01 May 2004
Surge shield
By Patrick McCurdy
Ins and outs of surge protection devices.
Today's process control environment is rich with advanced electronic technology that brings benefits such as faster data acquisition, real-time control, and fully automated factories. Unfortunately, a trade-off of the increased performance is the increased susceptibility of these electronic devices to voltage and current transient events. Such power surges are often the work of Mother Nature. Lightning, which by some estimates strikes nearly 40 million times annually in the U.S., is a leading cause of failure in process control systems. Although the most devastating source of transient voltage and current activity is lightning, other sources include static buildup, human error, inductive load switching, and utility capacitor switching.
Companies typically pay attention to protecting the alternating current (AC) power from such damaging power surges, but often the pathway to sensitive electronics lies with the signal and data lines. We think of these as the back door from which surges can enter a control system. The instrumentation and data cables, even though shielded, provide a perfect pathway to havoc in a control system. The minimum results are unreliable instrumentation readings and operation, with periodic failures. The worst-case result is completely destroyed controllers or instrumentation. Depending on the process, both results could have drastic effects on plant reliability and safety.
Circuit design
The basic design of a surge protection device (SPD) limits the magnitude of overvoltage transients to prevent equipment damage. Although quite a few component technologies address surge problems, including metal oxide varistors, silicon avalanche diodes (SADs), and gas discharge tubes (GDTs), typically the best solution involves a combination of these or a hybrid approach.
Generally SPDs are nonlinear devices. When subjected to a surge, they change from a high-impedance state (nearly open circuit) to a low-impedance state (nearly short circuit). A short circuit from line to ground reduces the voltage difference to zero. It also provides a safe path for surge current to flow rather than through the equipment. The equipment is spared the stress of the overvoltages and associated surge currents—in theory. When switches close during a surge, after the surge is diverted, in theory the switches would open again. This happens quickly—in microseconds. In practice, you should take great care and do some planning with circuit design and installation, or you will compromise the system's protection.
When protecting instrumentation signals and communication lines, account for proper circuit design to assure the SPD does not violate the integrity of the signal itself. Consider the structure of the I/O (analog or digital/floating-ground or ground-referenced), the nominal voltage of the signal, and especially in the case of data lines, the speed or frequency of the data rates. The following three figures show different hybrid circuits for different communication structures. (See next page.) The purpose of the hybrid design in these examples is to take advantage of the SAD's fast response time and low-clamping voltage, while coordinating with a high-powered GDT to address the high-surge current values that can be present with lightning events. The components work together to provide low-clamping voltage, fast response, and high-current discharge capability.
The configuration of the components depends on the structure of the I/O. In the case of analog I/O, SADs connect only line to line (or normal mode) so they do not violate the floating ground nature of the signal. For digital I/O (binary signal), SADs and GDTs connect directly to ground, as these are ground-referenced signals with common returns. Therefore, they should be protected in this way. Data signals, which have relatively high switching rates, require a slightly different approach. Here there is the added concern the SPD components should not add a capacitive effect—attenuating the normal data flow. So the circuit design for data applications typically involves a SAD circuit arranged in a bridge configuration, minimizing parasitic capacitance and thus attenuation.
One final SPD circuit design element to consider for instrumentation and data signals is the classification of the environment. Areas classified as hazardous require special certifications and in some cases special designs. SPDs for use in Class I, Division 2, hazardous areas by definition cannot be considered simple apparatus. That is because the nominal voltage rating in normal conditions is always greater than 1.2 volts. Therefore, for SPDs installed in Class I, Division 2, areas in normal enclosures, you will need to conduct special testing and certifications on the SPD to assure the device meets the National Electric Code and National Fire Protection Association parameters for these environments. The testing on the SPD for Class I, Division 2, environments involves measuring temperature rise characteristics and looking for sparking or arcing when tested to the manufacturer's normal load specifications. To know if an SPD has received Class I, Division 2, certifications, look for UL 1604 or FM 3611 approvals in the manufacturer's data sheets.
For Class I, Division 1, hazardous areas, you will often need a special design. For these environments, testers typically treat the SPD as part of an intrinsically safe (IS) circuit and rate it Exi. SPD design requirements should coincide with the requirements of IS devices. This accounts for specific limitations on inductance, capacitance, and voltage. SPDs for use in IS circuits should have isolation values between each protected line and the IS ground of at least 500 volts. This ensures the integrity of the IS ground, and DIN VDE 0165 requires it. SPDs for use in IS systems and Class I, Division 1, environments should have at least FM 3610 or UL 913 approval for use in such circuits.
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Grounding of SPDs for best performance
The installation instructions with most SPDs recommend connection to a good earth ground, using thick wire with low impedance that is as straight and short as possible. The reason? SPDs limit overvoltages by creating a low-impedance path to ground. To simplify the point, let's ignore normal-mode transient protection for now. In the case of lightning-induced surges, you want the surge current to divert to ground through the SPD, not a flashover somewhere else in the system. You also want the only voltage visible across the sensitive load to be the clamping voltage from the SPD.
It is important to cover impedance versus resistance of the ground connection, because transient events have a strong high-frequency component. While under normal signal or 60-hertz AC power conditions, the inductance part of the ground connection is usually negligible in practice, which leaves only resistance as a consideration. However, inductance becomes more important when discussing transient activity—where the waveforms have fast edges and sometimes oscillate in the 100 kilohertz and greater range. An earth connection adequate for normal mains supply frequency may not be suitable for proper SPD operation. Therefore, although it is important to use the cross-section diameter of wiring for the ground connections, its effect on performance is relatively minor when compared with the effect of the wire's length.
When carefully selected and correctly installed, surge protection devices are a key part of any critical process control application. The additional margins of safety and reliability often cover the component costs within the first year, providing a very quick return on investment. Important points to keep in mind when selecting SPDs for signal and data applications include:
- Is the circuit designed specifically for your I/O structure?
- Do the SPDs meet the certification requirements for the environment (hazardous areas)?
- Do the SPDs enable low-impedance ground connections?
- Is the equipment connected properly to the SPD?
Answering these questions will enable you to evaluate SPDs effectively to ensure you will operate at the highest level of safety.
Behind the byline
Patrick McCurdy is product marketing manager for Surge Suppression at Phoenix Contact Inc. in Harrisburg, Pa.
Three scenariosEach example below shows a generic electronic load that in reality could be a programmable logic controller, PC, distributed control system, analyzer, transmitter, or controller. The analyzed surge event is an 8-kiloampere, 8-by-20-microsecond (µs) surge current impulse event. IEC 60060-1 and IEEE C62.41 standardize this test waveform, and the industry widely accepts it as a typical value and waveshape present in the electromagnetic environment. Each scenario calculates what voltage will appear in common mode across the sensitive electronics. This voltage represents the voltage above the nominal voltage of the system and above the suppression voltage of the SPD. In all three scenarios the electronic equipment will function properly with no surge condition present. We can assume the SPD is a design with a grounding connection via the DIN-rail foot, which provides excellent low-impedance connections. However, we should also assume the DIN rail is isolated from any backplane grounding (to prove the point about the negative effect of impedance on SPD performance). General assumptions
Scenario 1 (least preferred)Equipment grounded separately from the SPD ground. This scenario examines the additional voltage that will appear across the sensitive electronics during a surge event shunted to ground by the SPD. In this scenario the foot element of the SPD makes electrical contact with the DIN rail and functions as the surge current discharge path. The grounding wire is then run from the SPD directly to a 0-volt system ground reference. When a surge is shunted by the SPD, a rise in potential from the earth will also affect the sensitive electronics—using a separate ground connection. We'll calculate the induced voltage the load sees by determining the voltage drop across the grounding of the SPD. Because we are looking at the voltage effects under a transient condition, we'll use the formulas applying transient values for time and current. Voltage stress on equipment (VE) = = VSPD + VTransient discharge through SPD = VSPD + L(di/dt) + R(It) = VSPD + 1m(1µH/m)(7,200A/8µs) + (0.0068Ω/m)(7,200A) = VSPD + 900V + 48.96V = VSPD + 948.9V The surge current causes a significant rise in the voltage across the equipment ground. Because the equipment ground and the SPD ground are at different points, a voltage difference of an additional 948.9 volts occurs across the equipment (common mode) above the voltage of the surge protection device. The SPD functioned as it was intended, but if the isolation voltage of the sensitive electronics was less than 948 volts, damage would still likely occur! Scenario 2 (better practice)Grounding jumper used between sensitive equipment and SPD. Both sensitive equipment and the SPD still have separate conductors (or pathways) to the system ground. You might not be able to avoid this, but you can improve the situation in scenario one by adding a jumper between the sensitive equipment ground and the SPD ground connection. This provides an additional pathway for the surge current to follow and will still cause a rise in voltage potential of the sensitive equipment relative to ground. However, the effect will be limited to the voltage across the jumper wire. As this can depend on the distance, you should minimize the wire length. In our example we assume the length of jumper wire is 0.5 meters (500 centimeters). We also assume the surge current will divide equally between the SPD ground wire and the jumper and equipment ground. Voltage stress on equipment (VE) = = VSPD + VJumper = VSPD + L(di/dt) + R(It) = -VSPD + 0.5m(1m)(1µH/m)(7,200A*0.5)/8µs + 0.5m(0.0068Ω/m)(7,200A*0.5) = VSPD + 225V+12.24V = VSPD + 237.2V By adding the jumper wire between the SPD ground and the equipment ground, the SPD and the equipment ground now share the surge current. Therefore, the voltage difference is limited to that across the jumper wire. The shorter this jumper wire is, the lower the additional voltage across the equipment will be. Scenario 3 (preferred practice)Sensitive electronic equipment is grounded through the SPD ground. We make the equipment ground of the sensitive electronic equipment through the SPD's ground connection to the system ground. Now in our calculation, the effect of the surge current shunting to ground only appears across the SPD. This is because the voltage rise of the system ground occurs equally with the SPD and the equipment; therefore, no potential difference occurs and we can see no additional voltage across the equipment. The voltage stress on the equipment is limited to the protection level of the SPD. Voltage stress on equipment (VE) = = VSPD + VL3 = VSPD + 0V = VSPD Clearly the preferred method of installing SPDs is to ground the sensitive equipment you are protecting through the SPD. This ensures the only transient voltage the load will see is that of the clamping voltage across the SPD itself. In any of the previous examples, you can enhance performance by using SPDs with a DIN-rail grounding foot, provided the mounting backplane is a suitable ground path (as well as the ground path of the equipment).
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