New Generation Surge Protectors Bring Benefits for I/O Systems
Vulnerability from lightning damage for measurement and control instrumentation in industries such as Water, Wastewater, Chemicals and Petro-chemicals, is a major concern to plant managers. Direct strikes, which cause damage to control systems, are relatively rare through the use of mechanical protection, such as ground rods, which act as an effective first line of defense. However, product damage and signal disturbances are much more frequently caused by transient surges as the result of remote strikes and cloud-to-cloud strikes. This problem is aggravated with the increased number of sensitive microprocessor-based electronics in field instrumentation and the growing use of fieldbus signaling. When the damage reaches through to the DCS I/O system, it can result in partial or complete plant shutdown. This makes the value of I/O surge protection of the upmost importance!
There are two primary surge formats – one with pluggable arrestors, the second in compact terminal format. The compact terminal format is ideal for process instrumentation installations. They include options and features, which improve the choice and breadth of applications and ensure high quality, ongoing protection through monitoring options and easy replacement. Following the direction of the latest IEC installation standard, both types automatically provide status indication or alarming to alert for arrestor replacement.
Progress in performance
In this latest generation of signaling surge protection devices (SPDs) some of the trusted basics are still in place. Common mode, (conductor protected to ground), remains the primary protection, and series mode, (conductor to conductor), is widely featured as well. The familiar voltage limiting components, Gas Discharge Tubes (GDTs), Metal Oxide Varistors (MOVs) and Avalanche Diodes (TAZs), usually in combination, are still the heart of the SPD.
Now, these arrestors are faster in operation (switching on), better insulators, and discharge higher transient currents. Two in combination is more effective than the previous combination of three. Also, the maximum surge discharge capability (Imax) is 20kA (8/20µs), even for the compact, terminal-sized SPD, compared with 5-10kA in the past. This doesn’t mean we expect it to discharge higher surges, although it can. It does mean the SPD will last longer i.e., will withstand many more 1kA or 2kA transients. In addition to this, protection for floating or grounded signal lines is offered, recognizing that many DCS I/O signals, especially analog, are floating (not ground referenced).
Once fitted, maintaining good ongoing surge protection has been a labor-intensive activity and for this reason, a less reliable one. The issue is that the arrestors, or voltage limiting components, gradually wear out. The more transients they suppress, or current surges they discharge, the less efficient they become until they degenerate into an interfering additional load in the loop to which they are fitted.
If failed SPDs in I/O systems are not recognized and replaced, users at best should expect measurement errors and poor control. It is therefore critical for the user to regularly monitor the SPD status, so the worn-out arrestors can be replaced in a timely fashion. The manual testing procedure has been a hassle however, and until recently, the SPD had to be disconnected from the measurement or control circuit while electrical checks were made to assess its condition. Large process plants can have thousands of analog and digital I/O loops to be checked. The involvement of the shift manager, coordination with the control room, generation of work permits, and switching to manual control for critical loops, all means that SPD monitoring is a complex maintenance function.
Recent innovations have simplified the maintenance of surge protectors in a variety of ways, based on the ability to automatically detect the degrading condition of a MOV, GDT or TAZ. The thermal disconnect (or trip), using a technology already well proven in power surge protection, is the key component that takes the arrestor out of the circuit when it degrades. The arrestor’s increased leakage current, as it becomes a less efficient insulator, increases the heat generated until the thermal trip operates. Whereas in power applications the trip would disconnect, opening the arrestor circuit to prevent high current flowing from the live conductor to the ground. In signaling applications the trip shorts the protected circuit to ground, and/or between conductors. This action is the subject of a recent mandate in IEC 62305, the international standard for the installation of signaling surge protectors. The goal of making the short circuit is to create a condition which will be noticed by the DCS as a “signal out of range”, generating a system alarm and thereby initiating a job request for a circuit check by a technician, who would then replace the arrestor. The additional benefit of the short circuit is to protect downstream equipment from further transient surges by providing a ready path of low resistance to ground.
Finding and fixing
When the technician arrives at the control panel, knowing which loop needs to be checked because the DCS has identified it, the arrestor-replacement job is simple. Colored markers on the SPD will verify the nominal circuit voltage of the device. Worst-case scenario, the technician will need to disconnect a few cables, test the SPD and re-connect. The SPDs fitted may have pluggable arrestors that confirm which item is to be replaced via their local LED display. Otherwise, with high-density, non-pluggable types the technician may look at the array of surge protectors and see their integral LEDs lit to show positive loop status. If any SPDs are found with LEDs fitted, but not lit, the technician may wish to test the loops. Lever disconnects on the SPD will make testing easy (again, without removing cables) by enabling a test voltage to be applied, which will turn on the LED if the arrestor is in good condition.
A valuable option with pluggable-arrestors is remote status signaling, where the SPD itself generates the call for the technician. This has been long used in power protection and only recently introduced to signaling protection. When the thermal trip is activated it operates the integral indicator and a switch to send a remote signal either into the DCS, the technician’s workshop, or both. To minimize the alarm cabling involved, the pluggable SPDs can be grouped up to ten in series, and connect to a single relay to initiate the remote alert.
Another new timesaving tool and an essential accessory for pluggable SPDs is the dedicated portable test unit. This device, developed to help fulfill the requirements of IEC 62305-3, which calls for periodic inspections for surge protectors, can be used for pluggable signal or power arrestors. The ultimate simplicity of unplugging the installed arrestor from it’s base, inserting it into the test unit, and reading the “OK/NOT OK” display makes the job a potential favorite for the technician – meaning of course that the job stands a good chance of being done!
Despite the additional protection these new SPDs offer, their lifetime is finite and hence protection is never complete. The number of transients occurring over a period of time, and their severity, are usually unknown, and therefore the condition of the arrestors are also unknown. Thus the best method, to effectively protect equipment in applications where lightning is a regular occurrence, is for plant managers to 1) fit suitable SPDs designed for easy testing and 2) test them regularly.