By Dan Bolland
The pictures below show a "fried" thermocouple input card and a GE MkV system on a critical refinery GTG that took a lightning hit. Thermocouples are a common route for lightning currents to enter DCS or other control systems. The author has been involved in five GE MkV lightning strike investigations. New installations now incorporate surge arrestors on the more sensitive circuits, as well as a better understanding of the need for bonding between the MkV control system and the GTG frame.
Multiple separate grounding systems are a common feature in lightning damage scenarios. Often there is a DCS ground, a safety ground, a lightning ground, and sometimes a computer ground. The correct answer is to have an integrated grounding scheme. Two overseas refineries modified their systems by integrating safety and DCS grounds and installing bonding between the control house and the plant. Since making these changes, there has been no lightning damage even during high storm activity. The picture on the left below shows separate instrument and safety grounds. On the right, some simple bonding is the best practice where safety and instrument grounds are bonded at one point.
Vendors of DCS and field instrumentation are often of little help when it comes to good lightning protection practice. DCS vendors want their own "clean earth" and field instrument suppliers will sell instruments with surge arrestors already fitted. Separate earthing systems lead to voltage differences during a lightning strike situation. At a large Louisiana refinery, DCS lightning damage occurred only on circuits where the latest installed field transmitters had "free" surge arrestors fitted. You have to know what you are doing with surge arrestors as they often cause lightning currents to be injected into the circuit. The transmitter may be OK, but the DCS at the other end is zapped.