Why use hardwired independent & and redundant localized control?

Back in the time, maybe beyond the era of the 80s when hardwired alarms were used for critical applications. Soon after the introduction of modern PLC and DCS, and implementation of IEC 61511 worldwide, the users had a choice of software-driven alarms configured from the thousands of analog inputs from the plant. The old rusty blind mechanical switches without diagnostics and display were passe though they had the advantage of a faster response time but the end users chose smart pressure transmitters over switches for the advantages of the transmitters known to everyone. Though smart transmitters' response time was very slow compared to the snap-acting switches, the users didn't want to compromise on the diagnostics and the data available from the field. Going forward, the smart transmitters designed to monitor the process were used in SIS with leading companies offering SIL 2 certification.

While the switches have faster response and low cost of installation, it can succumb to high vibration, process hunting, and spikes. The fast-responding switches can even respond to a spike and changeover. The smart transmitters designed for monitoring the process with typical response time of 100 msec (T63) added with a delay of analog input scan time in the CPU in the host can miss to register a spike and the trending in the host would still look a smooth curve.

United Electric came up with the world's first-ever 2-wire loop-powered transmitter with a safety relay output (SRO) with the industry's best SFF of 98.8% and the instrument serves the purpose of both. While the SRO helps the fast and localized control, the 4-20 mA can be used to compare the SRO action in the host.

At times, the users use SIL 2 trip amplifiers with SIL 2 pressure transmitters to create an independent shutdown system in remote locations.

The hardwire localized control can still be redudant to software driven controls. Just imagine a situation when the software gets corrupt and the software driven controls fail, the hardwired controls indepdent to the control system can be still a good redudant solution.

It is important to combine the technology to avoid common mode failures. What if the smart transmitters used in 2oo3 voting logic for an SIS, have an issue with the conformal coating on their motherboard and if all three transmitters were manufactured in the same batch and start drifting at the same time resulting into a false trip causing a huge production loss.

Some processes change so fast that the soft alarm-driven systems are unmatched to cope with the required response time. Such a situation demands a redundant hardwired control as it is localized and faster in controlling the abnormal process conditions.

The above all reason is the cost of hardware comes down with localized control philosophy. You may need only to run the signal cables up to the control room. While the hardwired control is directly from the device level to final control element, saving the huge costs of laying cables.