Understanding surge suppression

When thunderstorms occur, do you watch the natural pyrotechnics in awe and wonder? Or do you dread what could be happening back at the plant? Many plants are protected from the effects of lightning and other electrical surges by only luck and hope — thin comfort considering that North America is one of the most lightning-prone areas on the planet.

11/10/2004


When thunderstorms occur, do you watch the natural pyrotechnics in awe and wonder? Or do you dread what could be happening back at the plant?

Many plants are protected from the effects of lightning and other electrical surges by only luck and hope — thin comfort considering that North America is one of the most lightning-prone areas on the planet. It doesn't help that there are many unnatural sources of surges, too, including utility switching operations, noisy equipment, line faults, and other utility users that dump dirty power back onto the grid.

But it doesn't have to be that way. The science and engineering of surge suppression is advancing every year. Proven and reliable technologies protect industrial plants and equipment almost as simply as the surge strip protects your home computer.

Protecting communication and I/O systems

Obviously, power lines and electrical distribution paths within the plant require protection. But don't forget about the others! Each electrical conductor is a potential pathway for surges — not just the ac power lines. Industrial control, factory automation, and electronic communication applications typically use copper wire as medium. For example, a typical I/O system contains many electrical conductors (Fig. 1).


PLCs — the brains involved in many automation and control applications — communicate with sensors, actuators, and instrumentation with discrete digital signals (ON/OFF) or with analog signals (4%%MDASSML%%20 mA and 0%%MDASSML%%10 V). In industrial applications these low voltage communication signals are just as susceptible to surge conditions as the ac power. In traditional process industries (chemical, water and wastewater treatment, petrochemical, or gas/pipeline) the susceptibility is even greater because of the long outdoor cable runs and the effects of nearby or direct lightning strikes.

Also at risk are various communication systems such as RS-232, RS-485, telephone, and Ethernet. These systems use low voltage signals, which make the electronics inside the devices especially susceptible to spikes.

Surge suppression theory

Surge suppression modules limit the magnitude of overvoltage transients to prevent equipment damage. While the cause and prevention of surges has been known for some time, the proliferation of semiconductor devices in recent years has made surge suppression a necessity in industrial facilities.

Most surge protection devices are of the shunt type using components such as metal oxide varistors (MOVs), diodes, and gas tubes. Shunts do not absorb, block, "suck-up" or stop transients. They provide an alternate path to ground for transient energy to follow, so instead of hitting your industrial equipment, it is diverted away.

These surge suppression devices are nonlinear. That means that when subjected to a surge, they change from high impedance (nearly open circuit) to low impedance (nearly short circuit) limiting the voltage seen by the equipment. It also provides a safe path for surge current to flow instead of through the equipment. The equipment is spared the stress of the overvoltage and associated surge current — in theory (Fig. 2).


However, in practice, great care must be taken in how the surge current is diverted to ground or the protection of the system will be compromised!

A surge protection device functions three ways:

  1. It prevents voltage spikes from stressing electrical equipment

  2. It ensures that the surge current flows away from the equipment

  3. It prevents secondary, induced voltages (created from the surge current) from damaging the equipment. (see "How to calculate induced voltage")

    1. Primarily, the grounding methodology, rather than the surge suppression device, determines the effectiveness of the third function.

      The effects of surge current

      The fine print provided with most surge protection devices tells installers to connect to a good ground using thick wire that is as straight as possible, because surge arresters limit overvoltage by creating a short-circuit path to ground (we will ignore normal mode transient protection for the purposes of this discussion). The surge current is supposed to be diverted to ground through the surge protection device — not flashover somewhere else.

      A low-impedance path is critical for the surge protection device to work correctly. A typical result of using a high impedance ground is damaged equipment with no damage to the surge protection device itself. (See "How to calculate induced voltage").

      Proper surge protection device installation

      The path the surge current takes is equally important because a voltage drop is created whenever current flows through a conductor. A worst-case scenario is shown in Fig. 3. In this example, surge current follows a different grounding path from that of the PLC. During a surge, a voltage difference is created between the two ground points, subjecting the PLC to significant overvoltage.


      Fig. 4 shows a better practice. Connecting the ground paths of the surge protection device and the PLC to one point lessens the effect of induced voltage. While an improvement, the PLC will still experience a voltage spike.


      The example circuit in Fig. 5 uses a single-point ground to eliminate the problems associated with multiple ground paths. In the previous example, the common ground — although it is a single point — is independent of both the equipment and the surge protection device, which still provides an opportunity to develop a voltage drop. However, if the installation is grounded at the surge protection device location, its provided ground path, that voltage drop is eliminated and the equipment is protected.


      Conclusion

      When evaluating surge suppression devices, ensure that the manufacturer provides clear grounding instructions. Special emphasis should be placed on the practicality of establishing a grounding system. For example, in industrial control cabinets, DIN rail typically is used to mount control system components. The DIN rail often serves as a ground path. When installing surge protection devices, you should take advantage of the mechanical and electrical advantages of the DIN rail whenever possible.

      Surge protection devices can help keep your plant running without relying on luck. Surges travel through any electrical conductor — not just ac power lines — causing damage to sensitive electronic circuits connected to them. The use of surge protection devices on critical communication and I/O lines is highly recommended. If equipment is installed outdoors, surge suppression is necessary. Proper surge protection device installation and wiring is vital to their future performance.


      Author Information
      Mike Nager is the Industry Marketing Manager for Phoenix Contact Inc., Harrisburg, PA. He holds a BSEE from the University of Scranton and has 15 years experience in industrial control. Mike is also Director of ISA's Food and Pharmaceutical Division. He can be reached at 800-888-7388 or mnager@phoenixcon.com .


      How to calculate induced voltage

      The goal of surge suppression is to divert the energy of the surge away from the protected equipment. It is equally important to ensure that the induced voltage the equipment sees from the surge current is the same everywhere. In other words, if the surge protection device and protected equipment both experience a 500-V voltage rise, there is no voltage difference to cause trouble.

      There are three ways surge currents cause induced voltages:

      1. The clamping characteristic of the surge suppression element itself — With a gas tube as the clamping element, a fixed voltage of 20%%MDASSML%%60 V is created regardless of the current. With other devices, such as MOVs, the voltage created by the surge current is proportional to the amount of current that flows through it:

      V component = 20 V (gas tube)

      2. The dc resistance of the wire itself — Consider a surge current of 10,000 A flowing through a length of conductor (say, 1 m) that has 0.006Ω of resistance. The voltage induced by the dc resistance is calculated as:

      V resistive = I surge x R dc resistance = 10,000 A x 0.006 Ω = 60 V

      The protected equipment will experience a 60—V induced voltage spike because of the resistance of the cable. A surge of 60 V usually isn't considered much of a problem although it may affect sensitive circuits.

      3. The ac resistance, or more specifically, the inductance of the wire at high frequency — The front (leading edge) of a voltage surge is a fast-rising transient that is considered to reach 90% of its value within 8

      V inductive = L wire inductance x di (surge)/dt = 0.000001H x 9000 A/0.000008 sec = 720 V

      The total induced voltage is the sum of all three:

      V total = V component +V resistive +V inductive

      V total = 20 V+60 V+720 V= 800 V

       



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