handler where the message is processed and personnel notified. This type of system allows users to acknowledge alarms using an internet connection via mobile. These systems leverage the multitude of cell towers and satellites with fewer points of failure.
There is also a scenario where the aforementioned systems are used in conjunction with one another, usually where the standalone devices are used as a backup to the SCADA system.